440 lines
17 KiB
Rust
440 lines
17 KiB
Rust
// Copyright 2013-2014 The Rust Project Developers. See the COPYRIGHT
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// file at the top-level directory of this distribution and at
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// http://rust-lang.org/COPYRIGHT.
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//
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// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
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// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
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// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
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// option. This file may not be copied, modified, or distributed
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// except according to those terms.
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//
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// ignore-lexer-test FIXME #15679
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//! Utilities for formatting and printing strings
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//!
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//! This module contains the runtime support for the `format!` syntax extension.
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//! This macro is implemented in the compiler to emit calls to this module in
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//! order to format arguments at runtime into strings and streams.
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//!
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//! ## Usage
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//!
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//! The `format!` macro is intended to be familiar to those coming from C's
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//! printf/fprintf functions or Python's `str.format` function. In its current
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//! revision, the `format!` macro returns a `String` type which is the result of
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//! the formatting. In the future it will also be able to pass in a stream to
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//! format arguments directly while performing minimal allocations.
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//!
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//! Some examples of the `format!` extension are:
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//!
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//! ```rust
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//! # fn main() {
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//! format!("Hello"); // => "Hello"
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//! format!("Hello, {}!", "world"); // => "Hello, world!"
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//! format!("The number is {}", 1i); // => "The number is 1"
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//! format!("{}", (3i, 4i)); // => "(3, 4)"
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//! format!("{value}", value=4i); // => "4"
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//! format!("{} {}", 1i, 2u); // => "1 2"
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//! # }
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//! ```
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//!
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//! From these, you can see that the first argument is a format string. It is
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//! required by the compiler for this to be a string literal; it cannot be a
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//! variable passed in (in order to perform validity checking). The compiler
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//! will then parse the format string and determine if the list of arguments
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//! provided is suitable to pass to this format string.
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//!
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//! ### Positional parameters
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//!
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//! Each formatting argument is allowed to specify which value argument it's
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//! referencing, and if omitted it is assumed to be "the next argument". For
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//! example, the format string `{} {} {}` would take three parameters, and they
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//! would be formatted in the same order as they're given. The format string
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//! `{2} {1} {0}`, however, would format arguments in reverse order.
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//!
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//! Things can get a little tricky once you start intermingling the two types of
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//! positional specifiers. The "next argument" specifier can be thought of as an
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//! iterator over the argument. Each time a "next argument" specifier is seen,
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//! the iterator advances. This leads to behavior like this:
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//!
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//! ```rust
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//! format!("{1} {} {0} {}", 1i, 2i); // => "2 1 1 2"
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//! ```
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//!
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//! The internal iterator over the argument has not been advanced by the time
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//! the first `{}` is seen, so it prints the first argument. Then upon reaching
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//! the second `{}`, the iterator has advanced forward to the second argument.
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//! Essentially, parameters which explicitly name their argument do not affect
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//! parameters which do not name an argument in terms of positional specifiers.
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//!
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//! A format string is required to use all of its arguments, otherwise it is a
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//! compile-time error. You may refer to the same argument more than once in the
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//! format string, although it must always be referred to with the same type.
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//!
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//! ### Named parameters
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//!
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//! Rust itself does not have a Python-like equivalent of named parameters to a
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//! function, but the `format!` macro is a syntax extension which allows it to
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//! leverage named parameters. Named parameters are listed at the end of the
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//! argument list and have the syntax:
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//!
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//! ```text
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//! identifier '=' expression
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//! ```
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//!
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//! For example, the following `format!` expressions all use named argument:
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//!
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//! ```rust
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//! # fn main() {
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//! format!("{argument}", argument = "test"); // => "test"
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//! format!("{name} {}", 1i, name = 2i); // => "2 1"
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//! format!("{a} {c} {b}", a="a", b=(), c=3i); // => "a 3 ()"
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//! # }
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//! ```
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//!
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//! It is illegal to put positional parameters (those without names) after
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//! arguments which have names. Like with positional parameters, it is illegal
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//! to provide named parameters that are unused by the format string.
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//!
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//! ### Argument types
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//!
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//! Each argument's type is dictated by the format string. It is a requirement
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//! that every argument is only ever referred to by one type. For example, this
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//! is an invalid format string:
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//!
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//! ```text
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//! {0:x} {0:o}
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//! ```
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//!
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//! This is invalid because the first argument is both referred to as a
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//! hexidecimal as well as an
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//! octal.
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//!
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//! There are various parameters which do require a particular type, however.
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//! Namely if the syntax `{:.*}` is used, then the number of characters to print
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//! precedes the actual object being formatted, and the number of characters
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//! must have the type `uint`. Although a `uint` can be printed with `{}`, it is
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//! illegal to reference an argument as such. For example this is another
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//! invalid format string:
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//!
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//! ```text
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//! {:.*} {0}
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//! ```
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//!
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//! ### Formatting traits
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//!
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//! When requesting that an argument be formatted with a particular type, you
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//! are actually requesting that an argument ascribes to a particular trait.
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//! This allows multiple actual types to be formatted via `{:x}` (like `i8` as
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//! well as `int`). The current mapping of types to traits is:
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//!
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//! * *nothing* ⇒ `Show`
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//! * `o` ⇒ `Octal`
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//! * `x` ⇒ `LowerHex`
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//! * `X` ⇒ `UpperHex`
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//! * `p` ⇒ `Pointer`
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//! * `b` ⇒ `Binary`
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//! * `e` ⇒ `LowerExp`
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//! * `E` ⇒ `UpperExp`
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//!
