core/fmt/
mod.rs

1//! Utilities for formatting and printing strings.
2
3#![stable(feature = "rust1", since = "1.0.0")]
4
5use crate::cell::{Cell, Ref, RefCell, RefMut, SyncUnsafeCell, UnsafeCell};
6use crate::char::{EscapeDebugExtArgs, MAX_LEN_UTF8};
7use crate::marker::PhantomData;
8use crate::num::fmt as numfmt;
9use crate::ops::Deref;
10use crate::{iter, result, str};
11
12mod builders;
13#[cfg(not(no_fp_fmt_parse))]
14mod float;
15#[cfg(no_fp_fmt_parse)]
16mod nofloat;
17mod num;
18mod rt;
19
20#[stable(feature = "fmt_flags_align", since = "1.28.0")]
21#[rustc_diagnostic_item = "Alignment"]
22/// Possible alignments returned by `Formatter::align`
23#[derive(Copy, Clone, Debug, PartialEq, Eq)]
24pub enum Alignment {
25    #[stable(feature = "fmt_flags_align", since = "1.28.0")]
26    /// Indication that contents should be left-aligned.
27    Left,
28    #[stable(feature = "fmt_flags_align", since = "1.28.0")]
29    /// Indication that contents should be right-aligned.
30    Right,
31    #[stable(feature = "fmt_flags_align", since = "1.28.0")]
32    /// Indication that contents should be center-aligned.
33    Center,
34}
35
36#[stable(feature = "debug_builders", since = "1.2.0")]
37pub use self::builders::{DebugList, DebugMap, DebugSet, DebugStruct, DebugTuple};
38#[unstable(feature = "debug_closure_helpers", issue = "117729")]
39pub use self::builders::{FromFn, from_fn};
40
41/// The type returned by formatter methods.
42///
43/// # Examples
44///
45/// ```
46/// use std::fmt;
47///
48/// #[derive(Debug)]
49/// struct Triangle {
50///     a: f32,
51///     b: f32,
52///     c: f32
53/// }
54///
55/// impl fmt::Display for Triangle {
56///     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
57///         write!(f, "({}, {}, {})", self.a, self.b, self.c)
58///     }
59/// }
60///
61/// let pythagorean_triple = Triangle { a: 3.0, b: 4.0, c: 5.0 };
62///
63/// assert_eq!(format!("{pythagorean_triple}"), "(3, 4, 5)");
64/// ```
65#[stable(feature = "rust1", since = "1.0.0")]
66pub type Result = result::Result<(), Error>;
67
68/// The error type which is returned from formatting a message into a stream.
69///
70/// This type does not support transmission of an error other than that an error
71/// occurred. This is because, despite the existence of this error,
72/// string formatting is considered an infallible operation.
73/// `fmt()` implementors should not return this `Error` unless they received it from their
74/// [`Formatter`]. The only time your code should create a new instance of this
75/// error is when implementing `fmt::Write`, in order to cancel the formatting operation when
76/// writing to the underlying stream fails.
77///
78/// Any extra information must be arranged to be transmitted through some other means,
79/// such as storing it in a field to be consulted after the formatting operation has been
80/// cancelled. (For example, this is how [`std::io::Write::write_fmt()`] propagates IO errors
81/// during writing.)
82///
83/// This type, `fmt::Error`, should not be
84/// confused with [`std::io::Error`] or [`std::error::Error`], which you may also
85/// have in scope.
86///
87/// [`std::io::Error`]: ../../std/io/struct.Error.html
88/// [`std::io::Write::write_fmt()`]: ../../std/io/trait.Write.html#method.write_fmt
89/// [`std::error::Error`]: ../../std/error/trait.Error.html
90///
91/// # Examples
92///
93/// ```rust
94/// use std::fmt::{self, write};
95///
96/// let mut output = String::new();
97/// if let Err(fmt::Error) = write(&mut output, format_args!("Hello {}!", "world")) {
98///     panic!("An error occurred");
99/// }
100/// ```
101#[stable(feature = "rust1", since = "1.0.0")]
102#[derive(Copy, Clone, Debug, Default, Eq, Hash, Ord, PartialEq, PartialOrd)]
103pub struct Error;
104
105/// A trait for writing or formatting into Unicode-accepting buffers or streams.
106///
107/// This trait only accepts UTF-8–encoded data and is not [flushable]. If you only
108/// want to accept Unicode and you don't need flushing, you should implement this trait;
109/// otherwise you should implement [`std::io::Write`].
110///
111/// [`std::io::Write`]: ../../std/io/trait.Write.html
112/// [flushable]: ../../std/io/trait.Write.html#tymethod.flush
113#[stable(feature = "rust1", since = "1.0.0")]
114pub trait Write {
115    /// Writes a string slice into this writer, returning whether the write
116    /// succeeded.
117    ///
118    /// This method can only succeed if the entire string slice was successfully
119    /// written, and this method will not return until all data has been
120    /// written or an error occurs.
121    ///
122    /// # Errors
123    ///
124    /// This function will return an instance of [`std::fmt::Error`][Error] on error.
125    ///
126    /// The purpose of that error is to abort the formatting operation when the underlying
127    /// destination encounters some error preventing it from accepting more text;
128    /// in particular, it does not communicate any information about *what* error occurred.
129    /// It should generally be propagated rather than handled, at least when implementing
130    /// formatting traits.
131    ///
132    /// # Examples
133    ///
134    /// ```
135    /// use std::fmt::{Error, Write};
136    ///
137    /// fn writer<W: Write>(f: &mut W, s: &str) -> Result<(), Error> {
138    ///     f.write_str(s)
139    /// }
140    ///
141    /// let mut buf = String::new();
142    /// writer(&mut buf, "hola")?;
143    /// assert_eq!(&buf, "hola");
144    /// # std::fmt::Result::Ok(())
145    /// ```
146    #[stable(feature = "rust1", since = "1.0.0")]
147    fn write_str(&mut self, s: &str) -> Result;
148
149    /// Writes a [`char`] into this writer, returning whether the write succeeded.
150    ///
151    /// A single [`char`] may be encoded as more than one byte.
152    /// This method can only succeed if the entire byte sequence was successfully
153    /// written, and this method will not return until all data has been
154    /// written or an error occurs.
155    ///
156    /// # Errors
157    ///
158    /// This function will return an instance of [`Error`] on error.
159    ///
160    /// # Examples
161    ///
162    /// ```
163    /// use std::fmt::{Error, Write};
164    ///
165    /// fn writer<W: Write>(f: &mut W, c: char) -> Result<(), Error> {
166    ///     f.write_char(c)
167    /// }
168    ///
169    /// let mut buf = String::new();
170    /// writer(&mut buf, 'a')?;
171    /// writer(&mut buf, 'b')?;
172    /// assert_eq!(&buf, "ab");
173    /// # std::fmt::Result::Ok(())
174    /// ```
175    #[stable(feature = "fmt_write_char", since = "1.1.0")]
176    fn write_char(&mut self, c: char) -> Result {
177        self.write_str(c.encode_utf8(&mut [0; MAX_LEN_UTF8]))
178    }
179
180    /// Glue for usage of the [`write!`] macro with implementors of this trait.
181    ///
182    /// This method should generally not be invoked manually, but rather through
183    /// the [`write!`] macro itself.
184    ///
185    /// # Errors
186    ///
187    /// This function will return an instance of [`Error`] on error. Please see
188    /// [write_str](Write::write_str) for details.
189    ///
190    /// # Examples
191    ///
192    /// ```
193    /// use std::fmt::{Error, Write};
194    ///
195    /// fn writer<W: Write>(f: &mut W, s: &str) -> Result<(), Error> {
196    ///     f.write_fmt(format_args!("{s}"))
197    /// }
198    ///
199    /// let mut buf = String::new();
200    /// writer(&mut buf, "world")?;
201    /// assert_eq!(&buf, "world");
202    /// # std::fmt::Result::Ok(())
203    /// ```
204    #[stable(feature = "rust1", since = "1.0.0")]
205    fn write_fmt(&mut self, args: Arguments<'_>) -> Result {
206        // We use a specialization for `Sized` types to avoid an indirection
207        // through `&mut self`
208        trait SpecWriteFmt {
209            fn spec_write_fmt(self, args: Arguments<'_>) -> Result;
210        }
211
212        impl<W: Write + ?Sized> SpecWriteFmt for &mut W {
213            #[inline]
214            default fn spec_write_fmt(mut self, args: Arguments<'_>) -> Result {
215                if let Some(s) = args.as_statically_known_str() {
216                    self.write_str(s)
217                } else {
218                    write(&mut self, args)
219                }
220            }
221        }
222
223        impl<W: Write> SpecWriteFmt for &mut W {
224            #[inline]
225            fn spec_write_fmt(self, args: Arguments<'_>) -> Result {
226                if let Some(s) = args.as_statically_known_str() {
227                    self.write_str(s)
228                } else {
229                    write(self, args)
230                }
231            }
232        }
233
234        self.spec_write_fmt(args)
235    }
236}
237
238#[stable(feature = "fmt_write_blanket_impl", since = "1.4.0")]
239impl<W: Write + ?Sized> Write for &mut W {
240    fn write_str(&mut self, s: &str) -> Result {
241        (**self).write_str(s)
242    }
243
244    fn write_char(&mut self, c: char) -> Result {
245        (**self).write_char(c)
246    }
247
248    fn write_fmt(&mut self, args: Arguments<'_>) -> Result {
249        (**self).write_fmt(args)
250    }
251}
252
253/// The signedness of a [`Formatter`] (or of a [`FormattingOptions`]).
254#[derive(Copy, Clone, Debug, PartialEq, Eq)]
255#[unstable(feature = "formatting_options", issue = "118117")]
256pub enum Sign {
257    /// Represents the `+` flag.
258    Plus,
259    /// Represents the `-` flag.
260    Minus,
261}
262
263/// Specifies whether the [`Debug`] trait should use lower-/upper-case
264/// hexadecimal or normal integers.
265#[derive(Copy, Clone, Debug, PartialEq, Eq)]
266#[unstable(feature = "formatting_options", issue = "118117")]
267pub enum DebugAsHex {
268    /// Use lower-case hexadecimal integers for the `Debug` trait (like [the `x?` type](../../std/fmt/index.html#formatting-traits)).
269    Lower,
270    /// Use upper-case hexadecimal integers for the `Debug` trait (like [the `X?` type](../../std/fmt/index.html#formatting-traits)).
271    Upper,
272}
273
274/// Options for formatting.
275///
276/// `FormattingOptions` is a [`Formatter`] without an attached [`Write`] trait.
277/// It is mainly used to construct `Formatter` instances.
278#[derive(Copy, Clone, Debug, PartialEq, Eq)]
279#[unstable(feature = "formatting_options", issue = "118117")]
280pub struct FormattingOptions {
281    /// Flags, with the following bit fields:
282    ///
283    /// ```text
284    ///   31  30  29  28  27  26  25  24  23  22  21  20                              0
285    /// ┌───┬───────┬───┬───┬───┬───┬───┬───┬───┬───┬──────────────────────────────────┐
286    /// │ 1 │ align │ p │ w │ X?│ x?│'0'│ # │ - │ + │               fill               │
287    /// └───┴───────┴───┴───┴───┴───┴───┴───┴───┴───┴──────────────────────────────────┘
288    ///   │     │     │   │  └─┬───────────────────┘ └─┬──────────────────────────────┘
289    ///   │     │     │   │    │                       └─ The fill character (21 bits char).
290    ///   │     │     │   │    └─ The debug upper/lower hex, zero pad, alternate, and plus/minus flags.
291    ///   │     │     │   └─ Whether a width is set. (The value is stored separately.)
292    ///   │     │     └─ Whether a precision is set. (The value is stored separately.)
293    ///   │     ├─ 0: Align left. (<)
294    ///   │     ├─ 1: Align right. (>)
295    ///   │     ├─ 2: Align center. (^)
296    ///   │     └─ 3: Alignment not set. (default)
297    ///   └─ Always set.
298    ///      This makes it possible to distinguish formatting flags from
299    ///      a &str size when stored in (the upper bits of) the same field.
300    ///      (fmt::Arguments will make use of this property in the future.)
301    /// ```
302    // Note: This could use a special niche type with range 0x8000_0000..=0xfdd0ffff.
303    // It's unclear if that's useful, though.
304    flags: u32,
305    /// Width if width flag (bit 27) above is set. Otherwise, always 0.
306    width: u16,
307    /// Precision if precision flag (bit 28) above is set. Otherwise, always 0.
308    precision: u16,
309}
310
311// This needs to match with compiler/rustc_ast_lowering/src/format.rs.
312mod flags {
313    pub(super) const SIGN_PLUS_FLAG: u32 = 1 << 21;
314    pub(super) const SIGN_MINUS_FLAG: u32 = 1 << 22;
315    pub(super) const ALTERNATE_FLAG: u32 = 1 << 23;
316    pub(super) const SIGN_AWARE_ZERO_PAD_FLAG: u32 = 1 << 24;
317    pub(super) const DEBUG_LOWER_HEX_FLAG: u32 = 1 << 25;
318    pub(super) const DEBUG_UPPER_HEX_FLAG: u32 = 1 << 26;
319    pub(super) const WIDTH_FLAG: u32 = 1 << 27;
320    pub(super) const PRECISION_FLAG: u32 = 1 << 28;
321    pub(super) const ALIGN_BITS: u32 = 0b11 << 29;
322    pub(super) const ALIGN_LEFT: u32 = 0 << 29;
323    pub(super) const ALIGN_RIGHT: u32 = 1 << 29;
324    pub(super) const ALIGN_CENTER: u32 = 2 << 29;
325    pub(super) const ALIGN_UNKNOWN: u32 = 3 << 29;
326    pub(super) const ALWAYS_SET: u32 = 1 << 31;
327}
328
329impl FormattingOptions {
330    /// Construct a new `FormatterBuilder` with the supplied `Write` trait
331    /// object for output that is equivalent to the `{}` formatting
332    /// specifier:
333    ///
334    /// - no flags,
335    /// - filled with spaces,
336    /// - no alignment,
337    /// - no width,
338    /// - no precision, and
339    /// - no [`DebugAsHex`] output mode.
