1//! Utilities for formatting and printing `String`s.
2//!
3//! This module contains the runtime support for the [`format!`] syntax extension.
4//! This macro is implemented in the compiler to emit calls to this module in
5//! order to format arguments at runtime into strings.
6//!
7//! # Usage
8//!
9//! The [`format!`] macro is intended to be familiar to those coming from C's
10//! `printf`/`fprintf` functions or Python's `str.format` function.
11//!
12//! Some examples of the [`format!`] extension are:
13//!
14//! ```
15//! # #![allow(unused_must_use)]
16//! format!("Hello"); // => "Hello"
17//! format!("Hello, {}!", "world"); // => "Hello, world!"
18//! format!("The number is {}", 1); // => "The number is 1"
19//! format!("{:?}", (3, 4)); // => "(3, 4)"
20//! format!("{value}", value=4); // => "4"
21//! let people = "Rustaceans";
22//! format!("Hello {people}!"); // => "Hello Rustaceans!"
23//! format!("{} {}", 1, 2); // => "1 2"
24//! format!("{:04}", 42); // => "0042" with leading zeros
25//! format!("{:#?}", (100, 200)); // => "(
26//! // 100,
27//! // 200,
28//! // )"
29//! ```
30//!
31//! From these, you can see that the first argument is a format string. It is
32//! required by the compiler for this to be a string literal; it cannot be a
33//! variable passed in (in order to perform validity checking). The compiler
34//! will then parse the format string and determine if the list of arguments
35//! provided is suitable to pass to this format string.
36//!
37//! To convert a single value to a string, use the [`to_string`] method. This
38//! will use the [`Display`] formatting trait.
39//!
40//! ## Positional parameters
41//!
42//! Each formatting argument is allowed to specify which value argument it's
43//! referencing, and if omitted it is assumed to be "the next argument". For
44//! example, the format string `{} {} {}` would take three parameters, and they
45//! would be formatted in the same order as they're given. The format string
46//! `{2} {1} {0}`, however, would format arguments in reverse order.
47//!
48//! Things can get a little tricky once you start intermingling the two types of
49//! positional specifiers. The "next argument" specifier can be thought of as an
50//! iterator over the argument. Each time a "next argument" specifier is seen,
51//! the iterator advances. This leads to behavior like this:
52//!
53//! ```
54//! # #![allow(unused_must_use)]
55//! format!("{1} {} {0} {}", 1, 2); // => "2 1 1 2"
56//! ```
57//!
58//! The internal iterator over the argument has not been advanced by the time
59//! the first `{}` is seen, so it prints the first argument. Then upon reaching
60//! the second `{}`, the iterator has advanced forward to the second argument.
61//! Essentially, parameters that explicitly name their argument do not affect
62//! parameters that do not name an argument in terms of positional specifiers.
63//!
64//! A format string is required to use all of its arguments, otherwise it is a
65//! compile-time error. You may refer to the same argument more than once in the
66//! format string.
67//!
68//! ## Named parameters
69//!
70//! Rust itself does not have a Python-like equivalent of named parameters to a
71//! function, but the [`format!`] macro is a syntax extension that allows it to
72//! leverage named parameters. Named parameters are listed at the end of the
73//! argument list and have the syntax:
74//!
75//! ```text
76//! identifier '=' expression
77//! ```
78//!
79//! For example, the following [`format!`] expressions all use named arguments:
80//!
81//! ```
82//! # #![allow(unused_must_use)]
83//! format!("{argument}", argument = "test"); // => "test"
84//! format!("{name} {}", 1, name = 2); // => "2 1"
85//! format!("{a} {c} {b}", a="a", b='b', c=3); // => "a 3 b"
86//! ```
87//!
88//! If a named parameter does not appear in the argument list, `format!` will
89//! reference a variable with that name in the current scope.
90//!
91//! ```
92//! # #![allow(unused_must_use)]
93//! let argument = 2 + 2;
94//! format!("{argument}"); // => "4"
95//!
96//! fn make_string(a: u32, b: &str) -> String {
97//! format!("{b} {a}")
98//! }
99//! make_string(927, "label"); // => "label 927"
100//! ```
101//!
