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</pre><pre class="rust ">
<span class="comment">// Copyright 2013-2015 The Rust Project Developers. See the COPYRIGHT</span>
<span class="comment">// file at the top-level directory of this distribution and at</span>
<span class="comment">// http://rust-lang.org/COPYRIGHT.</span>
<span class="comment">//</span>
<span class="comment">// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or</span>
<span class="comment">// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license</span>
<span class="comment">// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your</span>
<span class="comment">// option. This file may not be copied, modified, or distributed</span>
<span class="comment">// except according to those terms.</span>
<span class="doccomment">//! Utilities for formatting and printing `String`s</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! This module contains the runtime support for the `format!` syntax extension.</span>
<span class="doccomment">//! This macro is implemented in the compiler to emit calls to this module in</span>
<span class="doccomment">//! order to format arguments at runtime into strings.</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! # Usage</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! The `format!` macro is intended to be familiar to those coming from C's</span>
<span class="doccomment">//! printf/fprintf functions or Python's `str.format` function.</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! Some examples of the `format!` extension are:</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! ```</span>
<span class="doccomment">//! format!("Hello"); // => "Hello"</span>
<span class="doccomment">//! format!("Hello, {}!", "world"); // => "Hello, world!"</span>
<span class="doccomment">//! format!("The number is {}", 1); // => "The number is 1"</span>
<span class="doccomment">//! format!("{:?}", (3, 4)); // => "(3, 4)"</span>
<span class="doccomment">//! format!("{value}", value=4); // => "4"</span>
<span class="doccomment">//! format!("{} {}", 1, 2); // => "1 2"</span>
<span class="doccomment">//! format!("{:04}", 42); // => "0042" with leading zeros</span>
<span class="doccomment">//! ```</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! From these, you can see that the first argument is a format string. It is</span>
<span class="doccomment">//! required by the compiler for this to be a string literal; it cannot be a</span>
<span class="doccomment">//! variable passed in (in order to perform validity checking). The compiler</span>
<span class="doccomment">//! will then parse the format string and determine if the list of arguments</span>
<span class="doccomment">//! provided is suitable to pass to this format string.</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! ## Positional parameters</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! Each formatting argument is allowed to specify which value argument it's</span>
<span class="doccomment">//! referencing, and if omitted it is assumed to be "the next argument". For</span>
<span class="doccomment">//! example, the format string `{} {} {}` would take three parameters, and they</span>
<span class="doccomment">//! would be formatted in the same order as they're given. The format string</span>
<span class="doccomment">//! `{2} {1} {0}`, however, would format arguments in reverse order.</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! Things can get a little tricky once you start intermingling the two types of</span>
<span class="doccomment">//! positional specifiers. The "next argument" specifier can be thought of as an</span>
<span class="doccomment">//! iterator over the argument. Each time a "next argument" specifier is seen,</span>
<span class="doccomment">//! the iterator advances. This leads to behavior like this:</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! ```</span>
<span class="doccomment">//! format!("{1} {} {0} {}", 1, 2); // => "2 1 1 2"</span>
<span class="doccomment">//! ```</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! The internal iterator over the argument has not been advanced by the time</span>
<span class="doccomment">//! the first `{}` is seen, so it prints the first argument. Then upon reaching</span>
<span class="doccomment">//! the second `{}`, the iterator has advanced forward to the second argument.</span>
<span class="doccomment">//! Essentially, parameters which explicitly name their argument do not affect</span>
<span class="doccomment">//! parameters which do not name an argument in terms of positional specifiers.</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! A format string is required to use all of its arguments, otherwise it is a</span>
<span class="doccomment">//! compile-time error. You may refer to the same argument more than once in the</span>
<span class="doccomment">//! format string.</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! ## Named parameters</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! Rust itself does not have a Python-like equivalent of named parameters to a</span>
<span class="doccomment">//! function, but the `format!` macro is a syntax extension which allows it to</span>
<span class="doccomment">//! leverage named parameters. Named parameters are listed at the end of the</span>