340    #[unstable(feature = "formatting_options", issue = "118117")]
341    pub const fn new() -> Self {
342        Self {
343            flags: ' ' as u32 | flags::ALIGN_UNKNOWN | flags::ALWAYS_SET,
344            width: 0,
345            precision: 0,
346        }
347    }
348
349    /// Sets or removes the sign (the `+` or the `-` flag).
350    ///
351    /// - `+`: This is intended for numeric types and indicates that the sign
352    /// should always be printed. By default only the negative sign of signed
353    /// values is printed, and the sign of positive or unsigned values is
354    /// omitted. This flag indicates that the correct sign (+ or -) should
355    /// always be printed.
356    /// - `-`: Currently not used
357    #[unstable(feature = "formatting_options", issue = "118117")]
358    pub fn sign(&mut self, sign: Option<Sign>) -> &mut Self {
359        let sign = match sign {
360            None => 0,
361            Some(Sign::Plus) => flags::SIGN_PLUS_FLAG,
362            Some(Sign::Minus) => flags::SIGN_MINUS_FLAG,
363        };
364        self.flags = self.flags & !(flags::SIGN_PLUS_FLAG | flags::SIGN_MINUS_FLAG) | sign;
365        self
366    }
367    /// Sets or unsets the `0` flag.
368    ///
369    /// This is used to indicate for integer formats that the padding to width should both be done with a 0 character as well as be sign-aware
370    #[unstable(feature = "formatting_options", issue = "118117")]
371    pub fn sign_aware_zero_pad(&mut self, sign_aware_zero_pad: bool) -> &mut Self {
372        if sign_aware_zero_pad {
373            self.flags |= flags::SIGN_AWARE_ZERO_PAD_FLAG;
374        } else {
375            self.flags &= !flags::SIGN_AWARE_ZERO_PAD_FLAG;
376        }
377        self
378    }
379    /// Sets or unsets the `#` flag.
380    ///
381    /// This flag indicates that the "alternate" form of printing should be
382    /// used. The alternate forms are:
383    /// - [`Debug`] : pretty-print the [`Debug`] formatting (adds linebreaks and indentation)
384    /// - [`LowerHex`] as well as [`UpperHex`] - precedes the argument with a `0x`
385    /// - [`Octal`] - precedes the argument with a `0b`
386    /// - [`Binary`] - precedes the argument with a `0o`
387    #[unstable(feature = "formatting_options", issue = "118117")]
388    pub fn alternate(&mut self, alternate: bool) -> &mut Self {
389        if alternate {
390            self.flags |= flags::ALTERNATE_FLAG;
391        } else {
392            self.flags &= !flags::ALTERNATE_FLAG;
393        }
394        self
395    }
396    /// Sets the fill character.
397    ///
398    /// The optional fill character and alignment is provided normally in
399    /// conjunction with the width parameter. This indicates that if the value
400    /// being formatted is smaller than width some extra characters will be
401    /// printed around it.
402    #[unstable(feature = "formatting_options", issue = "118117")]
403    pub fn fill(&mut self, fill: char) -> &mut Self {
404        self.flags = self.flags & (u32::MAX << 21) | fill as u32;
405        self
406    }
407    /// Sets or removes the alignment.
408    ///
409    /// The alignment specifies how the value being formatted should be
410    /// positioned if it is smaller than the width of the formatter.
411    #[unstable(feature = "formatting_options", issue = "118117")]
412    pub fn align(&mut self, align: Option<Alignment>) -> &mut Self {
413        let align: u32 = match align {
414            Some(Alignment::Left) => flags::ALIGN_LEFT,
415            Some(Alignment::Right) => flags::ALIGN_RIGHT,
416            Some(Alignment::Center) => flags::ALIGN_CENTER,
417            None => flags::ALIGN_UNKNOWN,
418        };
419        self.flags = self.flags & !flags::ALIGN_BITS | align;
420        self
421    }
422    /// Sets or removes the width.
423    ///
424    /// This is a parameter for the “minimum width” that the format should take
425    /// up. If the value’s string does not fill up this many characters, then
426    /// the padding specified by [`FormattingOptions::fill`]/[`FormattingOptions::align`]
427    /// will be used to take up the required space.
428    #[unstable(feature = "formatting_options", issue = "118117")]
429    pub fn width(&mut self, width: Option<u16>) -> &mut Self {
430        if let Some(width) = width {
431            self.flags |= flags::WIDTH_FLAG;
432            self.width = width;
433        } else {
434            self.flags &= !flags::WIDTH_FLAG;
435            self.width = 0;
436        }
437        self
438    }
439    /// Sets or removes the precision.
440    ///
441    /// - For non-numeric types, this can be considered a “maximum width”. If
442    /// the resulting string is longer than this width, then it is truncated
443    /// down to this many characters and that truncated value is emitted with
444    /// proper fill, alignment and width if those parameters are set.
445    /// - For integral types, this is ignored.
446    /// - For floating-point types, this indicates how many digits after the
447    /// decimal point should be printed.
448    #[unstable(feature = "formatting_options", issue = "118117")]
449    pub fn precision(&mut self, precision: Option<u16>) -> &mut Self {
450        if let Some(precision) = precision {
451            self.flags |= flags::PRECISION_FLAG;
452            self.precision = precision;
453        } else {
454            self.flags &= !flags::PRECISION_FLAG;
455            self.precision = 0;
456        }
457        self
458    }
459    /// Specifies whether the [`Debug`] trait should use lower-/upper-case
460    /// hexadecimal or normal integers
461    #[unstable(feature = "formatting_options", issue = "118117")]
462    pub fn debug_as_hex(&mut self, debug_as_hex: Option<DebugAsHex>) -> &mut Self {
463        let debug_as_hex = match debug_as_hex {
464            None => 0,
465            Some(DebugAsHex::Lower) => flags::DEBUG_LOWER_HEX_FLAG,
466            Some(DebugAsHex::Upper) => flags::DEBUG_UPPER_HEX_FLAG,
467        };
468        self.flags = self.flags & !(flags::DEBUG_LOWER_HEX_FLAG | flags::DEBUG_UPPER_HEX_FLAG)
469            | debug_as_hex;
470        self
471    }
472
473    /// Returns the current sign (the `+` or the `-` flag).
474    #[unstable(feature = "formatting_options", issue = "118117")]
475    pub const fn get_sign(&self) -> Option<Sign> {
476        if self.flags & flags::SIGN_PLUS_FLAG != 0 {
477            Some(Sign::Plus)
478        } else if self.flags & flags::SIGN_MINUS_FLAG != 0 {
479            Some(Sign::Minus)
480        } else {
481            None
482        }
483    }
484    /// Returns the current `0` flag.
485    #[unstable(feature = "formatting_options", issue = "118117")]
486    pub const fn get_sign_aware_zero_pad(&self) -> bool {
487        self.flags & flags::SIGN_AWARE_ZERO_PAD_FLAG != 0
488    }
489    /// Returns the current `#` flag.
490    #[unstable(feature = "formatting_options", issue = "118117")]
491    pub const fn get_alternate(&self) -> bool {
492        self.flags & flags::ALTERNATE_FLAG != 0
493    }
494    /// Returns the current fill character.
495    #[unstable(feature = "formatting_options", issue = "118117")]
496    pub const fn get_fill(&self) -> char {
497        // SAFETY: We only ever put a valid `char` in the lower 21 bits of the flags field.
498        unsafe { char::from_u32_unchecked(self.flags & 0x1FFFFF) }
499    }
500    /// Returns the current alignment.
501    #[unstable(feature = "formatting_options", issue = "118117")]
502    pub const fn get_align(&self) -> Option<Alignment> {
503        match self.flags & flags::ALIGN_BITS {
504            flags::ALIGN_LEFT => Some(Alignment::Left),
505            flags::ALIGN_RIGHT => Some(Alignment::Right),
506            flags::ALIGN_CENTER => Some(Alignment::Center),
507            _ => None,
508        }
509    }
510    /// Returns the current width.
511    #[unstable(feature = "formatting_options", issue = "118117")]
512    pub const fn get_width(&self) -> Option<u16> {
513        if self.flags & flags::WIDTH_FLAG != 0 { Some(self.width) } else { None }
514    }
515    /// Returns the current precision.
516    #[unstable(feature = "formatting_options", issue = "118117")]
517    pub const fn get_precision(&self) -> Option<u16> {
518        if self.flags & flags::PRECISION_FLAG != 0 { Some(self.precision) } else { None }
519    }
520    /// Returns the current precision.
521    #[unstable(feature = "formatting_options", issue = "118117")]
522    pub const fn get_debug_as_hex(&self) -> Option<DebugAsHex> {
523        if self.flags & flags::DEBUG_LOWER_HEX_FLAG != 0 {
524            Some(DebugAsHex::Lower)
525        } else if self.flags & flags::DEBUG_UPPER_HEX_FLAG != 0 {
526            Some(DebugAsHex::Upper)
527        } else {
528            None
529        }
530    }
531
532    /// Creates a [`Formatter`] that writes its output to the given [`Write`] trait.
533    ///
534    /// You may alternatively use [`Formatter::new()`].
535    #[unstable(feature = "formatting_options", issue = "118117")]
536    pub fn create_formatter<'a>(self, write: &'a mut (dyn Write + 'a)) -> Formatter<'a> {
537        Formatter { options: self, buf: write }
538    }
539}
540
541#[unstable(feature = "formatting_options", issue = "118117")]
542impl Default for FormattingOptions {
543    /// Same as [`FormattingOptions::new()`].
544    fn default() -> Self {
545        // The `#[derive(Default)]` implementation would set `fill` to `\0` instead of space.
546        Self::new()
547    }
548}
549
550/// Configuration for formatting.
551///
552/// A `Formatter` represents various options related to formatting. Users do not
553/// construct `Formatter`s directly; a mutable reference to one is passed to
554/// the `fmt` method of all formatting traits, like [`Debug`] and [`Display`].
555///
556/// To interact with a `Formatter`, you'll call various methods to change the
557/// various options related to formatting. For examples, please see the
558/// documentation of the methods defined on `Formatter` below.
559#[allow(missing_debug_implementations)]
560#[stable(feature = "rust1", since = "1.0.0")]
561#[rustc_diagnostic_item = "Formatter"]
562pub struct Formatter<'a> {
563    options: FormattingOptions,
564
565    buf: &'a mut (dyn Write + 'a),
566}
567
568impl<'a> Formatter<'a> {
569    /// Creates a new formatter with given [`FormattingOptions`].
570    ///
571    /// If `write` is a reference to a formatter, it is recommended to use
572    /// [`Formatter::with_options`] instead as this can borrow the underlying
573    /// `write`, thereby bypassing one layer of indirection.
574    ///
575    /// You may alternatively use [`FormattingOptions::create_formatter()`].
576    #[unstable(feature = "formatting_options", issue = "118117")]
577    pub fn new(write: &'a mut (dyn Write + 'a), options: FormattingOptions) -> Self {
578        Formatter { options, buf: write }
579    }
580
581    /// Creates a new formatter based on this one with given [`FormattingOptions`].
582    #[unstable(feature = "formatting_options", issue = "118117")]
583    pub fn with_options<'b>(&'b mut self, options: FormattingOptions) -> Formatter<'b> {
584        Formatter { options, buf: self.buf }
585    }
586}
587
588/// This structure represents a safely precompiled version of a format string
589/// and its arguments. This cannot be generated at runtime because it cannot
590/// safely be done, so no constructors are given and the fields are private
591/// to prevent modification.
592///
593/// The [`format_args!`] macro will safely create an instance of this structure.
594/// The macro validates the format string at compile-time so usage of the
595/// [`write()`] and [`format()`] functions can be safely performed.
596///
597/// You can use the `Arguments<'a>` that [`format_args!`] returns in `Debug`
598/// and `Display` contexts as seen below. The example also shows that `Debug`
599/// and `Display` format to the same thing: the interpolated format string
600/// in `format_args!`.
601///
602/// ```rust
603/// let debug = format!("{:?}", format_args!("{} foo {:?}", 1, 2));
604/// let display = format!("{}", format_args!("{} foo {:?}", 1, 2));
605/// assert_eq!("1 foo 2", display);
606/// assert_eq!(display, debug);
607/// ```
608///
609/// [`format()`]: ../../std/fmt/fn.format.html
610#[lang = "format_arguments"]
611#[stable(feature = "rust1", since = "1.0.0")]
612#[derive(Copy, Clone)]
613pub struct Arguments<'a> {
614    // Format string pieces to print.
615    pieces: &'a [&'static str],
616
617    // Placeholder specs, or `None` if all specs are default (as in "{}{}").
618    fmt: Option<&'a [rt::Placeholder]>,
619
620    // Dynamic arguments for interpolation, to be interleaved with string
621    // pieces. (Every argument is preceded by a string piece.)
622    args: &'a [rt::Argument<'a>],
623}
624
625/// Used by the format_args!() macro to create a fmt::Arguments object.
626#[doc(hidden)]
627#[unstable(feature = "fmt_internals", issue = "none")]
628impl<'a> Arguments<'a> {
629    #[inline]
630    pub const fn new_const<const N: usize>(pieces: &'a [&'static str; N]) -> Self {
631        const { assert!(N <= 1) };
632        Arguments { pieces, fmt: None, args: &[] }
633    }
634
635    /// When using the format_args!() macro, this function is used to generate the
636    /// Arguments structure.
637    #[inline]
638    pub const fn new_v1<const P: usize, const A: usize>(
639        pieces: &'a [&'static str; P],
640        args: &'a [rt::Argument<'a>; A],
641    ) -> Arguments<'a> {
642        const { assert!(P >= A && P <= A + 1, "invalid args") }
643        Arguments { pieces, fmt: None, args }
644    }
645
646    /// Specifies nonstandard formatting parameters.
647    ///
648    /// An `rt::UnsafeArg` is required because the following invariants must be held
649    /// in order for this function to be safe:
650    /// 1. The `pieces` slice must be at least as long as `fmt`.
651    /// 2. Every `rt::Placeholder::position` value within `fmt` must be a valid index of `args`.
652    /// 3. Every `rt::Count::Param` within `fmt` must contain a valid index of `args`.