102//! It is not valid to put positional parameters (those without names) after
103//! arguments that have names. Like with positional parameters, it is not
104//! valid to provide named parameters that are unused by the format string.
105//!
106//! # Formatting Parameters
107//!
108//! Each argument being formatted can be transformed by a number of formatting
109//! parameters (corresponding to `format_spec` in [the syntax](#syntax)). These
110//! parameters affect the string representation of what's being formatted.
111//!
112//! The colon `:` in format syntax divides identifier of the input data and
113//! the formatting options, the colon itself does not change anything, only
114//! introduces the options.
115//!
116//! ```
117//! let a = 5;
118//! let b = &a;
119//! println!("{a:e} {b:p}"); // => 5e0 0x7ffe37b7273c
120//! ```
121//!
122//! ## Width
123//!
124//! ```
125//! // All of these print "Hello x !"
126//! println!("Hello {:5}!", "x");
127//! println!("Hello {:1$}!", "x", 5);
128//! println!("Hello {1:0$}!", 5, "x");
129//! println!("Hello {:width$}!", "x", width = 5);
130//! let width = 5;
131//! println!("Hello {:width$}!", "x");
132//! ```
133//!
134//! This is a parameter for the "minimum width" that the format should take up.
135//! If the value's string does not fill up this many characters, then the
136//! padding specified by fill/alignment will be used to take up the required
137//! space (see below).
138//!
139//! The value for the width can also be provided as a [`usize`] in the list of
140//! parameters by adding a postfix `$`, indicating that the second argument is
141//! a [`usize`] specifying the width.
142//!
143//! Referring to an argument with the dollar syntax does not affect the "next
144//! argument" counter, so it's usually a good idea to refer to arguments by
145//! position, or use named arguments.
146//!
147//! ## Fill/Alignment
148//!
149//! ```
150//! assert_eq!(format!("Hello {:<5}!", "x"), "Hello x !");
151//! assert_eq!(format!("Hello {:-<5}!", "x"), "Hello x----!");
152//! assert_eq!(format!("Hello {:^5}!", "x"), "Hello x !");
153//! assert_eq!(format!("Hello {:>5}!", "x"), "Hello x!");
154//! ```
155//!
156//! The optional fill character and alignment is provided normally in conjunction with the
157//! [`width`](#width) parameter. It must be defined before `width`, right after the `:`.
158//! This indicates that if the value being formatted is smaller than
159//! `width` some extra characters will be printed around it.
160//! Filling comes in the following variants for different alignments:
161//!
162//! * `[fill]<` - the argument is left-aligned in `width` columns
163//! * `[fill]^` - the argument is center-aligned in `width` columns
164//! * `[fill]>` - the argument is right-aligned in `width` columns
165//!
166//! The default [fill/alignment](#fillalignment) for non-numerics is a space and
167//! left-aligned. The
168//! default for numeric formatters is also a space character but with right-alignment. If
169//! the `0` flag (see below) is specified for numerics, then the implicit fill character is
170//! `0`.
171//!
172//! Note that alignment might not be implemented by some types. In particular, it
173//! is not generally implemented for the `Debug` trait. A good way to ensure
174//! padding is applied is to format your input, then pad this resulting string
175//! to obtain your output:
176//!
177//! ```
178//! println!("Hello {:^15}!", format!("{:?}", Some("hi"))); // => "Hello Some("hi") !"
179//! ```
180//!
181//! ## Sign/`#`/`0`
182//!
183//! ```
184//! assert_eq!(format!("Hello {:+}!", 5), "Hello +5!");
185//! assert_eq!(format!("{:#x}!", 27), "0x1b!");
186//! assert_eq!(format!("Hello {:05}!", 5), "Hello 00005!");
187//! assert_eq!(format!("Hello {:05}!", -5), "Hello -0005!");
188//! assert_eq!(format!("{:#010x}!", 27), "0x0000001b!");
189//! ```
190//!
191//! These are all flags altering the behavior of the formatter.
192//!
193//! * `+` - This is intended for numeric types and indicates that the sign
194//! should always be printed. By default only the negative sign of signed values
195//! is printed, and the sign of positive or unsigned values is omitted.