<span class="doccomment">//! argument list and have the syntax:</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! ```text</span>
<span class="doccomment">//! identifier '=' expression</span>
<span class="doccomment">//! ```</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! For example, the following `format!` expressions all use named argument:</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! ```</span>
<span class="doccomment">//! format!("{argument}", argument = "test"); // => "test"</span>
<span class="doccomment">//! format!("{name} {}", 1, name = 2); // => "2 1"</span>
<span class="doccomment">//! format!("{a} {c} {b}", a="a", b='b', c=3); // => "a 3 b"</span>
<span class="doccomment">//! ```</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! It is not valid to put positional parameters (those without names) after</span>
<span class="doccomment">//! arguments which have names. Like with positional parameters, it is not</span>
<span class="doccomment">//! valid to provide named parameters that are unused by the format string.</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! ## Argument types</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! Each argument's type is dictated by the format string.</span>
<span class="doccomment">//! There are various parameters which require a particular type, however.</span>
<span class="doccomment">//! An example is the `{:.*}` syntax, which sets the number of decimal places</span>
<span class="doccomment">//! in floating-point types:</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! ```</span>
<span class="doccomment">//! let formatted_number = format!("{:.*}", 2, 1.234567);</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! assert_eq!("1.23", formatted_number)</span>
<span class="doccomment">//! ```</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! If this syntax is used, then the number of characters to print precedes the</span>
<span class="doccomment">//! actual object being formatted, and the number of characters must have the</span>
<span class="doccomment">//! type `usize`.</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! ## Formatting traits</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! When requesting that an argument be formatted with a particular type, you</span>
<span class="doccomment">//! are actually requesting that an argument ascribes to a particular trait.</span>
<span class="doccomment">//! This allows multiple actual types to be formatted via `{:x}` (like `i8` as</span>
<span class="doccomment">//! well as `isize`). The current mapping of types to traits is:</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! * *nothing* ⇒ [`Display`](trait.Display.html)</span>
<span class="doccomment">//! * `?` ⇒ [`Debug`](trait.Debug.html)</span>
<span class="doccomment">//! * `o` ⇒ [`Octal`](trait.Octal.html)</span>
<span class="doccomment">//! * `x` ⇒ [`LowerHex`](trait.LowerHex.html)</span>
<span class="doccomment">//! * `X` ⇒ [`UpperHex`](trait.UpperHex.html)</span>
<span class="doccomment">//! * `p` ⇒ [`Pointer`](trait.Pointer.html)</span>
<span class="doccomment">//! * `b` ⇒ [`Binary`](trait.Binary.html)</span>
<span class="doccomment">//! * `e` ⇒ [`LowerExp`](trait.LowerExp.html)</span>
<span class="doccomment">//! * `E` ⇒ [`UpperExp`](trait.UpperExp.html)</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! What this means is that any type of argument which implements the</span>
<span class="doccomment">//! `fmt::Binary` trait can then be formatted with `{:b}`. Implementations</span>
<span class="doccomment">//! are provided for these traits for a number of primitive types by the</span>
<span class="doccomment">//! standard library as well. If no format is specified (as in `{}` or `{:6}`),</span>
<span class="doccomment">//! then the format trait used is the `Display` trait.</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! When implementing a format trait for your own type, you will have to</span>
<span class="doccomment">//! implement a method of the signature:</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! ```</span>
<span class="doccomment">//! # #![allow(dead_code)]</span>
<span class="doccomment">//! # use std::fmt;</span>
<span class="doccomment">//! # struct Foo; // our custom type</span>
<span class="doccomment">//! # impl fmt::Display for Foo {</span>
<span class="doccomment">//! fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {</span>
<span class="doccomment">//! # write!(f, "testing, testing")</span>
<span class="doccomment">//! # } }</span>
<span class="doccomment">//! ```</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! Your type will be passed as `self` by-reference, and then the function</span>
<span class="doccomment">//! should emit output into the `f.buf` stream. It is up to each format trait</span>
<span class="doccomment">//! implementation to correctly adhere to the requested formatting parameters.</span>
<span class="doccomment">//! The values of these parameters will be listed in the fields of the</span>
<span class="doccomment">//! `Formatter` struct. In order to help with this, the `Formatter` struct also</span>
<span class="doccomment">//! provides some helper methods.</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! Additionally, the return value of this function is `fmt::Result` which is a</span>