653    #[inline]
654    pub const fn new_v1_formatted(
655        pieces: &'a [&'static str],
656        args: &'a [rt::Argument<'a>],
657        fmt: &'a [rt::Placeholder],
658        _unsafe_arg: rt::UnsafeArg,
659    ) -> Arguments<'a> {
660        Arguments { pieces, fmt: Some(fmt), args }
661    }
662
663    /// Estimates the length of the formatted text.
664    ///
665    /// This is intended to be used for setting initial `String` capacity
666    /// when using `format!`. Note: this is neither the lower nor upper bound.
667    #[inline]
668    pub fn estimated_capacity(&self) -> usize {
669        let pieces_length: usize = self.pieces.iter().map(|x| x.len()).sum();
670
671        if self.args.is_empty() {
672            pieces_length
673        } else if !self.pieces.is_empty() && self.pieces[0].is_empty() && pieces_length < 16 {
674            // If the format string starts with an argument,
675            // don't preallocate anything, unless length
676            // of pieces is significant.
677            0
678        } else {
679            // There are some arguments, so any additional push
680            // will reallocate the string. To avoid that,
681            // we're "pre-doubling" the capacity here.
682            pieces_length.checked_mul(2).unwrap_or(0)
683        }
684    }
685}
686
687impl<'a> Arguments<'a> {
688    /// Gets the formatted string, if it has no arguments to be formatted at runtime.
689    ///
690    /// This can be used to avoid allocations in some cases.
691    ///
692    /// # Guarantees
693    ///
694    /// For `format_args!("just a literal")`, this function is guaranteed to
695    /// return `Some("just a literal")`.
696    ///
697    /// For most cases with placeholders, this function will return `None`.
698    ///
699    /// However, the compiler may perform optimizations that can cause this
700    /// function to return `Some(_)` even if the format string contains
701    /// placeholders. For example, `format_args!("Hello, {}!", "world")` may be
702    /// optimized to `format_args!("Hello, world!")`, such that `as_str()`
703    /// returns `Some("Hello, world!")`.
704    ///
705    /// The behavior for anything but the trivial case (without placeholders)
706    /// is not guaranteed, and should not be relied upon for anything other
707    /// than optimization.
708    ///
709    /// # Examples
710    ///
711    /// ```rust
712    /// use std::fmt::Arguments;
713    ///
714    /// fn write_str(_: &str) { /* ... */ }
715    ///
716    /// fn write_fmt(args: &Arguments<'_>) {
717    ///     if let Some(s) = args.as_str() {
718    ///         write_str(s)
719    ///     } else {
720    ///         write_str(&args.to_string());
721    ///     }
722    /// }
723    /// ```
724    ///
725    /// ```rust
726    /// assert_eq!(format_args!("hello").as_str(), Some("hello"));
727    /// assert_eq!(format_args!("").as_str(), Some(""));
728    /// assert_eq!(format_args!("{:?}", std::env::current_dir()).as_str(), None);
729    /// ```
730    #[stable(feature = "fmt_as_str", since = "1.52.0")]
731    #[rustc_const_stable(feature = "const_arguments_as_str", since = "1.84.0")]
732    #[must_use]
733    #[inline]
734    pub const fn as_str(&self) -> Option<&'static str> {
735        match (self.pieces, self.args) {
736            ([], []) => Some(""),
737            ([s], []) => Some(s),
738            _ => None,
739        }
740    }
741
742    /// Same as [`Arguments::as_str`], but will only return `Some(s)` if it can be determined at compile time.
743    #[unstable(feature = "fmt_internals", reason = "internal to standard library", issue = "none")]
744    #[must_use]
745    #[inline]
746    #[doc(hidden)]
747    pub fn as_statically_known_str(&self) -> Option<&'static str> {
748        let s = self.as_str();
749        if core::intrinsics::is_val_statically_known(s.is_some()) { s } else { None }
750    }
751}
752
753// Manually implementing these results in better error messages.
754#[stable(feature = "rust1", since = "1.0.0")]
755impl !Send for Arguments<'_> {}
756#[stable(feature = "rust1", since = "1.0.0")]
757impl !Sync for Arguments<'_> {}
758
759#[stable(feature = "rust1", since = "1.0.0")]
760impl Debug for Arguments<'_> {
761    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result {
762        Display::fmt(self, fmt)
763    }
764}
765
766#[stable(feature = "rust1", since = "1.0.0")]
767impl Display for Arguments<'_> {
768    fn fmt(&self, fmt: &mut Formatter<'_>) -> Result {
769        write(fmt.buf, *self)
770    }
771}
772
773/// `?` formatting.
774///
775/// `Debug` should format the output in a programmer-facing, debugging context.
776///
777/// Generally speaking, you should just `derive` a `Debug` implementation.
778///
779/// When used with the alternate format specifier `#?`, the output is pretty-printed.
780///
781/// For more information on formatters, see [the module-level documentation][module].
782///
783/// [module]: ../../std/fmt/index.html
784///
785/// This trait can be used with `#[derive]` if all fields implement `Debug`. When
786/// `derive`d for structs, it will use the name of the `struct`, then `{`, then a
787/// comma-separated list of each field's name and `Debug` value, then `}`. For
788/// `enum`s, it will use the name of the variant and, if applicable, `(`, then the
789/// `Debug` values of the fields, then `)`.
790///
791/// # Stability
792///
793/// Derived `Debug` formats are not stable, and so may change with future Rust
794/// versions. Additionally, `Debug` implementations of types provided by the
795/// standard library (`std`, `core`, `alloc`, etc.) are not stable, and
796/// may also change with future Rust versions.
797///
798/// # Examples
799///
800/// Deriving an implementation:
801///
802/// ```
803/// #[derive(Debug)]
804/// struct Point {
805///     x: i32,
806///     y: i32,
807/// }
808///
809/// let origin = Point { x: 0, y: 0 };
810///
811/// assert_eq!(
812///     format!("The origin is: {origin:?}"),
813///     "The origin is: Point { x: 0, y: 0 }",
814/// );
815/// ```
816///
817/// Manually implementing:
818///
819/// ```
820/// use std::fmt;
821///
822/// struct Point {
823///     x: i32,
824///     y: i32,
825/// }
826///
827/// impl fmt::Debug for Point {
828///     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
829///         f.debug_struct("Point")
830///          .field("x", &self.x)
831///          .field("y", &self.y)
832///          .finish()
833///     }
834/// }
835///
836/// let origin = Point { x: 0, y: 0 };
837///
838/// assert_eq!(
839///     format!("The origin is: {origin:?}"),
840///     "The origin is: Point { x: 0, y: 0 }",
841/// );
842/// ```
843///
844/// There are a number of helper methods on the [`Formatter`] struct to help you with manual
845/// implementations, such as [`debug_struct`].
846///
847/// [`debug_struct`]: Formatter::debug_struct
848///
849/// Types that do not wish to use the standard suite of debug representations
850/// provided by the `Formatter` trait (`debug_struct`, `debug_tuple`,
851/// `debug_list`, `debug_set`, `debug_map`) can do something totally custom by
852/// manually writing an arbitrary representation to the `Formatter`.
853///
854/// ```
855/// # use std::fmt;
856/// # struct Point {
857/// #     x: i32,
858/// #     y: i32,
859/// # }
860/// #
861/// impl fmt::Debug for Point {
862///     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
863///         write!(f, "Point [{} {}]", self.x, self.y)
864///     }
865/// }
866/// ```
867///
868/// `Debug` implementations using either `derive` or the debug builder API
869/// on [`Formatter`] support pretty-printing using the alternate flag: `{:#?}`.
870///
871/// Pretty-printing with `#?`:
872///
873/// ```
874/// #[derive(Debug)]
875/// struct Point {
876///     x: i32,
877///     y: i32,
878/// }
879///
880/// let origin = Point { x: 0, y: 0 };
881///
882/// let expected = "The origin is: Point {
883///     x: 0,
884///     y: 0,
885/// }";
886/// assert_eq!(format!("The origin is: {origin:#?}"), expected);
887/// ```
888
889#[stable(feature = "rust1", since = "1.0.0")]
890#[rustc_on_unimplemented(
891    on(
892        crate_local,
893        label = "`{Self}` cannot be formatted using `{{:?}}`",
894        note = "add `#[derive(Debug)]` to `{Self}` or manually `impl {Debug} for {Self}`"
895    ),
896    message = "`{Self}` doesn't implement `{Debug}`",
897    label = "`{Self}` cannot be formatted using `{{:?}}` because it doesn't implement `{Debug}`"
898)]
899#[doc(alias = "{:?}")]
900#[rustc_diagnostic_item = "Debug"]
901#[rustc_trivial_field_reads]
902pub trait Debug {
903    #[doc = include_str!("fmt_trait_method_doc.md")]
904    ///
905    /// # Examples
906    ///
907    /// ```
908    /// use std::fmt;
909    ///
910    /// struct Position {
911    ///     longitude: f32,
912    ///     latitude: f32,
913    /// }
914    ///
915    /// impl fmt::Debug for Position {
916    ///     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
917    ///         f.debug_tuple("")
918    ///          .field(&self.longitude)
919    ///          .field(&self.latitude)
920    ///          .finish()
921    ///     }
922    /// }
923    ///
924    /// let position = Position { longitude: 1.987, latitude: 2.983 };
925    /// assert_eq!(format!("{position:?}"), "(1.987, 2.983)");
926    ///
927    /// assert_eq!(format!("{position:#?}"), "(
928    ///     1.987,
929    ///     2.983,
930    /// )");
931    /// ```
932    #[stable(feature = "rust1", since = "1.0.0")]
933    fn fmt(&self, f: &mut Formatter<'_>) -> Result;
934}
935
936// Separate module to reexport the macro `Debug` from prelude without the trait `Debug`.
937pub(crate) mod macros {
938    /// Derive macro generating an impl of the trait `Debug`.
939    #[rustc_builtin_macro]
940    #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
941    #[allow_internal_unstable(core_intrinsics, fmt_helpers_for_derive)]
942    pub macro Debug($item:item) {
943        /* compiler built-in */
944    }
945}
946#[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
947#[doc(inline)]
948pub use macros::Debug;
949
950/// Format trait for an empty format, `{}`.
951///
952/// Implementing this trait for a type will automatically implement the
953/// [`ToString`][tostring] trait for the type, allowing the usage
954/// of the [`.to_string()`][tostring_function] method. Prefer implementing
955/// the `Display` trait for a type, rather than [`ToString`][tostring].
956///
957/// `Display` is similar to [`Debug`], but `Display` is for user-facing
958/// output, and so cannot be derived.
959///
960/// For more information on formatters, see [the module-level documentation][module].
961///
962/// [module]: ../../std/fmt/index.html
963/// [tostring]: ../../std/string/trait.ToString.html
964/// [tostring_function]: ../../std/string/trait.ToString.html#tymethod.to_string
965///
966/// # Internationalization
967///
968/// Because a type can only have one `Display` implementation, it is often preferable
969/// to only implement `Display` when there is a single most "obvious" way that
970/// values can be formatted as text. This could mean formatting according to the
971/// "invariant" culture and "undefined" locale, or it could mean that the type
972/// display is designed for a specific culture/locale, such as developer logs.
973///
974/// If not all values have a justifiably canonical textual format or if you want
975/// to support alternative formats not covered by the standard set of possible
976/// [formatting traits], the most flexible approach is display adapters: methods
977/// like [`str::escape_default`] or [`Path::display`] which create a wrapper
978/// implementing `Display` to output the specific display format.
979///
980/// [formatting traits]: ../../std/fmt/index.html#formatting-traits
981/// [`Path::display`]: ../../std/path/struct.Path.html#method.display
982///
983/// # Examples
984///
985/// Implementing `Display` on a type:
986///
987/// ```
988/// use std::fmt;
989///
990/// struct Point {
991///     x: i32,
992///     y: i32,
993/// }
994///
995/// impl fmt::Display for Point {
996///     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
997///         write!(f, "({}, {})", self.x, self.y)
998///     }
999/// }
1000///
1001/// let origin = Point { x: 0, y: 0 };
1002///
1003/// assert_eq!(format!("The origin is: {origin}"), "The origin is: (0, 0)");
1004/// ```
1005#[rustc_on_unimplemented(
1006    on(
1007        any(_Self = "std::path::Path", _Self = "std::path::PathBuf"),
1008        label = "`{Self}` cannot be formatted with the default formatter; call `.display()` on it",
1009        note = "call `.display()` or `.to_string_lossy()` to safely print paths, \
1010                as they may contain non-Unicode data"
1011    ),
1012    message = "`{Self}` doesn't implement `{Display}`",
1013    label = "`{Self}` cannot be formatted with the default formatter",
1014    note = "in format strings you may be able to use `{{:?}}` (or {{:#?}} for pretty-print) instead"
1015)]
1016#[doc(alias = "{}")]
1017#[rustc_diagnostic_item = "Display"]
1018#[stable(feature = "rust1", since = "1.0.0")]
1019pub trait Display {
1020    #[doc = include_str!("fmt_trait_method_doc.md")]
1021    ///
1022    /// # Examples
1023    ///
1024    /// ```
1025    /// use std::fmt;
1026    ///
1027    /// struct Position {
1028    ///     longitude: f32,
1029    ///     latitude: f32,
1030    /// }
1031    ///
1032    /// impl fmt::Display for Position {
1033    ///     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1034    ///         write!(f, "({}, {})", self.longitude, self.latitude)
1035    ///     }
1036    /// }
1037    ///
1038    /// assert_eq!(
1039    ///     "(1.987, 2.983)",
1040    ///     format!("{}", Position { longitude: 1.987, latitude: 2.983, }),
1041    /// );
1042    /// ```
1043    #[stable(feature = "rust1", since = "1.0.0")]
1044    fn fmt(&self, f: &mut Formatter<'_>) -> Result;
1045}
1046
1047/// `o` formatting.
1048///
1049/// The `Octal` trait should format its output as a number in base-8.
1050///
1051/// For primitive signed integers (`i8` to `i128`, and `isize`),
1052/// negative values are formatted as the two’s complement representation.
1053///
1054/// The alternate flag, `#`, adds a `0o` in front of the output.
1055///
1056/// For more information on formatters, see [the module-level documentation][module].