196//! This flag indicates that the correct sign (`+` or `-`) should always be printed.
197//! * `-` - Currently not used
198//! * `#` - This flag indicates that the "alternate" form of printing should
199//! be used. The alternate forms are:
200//! * `#?` - pretty-print the [`Debug`] formatting (adds linebreaks and indentation)
201//! * `#x` - precedes the argument with a `0x`
202//! * `#X` - precedes the argument with a `0x`
203//! * `#b` - precedes the argument with a `0b`
204//! * `#o` - precedes the argument with a `0o`
205//!
206//! See [Formatting traits](#formatting-traits) for a description of what the `?`, `x`, `X`,
207//! `b`, and `o` flags do.
208//!
209//! * `0` - This is used to indicate for integer formats that the padding to `width` should
210//! both be done with a `0` character as well as be sign-aware. A format
211//! like `{:08}` would yield `00000001` for the integer `1`, while the
212//! same format would yield `-0000001` for the integer `-1`. Notice that
213//! the negative version has one fewer zero than the positive version.
214//! Note that padding zeros are always placed after the sign (if any)
215//! and before the digits. When used together with the `#` flag, a similar
216//! rule applies: padding zeros are inserted after the prefix but before
217//! the digits. The prefix is included in the total width.
218//! This flag overrides the [fill character and alignment flag](#fillalignment).
219//!
220//! ## Precision
221//!
222//! For non-numeric types, this can be considered a "maximum width". If the resulting string is
223//! longer than this width, then it is truncated down to this many characters and that truncated
224//! value is emitted with proper `fill`, `alignment` and `width` if those parameters are set.
225//!
226//! For integral types, this is ignored.
227//!
228//! For floating-point types, this indicates how many digits after the decimal point should be
229//! printed.
230//!
231//! There are three possible ways to specify the desired `precision`:
232//!
233//! 1. An integer `.N`:
234//!
235//! the integer `N` itself is the precision.
236//!
237//! 2. An integer or name followed by dollar sign `.N$`:
238//!
239//! use format *argument* `N` (which must be a `usize`) as the precision.
240//!
241//! 3. An asterisk `.*`:
242//!
243//! `.*` means that this `{...}` is associated with *two* format inputs rather than one:
244//! - If a format string in the fashion of `{:<spec>.*}` is used, then the first input holds
245//! the `usize` precision, and the second holds the value to print.
246//! - If a format string in the fashion of `{<arg>:<spec>.*}` is used, then the `<arg>` part
247//! refers to the value to print, and the `precision` is taken like it was specified with an
248//! omitted positional parameter (`{}` instead of `{<arg>:}`).
249//!
250//! For example, the following calls all print the same thing `Hello x is 0.01000`:
251//!
252//! ```
253//! // Hello {arg 0 ("x")} is {arg 1 (0.01) with precision specified inline (5)}
254//! println!("Hello {0} is {1:.5}", "x", 0.01);
255//!
256//! // Hello {arg 1 ("x")} is {arg 2 (0.01) with precision specified in arg 0 (5)}
257//! println!("Hello {1} is {2:.0$}", 5, "x", 0.01);
258//!
259//! // Hello {arg 0 ("x")} is {arg 2 (0.01) with precision specified in arg 1 (5)}
260//! println!("Hello {0} is {2:.1$}", "x", 5, 0.01);
261//!
262//! // Hello {next arg -> arg 0 ("x")} is {second of next two args -> arg 2 (0.01) with precision
263//! // specified in first of next two args -> arg 1 (5)}
264//! println!("Hello {} is {:.*}", "x", 5, 0.01);
265//!
266//! // Hello {arg 1 ("x")} is {arg 2 (0.01) with precision
267//! // specified in next arg -> arg 0 (5)}
268//! println!("Hello {1} is {2:.*}", 5, "x", 0.01);
269//!
270//! // Hello {next arg -> arg 0 ("x")} is {arg 2 (0.01) with precision
271//! // specified in next arg -> arg 1 (5)}
272//! println!("Hello {} is {2:.*}", "x", 5, 0.01);
273//!