<span class="doccomment">//! type alias of `Result<(), std::fmt::Error>`. Formatting implementations</span>
<span class="doccomment">//! should ensure that they propagate errors from the `Formatter` (e.g., when</span>
<span class="doccomment">//! calling `write!`) however, they should never return errors spuriously. That</span>
<span class="doccomment">//! is, a formatting implementation must and may only return an error if the</span>
<span class="doccomment">//! passed-in `Formatter` returns an error. This is because, contrary to what</span>
<span class="doccomment">//! the function signature might suggest, string formatting is an infallible</span>
<span class="doccomment">//! operation. This function only returns a result because writing to the</span>
<span class="doccomment">//! underlying stream might fail and it must provide a way to propagate the fact</span>
<span class="doccomment">//! that an error has occurred back up the stack.</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! An example of implementing the formatting traits would look</span>
<span class="doccomment">//! like:</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! ```</span>
<span class="doccomment">//! use std::fmt;</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! #[derive(Debug)]</span>
<span class="doccomment">//! struct Vector2D {</span>
<span class="doccomment">//! x: isize,</span>
<span class="doccomment">//! y: isize,</span>
<span class="doccomment">//! }</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! impl fmt::Display for Vector2D {</span>
<span class="doccomment">//! fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {</span>
<span class="doccomment">//! // The `f` value implements the `Write` trait, which is what the</span>
<span class="doccomment">//! // write! macro is expecting. Note that this formatting ignores the</span>
<span class="doccomment">//! // various flags provided to format strings.</span>
<span class="doccomment">//! write!(f, "({}, {})", self.x, self.y)</span>
<span class="doccomment">//! }</span>
<span class="doccomment">//! }</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! // Different traits allow different forms of output of a type. The meaning</span>
<span class="doccomment">//! // of this format is to print the magnitude of a vector.</span>
<span class="doccomment">//! impl fmt::Binary for Vector2D {</span>
<span class="doccomment">//! fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {</span>
<span class="doccomment">//! let magnitude = (self.x * self.x + self.y * self.y) as f64;</span>
<span class="doccomment">//! let magnitude = magnitude.sqrt();</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! // Respect the formatting flags by using the helper method</span>
<span class="doccomment">//! // `pad_integral` on the Formatter object. See the method</span>
<span class="doccomment">//! // documentation for details, and the function `pad` can be used</span>
<span class="doccomment">//! // to pad strings.</span>
<span class="doccomment">//! let decimals = f.precision().unwrap_or(3);</span>
<span class="doccomment">//! let string = format!("{:.*}", decimals, magnitude);</span>
<span class="doccomment">//! f.pad_integral(true, "", &string)</span>
<span class="doccomment">//! }</span>
<span class="doccomment">//! }</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! fn main() {</span>
<span class="doccomment">//! let myvector = Vector2D { x: 3, y: 4 };</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! println!("{}", myvector); // => "(3, 4)"</span>
<span class="doccomment">//! println!("{:?}", myvector); // => "Vector2D {x: 3, y:4}"</span>
<span class="doccomment">//! println!("{:10.3b}", myvector); // => " 5.000"</span>
<span class="doccomment">//! }</span>
<span class="doccomment">//! ```</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! ### `fmt::Display` vs `fmt::Debug`</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! These two formatting traits have distinct purposes:</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! - `fmt::Display` implementations assert that the type can be faithfully</span>
<span class="doccomment">//! represented as a UTF-8 string at all times. It is **not** expected that</span>
<span class="doccomment">//! all types implement the `Display` trait.</span>
<span class="doccomment">//! - `fmt::Debug` implementations should be implemented for **all** public types.</span>
<span class="doccomment">//! Output will typically represent the internal state as faithfully as possible.</span>
<span class="doccomment">//! The purpose of the `Debug` trait is to facilitate debugging Rust code. In</span>
<span class="doccomment">//! most cases, using `#[derive(Debug)]` is sufficient and recommended.</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! Some examples of the output from both traits:</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! ```</span>
<span class="doccomment">//! assert_eq!(format!("{} {:?}", 3, 4), "3 4");</span>
<span class="doccomment">//! assert_eq!(format!("{} {:?}", 'a', 'b'), "a 'b'");</span>
<span class="doccomment">//! assert_eq!(format!("{} {:?}", "foo\n", "bar\n"), "foo\n \"bar\\n\"");</span>
<span class="doccomment">//! ```</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! ## Related macros</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! There are a number of related macros in the `format!` family. The ones that</span>