1057///
1058/// [module]: ../../std/fmt/index.html
1059///
1060/// # Examples
1061///
1062/// Basic usage with `i32`:
1063///
1064/// ```
1065/// let x = 42; // 42 is '52' in octal
1066///
1067/// assert_eq!(format!("{x:o}"), "52");
1068/// assert_eq!(format!("{x:#o}"), "0o52");
1069///
1070/// assert_eq!(format!("{:o}", -16), "37777777760");
1071/// ```
1072///
1073/// Implementing `Octal` on a type:
1074///
1075/// ```
1076/// use std::fmt;
1077///
1078/// struct Length(i32);
1079///
1080/// impl fmt::Octal for Length {
1081///     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1082///         let val = self.0;
1083///
1084///         fmt::Octal::fmt(&val, f) // delegate to i32's implementation
1085///     }
1086/// }
1087///
1088/// let l = Length(9);
1089///
1090/// assert_eq!(format!("l as octal is: {l:o}"), "l as octal is: 11");
1091///
1092/// assert_eq!(format!("l as octal is: {l:#06o}"), "l as octal is: 0o0011");
1093/// ```
1094#[stable(feature = "rust1", since = "1.0.0")]
1095pub trait Octal {
1096    #[doc = include_str!("fmt_trait_method_doc.md")]
1097    #[stable(feature = "rust1", since = "1.0.0")]
1098    fn fmt(&self, f: &mut Formatter<'_>) -> Result;
1099}
1100
1101/// `b` formatting.
1102///
1103/// The `Binary` trait should format its output as a number in binary.
1104///
1105/// For primitive signed integers ([`i8`] to [`i128`], and [`isize`]),
1106/// negative values are formatted as the two’s complement representation.
1107///
1108/// The alternate flag, `#`, adds a `0b` in front of the output.
1109///
1110/// For more information on formatters, see [the module-level documentation][module].
1111///
1112/// [module]: ../../std/fmt/index.html
1113///
1114/// # Examples
1115///
1116/// Basic usage with [`i32`]:
1117///
1118/// ```
1119/// let x = 42; // 42 is '101010' in binary
1120///
1121/// assert_eq!(format!("{x:b}"), "101010");
1122/// assert_eq!(format!("{x:#b}"), "0b101010");
1123///
1124/// assert_eq!(format!("{:b}", -16), "11111111111111111111111111110000");
1125/// ```
1126///
1127/// Implementing `Binary` on a type:
1128///
1129/// ```
1130/// use std::fmt;
1131///
1132/// struct Length(i32);
1133///
1134/// impl fmt::Binary for Length {
1135///     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1136///         let val = self.0;
1137///
1138///         fmt::Binary::fmt(&val, f) // delegate to i32's implementation
1139///     }
1140/// }
1141///
1142/// let l = Length(107);
1143///
1144/// assert_eq!(format!("l as binary is: {l:b}"), "l as binary is: 1101011");
1145///
1146/// assert_eq!(
1147///     // Note that the `0b` prefix added by `#` is included in the total width, so we
1148///     // need to add two to correctly display all 32 bits.
1149///     format!("l as binary is: {l:#034b}"),
1150///     "l as binary is: 0b00000000000000000000000001101011"
1151/// );
1152/// ```
1153#[stable(feature = "rust1", since = "1.0.0")]
1154pub trait Binary {
1155    #[doc = include_str!("fmt_trait_method_doc.md")]
1156    #[stable(feature = "rust1", since = "1.0.0")]
1157    fn fmt(&self, f: &mut Formatter<'_>) -> Result;
1158}
1159
1160/// `x` formatting.
1161///
1162/// The `LowerHex` trait should format its output as a number in hexadecimal, with `a` through `f`
1163/// in lower case.
1164///
1165/// For primitive signed integers (`i8` to `i128`, and `isize`),
1166/// negative values are formatted as the two’s complement representation.
1167///
1168/// The alternate flag, `#`, adds a `0x` in front of the output.
1169///
1170/// For more information on formatters, see [the module-level documentation][module].
1171///
1172/// [module]: ../../std/fmt/index.html
1173///
1174/// # Examples
1175///
1176/// Basic usage with `i32`:
1177///
1178/// ```
1179/// let y = 42; // 42 is '2a' in hex
1180///
1181/// assert_eq!(format!("{y:x}"), "2a");
1182/// assert_eq!(format!("{y:#x}"), "0x2a");
1183///
1184/// assert_eq!(format!("{:x}", -16), "fffffff0");
1185/// ```
1186///
1187/// Implementing `LowerHex` on a type:
1188///
1189/// ```
1190/// use std::fmt;
1191///
1192/// struct Length(i32);
1193///
1194/// impl fmt::LowerHex for Length {
1195///     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1196///         let val = self.0;
1197///
1198///         fmt::LowerHex::fmt(&val, f) // delegate to i32's implementation
1199///     }
1200/// }
1201///
1202/// let l = Length(9);
1203///
1204/// assert_eq!(format!("l as hex is: {l:x}"), "l as hex is: 9");
1205///
1206/// assert_eq!(format!("l as hex is: {l:#010x}"), "l as hex is: 0x00000009");
1207/// ```
1208#[stable(feature = "rust1", since = "1.0.0")]
1209pub trait LowerHex {
1210    #[doc = include_str!("fmt_trait_method_doc.md")]
1211    #[stable(feature = "rust1", since = "1.0.0")]
1212    fn fmt(&self, f: &mut Formatter<'_>) -> Result;
1213}
1214
1215/// `X` formatting.
1216///
1217/// The `UpperHex` trait should format its output as a number in hexadecimal, with `A` through `F`
1218/// in upper case.
1219///
1220/// For primitive signed integers (`i8` to `i128`, and `isize`),
1221/// negative values are formatted as the two’s complement representation.
1222///
1223/// The alternate flag, `#`, adds a `0x` in front of the output.
1224///
1225/// For more information on formatters, see [the module-level documentation][module].
1226///
1227/// [module]: ../../std/fmt/index.html
1228///
1229/// # Examples
1230///
1231/// Basic usage with `i32`:
1232///
1233/// ```
1234/// let y = 42; // 42 is '2A' in hex
1235///
1236/// assert_eq!(format!("{y:X}"), "2A");
1237/// assert_eq!(format!("{y:#X}"), "0x2A");
1238///
1239/// assert_eq!(format!("{:X}", -16), "FFFFFFF0");
1240/// ```
1241///
1242/// Implementing `UpperHex` on a type:
1243///
1244/// ```
1245/// use std::fmt;
1246///
1247/// struct Length(i32);
1248///
1249/// impl fmt::UpperHex for Length {
1250///     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1251///         let val = self.0;
1252///
1253///         fmt::UpperHex::fmt(&val, f) // delegate to i32's implementation
1254///     }
1255/// }
1256///
1257/// let l = Length(i32::MAX);
1258///
1259/// assert_eq!(format!("l as hex is: {l:X}"), "l as hex is: 7FFFFFFF");
1260///
1261/// assert_eq!(format!("l as hex is: {l:#010X}"), "l as hex is: 0x7FFFFFFF");
1262/// ```
1263#[stable(feature = "rust1", since = "1.0.0")]
1264pub trait UpperHex {
1265    #[doc = include_str!("fmt_trait_method_doc.md")]
1266    #[stable(feature = "rust1", since = "1.0.0")]
1267    fn fmt(&self, f: &mut Formatter<'_>) -> Result;
1268}
1269
1270/// `p` formatting.
1271///
1272/// The `Pointer` trait should format its output as a memory location. This is commonly presented
1273/// as hexadecimal. For more information on formatters, see [the module-level documentation][module].
1274///
1275/// Printing of pointers is not a reliable way to discover how Rust programs are implemented.
1276/// The act of reading an address changes the program itself, and may change how the data is represented
1277/// in memory, and may affect which optimizations are applied to the code.
1278///
1279/// The printed pointer values are not guaranteed to be stable nor unique identifiers of objects.
1280/// Rust allows moving values to different memory locations, and may reuse the same memory locations
1281/// for different purposes.
1282///
1283/// There is no guarantee that the printed value can be converted back to a pointer.
1284///
1285/// [module]: ../../std/fmt/index.html
1286///
1287/// # Examples
1288///
1289/// Basic usage with `&i32`:
1290///
1291/// ```
1292/// let x = &42;
1293///
1294/// let address = format!("{x:p}"); // this produces something like '0x7f06092ac6d0'
1295/// ```
1296///
1297/// Implementing `Pointer` on a type:
1298///
1299/// ```
1300/// use std::fmt;
1301///
1302/// struct Length(i32);
1303///
1304/// impl fmt::Pointer for Length {
1305///     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1306///         // use `as` to convert to a `*const T`, which implements Pointer, which we can use
1307///
1308///         let ptr = self as *const Self;
1309///         fmt::Pointer::fmt(&ptr, f)
1310///     }
1311/// }
1312///
1313/// let l = Length(42);
1314///
1315/// println!("l is in memory here: {l:p}");
1316///
1317/// let l_ptr = format!("{l:018p}");
1318/// assert_eq!(l_ptr.len(), 18);
1319/// assert_eq!(&l_ptr[..2], "0x");
1320/// ```
1321#[stable(feature = "rust1", since = "1.0.0")]
1322#[rustc_diagnostic_item = "Pointer"]
1323pub trait Pointer {
1324    #[doc = include_str!("fmt_trait_method_doc.md")]
1325    #[stable(feature = "rust1", since = "1.0.0")]
1326    fn fmt(&self, f: &mut Formatter<'_>) -> Result;
1327}
1328
1329/// `e` formatting.
1330///
1331/// The `LowerExp` trait should format its output in scientific notation with a lower-case `e`.
1332///
1333/// For more information on formatters, see [the module-level documentation][module].
1334///
1335/// [module]: ../../std/fmt/index.html
1336///
1337/// # Examples
1338///
1339/// Basic usage with `f64`:
1340///
1341/// ```
1342/// let x = 42.0; // 42.0 is '4.2e1' in scientific notation
1343///
1344/// assert_eq!(format!("{x:e}"), "4.2e1");
1345/// ```
1346///
1347/// Implementing `LowerExp` on a type:
1348///
1349/// ```
1350/// use std::fmt;
1351///
1352/// struct Length(i32);
1353///
1354/// impl fmt::LowerExp for Length {
1355///     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1356///         let val = f64::from(self.0);
1357///         fmt::LowerExp::fmt(&val, f) // delegate to f64's implementation
1358///     }
1359/// }
1360///
1361/// let l = Length(100);
1362///
1363/// assert_eq!(
1364///     format!("l in scientific notation is: {l:e}"),
1365///     "l in scientific notation is: 1e2"
1366/// );
1367///
1368/// assert_eq!(
1369///     format!("l in scientific notation is: {l:05e}"),
1370///     "l in scientific notation is: 001e2"
1371/// );
1372/// ```
1373#[stable(feature = "rust1", since = "1.0.0")]
1374pub trait LowerExp {
1375    #[doc = include_str!("fmt_trait_method_doc.md")]
1376    #[stable(feature = "rust1", since = "1.0.0")]
1377    fn fmt(&self, f: &mut Formatter<'_>) -> Result;
1378}
1379
1380/// `E` formatting.
1381///
1382/// The `UpperExp` trait should format its output in scientific notation with an upper-case `E`.
1383///
1384/// For more information on formatters, see [the module-level documentation][module].
1385///
1386/// [module]: ../../std/fmt/index.html
1387///
1388/// # Examples
1389///
1390/// Basic usage with `f64`:
1391///
1392/// ```
1393/// let x = 42.0; // 42.0 is '4.2E1' in scientific notation
1394///
1395/// assert_eq!(format!("{x:E}"), "4.2E1");
1396/// ```
1397///
1398/// Implementing `UpperExp` on a type:
1399///
1400/// ```
1401/// use std::fmt;
1402///
1403/// struct Length(i32);
1404///
1405/// impl fmt::UpperExp for Length {
1406///     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1407///         let val = f64::from(self.0);
1408///         fmt::UpperExp::fmt(&val, f) // delegate to f64's implementation
1409///     }
1410/// }
1411///
1412/// let l = Length(100);
1413///
1414/// assert_eq!(
1415///     format!("l in scientific notation is: {l:E}"),
1416///     "l in scientific notation is: 1E2"
1417/// );
1418///
1419/// assert_eq!(
1420///     format!("l in scientific notation is: {l:05E}"),
1421///     "l in scientific notation is: 001E2"
1422/// );
1423/// ```
1424#[stable(feature = "rust1", since = "1.0.0")]
1425pub trait UpperExp {
1426    #[doc = include_str!("fmt_trait_method_doc.md")]
1427    #[stable(feature = "rust1", since = "1.0.0")]
1428    fn fmt(&self, f: &mut Formatter<'_>) -> Result;
1429}
1430
1431/// Takes an output stream and an `Arguments` struct that can be precompiled with
1432/// the `format_args!` macro.
1433///
1434/// The arguments will be formatted according to the specified format string
1435/// into the output stream provided.
1436///
1437/// # Examples
1438///
1439/// Basic usage:
1440///
1441/// ```
1442/// use std::fmt;
1443///
1444/// let mut output = String::new();
1445/// fmt::write(&mut output, format_args!("Hello {}!", "world"))
1446///     .expect("Error occurred while trying to write in String");
1447/// assert_eq!(output, "Hello world!");
1448/// ```
1449///
1450/// Please note that using [`write!`] might be preferable. Example:
1451///
1452/// ```
1453/// use std::fmt::Write;
1454///
1455/// let mut output = String::new();
1456/// write!(&mut output, "Hello {}!", "world")
1457///     .expect("Error occurred while trying to write in String");
1458/// assert_eq!(output, "Hello world!");
1459/// ```
1460///
1461/// [`write!`]: crate::write!
1462#[stable(feature = "rust1", since = "1.0.0")]
1463pub fn write(output: &mut dyn Write, args: Arguments<'_>) -> Result {
1464    let mut formatter = Formatter::new(output, FormattingOptions::new());
1465    let mut idx = 0;
1466
1467    match args.fmt {
1468        None => {
1469            // We can use default formatting parameters for all arguments.