274//! // Hello {next arg -> arg 0 ("x")} is {arg "number" (0.01) with precision specified
275//! // in arg "prec" (5)}
276//! println!("Hello {} is {number:.prec$}", "x", prec = 5, number = 0.01);
277//! ```
278//!
279//! While these:
280//!
281//! ```
282//! println!("{}, `{name:.*}` has 3 fractional digits", "Hello", 3, name=1234.56);
283//! println!("{}, `{name:.*}` has 3 characters", "Hello", 3, name="1234.56");
284//! println!("{}, `{name:>8.*}` has 3 right-aligned characters", "Hello", 3, name="1234.56");
285//! ```
286//!
287//! print three significantly different things:
288//!
289//! ```text
290//! Hello, `1234.560` has 3 fractional digits
291//! Hello, `123` has 3 characters
292//! Hello, ` 123` has 3 right-aligned characters
293//! ```
294//!
295//! When truncating these values, Rust uses [round half-to-even](https://en.wikipedia.org/wiki/Rounding#Rounding_half_to_even),
296//! which is the default rounding mode in IEEE 754.
297//! For example,
298//!
299//! ```
300//! print!("{0:.1$e}", 12345, 3);
301//! print!("{0:.1$e}", 12355, 3);
302//! ```
303//!
304//! Would return:
305//!
306//! ```text
307//! 1.234e4
308//! 1.236e4
309//! ```
310//!
311//! ## Localization
312//!
313//! In some programming languages, the behavior of string formatting functions
314//! depends on the operating system's locale setting. The format functions
315//! provided by Rust's standard library do not have any concept of locale and
316//! will produce the same results on all systems regardless of user
317//! configuration.
318//!
319//! For example, the following code will always print `1.5` even if the system
320//! locale uses a decimal separator other than a dot.
321//!
322//! ```
323//! println!("The value is {}", 1.5);
324//! ```
325//!
326//! # Escaping
327//!
328//! The literal characters `{` and `}` may be included in a string by preceding
329//! them with the same character. For example, the `{` character is escaped with
330//! `{{` and the `}` character is escaped with `}}`.
331//!
332//! ```
333//! assert_eq!(format!("Hello {{}}"), "Hello {}");
334//! assert_eq!(format!("{{ Hello"), "{ Hello");
335//! ```
336//!
337//! # Syntax
338//!
339//! To summarize, here you can find the full grammar of format strings.
340//! The syntax for the formatting language used is drawn from other languages,
341//! so it should not be too alien. Arguments are formatted with Python-like
342//! syntax, meaning that arguments are surrounded by `{}` instead of the C-like
343//! `%`. The actual grammar for the formatting syntax is:
344//!
345//! ```text
346//! format_string := text [ maybe_format text ] *
347//! maybe_format := '{' '{' | '}' '}' | format
348//! format := '{' [ argument ] [ ':' format_spec ] [ ws ] * '}'
349//! argument := integer | identifier
350//!
351//! format_spec := [[fill]align][sign]['#']['0'][width]['.' precision]type
352//! fill := character
353//! align := '<' | '^' | '>'
354//! sign := '+' | '-'
355//! width := count
356//! precision := count | '*'
357//! type := '' | '?' | 'x?' | 'X?' | identifier
358//! count := parameter | integer
359//! parameter := argument '$'
360//! ```
361//! In the above grammar,
362//! - `text` must not contain any `'{'` or `'}'` characters,
363//! - `ws` is any character for which [`char::is_whitespace`] returns `true`, has no semantic
364//! meaning and is completely optional,
365//! - `integer` is a decimal integer that may contain leading zeroes and must fit into an `usize` and
366//! - `identifier` is an `IDENTIFIER_OR_KEYWORD` (not an `IDENTIFIER`) as defined by the [Rust language reference](https://doc.rust-lang.org/reference/identifiers.html).
367//!
368//! # Formatting traits
369//!
370//! When requesting that an argument be formatted with a particular type, you
371//! are actually requesting that an argument ascribes to a particular trait.