<span class="doccomment">//! are currently implemented are:</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! ```ignore</span>
<span class="doccomment">//! format! // described above</span>
<span class="doccomment">//! write! // first argument is a &mut io::Write, the destination</span>
<span class="doccomment">//! writeln! // same as write but appends a newline</span>
<span class="doccomment">//! print! // the format string is printed to the standard output</span>
<span class="doccomment">//! println! // same as print but appends a newline</span>
<span class="doccomment">//! format_args! // described below.</span>
<span class="doccomment">//! ```</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! ### `write!`</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! This and `writeln` are two macros which are used to emit the format string</span>
<span class="doccomment">//! to a specified stream. This is used to prevent intermediate allocations of</span>
<span class="doccomment">//! format strings and instead directly write the output. Under the hood, this</span>
<span class="doccomment">//! function is actually invoking the `write_fmt` function defined on the</span>
<span class="doccomment">//! `std::io::Write` trait. Example usage is:</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! ```</span>
<span class="doccomment">//! # #![allow(unused_must_use)]</span>
<span class="doccomment">//! use std::io::Write;</span>
<span class="doccomment">//! let mut w = Vec::new();</span>
<span class="doccomment">//! write!(&mut w, "Hello {}!", "world");</span>
<span class="doccomment">//! ```</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! ### `print!`</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! This and `println` emit their output to stdout. Similarly to the `write!`</span>
<span class="doccomment">//! macro, the goal of these macros is to avoid intermediate allocations when</span>
<span class="doccomment">//! printing output. Example usage is:</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! ```</span>
<span class="doccomment">//! print!("Hello {}!", "world");</span>
<span class="doccomment">//! println!("I have a newline {}", "character at the end");</span>
<span class="doccomment">//! ```</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! ### `format_args!`</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! This is a curious macro which is used to safely pass around</span>
<span class="doccomment">//! an opaque object describing the format string. This object</span>
<span class="doccomment">//! does not require any heap allocations to create, and it only</span>
<span class="doccomment">//! references information on the stack. Under the hood, all of</span>
<span class="doccomment">//! the related macros are implemented in terms of this. First</span>
<span class="doccomment">//! off, some example usage is:</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! ```</span>
<span class="doccomment">//! # #![allow(unused_must_use)]</span>
<span class="doccomment">//! use std::fmt;</span>
<span class="doccomment">//! use std::io::{self, Write};</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! let mut some_writer = io::stdout();</span>
<span class="doccomment">//! write!(&mut some_writer, "{}", format_args!("print with a {}", "macro"));</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! fn my_fmt_fn(args: fmt::Arguments) {</span>
<span class="doccomment">//! write!(&mut io::stdout(), "{}", args);</span>
<span class="doccomment">//! }</span>
<span class="doccomment">//! my_fmt_fn(format_args!(", or a {} too", "function"));</span>
<span class="doccomment">//! ```</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! The result of the `format_args!` macro is a value of type `fmt::Arguments`.</span>
<span class="doccomment">//! This structure can then be passed to the `write` and `format` functions</span>
<span class="doccomment">//! inside this module in order to process the format string.</span>
<span class="doccomment">//! The goal of this macro is to even further prevent intermediate allocations</span>
<span class="doccomment">//! when dealing formatting strings.</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! For example, a logging library could use the standard formatting syntax, but</span>
<span class="doccomment">//! it would internally pass around this structure until it has been determined</span>
<span class="doccomment">//! where output should go to.</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! # Syntax</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! The syntax for the formatting language used is drawn from other languages,</span>
<span class="doccomment">//! so it should not be too alien. Arguments are formatted with Python-like</span>
<span class="doccomment">//! syntax, meaning that arguments are surrounded by `{}` instead of the C-like</span>
<span class="doccomment">//! `%`. The actual grammar for the formatting syntax is:</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! ```text</span>
<span class="doccomment">//! format_string := <text> [ maybe-format <text> ] *</span>
<span class="doccomment">//! maybe-format := '{' '{' | '}' '}' | <format></span>
<span class="doccomment">//! format := '{' [ argument ] [ ':' format_spec ] '}'</span>