1470            for (i, arg) in args.args.iter().enumerate() {
1471                // SAFETY: args.args and args.pieces come from the same Arguments,
1472                // which guarantees the indexes are always within bounds.
1473                let piece = unsafe { args.pieces.get_unchecked(i) };
1474                if !piece.is_empty() {
1475                    formatter.buf.write_str(*piece)?;
1476                }
1477
1478                // SAFETY: There are no formatting parameters and hence no
1479                // count arguments.
1480                unsafe {
1481                    arg.fmt(&mut formatter)?;
1482                }
1483                idx += 1;
1484            }
1485        }
1486        Some(fmt) => {
1487            // Every spec has a corresponding argument that is preceded by
1488            // a string piece.
1489            for (i, arg) in fmt.iter().enumerate() {
1490                // SAFETY: fmt and args.pieces come from the same Arguments,
1491                // which guarantees the indexes are always within bounds.
1492                let piece = unsafe { args.pieces.get_unchecked(i) };
1493                if !piece.is_empty() {
1494                    formatter.buf.write_str(*piece)?;
1495                }
1496                // SAFETY: arg and args.args come from the same Arguments,
1497                // which guarantees the indexes are always within bounds.
1498                unsafe { run(&mut formatter, arg, args.args) }?;
1499                idx += 1;
1500            }
1501        }
1502    }
1503
1504    // There can be only one trailing string piece left.
1505    if let Some(piece) = args.pieces.get(idx) {
1506        formatter.buf.write_str(*piece)?;
1507    }
1508
1509    Ok(())
1510}
1511
1512unsafe fn run(fmt: &mut Formatter<'_>, arg: &rt::Placeholder, args: &[rt::Argument<'_>]) -> Result {
1513    let (width, precision) =
1514        // SAFETY: arg and args come from the same Arguments,
1515        // which guarantees the indexes are always within bounds.
1516        unsafe { (getcount(args, &arg.width), getcount(args, &arg.precision)) };
1517
1518    let options = FormattingOptions { flags: arg.flags, width, precision };
1519
1520    // Extract the correct argument
1521    debug_assert!(arg.position < args.len());
1522    // SAFETY: arg and args come from the same Arguments,
1523    // which guarantees its index is always within bounds.
1524    let value = unsafe { args.get_unchecked(arg.position) };
1525
1526    // Set all the formatting options.
1527    fmt.options = options;
1528
1529    // Then actually do some printing
1530    // SAFETY: this is a placeholder argument.
1531    unsafe { value.fmt(fmt) }
1532}
1533
1534unsafe fn getcount(args: &[rt::Argument<'_>], cnt: &rt::Count) -> u16 {
1535    match *cnt {
1536        rt::Count::Is(n) => n,
1537        rt::Count::Implied => 0,
1538        rt::Count::Param(i) => {
1539            debug_assert!(i < args.len());
1540            // SAFETY: cnt and args come from the same Arguments,
1541            // which guarantees this index is always within bounds.
1542            unsafe { args.get_unchecked(i).as_u16().unwrap_unchecked() }
1543        }
1544    }
1545}
1546
1547/// Padding after the end of something. Returned by `Formatter::padding`.
1548#[must_use = "don't forget to write the post padding"]
1549pub(crate) struct PostPadding {
1550    fill: char,
1551    padding: u16,
1552}
1553
1554impl PostPadding {
1555    fn new(fill: char, padding: u16) -> PostPadding {
1556        PostPadding { fill, padding }
1557    }
1558
1559    /// Writes this post padding.
1560    pub(crate) fn write(self, f: &mut Formatter<'_>) -> Result {
1561        for _ in 0..self.padding {
1562            f.buf.write_char(self.fill)?;
1563        }
1564        Ok(())
1565    }
1566}
1567
1568impl<'a> Formatter<'a> {
1569    fn wrap_buf<'b, 'c, F>(&'b mut self, wrap: F) -> Formatter<'c>
1570    where
1571        'b: 'c,
1572        F: FnOnce(&'b mut (dyn Write + 'b)) -> &'c mut (dyn Write + 'c),
1573    {
1574        Formatter {
1575            // We want to change this
1576            buf: wrap(self.buf),
1577
1578            // And preserve these
1579            options: self.options,
1580        }
1581    }
1582
1583    // Helper methods used for padding and processing formatting arguments that
1584    // all formatting traits can use.
1585
1586    /// Performs the correct padding for an integer which has already been
1587    /// emitted into a str. The str should *not* contain the sign for the
1588    /// integer, that will be added by this method.
1589    ///
1590    /// # Arguments
1591    ///
1592    /// * is_nonnegative - whether the original integer was either positive or zero.
1593    /// * prefix - if the '#' character (Alternate) is provided, this
1594    ///   is the prefix to put in front of the number.
1595    /// * buf - the byte array that the number has been formatted into
1596    ///
1597    /// This function will correctly account for the flags provided as well as
1598    /// the minimum width. It will not take precision into account.
1599    ///
1600    /// # Examples
1601    ///
1602    /// ```
1603    /// use std::fmt;
1604    ///
1605    /// struct Foo { nb: i32 }
1606    ///
1607    /// impl Foo {
1608    ///     fn new(nb: i32) -> Foo {
1609    ///         Foo {
1610    ///             nb,
1611    ///         }
1612    ///     }
1613    /// }
1614    ///
1615    /// impl fmt::Display for Foo {
1616    ///     fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
1617    ///         // We need to remove "-" from the number output.
1618    ///         let tmp = self.nb.abs().to_string();
1619    ///
1620    ///         formatter.pad_integral(self.nb >= 0, "Foo ", &tmp)
1621    ///     }
1622    /// }
1623    ///
1624    /// assert_eq!(format!("{}", Foo::new(2)), "2");
1625    /// assert_eq!(format!("{}", Foo::new(-1)), "-1");
1626    /// assert_eq!(format!("{}", Foo::new(0)), "0");
1627    /// assert_eq!(format!("{:#}", Foo::new(-1)), "-Foo 1");
1628    /// assert_eq!(format!("{:0>#8}", Foo::new(-1)), "00-Foo 1");
1629    /// ```
1630    #[stable(feature = "rust1", since = "1.0.0")]
1631    pub fn pad_integral(&mut self, is_nonnegative: bool, prefix: &str, buf: &str) -> Result {
1632        let mut width = buf.len();
1633
1634        let mut sign = None;
1635        if !is_nonnegative {
1636            sign = Some('-');
1637            width += 1;
1638        } else if self.sign_plus() {
1639            sign = Some('+');
1640            width += 1;
1641        }
1642
1643        let prefix = if self.alternate() {
1644            width += prefix.chars().count();
1645            Some(prefix)
1646        } else {
1647            None
1648        };
1649
1650        // Writes the sign if it exists, and then the prefix if it was requested
1651        #[inline(never)]
1652        fn write_prefix(f: &mut Formatter<'_>, sign: Option<char>, prefix: Option<&str>) -> Result {
1653            if let Some(c) = sign {
1654                f.buf.write_char(c)?;
1655            }
1656            if let Some(prefix) = prefix { f.buf.write_str(prefix) } else { Ok(()) }
1657        }
1658
1659        // The `width` field is more of a `min-width` parameter at this point.
1660        let min = self.options.width;
1661        if width >= usize::from(min) {
1662            // We're over the minimum width, so then we can just write the bytes.
1663            write_prefix(self, sign, prefix)?;
1664            self.buf.write_str(buf)
1665        } else if self.sign_aware_zero_pad() {
1666            // The sign and prefix goes before the padding if the fill character
1667            // is zero
1668            let old_options = self.options;
1669            self.options.fill('0').align(Some(Alignment::Right));
1670            write_prefix(self, sign, prefix)?;
1671            let post_padding = self.padding(min - width as u16, Alignment::Right)?;
1672            self.buf.write_str(buf)?;
1673            post_padding.write(self)?;
1674            self.options = old_options;
1675            Ok(())
1676        } else {
1677            // Otherwise, the sign and prefix goes after the padding
1678            let post_padding = self.padding(min - width as u16, Alignment::Right)?;
1679            write_prefix(self, sign, prefix)?;
1680            self.buf.write_str(buf)?;
1681            post_padding.write(self)
1682        }
1683    }
1684
1685    /// Takes a string slice and emits it to the internal buffer after applying
1686    /// the relevant formatting flags specified.
1687    ///
1688    /// The flags recognized for generic strings are:
1689    ///
1690    /// * width - the minimum width of what to emit
1691    /// * fill/align - what to emit and where to emit it if the string
1692    ///                provided needs to be padded
1693    /// * precision - the maximum length to emit, the string is truncated if it
1694    ///               is longer than this length
1695    ///
1696    /// Notably this function ignores the `flag` parameters.
1697    ///
1698    /// # Examples
1699    ///
1700    /// ```
1701    /// use std::fmt;
1702    ///
1703    /// struct Foo;
1704    ///
1705    /// impl fmt::Display for Foo {
1706    ///     fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
1707    ///         formatter.pad("Foo")
1708    ///     }
1709    /// }
1710    ///
1711    /// assert_eq!(format!("{Foo:<4}"), "Foo ");
1712    /// assert_eq!(format!("{Foo:0>4}"), "0Foo");
1713    /// ```
1714    #[stable(feature = "rust1", since = "1.0.0")]
1715    pub fn pad(&mut self, s: &str) -> Result {
1716        // Make sure there's a fast path up front.
1717        if self.options.flags & (flags::WIDTH_FLAG | flags::PRECISION_FLAG) == 0 {
1718            return self.buf.write_str(s);
1719        }
1720
1721        // The `precision` field can be interpreted as a maximum width for the
1722        // string being formatted.
1723        let (s, char_count) = if let Some(max_char_count) = self.options.get_precision() {
1724            let mut iter = s.char_indices();
1725            let remaining = match iter.advance_by(usize::from(max_char_count)) {
1726                Ok(()) => 0,
1727                Err(remaining) => remaining.get(),
1728            };
1729            // SAFETY: The offset of `.char_indices()` is guaranteed to be
1730            // in-bounds and between character boundaries.
1731            let truncated = unsafe { s.get_unchecked(..iter.offset()) };
1732            (truncated, usize::from(max_char_count) - remaining)
1733        } else {
1734            // Use the optimized char counting algorithm for the full string.
1735            (s, s.chars().count())
1736        };
1737
1738        // The `width` field is more of a minimum width parameter at this point.
1739        if char_count < usize::from(self.options.width) {
1740            // If we're under the minimum width, then fill up the minimum width
1741            // with the specified string + some alignment.
1742            let post_padding =
1743                self.padding(self.options.width - char_count as u16, Alignment::Left)?;
1744            self.buf.write_str(s)?;
1745            post_padding.write(self)
1746        } else {
1747            // If we're over the minimum width or there is no minimum width, we
1748            // can just emit the string.
1749            self.buf.write_str(s)
1750        }
1751    }
1752
1753    /// Writes the pre-padding and returns the unwritten post-padding.
1754    ///
1755    /// Callers are responsible for ensuring post-padding is written after the
1756    /// thing that is being padded.
1757    pub(crate) fn padding(
1758        &mut self,
1759        padding: u16,
1760        default: Alignment,
1761    ) -> result::Result<PostPadding, Error> {
1762        let align = self.options.get_align().unwrap_or(default);
1763        let fill = self.options.get_fill();
1764
1765        let padding_left = match align {
1766            Alignment::Left => 0,
1767            Alignment::Right => padding,
1768            Alignment::Center => padding / 2,
1769        };
1770
1771        for _ in 0..padding_left {
1772            self.buf.write_char(fill)?;
1773        }
1774
1775        Ok(PostPadding::new(fill, padding - padding_left))
1776    }
1777
1778    /// Takes the formatted parts and applies the padding.
1779    ///
1780    /// Assumes that the caller already has rendered the parts with required precision,
1781    /// so that `self.precision` can be ignored.
1782    ///
1783    /// # Safety
1784    ///
1785    /// Any `numfmt::Part::Copy` parts in `formatted` must contain valid UTF-8.
1786    unsafe fn pad_formatted_parts(&mut self, formatted: &numfmt::Formatted<'_>) -> Result {
1787        if self.options.width == 0 {
1788            // this is the common case and we take a shortcut
1789            // SAFETY: Per the precondition.
1790            unsafe { self.write_formatted_parts(formatted) }
1791        } else {
1792            // for the sign-aware zero padding, we render the sign first and
1793            // behave as if we had no sign from the beginning.
1794            let mut formatted = formatted.clone();
1795            let mut width = self.options.width;
1796            let old_options = self.options;
1797            if self.sign_aware_zero_pad() {
1798                // a sign always goes first
1799                let sign = formatted.sign;
1800                self.buf.write_str(sign)?;
1801
1802                // remove the sign from the formatted parts
1803                formatted.sign = "";
1804                width = width.saturating_sub(sign.len() as u16);
1805                self.options.fill('0').align(Some(Alignment::Right));
1806            }
1807
1808            // remaining parts go through the ordinary padding process.
1809            let len = formatted.len();
1810            let ret = if usize::from(width) <= len {
1811                // no padding
1812                // SAFETY: Per the precondition.
1813                unsafe { self.write_formatted_parts(&formatted) }
1814            } else {
1815                let post_padding = self.padding(width - len as u16, Alignment::Right)?;
1816                // SAFETY: Per the precondition.
1817                unsafe {
1818                    self.write_formatted_parts(&formatted)?;
1819                }
1820                post_padding.write(self)
1821            };
1822            self.options = old_options;
1823            ret
1824        }
1825    }
1826
1827    /// # Safety
1828    ///
1829    /// Any `numfmt::Part::Copy` parts in `formatted` must contain valid UTF-8.
1830    unsafe fn write_formatted_parts(&mut self, formatted: &numfmt::Formatted<'_>) -> Result {
1831        unsafe fn write_bytes(buf: &mut dyn Write, s: &[u8]) -> Result {
1832            // SAFETY: This is used for `numfmt::Part::Num` and `numfmt::Part::Copy`.
1833            // It's safe to use for `numfmt::Part::Num` since every char `c` is between
1834            // `b'0'` and `b'9'`, which means `s` is valid UTF-8. It's safe to use for
1835            // `numfmt::Part::Copy` due to this function's precondition.