372//! This allows multiple actual types to be formatted via `{:x}` (like [`i8`] as
373//! well as [`isize`]). The current mapping of types to traits is:
374//!
375//! * *nothing* ⇒ [`Display`]
376//! * `?` ⇒ [`Debug`]
377//! * `x?` ⇒ [`Debug`] with lower-case hexadecimal integers
378//! * `X?` ⇒ [`Debug`] with upper-case hexadecimal integers
379//! * `o` ⇒ [`Octal`]
380//! * `x` ⇒ [`LowerHex`]
381//! * `X` ⇒ [`UpperHex`]
382//! * `p` ⇒ [`Pointer`]
383//! * `b` ⇒ [`Binary`]
384//! * `e` ⇒ [`LowerExp`]
385//! * `E` ⇒ [`UpperExp`]
386//!
387//! What this means is that any type of argument which implements the
388//! [`fmt::Binary`][`Binary`] trait can then be formatted with `{:b}`. Implementations
389//! are provided for these traits for a number of primitive types by the
390//! standard library as well. If no format is specified (as in `{}` or `{:6}`),
391//! then the format trait used is the [`Display`] trait.
392//!
393//! When implementing a format trait for your own type, you will have to
394//! implement a method of the signature:
395//!
396//! ```
397//! # #![allow(dead_code)]
398//! # use std::fmt;
399//! # struct Foo; // our custom type
400//! # impl fmt::Display for Foo {
401//! fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
402//! # write!(f, "testing, testing")
403//! # } }
404//! ```
405//!
406//! Your type will be passed as `self` by-reference, and then the function
407//! should emit output into the Formatter `f` which implements `fmt::Write`. It is up to each
408//! format trait implementation to correctly adhere to the requested formatting parameters.
409//! The values of these parameters can be accessed with methods of the
410//! [`Formatter`] struct. In order to help with this, the [`Formatter`] struct also
411//! provides some helper methods.
412//!
413//! Additionally, the return value of this function is [`fmt::Result`] which is a
414//! type alias of <code>[Result]<(), [std::fmt::Error]></code>. Formatting implementations
415//! should ensure that they propagate errors from the [`Formatter`] (e.g., when
416//! calling [`write!`]). However, they should never return errors spuriously. That
417//! is, a formatting implementation must and may only return an error if the
418//! passed-in [`Formatter`] returns an error. This is because, contrary to what
419//! the function signature might suggest, string formatting is an infallible
420//! operation. This function only returns a [`Result`] because writing to the
421//! underlying stream might fail and it must provide a way to propagate the fact
422//! that an error has occurred back up the stack.
423//!
424//! An example of implementing the formatting traits would look
425//! like:
426//!
427//! ```
428//! use std::fmt;
429//!
430//! #[derive(Debug)]
431//! struct Vector2D {
432//! x: isize,
433//! y: isize,
434//! }
435//!
436//! impl fmt::Display for Vector2D {
437//! fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
438//! // The `f` value implements the `Write` trait, which is what the
439//! // write! macro is expecting. Note that this formatting ignores the
440//! // various flags provided to format strings.
441//! write!(f, "({}, {})", self.x, self.y)
442//! }
443//! }
444//!
445//! // Different traits allow different forms of output of a type. The meaning
446//! // of this format is to print the magnitude of a vector.
447//! impl fmt::Binary for Vector2D {
448//! fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
449//! let magnitude = (self.x * self.x + self.y * self.y) as f64;
450//! let magnitude = magnitude.sqrt();
451//!
452//! // Respect the formatting flags by using the helper method
453//! // `pad_integral` on the Formatter object. See the method
454//! // documentation for details, and the function `pad` can be used
455//! // to pad strings.
456//! let decimals = f.precision().unwrap_or(3);
457//! let string = format!("{magnitude:.decimals$}");
458//! f.pad_integral(true, "", &string)
459//! }
460//! }
461//!
462//! fn main() {
463//! let myvector = Vector2D { x: 3, y: 4 };
464//!
465//! println!("{myvector}"); // => "(3, 4)"
466//! println!("{myvector:?}"); // => "Vector2D {x: 3, y:4}"
467//! println!("{myvector:10.3b}"); // => " 5.000"
468//! }
469//! ```
470//!