<span class="doccomment">//! argument := integer | identifier</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! format_spec := [[fill]align][sign]['#']['0'][width]['.' precision][type]</span>
<span class="doccomment">//! fill := character</span>
<span class="doccomment">//! align := '<' | '^' | '>'</span>
<span class="doccomment">//! sign := '+' | '-'</span>
<span class="doccomment">//! width := count</span>
<span class="doccomment">//! precision := count | '*'</span>
<span class="doccomment">//! type := identifier | ''</span>
<span class="doccomment">//! count := parameter | integer</span>
<span class="doccomment">//! parameter := argument '$'</span>
<span class="doccomment">//! ```</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! # Formatting Parameters</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! Each argument being formatted can be transformed by a number of formatting</span>
<span class="doccomment">//! parameters (corresponding to `format_spec` in the syntax above). These</span>
<span class="doccomment">//! parameters affect the string representation of what's being formatted. This</span>
<span class="doccomment">//! syntax draws heavily from Python's, so it may seem a bit familiar.</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! ## Fill/Alignment</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! The fill character is provided normally in conjunction with the `width`</span>
<span class="doccomment">//! parameter. This indicates that if the value being formatted is smaller than</span>
<span class="doccomment">//! `width` some extra characters will be printed around it. The extra</span>
<span class="doccomment">//! characters are specified by `fill`, and the alignment can be one of the</span>
<span class="doccomment">//! following options:</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! * `<` - the argument is left-aligned in `width` columns</span>
<span class="doccomment">//! * `^` - the argument is center-aligned in `width` columns</span>
<span class="doccomment">//! * `>` - the argument is right-aligned in `width` columns</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! Note that alignment may not be implemented by some types. A good way</span>
<span class="doccomment">//! to ensure padding is applied is to format your input, then use this</span>
<span class="doccomment">//! resulting string to pad your output.</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! ## Sign/`#`/`0`</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! These can all be interpreted as flags for a particular formatter.</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! * `+` - This is intended for numeric types and indicates that the sign</span>
<span class="doccomment">//! should always be printed. Positive signs are never printed by</span>
<span class="doccomment">//! default, and the negative sign is only printed by default for the</span>
<span class="doccomment">//! `Signed` trait. This flag indicates that the correct sign (`+` or `-`)</span>
<span class="doccomment">//! should always be printed.</span>
<span class="doccomment">//! * `-` - Currently not used</span>
<span class="doccomment">//! * `#` - This flag is indicates that the "alternate" form of printing should</span>
<span class="doccomment">//! be used. The alternate forms are:</span>
<span class="doccomment">//! * `#?` - pretty-print the `Debug` formatting</span>
<span class="doccomment">//! * `#x` - precedes the argument with a `0x`</span>
<span class="doccomment">//! * `#X` - precedes the argument with a `0x`</span>
<span class="doccomment">//! * `#b` - precedes the argument with a `0b`</span>
<span class="doccomment">//! * `#o` - precedes the argument with a `0o`</span>
<span class="doccomment">//! * `0` - This is used to indicate for integer formats that the padding should</span>
<span class="doccomment">//! both be done with a `0` character as well as be sign-aware. A format</span>
<span class="doccomment">//! like `{:08}` would yield `00000001` for the integer `1`, while the</span>
<span class="doccomment">//! same format would yield `-0000001` for the integer `-1`. Notice that</span>
<span class="doccomment">//! the negative version has one fewer zero than the positive version.</span>
<span class="doccomment">//! Note that padding zeroes are always placed after the sign (if any)</span>
<span class="doccomment">//! and before the digits. When used together with the `#` flag, a similar</span>
<span class="doccomment">//! rule applies: padding zeroes are inserted after the prefix but before</span>
<span class="doccomment">//! the digits.</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! ## Width</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! This is a parameter for the "minimum width" that the format should take up.</span>
<span class="doccomment">//! If the value's string does not fill up this many characters, then the</span>
<span class="doccomment">//! padding specified by fill/alignment will be used to take up the required</span>
<span class="doccomment">//! space.</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! The default fill/alignment for non-numerics is a space and left-aligned. The</span>
<span class="doccomment">//! defaults for numeric formatters is also a space but with right-alignment. If</span>
<span class="doccomment">//! the `0` flag is specified for numerics, then the implicit fill character is</span>