1836            buf.write_str(unsafe { str::from_utf8_unchecked(s) })
1837        }
1838
1839        if !formatted.sign.is_empty() {
1840            self.buf.write_str(formatted.sign)?;
1841        }
1842        for part in formatted.parts {
1843            match *part {
1844                numfmt::Part::Zero(mut nzeroes) => {
1845                    const ZEROES: &str = // 64 zeroes
1846                        "0000000000000000000000000000000000000000000000000000000000000000";
1847                    while nzeroes > ZEROES.len() {
1848                        self.buf.write_str(ZEROES)?;
1849                        nzeroes -= ZEROES.len();
1850                    }
1851                    if nzeroes > 0 {
1852                        self.buf.write_str(&ZEROES[..nzeroes])?;
1853                    }
1854                }
1855                numfmt::Part::Num(mut v) => {
1856                    let mut s = [0; 5];
1857                    let len = part.len();
1858                    for c in s[..len].iter_mut().rev() {
1859                        *c = b'0' + (v % 10) as u8;
1860                        v /= 10;
1861                    }
1862                    // SAFETY: Per the precondition.
1863                    unsafe {
1864                        write_bytes(self.buf, &s[..len])?;
1865                    }
1866                }
1867                // SAFETY: Per the precondition.
1868                numfmt::Part::Copy(buf) => unsafe {
1869                    write_bytes(self.buf, buf)?;
1870                },
1871            }
1872        }
1873        Ok(())
1874    }
1875
1876    /// Writes some data to the underlying buffer contained within this
1877    /// formatter.
1878    ///
1879    /// # Examples
1880    ///
1881    /// ```
1882    /// use std::fmt;
1883    ///
1884    /// struct Foo;
1885    ///
1886    /// impl fmt::Display for Foo {
1887    ///     fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
1888    ///         formatter.write_str("Foo")
1889    ///         // This is equivalent to:
1890    ///         // write!(formatter, "Foo")
1891    ///     }
1892    /// }
1893    ///
1894    /// assert_eq!(format!("{Foo}"), "Foo");
1895    /// assert_eq!(format!("{Foo:0>8}"), "Foo");
1896    /// ```
1897    #[stable(feature = "rust1", since = "1.0.0")]
1898    pub fn write_str(&mut self, data: &str) -> Result {
1899        self.buf.write_str(data)
1900    }
1901
1902    /// Glue for usage of the [`write!`] macro with implementors of this trait.
1903    ///
1904    /// This method should generally not be invoked manually, but rather through
1905    /// the [`write!`] macro itself.
1906    ///
1907    /// Writes some formatted information into this instance.
1908    ///
1909    /// # Examples
1910    ///
1911    /// ```
1912    /// use std::fmt;
1913    ///
1914    /// struct Foo(i32);
1915    ///
1916    /// impl fmt::Display for Foo {
1917    ///     fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
1918    ///         formatter.write_fmt(format_args!("Foo {}", self.0))
1919    ///     }
1920    /// }
1921    ///
1922    /// assert_eq!(format!("{}", Foo(-1)), "Foo -1");
1923    /// assert_eq!(format!("{:0>8}", Foo(2)), "Foo 2");
1924    /// ```
1925    #[stable(feature = "rust1", since = "1.0.0")]
1926    #[inline]
1927    pub fn write_fmt(&mut self, fmt: Arguments<'_>) -> Result {
1928        if let Some(s) = fmt.as_statically_known_str() {
1929            self.buf.write_str(s)
1930        } else {
1931            write(self.buf, fmt)
1932        }
1933    }
1934
1935    /// Returns flags for formatting.
1936    #[must_use]
1937    #[stable(feature = "rust1", since = "1.0.0")]
1938    #[deprecated(
1939        since = "1.24.0",
1940        note = "use the `sign_plus`, `sign_minus`, `alternate`, \
1941                or `sign_aware_zero_pad` methods instead"
1942    )]
1943    pub fn flags(&self) -> u32 {
1944        // Extract the debug upper/lower hex, zero pad, alternate, and plus/minus flags
1945        // to stay compatible with older versions of Rust.
1946        self.options.flags >> 21 & 0x3F
1947    }
1948
1949    /// Returns the character used as 'fill' whenever there is alignment.
1950    ///
1951    /// # Examples
1952    ///
1953    /// ```
1954    /// use std::fmt;
1955    ///
1956    /// struct Foo;
1957    ///
1958    /// impl fmt::Display for Foo {
1959    ///     fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
1960    ///         let c = formatter.fill();
1961    ///         if let Some(width) = formatter.width() {
1962    ///             for _ in 0..width {
1963    ///                 write!(formatter, "{c}")?;
1964    ///             }
1965    ///             Ok(())
1966    ///         } else {
1967    ///             write!(formatter, "{c}")
1968    ///         }
1969    ///     }
1970    /// }
1971    ///
1972    /// // We set alignment to the right with ">".
1973    /// assert_eq!(format!("{Foo:G>3}"), "GGG");
1974    /// assert_eq!(format!("{Foo:t>6}"), "tttttt");
1975    /// ```
1976    #[must_use]
1977    #[stable(feature = "fmt_flags", since = "1.5.0")]
1978    pub fn fill(&self) -> char {
1979        self.options.get_fill()
1980    }
1981
1982    /// Returns a flag indicating what form of alignment was requested.
1983    ///
1984    /// # Examples
1985    ///
1986    /// ```
1987    /// use std::fmt::{self, Alignment};
1988    ///
1989    /// struct Foo;
1990    ///
1991    /// impl fmt::Display for Foo {
1992    ///     fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
1993    ///         let s = if let Some(s) = formatter.align() {
1994    ///             match s {
1995    ///                 Alignment::Left    => "left",
1996    ///                 Alignment::Right   => "right",
1997    ///                 Alignment::Center  => "center",
1998    ///             }
1999    ///         } else {
2000    ///             "into the void"
2001    ///         };
2002    ///         write!(formatter, "{s}")
2003    ///     }
2004    /// }
2005    ///
2006    /// assert_eq!(format!("{Foo:<}"), "left");
2007    /// assert_eq!(format!("{Foo:>}"), "right");
2008    /// assert_eq!(format!("{Foo:^}"), "center");
2009    /// assert_eq!(format!("{Foo}"), "into the void");
2010    /// ```
2011    #[must_use]
2012    #[stable(feature = "fmt_flags_align", since = "1.28.0")]
2013    pub fn align(&self) -> Option<Alignment> {
2014        self.options.get_align()
2015    }
2016
2017    /// Returns the optionally specified integer width that the output should be.
2018    ///
2019    /// # Examples
2020    ///
2021    /// ```
2022    /// use std::fmt;
2023    ///
2024    /// struct Foo(i32);
2025    ///
2026    /// impl fmt::Display for Foo {
2027    ///     fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
2028    ///         if let Some(width) = formatter.width() {
2029    ///             // If we received a width, we use it
2030    ///             write!(formatter, "{:width$}", format!("Foo({})", self.0), width = width)
2031    ///         } else {
2032    ///             // Otherwise we do nothing special
2033    ///             write!(formatter, "Foo({})", self.0)
2034    ///         }
2035    ///     }
2036    /// }
2037    ///
2038    /// assert_eq!(format!("{:10}", Foo(23)), "Foo(23)   ");
2039    /// assert_eq!(format!("{}", Foo(23)), "Foo(23)");
2040    /// ```
2041    #[must_use]
2042    #[stable(feature = "fmt_flags", since = "1.5.0")]
2043    pub fn width(&self) -> Option<usize> {
2044        if self.options.flags & flags::WIDTH_FLAG == 0 {
2045            None
2046        } else {
2047            Some(self.options.width as usize)
2048        }
2049    }
2050
2051    /// Returns the optionally specified precision for numeric types.
2052    /// Alternatively, the maximum width for string types.
2053    ///
2054    /// # Examples
2055    ///
2056    /// ```
2057    /// use std::fmt;
2058    ///
2059    /// struct Foo(f32);
2060    ///
2061    /// impl fmt::Display for Foo {
2062    ///     fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
2063    ///         if let Some(precision) = formatter.precision() {
2064    ///             // If we received a precision, we use it.
2065    ///             write!(formatter, "Foo({1:.*})", precision, self.0)
2066    ///         } else {
2067    ///             // Otherwise we default to 2.
2068    ///             write!(formatter, "Foo({:.2})", self.0)
2069    ///         }
2070    ///     }
2071    /// }
2072    ///
2073    /// assert_eq!(format!("{:.4}", Foo(23.2)), "Foo(23.2000)");
2074    /// assert_eq!(format!("{}", Foo(23.2)), "Foo(23.20)");
2075    /// ```
2076    #[must_use]
2077    #[stable(feature = "fmt_flags", since = "1.5.0")]
2078    pub fn precision(&self) -> Option<usize> {
2079        if self.options.flags & flags::PRECISION_FLAG == 0 {
2080            None
2081        } else {
2082            Some(self.options.precision as usize)
2083        }
2084    }
2085
2086    /// Determines if the `+` flag was specified.
2087    ///
2088    /// # Examples
2089    ///
2090    /// ```
2091    /// use std::fmt;
2092    ///
2093    /// struct Foo(i32);
2094    ///
2095    /// impl fmt::Display for Foo {
2096    ///     fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
2097    ///         if formatter.sign_plus() {
2098    ///             write!(formatter,
2099    ///                    "Foo({}{})",
2100    ///                    if self.0 < 0 { '-' } else { '+' },
2101    ///                    self.0.abs())
2102    ///         } else {
2103    ///             write!(formatter, "Foo({})", self.0)
2104    ///         }
2105    ///     }
2106    /// }
2107    ///
2108    /// assert_eq!(format!("{:+}", Foo(23)), "Foo(+23)");
2109    /// assert_eq!(format!("{:+}", Foo(-23)), "Foo(-23)");
2110    /// assert_eq!(format!("{}", Foo(23)), "Foo(23)");
2111    /// ```
2112    #[must_use]
2113    #[stable(feature = "fmt_flags", since = "1.5.0")]
2114    pub fn sign_plus(&self) -> bool {
2115        self.options.flags & flags::SIGN_PLUS_FLAG != 0
2116    }
2117
2118    /// Determines if the `-` flag was specified.
2119    ///
2120    /// # Examples
2121    ///
2122    /// ```
2123    /// use std::fmt;
2124    ///
2125    /// struct Foo(i32);
2126    ///
2127    /// impl fmt::Display for Foo {
2128    ///     fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
2129    ///         if formatter.sign_minus() {
2130    ///             // You want a minus sign? Have one!
2131    ///             write!(formatter, "-Foo({})", self.0)
2132    ///         } else {
2133    ///             write!(formatter, "Foo({})", self.0)
2134    ///         }
2135    ///     }
2136    /// }
2137    ///
2138    /// assert_eq!(format!("{:-}", Foo(23)), "-Foo(23)");
2139    /// assert_eq!(format!("{}", Foo(23)), "Foo(23)");
2140    /// ```
2141    #[must_use]
2142    #[stable(feature = "fmt_flags", since = "1.5.0")]
2143    pub fn sign_minus(&self) -> bool {
2144        self.options.flags & flags::SIGN_MINUS_FLAG != 0
2145    }
2146
2147    /// Determines if the `#` flag was specified.
2148    ///
2149    /// # Examples
2150    ///
2151    /// ```
2152    /// use std::fmt;
2153    ///
2154    /// struct Foo(i32);
2155    ///
2156    /// impl fmt::Display for Foo {
2157    ///     fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
2158    ///         if formatter.alternate() {
2159    ///             write!(formatter, "Foo({})", self.0)
2160    ///         } else {
2161    ///             write!(formatter, "{}", self.0)
2162    ///         }
2163    ///     }
2164    /// }
2165    ///
2166    /// assert_eq!(format!("{:#}", Foo(23)), "Foo(23)");
2167    /// assert_eq!(format!("{}", Foo(23)), "23");
2168    /// ```
2169    #[must_use]
2170    #[stable(feature = "fmt_flags", since = "1.5.0")]
2171    pub fn alternate(&self) -> bool {
2172        self.options.flags & flags::ALTERNATE_FLAG != 0
2173    }
2174
2175    /// Determines if the `0` flag was specified.
2176    ///
2177    /// # Examples
2178    ///
2179    /// ```
2180    /// use std::fmt;
2181    ///
2182    /// struct Foo(i32);
2183    ///
2184    /// impl fmt::Display for Foo {
2185    ///     fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
2186    ///         assert!(formatter.sign_aware_zero_pad());
2187    ///         assert_eq!(formatter.width(), Some(4));
2188    ///         // We ignore the formatter's options.
2189    ///         write!(formatter, "{}", self.0)
2190    ///     }
2191    /// }
2192    ///
2193    /// assert_eq!(format!("{:04}", Foo(23)), "23");
2194    /// ```
2195    #[must_use]
2196    #[stable(feature = "fmt_flags", since = "1.5.0")]
2197    pub fn sign_aware_zero_pad(&self) -> bool {
2198        self.options.flags & flags::SIGN_AWARE_ZERO_PAD_FLAG != 0
2199    }
2200
2201    // FIXME: Decide what public API we want for these two flags.
2202    // https://github.com/rust-lang/rust/issues/48584
2203    fn debug_lower_hex(&self) -> bool {
2204        self.options.flags & flags::DEBUG_LOWER_HEX_FLAG != 0
2205    }
2206    fn debug_upper_hex(&self) -> bool {
2207        self.options.flags & flags::DEBUG_UPPER_HEX_FLAG != 0
2208    }
2209
2210    /// Creates a [`DebugStruct`] builder designed to assist with creation of
2211    /// [`fmt::Debug`] implementations for structs.