471//! ### `fmt::Display` vs `fmt::Debug`
472//!
473//! These two formatting traits have distinct purposes:
474//!
475//! - [`fmt::Display`][`Display`] implementations assert that the type can be faithfully
476//! represented as a UTF-8 string at all times. It is **not** expected that
477//! all types implement the [`Display`] trait.
478//! - [`fmt::Debug`][`Debug`] implementations should be implemented for **all** public types.
479//! Output will typically represent the internal state as faithfully as possible.
480//! The purpose of the [`Debug`] trait is to facilitate debugging Rust code. In
481//! most cases, using `#[derive(Debug)]` is sufficient and recommended.
482//!
483//! Some examples of the output from both traits:
484//!
485//! ```
486//! assert_eq!(format!("{} {:?}", 3, 4), "3 4");
487//! assert_eq!(format!("{} {:?}", 'a', 'b'), "a 'b'");
488//! assert_eq!(format!("{} {:?}", "foo\n", "bar\n"), "foo\n \"bar\\n\"");
489//! ```
490//!
491//! # Related macros
492//!
493//! There are a number of related macros in the [`format!`] family. The ones that
494//! are currently implemented are:
495//!
496//! ```ignore (only-for-syntax-highlight)
497//! format! // described above
498//! write! // first argument is either a &mut io::Write or a &mut fmt::Write, the destination
499//! writeln! // same as write but appends a newline
500//! print! // the format string is printed to the standard output
501//! println! // same as print but appends a newline
502//! eprint! // the format string is printed to the standard error
503//! eprintln! // same as eprint but appends a newline
504//! format_args! // described below.
505//! ```
506//!
507//! ### `write!`
508//!
509//! [`write!`] and [`writeln!`] are two macros which are used to emit the format string
510//! to a specified stream. This is used to prevent intermediate allocations of
511//! format strings and instead directly write the output. Under the hood, this
512//! function is actually invoking the [`write_fmt`] function defined on the
513//! [`std::io::Write`] and the [`std::fmt::Write`] trait. Example usage is:
514//!
515//! ```
516//! # #![allow(unused_must_use)]
517//! use std::io::Write;
518//! let mut w = Vec::new();
519//! write!(&mut w, "Hello {}!", "world");
520//! ```
521//!
522//! ### `print!`
523//!
524//! This and [`println!`] emit their output to stdout. Similarly to the [`write!`]
525//! macro, the goal of these macros is to avoid intermediate allocations when
526//! printing output. Example usage is:
527//!
528//! ```
529//! print!("Hello {}!", "world");
530//! println!("I have a newline {}", "character at the end");
531//! ```
532//! ### `eprint!`
533//!
534//! The [`eprint!`] and [`eprintln!`] macros are identical to
535//! [`print!`] and [`println!`], respectively, except they emit their
536//! output to stderr.
537//!
538//! ### `format_args!`
539//!
540//! [`format_args!`] is a curious macro used to safely pass around
541//! an opaque object describing the format string. This object
542//! does not require any heap allocations to create, and it only
543//! references information on the stack. Under the hood, all of
544//! the related macros are implemented in terms of this. First
545//! off, some example usage is:
546//!
547//! ```
548//! # #![allow(unused_must_use)]
549//! use std::fmt;
550//! use std::io::{self, Write};
551//!
552//! let mut some_writer = io::stdout();
553//! write!(&mut some_writer, "{}", format_args!("print with a {}", "macro"));
554//!
555//! fn my_fmt_fn(args: fmt::Arguments<'_>) {
556//! write!(&mut io::stdout(), "{args}");
557//! }
558//! my_fmt_fn(format_args!(", or a {} too", "function"));
559//! ```
560//!
561//! The result of the [`format_args!`] macro is a value of type [`fmt::Arguments`].
562//! This structure can then be passed to the [`write`] and [`format`] functions
563//! inside this module in order to process the format string.
564//! The goal of this macro is to even further prevent intermediate allocations
565//! when dealing with formatting strings.
566//!