<span class="doccomment">//! `0`.</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! The value for the width can also be provided as a `usize` in the list of</span>
<span class="doccomment">//! parameters by using the dollar syntax indicating that the second argument is</span>
<span class="doccomment">//! a `usize` specifying the width, for example:</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! ```</span>
<span class="doccomment">//! // All of these print "Hello x !"</span>
<span class="doccomment">//! println!("Hello {:5}!", "x");</span>
<span class="doccomment">//! println!("Hello {:1$}!", "x", 5);</span>
<span class="doccomment">//! println!("Hello {1:0$}!", 5, "x");</span>
<span class="doccomment">//! println!("Hello {:width$}!", "x", width = 5);</span>
<span class="doccomment">//! ```</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! Referring to an argument with the dollar syntax does not affect the "next</span>
<span class="doccomment">//! argument" counter, so it's usually a good idea to refer to arguments by</span>
<span class="doccomment">//! position, or use named arguments.</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! ## Precision</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! For non-numeric types, this can be considered a "maximum width". If the resulting string is</span>
<span class="doccomment">//! longer than this width, then it is truncated down to this many characters and that truncated</span>
<span class="doccomment">//! value is emitted with proper `fill`, `alignment` and `width` if those parameters are set.</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! For integral types, this is ignored.</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! For floating-point types, this indicates how many digits after the decimal point should be</span>
<span class="doccomment">//! printed.</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! There are three possible ways to specify the desired `precision`:</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! 1. An integer `.N`:</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! the integer `N` itself is the precision.</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! 2. An integer or name followed by dollar sign `.N$`:</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! use format *argument* `N` (which must be a `usize`) as the precision.</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! 3. An asterisk `.*`:</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! `.*` means that this `{...}` is associated with *two* format inputs rather than one: the</span>
<span class="doccomment">//! first input holds the `usize` precision, and the second holds the value to print. Note that</span>
<span class="doccomment">//! in this case, if one uses the format string `{<arg>:<spec>.*}`, then the `<arg>` part refers</span>
<span class="doccomment">//! to the *value* to print, and the `precision` must come in the input preceding `<arg>`.</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! For example, the following calls all print the same thing `Hello x is 0.01000`:</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! ```</span>
<span class="doccomment">//! // Hello {arg 0 ("x")} is {arg 1 (0.01) with precision specified inline (5)}</span>
<span class="doccomment">//! println!("Hello {0} is {1:.5}", "x", 0.01);</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! // Hello {arg 1 ("x")} is {arg 2 (0.01) with precision specified in arg 0 (5)}</span>
<span class="doccomment">//! println!("Hello {1} is {2:.0$}", 5, "x", 0.01);</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! // Hello {arg 0 ("x")} is {arg 2 (0.01) with precision specified in arg 1 (5)}</span>
<span class="doccomment">//! println!("Hello {0} is {2:.1$}", "x", 5, 0.01);</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! // Hello {next arg ("x")} is {second of next two args (0.01) with precision</span>
<span class="doccomment">//! // specified in first of next two args (5)}</span>
<span class="doccomment">//! println!("Hello {} is {:.*}", "x", 5, 0.01);</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! // Hello {next arg ("x")} is {arg 2 (0.01) with precision</span>
<span class="doccomment">//! // specified in its predecessor (5)}</span>
<span class="doccomment">//! println!("Hello {} is {2:.*}", "x", 5, 0.01);</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! // Hello {next arg ("x")} is {arg "number" (0.01) with precision specified</span>
<span class="doccomment">//! // in arg "prec" (5)}</span>
<span class="doccomment">//! println!("Hello {} is {number:.prec$}", "x", prec = 5, number = 0.01);</span>
<span class="doccomment">//! ```</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! While these:</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! ```</span>
<span class="doccomment">//! println!("{}, `{name:.*}` has 3 fractional digits", "Hello", 3, name=1234.56);</span>
<span class="doccomment">//! println!("{}, `{name:.*}` has 3 characters", "Hello", 3, name="1234.56");</span>
<span class="doccomment">//! println!("{}, `{name:>8.*}` has 3 right-aligned characters", "Hello", 3, name="1234.56");</span>
<span class="doccomment">//! ```</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! print two significantly different things:</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! ```text</span>