2212    ///
2213    /// [`fmt::Debug`]: self::Debug
2214    ///
2215    /// # Examples
2216    ///
2217    /// ```rust
2218    /// use std::fmt;
2219    /// use std::net::Ipv4Addr;
2220    ///
2221    /// struct Foo {
2222    ///     bar: i32,
2223    ///     baz: String,
2224    ///     addr: Ipv4Addr,
2225    /// }
2226    ///
2227    /// impl fmt::Debug for Foo {
2228    ///     fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
2229    ///         fmt.debug_struct("Foo")
2230    ///             .field("bar", &self.bar)
2231    ///             .field("baz", &self.baz)
2232    ///             .field("addr", &format_args!("{}", self.addr))
2233    ///             .finish()
2234    ///     }
2235    /// }
2236    ///
2237    /// assert_eq!(
2238    ///     "Foo { bar: 10, baz: \"Hello World\", addr: 127.0.0.1 }",
2239    ///     format!("{:?}", Foo {
2240    ///         bar: 10,
2241    ///         baz: "Hello World".to_string(),
2242    ///         addr: Ipv4Addr::new(127, 0, 0, 1),
2243    ///     })
2244    /// );
2245    /// ```
2246    #[stable(feature = "debug_builders", since = "1.2.0")]
2247    pub fn debug_struct<'b>(&'b mut self, name: &str) -> DebugStruct<'b, 'a> {
2248        builders::debug_struct_new(self, name)
2249    }
2250
2251    /// Shrinks `derive(Debug)` code, for faster compilation and smaller
2252    /// binaries. `debug_struct_fields_finish` is more general, but this is
2253    /// faster for 1 field.
2254    #[doc(hidden)]
2255    #[unstable(feature = "fmt_helpers_for_derive", issue = "none")]
2256    pub fn debug_struct_field1_finish<'b>(
2257        &'b mut self,
2258        name: &str,
2259        name1: &str,
2260        value1: &dyn Debug,
2261    ) -> Result {
2262        let mut builder = builders::debug_struct_new(self, name);
2263        builder.field(name1, value1);
2264        builder.finish()
2265    }
2266
2267    /// Shrinks `derive(Debug)` code, for faster compilation and smaller
2268    /// binaries. `debug_struct_fields_finish` is more general, but this is
2269    /// faster for 2 fields.
2270    #[doc(hidden)]
2271    #[unstable(feature = "fmt_helpers_for_derive", issue = "none")]
2272    pub fn debug_struct_field2_finish<'b>(
2273        &'b mut self,
2274        name: &str,
2275        name1: &str,
2276        value1: &dyn Debug,
2277        name2: &str,
2278        value2: &dyn Debug,
2279    ) -> Result {
2280        let mut builder = builders::debug_struct_new(self, name);
2281        builder.field(name1, value1);
2282        builder.field(name2, value2);
2283        builder.finish()
2284    }
2285
2286    /// Shrinks `derive(Debug)` code, for faster compilation and smaller
2287    /// binaries. `debug_struct_fields_finish` is more general, but this is
2288    /// faster for 3 fields.
2289    #[doc(hidden)]
2290    #[unstable(feature = "fmt_helpers_for_derive", issue = "none")]
2291    pub fn debug_struct_field3_finish<'b>(
2292        &'b mut self,
2293        name: &str,
2294        name1: &str,
2295        value1: &dyn Debug,
2296        name2: &str,
2297        value2: &dyn Debug,
2298        name3: &str,
2299        value3: &dyn Debug,
2300    ) -> Result {
2301        let mut builder = builders::debug_struct_new(self, name);
2302        builder.field(name1, value1);
2303        builder.field(name2, value2);
2304        builder.field(name3, value3);
2305        builder.finish()
2306    }
2307
2308    /// Shrinks `derive(Debug)` code, for faster compilation and smaller
2309    /// binaries. `debug_struct_fields_finish` is more general, but this is
2310    /// faster for 4 fields.
2311    #[doc(hidden)]
2312    #[unstable(feature = "fmt_helpers_for_derive", issue = "none")]
2313    pub fn debug_struct_field4_finish<'b>(
2314        &'b mut self,
2315        name: &str,
2316        name1: &str,
2317        value1: &dyn Debug,
2318        name2: &str,
2319        value2: &dyn Debug,
2320        name3: &str,
2321        value3: &dyn Debug,
2322        name4: &str,
2323        value4: &dyn Debug,
2324    ) -> Result {
2325        let mut builder = builders::debug_struct_new(self, name);
2326        builder.field(name1, value1);
2327        builder.field(name2, value2);
2328        builder.field(name3, value3);
2329        builder.field(name4, value4);
2330        builder.finish()
2331    }
2332
2333    /// Shrinks `derive(Debug)` code, for faster compilation and smaller
2334    /// binaries. `debug_struct_fields_finish` is more general, but this is
2335    /// faster for 5 fields.
2336    #[doc(hidden)]
2337    #[unstable(feature = "fmt_helpers_for_derive", issue = "none")]
2338    pub fn debug_struct_field5_finish<'b>(
2339        &'b mut self,
2340        name: &str,
2341        name1: &str,
2342        value1: &dyn Debug,
2343        name2: &str,
2344        value2: &dyn Debug,
2345        name3: &str,
2346        value3: &dyn Debug,
2347        name4: &str,
2348        value4: &dyn Debug,
2349        name5: &str,
2350        value5: &dyn Debug,
2351    ) -> Result {
2352        let mut builder = builders::debug_struct_new(self, name);
2353        builder.field(name1, value1);
2354        builder.field(name2, value2);
2355        builder.field(name3, value3);
2356        builder.field(name4, value4);
2357        builder.field(name5, value5);
2358        builder.finish()
2359    }
2360
2361    /// Shrinks `derive(Debug)` code, for faster compilation and smaller binaries.
2362    /// For the cases not covered by `debug_struct_field[12345]_finish`.
2363    #[doc(hidden)]
2364    #[unstable(feature = "fmt_helpers_for_derive", issue = "none")]
2365    pub fn debug_struct_fields_finish<'b>(
2366        &'b mut self,
2367        name: &str,
2368        names: &[&str],
2369        values: &[&dyn Debug],
2370    ) -> Result {
2371        assert_eq!(names.len(), values.len());
2372        let mut builder = builders::debug_struct_new(self, name);
2373        for (name, value) in iter::zip(names, values) {
2374            builder.field(name, value);
2375        }
2376        builder.finish()
2377    }
2378
2379    /// Creates a `DebugTuple` builder designed to assist with creation of
2380    /// `fmt::Debug` implementations for tuple structs.
2381    ///
2382    /// # Examples
2383    ///
2384    /// ```rust
2385    /// use std::fmt;
2386    /// use std::marker::PhantomData;
2387    ///
2388    /// struct Foo<T>(i32, String, PhantomData<T>);
2389    ///
2390    /// impl<T> fmt::Debug for Foo<T> {
2391    ///     fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
2392    ///         fmt.debug_tuple("Foo")
2393    ///             .field(&self.0)
2394    ///             .field(&self.1)
2395    ///             .field(&format_args!("_"))
2396    ///             .finish()
2397    ///     }
2398    /// }
2399    ///
2400    /// assert_eq!(
2401    ///     "Foo(10, \"Hello\", _)",
2402    ///     format!("{:?}", Foo(10, "Hello".to_string(), PhantomData::<u8>))
2403    /// );
2404    /// ```
2405    #[stable(feature = "debug_builders", since = "1.2.0")]
2406    pub fn debug_tuple<'b>(&'b mut self, name: &str) -> DebugTuple<'b, 'a> {
2407        builders::debug_tuple_new(self, name)
2408    }
2409
2410    /// Shrinks `derive(Debug)` code, for faster compilation and smaller
2411    /// binaries. `debug_tuple_fields_finish` is more general, but this is faster
2412    /// for 1 field.
2413    #[doc(hidden)]
2414    #[unstable(feature = "fmt_helpers_for_derive", issue = "none")]
2415    pub fn debug_tuple_field1_finish<'b>(&'b mut self, name: &str, value1: &dyn Debug) -> Result {
2416        let mut builder = builders::debug_tuple_new(self, name);
2417        builder.field(value1);
2418        builder.finish()
2419    }
2420
2421    /// Shrinks `derive(Debug)` code, for faster compilation and smaller
2422    /// binaries. `debug_tuple_fields_finish` is more general, but this is faster
2423    /// for 2 fields.
2424    #[doc(hidden)]
2425    #[unstable(feature = "fmt_helpers_for_derive", issue = "none")]
2426    pub fn debug_tuple_field2_finish<'b>(
2427        &'b mut self,
2428        name: &str,
2429        value1: &dyn Debug,
2430        value2: &dyn Debug,
2431    ) -> Result {
2432        let mut builder = builders::debug_tuple_new(self, name);
2433        builder.field(value1);
2434        builder.field(value2);
2435        builder.finish()
2436    }
2437
2438    /// Shrinks `derive(Debug)` code, for faster compilation and smaller
2439    /// binaries. `debug_tuple_fields_finish` is more general, but this is faster
2440    /// for 3 fields.
2441    #[doc(hidden)]
2442    #[unstable(feature = "fmt_helpers_for_derive", issue = "none")]
2443    pub fn debug_tuple_field3_finish<'b>(
2444        &'b mut self,
2445        name: &str,
2446        value1: &dyn Debug,
2447        value2: &dyn Debug,
2448        value3: &dyn Debug,
2449    ) -> Result {
2450        let mut builder = builders::debug_tuple_new(self, name);
2451        builder.field(value1);
2452        builder.field(value2);
2453        builder.field(value3);
2454        builder.finish()
2455    }
2456
2457    /// Shrinks `derive(Debug)` code, for faster compilation and smaller
2458    /// binaries. `debug_tuple_fields_finish` is more general, but this is faster
2459    /// for 4 fields.
2460    #[doc(hidden)]
2461    #[unstable(feature = "fmt_helpers_for_derive", issue = "none")]
2462    pub fn debug_tuple_field4_finish<'b>(
2463        &'b mut self,
2464        name: &str,
2465        value1: &dyn Debug,
2466        value2: &dyn Debug,
2467        value3: &dyn Debug,
2468        value4: &dyn Debug,
2469    ) -> Result {
2470        let mut builder = builders::debug_tuple_new(self, name);
2471        builder.field(value1);
2472        builder.field(value2);
2473        builder.field(value3);
2474        builder.field(value4);
2475        builder.finish()
2476    }
2477
2478    /// Shrinks `derive(Debug)` code, for faster compilation and smaller
2479    /// binaries. `debug_tuple_fields_finish` is more general, but this is faster
2480    /// for 5 fields.
2481    #[doc(hidden)]
2482    #[unstable(feature = "fmt_helpers_for_derive", issue = "none")]
2483    pub fn debug_tuple_field5_finish<'b>(
2484        &'b mut self,
2485        name: &str,
2486        value1: &dyn Debug,
2487        value2: &dyn Debug,
2488        value3: &dyn Debug,
2489        value4: &dyn Debug,
2490        value5: &dyn Debug,
2491    ) -> Result {
2492        let mut builder = builders::debug_tuple_new(self, name);
2493        builder.field(value1);
2494        builder.field(value2);
2495        builder.field(value3);
2496        builder.field(value4);
2497        builder.field(value5);
2498        builder.finish()
2499    }
2500
2501    /// Shrinks `derive(Debug)` code, for faster compilation and smaller
2502    /// binaries. For the cases not covered by `debug_tuple_field[12345]_finish`.
2503    #[doc(hidden)]
2504    #[unstable(feature = "fmt_helpers_for_derive", issue = "none")]
2505    pub fn debug_tuple_fields_finish<'b>(
2506        &'b mut self,
2507        name: &str,
2508        values: &[&dyn Debug],
2509    ) -> Result {
2510        let mut builder = builders::debug_tuple_new(self, name);
2511        for value in values {
2512            builder.field(value);
2513        }
2514        builder.finish()
2515    }
2516
2517    /// Creates a `DebugList` builder designed to assist with creation of
2518    /// `fmt::Debug` implementations for list-like structures.
2519    ///
2520    /// # Examples
2521    ///
2522    /// ```rust
2523    /// use std::fmt;
2524    ///
2525    /// struct Foo(Vec<i32>);
2526    ///
2527    /// impl fmt::Debug for Foo {
2528    ///     fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
2529    ///         fmt.debug_list().entries(self.0.iter()).finish()
2530    ///     }
2531    /// }
2532    ///
2533    /// assert_eq!(format!("{:?}", Foo(vec![10, 11])), "[10, 11]");
2534    /// ```
2535    #[stable(feature = "debug_builders", since = "1.2.0")]
2536    pub fn debug_list<'b>(&'b mut self) -> DebugList<'b, 'a> {
2537        builders::debug_list_new(self)
2538    }
2539
2540    /// Creates a `DebugSet` builder designed to assist with creation of
2541    /// `fmt::Debug` implementations for set-like structures.
2542    ///
2543    /// # Examples
2544    ///
2545    /// ```rust
2546    /// use std::fmt;
2547    ///
2548    /// struct Foo(Vec<i32>);
2549    ///
2550    /// impl fmt::Debug for Foo {
2551    ///     fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
2552    ///         fmt.debug_set().entries(self.0.iter()).finish()
2553    ///     }
2554    /// }
2555    ///
2556    /// assert_eq!(format!("{:?}", Foo(vec![10, 11])), "{10, 11}");
2557    /// ```
2558    ///
2559    /// [`format_args!`]: crate::format_args
2560    ///
2561    /// In this more complex example, we use [`format_args!`] and `.debug_set()`
2562    /// to build a list of match arms:
2563    ///
2564    /// ```rust
2565    /// use std::fmt;
2566    ///
2567    /// struct Arm<'a, L, R>(&'a (L, R));
2568    /// struct Table<'a, K, V>(&'a [(K, V)], V);
2569    ///
2570    /// impl<'a, L, R> fmt::Debug for Arm<'a, L, R>
2571    /// where
2572    ///     L: 'a + fmt::Debug, R: 'a + fmt::Debug
2573    /// {
2574    ///     fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
2575    ///         L::fmt(&(self.0).0, fmt)?;
2576    ///         fmt.write_str(" => ")?;
2577    ///         R::fmt(&(self.0).1, fmt)
2578    ///     }
2579    /// }
2580    ///
2581    /// impl<'a, K, V> fmt::Debug for Table<'a, K, V>
2582    /// where
2583    ///     K: 'a + fmt::Debug, V: 'a + fmt::Debug
2584    /// {
2585    ///     fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
2586    ///         fmt.debug_set()
2587    ///         .entries(self.0.iter().map(Arm))
2588    ///         .entry(&Arm(&(format_args!("_"), &self.1)))
2589    ///         .finish()
2590    ///     }
2591    /// }
2592    /// ```
2593    #[stable(feature = "debug_builders", since = "1.2.0")]
2594    pub fn debug_set<'b>(&'b mut self) -> DebugSet<'b, 'a> {
2595        builders::debug_set_new(self)
2596    }
2597
2598    /// Creates a `DebugMap` builder designed to assist with creation of
2599    /// `fmt::Debug` implementations for map-like structures.