567//! For example, a logging library could use the standard formatting syntax, but
568//! it would internally pass around this structure until it has been determined
569//! where output should go to.
570//!
571//! [`fmt::Result`]: Result "fmt::Result"
572//! [Result]: core::result::Result "std::result::Result"
573//! [std::fmt::Error]: Error "fmt::Error"
574//! [`write`]: write() "fmt::write"
575//! [`to_string`]: crate::string::ToString::to_string "ToString::to_string"
576//! [`write_fmt`]: ../../std/io/trait.Write.html#method.write_fmt
577//! [`std::io::Write`]: ../../std/io/trait.Write.html
578//! [`std::fmt::Write`]: ../../std/fmt/trait.Write.html
579//! [`print!`]: ../../std/macro.print.html "print!"
580//! [`println!`]: ../../std/macro.println.html "println!"
581//! [`eprint!`]: ../../std/macro.eprint.html "eprint!"
582//! [`eprintln!`]: ../../std/macro.eprintln.html "eprintln!"
583//! [`format_args!`]: ../../std/macro.format_args.html "format_args!"
584//! [`fmt::Arguments`]: Arguments "fmt::Arguments"
585//! [`format`]: format() "fmt::format"
586
587#![stable(feature = "rust1", since = "1.0.0")]
588
589#[stable(feature = "fmt_flags_align", since = "1.28.0")]
590pub use core::fmt::Alignment;
591#[stable(feature = "rust1", since = "1.0.0")]
592pub use core::fmt::Error;
593#[stable(feature = "rust1", since = "1.0.0")]
594pub use core::fmt::{Arguments, write};
595#[stable(feature = "rust1", since = "1.0.0")]
596pub use core::fmt::{Binary, Octal};
597#[stable(feature = "rust1", since = "1.0.0")]
598pub use core::fmt::{Debug, Display};
599#[unstable(feature = "formatting_options", issue = "118117")]
600pub use core::fmt::{DebugAsHex, FormattingOptions, Sign};
601#[stable(feature = "rust1", since = "1.0.0")]
602pub use core::fmt::{DebugList, DebugMap, DebugSet, DebugStruct, DebugTuple};
603#[stable(feature = "rust1", since = "1.0.0")]
604pub use core::fmt::{Formatter, Result, Write};
605#[unstable(feature = "debug_closure_helpers", issue = "117729")]
606pub use core::fmt::{FromFn, from_fn};
607#[stable(feature = "rust1", since = "1.0.0")]
608pub use core::fmt::{LowerExp, UpperExp};
609#[stable(feature = "rust1", since = "1.0.0")]
610pub use core::fmt::{LowerHex, Pointer, UpperHex};
611
612#[cfg(not(no_global_oom_handling))]
613use crate::string;
614
615/// Takes an [`Arguments`] struct and returns the resulting formatted string.
616///
617/// The [`Arguments`] instance can be created with the [`format_args!`] macro.
618///
619/// # Examples
620///
621/// Basic usage:
622///
623/// ```
624/// use std::fmt;
625///
626/// let s = fmt::format(format_args!("Hello, {}!", "world"));
627/// assert_eq!(s, "Hello, world!");
628/// ```
629///
630/// Please note that using [`format!`] might be preferable.
631/// Example:
632///
633/// ```
634/// let s = format!("Hello, {}!", "world");
635/// assert_eq!(s, "Hello, world!");
636/// ```
637///
638/// [`format_args!`]: core::format_args
639/// [`format!`]: crate::format
640#[cfg(not(no_global_oom_handling))]
641#[must_use]
642#[stable(feature = "rust1", since = "1.0.0")]
643#[inline]
644pub fn format(args: Arguments<'_>) -> string::String {
645 fn format_inner(args: Arguments<'_>) -> string::String {
646 let capacity: usize = args.estimated_capacity();
647 let mut output: String = string::String::with_capacity(capacity);
648 output
649 .write_fmt(args)
650 .expect(msg:"a formatting trait implementation returned an error when the underlying stream did not");
651 output
652 }
653
654 args.as_str().map_or_else(|| format_inner(args), f:crate::borrow::ToOwned::to_owned)
655}
656