<span class="doccomment">//! Hello, `1234.560` has 3 fractional digits</span>
<span class="doccomment">//! Hello, `123` has 3 characters</span>
<span class="doccomment">//! Hello, ` 123` has 3 right-aligned characters</span>
<span class="doccomment">//! ```</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! # Escaping</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! The literal characters `{` and `}` may be included in a string by preceding</span>
<span class="doccomment">//! them with the same character. For example, the `{` character is escaped with</span>
<span class="doccomment">//! `{{` and the `}` character is escaped with `}}`.</span>
<span class="attribute">#<span class="op">!</span>[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="attribute">#[<span class="ident">unstable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"fmt_internals"</span>, <span class="ident">issue</span> <span class="op">=</span> <span class="string">"0"</span>)]</span>
<span class="kw">pub</span> <span class="kw">use</span> <span class="ident">core</span>::<span class="ident">fmt</span>::<span class="ident">rt</span>;
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="kw">pub</span> <span class="kw">use</span> <span class="ident">core</span>::<span class="ident">fmt</span>::{<span class="ident">Formatter</span>, <span class="prelude-ty">Result</span>, <span class="ident">Write</span>};
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="kw">pub</span> <span class="kw">use</span> <span class="ident">core</span>::<span class="ident">fmt</span>::{<span class="ident">Octal</span>, <span class="ident">Binary</span>};
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="kw">pub</span> <span class="kw">use</span> <span class="ident">core</span>::<span class="ident">fmt</span>::{<span class="ident">Display</span>, <span class="ident">Debug</span>};
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="kw">pub</span> <span class="kw">use</span> <span class="ident">core</span>::<span class="ident">fmt</span>::{<span class="ident">LowerHex</span>, <span class="ident">UpperHex</span>, <span class="ident">Pointer</span>};
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="kw">pub</span> <span class="kw">use</span> <span class="ident">core</span>::<span class="ident">fmt</span>::{<span class="ident">LowerExp</span>, <span class="ident">UpperExp</span>};
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="kw">pub</span> <span class="kw">use</span> <span class="ident">core</span>::<span class="ident">fmt</span>::<span class="ident">Error</span>;
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="kw">pub</span> <span class="kw">use</span> <span class="ident">core</span>::<span class="ident">fmt</span>::{<span class="ident">ArgumentV1</span>, <span class="ident">Arguments</span>, <span class="ident">write</span>};
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="kw">pub</span> <span class="kw">use</span> <span class="ident">core</span>::<span class="ident">fmt</span>::{<span class="ident">DebugList</span>, <span class="ident">DebugMap</span>, <span class="ident">DebugSet</span>, <span class="ident">DebugStruct</span>, <span class="ident">DebugTuple</span>};
<span class="kw">use</span> <span class="ident">string</span>;
<span class="doccomment">/// The format function takes a precompiled format string and a list of</span>
<span class="doccomment">/// arguments, to return the resulting formatted string.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Arguments</span>
<span class="doccomment">///</span>
<span class="doccomment">/// * args - a structure of arguments generated via the `format_args!` macro.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Basic usage:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// use std::fmt;</span>
<span class="doccomment">///</span>
<span class="doccomment">/// let s = fmt::format(format_args!("Hello, {}!", "world"));</span>
<span class="doccomment">/// assert_eq!(s, "Hello, world!");</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Please note that using [`format!`][format!] might be preferrable.</span>
<span class="doccomment">/// Example:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// let s = format!("Hello, {}!", "world");</span>
<span class="doccomment">/// assert_eq!(s, "Hello, world!");</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [format!]: ../macro.format.html</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">format</span>(<span class="ident">args</span>: <span class="ident">Arguments</span>) <span class="op">-></span> <span class="ident">string</span>::<span class="ident">String</span> {
<span class="kw">let</span> <span class="ident">capacity</span> <span class="op">=</span> <span class="ident">args</span>.<span class="ident">estimated_capacity</span>();
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">output</span> <span class="op">=</span> <span class="ident">string</span>::<span class="ident">String</span>::<span class="ident">with_capacity</span>(<span class="ident">capacity</span>);
<span class="ident">output</span>.<span class="ident">write_fmt</span>(<span class="ident">args</span>)
.<span class="ident">expect</span>(<span class="string">"a formatting trait implementation returned an error"</span>);
<span class="ident">output</span>
}
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