2600    ///
2601    /// # Examples
2602    ///
2603    /// ```rust
2604    /// use std::fmt;
2605    ///
2606    /// struct Foo(Vec<(String, i32)>);
2607    ///
2608    /// impl fmt::Debug for Foo {
2609    ///     fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
2610    ///         fmt.debug_map().entries(self.0.iter().map(|&(ref k, ref v)| (k, v))).finish()
2611    ///     }
2612    /// }
2613    ///
2614    /// assert_eq!(
2615    ///     format!("{:?}",  Foo(vec![("A".to_string(), 10), ("B".to_string(), 11)])),
2616    ///     r#"{"A": 10, "B": 11}"#
2617    ///  );
2618    /// ```
2619    #[stable(feature = "debug_builders", since = "1.2.0")]
2620    pub fn debug_map<'b>(&'b mut self) -> DebugMap<'b, 'a> {
2621        builders::debug_map_new(self)
2622    }
2623
2624    /// Returns the sign of this formatter (`+` or `-`).
2625    #[unstable(feature = "formatting_options", issue = "118117")]
2626    pub const fn sign(&self) -> Option<Sign> {
2627        self.options.get_sign()
2628    }
2629
2630    /// Returns the formatting options this formatter corresponds to.
2631    #[unstable(feature = "formatting_options", issue = "118117")]
2632    pub const fn options(&self) -> FormattingOptions {
2633        self.options
2634    }
2635}
2636
2637#[stable(since = "1.2.0", feature = "formatter_write")]
2638impl Write for Formatter<'_> {
2639    fn write_str(&mut self, s: &str) -> Result {
2640        self.buf.write_str(s)
2641    }
2642
2643    fn write_char(&mut self, c: char) -> Result {
2644        self.buf.write_char(c)
2645    }
2646
2647    #[inline]
2648    fn write_fmt(&mut self, args: Arguments<'_>) -> Result {
2649        if let Some(s) = args.as_statically_known_str() {
2650            self.buf.write_str(s)
2651        } else {
2652            write(self.buf, args)
2653        }
2654    }
2655}
2656
2657#[stable(feature = "rust1", since = "1.0.0")]
2658impl Display for Error {
2659    fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2660        Display::fmt("an error occurred when formatting an argument", f)
2661    }
2662}
2663
2664// Implementations of the core formatting traits
2665
2666macro_rules! fmt_refs {
2667    ($($tr:ident),*) => {
2668        $(
2669        #[stable(feature = "rust1", since = "1.0.0")]
2670        impl<T: ?Sized + $tr> $tr for &T {
2671            fn fmt(&self, f: &mut Formatter<'_>) -> Result { $tr::fmt(&**self, f) }
2672        }
2673        #[stable(feature = "rust1", since = "1.0.0")]
2674        impl<T: ?Sized + $tr> $tr for &mut T {
2675            fn fmt(&self, f: &mut Formatter<'_>) -> Result { $tr::fmt(&**self, f) }
2676        }
2677        )*
2678    }
2679}
2680
2681fmt_refs! { Debug, Display, Octal, Binary, LowerHex, UpperHex, LowerExp, UpperExp }
2682
2683#[unstable(feature = "never_type", issue = "35121")]
2684impl Debug for ! {
2685    #[inline]
2686    fn fmt(&self, _: &mut Formatter<'_>) -> Result {
2687        *self
2688    }
2689}
2690
2691#[unstable(feature = "never_type", issue = "35121")]
2692impl Display for ! {
2693    #[inline]
2694    fn fmt(&self, _: &mut Formatter<'_>) -> Result {
2695        *self
2696    }
2697}
2698
2699#[stable(feature = "rust1", since = "1.0.0")]
2700impl Debug for bool {
2701    #[inline]
2702    fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2703        Display::fmt(self, f)
2704    }
2705}
2706
2707#[stable(feature = "rust1", since = "1.0.0")]
2708impl Display for bool {
2709    fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2710        Display::fmt(if *self { "true" } else { "false" }, f)
2711    }
2712}
2713
2714#[stable(feature = "rust1", since = "1.0.0")]
2715impl Debug for str {
2716    fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2717        f.write_char('"')?;
2718
2719        // substring we know is printable
2720        let mut printable_range = 0..0;
2721
2722        fn needs_escape(b: u8) -> bool {
2723            b > 0x7E || b < 0x20 || b == b'\\' || b == b'"'
2724        }
2725
2726        // the loop here first skips over runs of printable ASCII as a fast path.
2727        // other chars (unicode, or ASCII that needs escaping) are then handled per-`char`.
2728        let mut rest = self;
2729        while rest.len() > 0 {
2730            let Some(non_printable_start) = rest.as_bytes().iter().position(|&b| needs_escape(b))
2731            else {
2732                printable_range.end += rest.len();
2733                break;
2734            };
2735
2736            printable_range.end += non_printable_start;
2737            // SAFETY: the position was derived from an iterator, so is known to be within bounds, and at a char boundary
2738            rest = unsafe { rest.get_unchecked(non_printable_start..) };
2739
2740            let mut chars = rest.chars();
2741            if let Some(c) = chars.next() {
2742                let esc = c.escape_debug_ext(EscapeDebugExtArgs {
2743                    escape_grapheme_extended: true,
2744                    escape_single_quote: false,
2745                    escape_double_quote: true,
2746                });
2747                if esc.len() != 1 {
2748                    f.write_str(&self[printable_range.clone()])?;
2749                    Display::fmt(&esc, f)?;
2750                    printable_range.start = printable_range.end + c.len_utf8();
2751                }
2752                printable_range.end += c.len_utf8();
2753            }
2754            rest = chars.as_str();
2755        }
2756
2757        f.write_str(&self[printable_range])?;
2758
2759        f.write_char('"')
2760    }
2761}
2762
2763#[stable(feature = "rust1", since = "1.0.0")]
2764impl Display for str {
2765    fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2766        f.pad(self)
2767    }
2768}
2769
2770#[stable(feature = "rust1", since = "1.0.0")]
2771impl Debug for char {
2772    fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2773        f.write_char('\'')?;
2774        let esc = self.escape_debug_ext(EscapeDebugExtArgs {
2775            escape_grapheme_extended: true,
2776            escape_single_quote: true,
2777            escape_double_quote: false,
2778        });
2779        Display::fmt(&esc, f)?;
2780        f.write_char('\'')
2781    }
2782}
2783
2784#[stable(feature = "rust1", since = "1.0.0")]
2785impl Display for char {
2786    fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2787        if f.options.flags & (flags::WIDTH_FLAG | flags::PRECISION_FLAG) == 0 {
2788            f.write_char(*self)
2789        } else {
2790            f.pad(self.encode_utf8(&mut [0; MAX_LEN_UTF8]))
2791        }
2792    }
2793}
2794
2795#[stable(feature = "rust1", since = "1.0.0")]
2796impl<T: ?Sized> Pointer for *const T {
2797    fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2798        if <<T as core::ptr::Pointee>::Metadata as core::unit::IsUnit>::is_unit() {
2799            pointer_fmt_inner(self.expose_provenance(), f)
2800        } else {
2801            f.debug_struct("Pointer")
2802                .field_with("addr", |f| pointer_fmt_inner(self.expose_provenance(), f))
2803                .field("metadata", &core::ptr::metadata(*self))
2804                .finish()
2805        }
2806    }
2807}
2808
2809/// Since the formatting will be identical for all pointer types, uses a
2810/// non-monomorphized implementation for the actual formatting to reduce the
2811/// amount of codegen work needed.
2812///
2813/// This uses `ptr_addr: usize` and not `ptr: *const ()` to be able to use this for
2814/// `fn(...) -> ...` without using [problematic] "Oxford Casts".
2815///
2816/// [problematic]: https://github.com/rust-lang/rust/issues/95489
2817pub(crate) fn pointer_fmt_inner(ptr_addr: usize, f: &mut Formatter<'_>) -> Result {
2818    let old_options = f.options;
2819
2820    // The alternate flag is already treated by LowerHex as being special-
2821    // it denotes whether to prefix with 0x. We use it to work out whether
2822    // or not to zero extend, and then unconditionally set it to get the
2823    // prefix.
2824    if f.options.get_alternate() {
2825        f.options.sign_aware_zero_pad(true);
2826
2827        if f.options.get_width().is_none() {
2828            f.options.width(Some((usize::BITS / 4) as u16 + 2));
2829        }
2830    }
2831    f.options.alternate(true);
2832
2833    let ret = LowerHex::fmt(&ptr_addr, f);
2834
2835    f.options = old_options;
2836
2837    ret
2838}
2839
2840#[stable(feature = "rust1", since = "1.0.0")]
2841impl<T: ?Sized> Pointer for *mut T {
2842    fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2843        Pointer::fmt(&(*self as *const T), f)
2844    }
2845}
2846
2847#[stable(feature = "rust1", since = "1.0.0")]
2848impl<T: ?Sized> Pointer for &T {
2849    fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2850        Pointer::fmt(&(*self as *const T), f)
2851    }
2852}
2853
2854#[stable(feature = "rust1", since = "1.0.0")]
2855impl<T: ?Sized> Pointer for &mut T {
2856    fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2857        Pointer::fmt(&(&**self as *const T), f)
2858    }
2859}
2860
2861// Implementation of Display/Debug for various core types
2862
2863#[stable(feature = "rust1", since = "1.0.0")]
2864impl<T: ?Sized> Debug for *const T {
2865    fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2866        Pointer::fmt(self, f)
2867    }
2868}
2869#[stable(feature = "rust1", since = "1.0.0")]
2870impl<T: ?Sized> Debug for *mut T {
2871    fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2872        Pointer::fmt(self, f)
2873    }
2874}
2875
2876macro_rules! peel {
2877    ($name:ident, $($other:ident,)*) => (tuple! { $($other,)* })
2878}
2879
2880macro_rules! tuple {
2881    () => ();
2882    ( $($name:ident,)+ ) => (
2883        maybe_tuple_doc! {
2884            $($name)+ @
2885            #[stable(feature = "rust1", since = "1.0.0")]
2886            impl<$($name:Debug),+> Debug for ($($name,)+) where last_type!($($name,)+): ?Sized {
2887                #[allow(non_snake_case, unused_assignments)]
2888                fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2889                    let mut builder = f.debug_tuple("");
2890                    let ($(ref $name,)+) = *self;
2891                    $(
2892                        builder.field(&$name);
2893                    )+
2894
2895                    builder.finish()
2896                }
2897            }
2898        }
2899        peel! { $($name,)+ }
2900    )
2901}
2902
2903macro_rules! maybe_tuple_doc {
2904    ($a:ident @ #[$meta:meta] $item:item) => {
2905        #[doc(fake_variadic)]
2906        #[doc = "This trait is implemented for tuples up to twelve items long."]
2907        #[$meta]
2908        $item
2909    };
2910    ($a:ident $($rest_a:ident)+ @ #[$meta:meta] $item:item) => {
2911        #[doc(hidden)]
2912        #[$meta]
2913        $item
2914    };
2915}
2916
2917macro_rules! last_type {
2918    ($a:ident,) => { $a };
2919    ($a:ident, $($rest_a:ident,)+) => { last_type!($($rest_a,)+) };
2920}
2921
2922tuple! { E, D, C, B, A, Z, Y, X, W, V, U, T, }
2923
2924#[stable(feature = "rust1", since = "1.0.0")]
2925impl<T: Debug> Debug for [T] {
2926    fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2927        f.debug_list().entries(self.iter()).finish()
2928    }
2929}
2930
2931#[stable(feature = "rust1", since = "1.0.0")]
2932impl Debug for () {
2933    #[inline]
2934    fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2935        f.pad("()")
2936    }
2937}
2938#[stable(feature = "rust1", since = "1.0.0")]
2939impl<T: ?Sized> Debug for PhantomData<T> {
2940    fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2941        write!(f, "PhantomData<{}>", crate::any::type_name::<T>())
2942    }
2943}
2944
2945#[stable(feature = "rust1", since = "1.0.0")]
2946impl<T: Copy + Debug> Debug for Cell<T> {
2947    fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2948        f.debug_struct("Cell").field("value", &self.get()).finish()
2949    }
2950}
2951
2952#[stable(feature = "rust1", since = "1.0.0")]
2953impl<T: ?Sized + Debug> Debug for RefCell<T> {
2954    fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2955        let mut d = f.debug_struct("RefCell");
2956        match self.try_borrow() {
2957            Ok(borrow) => d.field("value", &borrow),
2958            Err(_) => d.field("value", &format_args!("<borrowed>")),
2959        };
2960        d.finish()
2961    }
2962}
2963
2964#[stable(feature = "rust1", since = "1.0.0")]
2965impl<T: ?Sized + Debug> Debug for Ref<'_, T> {
2966    fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2967        Debug::fmt(&**self, f)
2968    }
2969}
2970
2971#[stable(feature = "rust1", since = "1.0.0")]
2972impl<T: ?Sized + Debug> Debug for RefMut<'_, T> {
2973    fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2974        Debug::fmt(&*(self.deref()), f)
2975    }
2976}
2977
2978#[stable(feature = "core_impl_debug", since = "1.9.0")]
2979impl<T: ?Sized> Debug for UnsafeCell<T> {
2980    fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2981        f.debug_struct("UnsafeCell").finish_non_exhaustive()
2982    }
2983}
2984
2985#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
2986impl<T: ?Sized> Debug for SyncUnsafeCell<T> {
2987    fn fmt(&self, f: &mut Formatter<'_>) -> Result {
2988        f.debug_struct("SyncUnsafeCell").finish_non_exhaustive()
2989    }
2990}
2991
2992// If you expected tests to be here, look instead at coretests/tests/fmt/;
2993// it's a lot easier than creating all of the rt::Piece structures here.
2994// There are also tests in alloctests/tests/fmt.rs, for those that need allocations.