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//! What this means is that any type of argument which implements the
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//! `std::fmt::Binary` trait can then be formatted with `{:b}`. Implementations
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//! are provided for these traits for a number of primitive types by the
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//! standard library as well. If no format is specified (as in `{}` or `{:6}`),
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//! then the format trait used is the `Show` trait. This is one of the more
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//! commonly implemented traits when formatting a custom type.
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//!
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//! When implementing a format trait for your own type, you will have to
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//! implement a method of the signature:
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//!
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//! ```rust
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//! # use std::fmt;
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//! # struct Foo; // our custom type
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//! # impl fmt::Show for Foo {
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//! fn fmt(&self, f: &mut std::fmt::Formatter) -> fmt::Result {
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//! # write!(f, "testing, testing")
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//! # } }
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//! ```
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//!
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//! Your type will be passed as `self` by-reference, and then the function
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//! should emit output into the `f.buf` stream. It is up to each format trait
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//! implementation to correctly adhere to the requested formatting parameters.
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//! The values of these parameters will be listed in the fields of the
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//! `Formatter` struct. In order to help with this, the `Formatter` struct also
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//! provides some helper methods.
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//!
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//! Additionally, the return value of this function is `fmt::Result` which is a
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//! typedef to `Result<(), IoError>` (also known as `IoResult<()>`). Formatting
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//! implementations should ensure that they return errors from `write!`
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//! correctly (propagating errors upward).
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//!
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//! An example of implementing the formatting traits would look
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//! like:
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//!
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//! ```rust
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//! use std::fmt;
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//! use std::f64;
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//! use std::num::Float;
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//!
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//! struct Vector2D {
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//! x: int,
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//! y: int,
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//! }
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//!
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//! impl fmt::Show for Vector2D {
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//! fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
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//! // The `f` value implements the `Writer` trait, which is what the
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//! // write! macro is expecting. Note that this formatting ignores the
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//! // various flags provided to format strings.
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//! write!(f, "({}, {})", self.x, self.y)
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//! }
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//! }
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//!
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//! // Different traits allow different forms of output of a type. The meaning
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//! // of this format is to print the magnitude of a vector.
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//! impl fmt::Binary for Vector2D {
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//! fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
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//! let magnitude = (self.x * self.x + self.y * self.y) as f64;
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//! let magnitude = magnitude.sqrt();
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//!
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//! // Respect the formatting flags by using the helper method
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//! // `pad_integral` on the Formatter object. See the method documentation
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//! // for details, and the function `pad` can be used to pad strings.
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//! let decimals = f.precision().unwrap_or(3);
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//! let string = f64::to_str_exact(magnitude, decimals);
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//! f.pad_integral(true, "", string.as_bytes())
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//! }
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//! }
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//!
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//! fn main() {
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//! let myvector = Vector2D { x: 3, y: 4 };
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//!
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//! println!("{}", myvector); // => "(3, 4)"
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//! println!("{:10.3b}", myvector); // => " 5.000"
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//! }
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//! ```
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//!
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//! ### Related macros
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//!
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//! There are a number of related macros in the `format!` family. The ones that
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//! are currently implemented are:
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//!
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//! ```ignore
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//! format! // described above
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//! write! // first argument is a &mut io::Writer, the destination
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//! writeln! // same as write but appends a newline
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//! print! // the format string is printed to the standard output
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//! println! // same as print but appends a newline
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//! format_args! // described below.
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//! ```
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//!
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//! #### `write!`
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//!
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//! This and `writeln` are two macros which are used to emit the format string
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//! to a specified stream. This is used to prevent intermediate allocations of
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//! format strings and instead directly write the output. Under the hood, this
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//! function is actually invoking the `write` function defined in this module.
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//! Example usage is:
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//!
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//! ```rust
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//! # #![allow(unused_must_use)]
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//! use std::io;
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//!
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//! let mut w = Vec::new();
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//! write!(&mut w as &mut io::Writer, "Hello {}!", "world");
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//! ```
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//!
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//! #### `print!`
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//!
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//! This and `println` emit their output to stdout. Similarly to the `write!`
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//! macro, the goal of these macros is to avoid intermediate allocations when
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//! printing output. Example usage is:
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//!
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//! ```rust
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//! print!("Hello {}!", "world");
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//! println!("I have a newline {}", "character at the end");
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//! ```
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//!
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//! #### `format_args!`
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//! This is a curious macro which is used to safely pass around
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//! an opaque object describing the format string. This object
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//! does not require any heap allocations to create, and it only
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//! references information on the stack. Under the hood, all of
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//! the related macros are implemented in terms of this. First
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//! off, some example usage is:
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//!
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//! ```
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//! use std::fmt;
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//! use std::io;
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//!
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//! # #[allow(unused_must_use)]
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//! # fn main() {
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//! fmt::format(format_args!("this returns {}", "String"));
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//!
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//! let some_writer: &mut io::Writer = &mut io::stdout();
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//! write!(some_writer, "{}", format_args!("print with a {}", "macro"));
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//!
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//! fn my_fmt_fn(args: fmt::Arguments) {
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//! write!(&mut io::stdout(), "{}", args);
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//! }
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//! my_fmt_fn(format_args!("or a {} too", "function"));
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//! # }
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//! ```
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//!
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//! The result of the `format_args!` macro is a value of type `fmt::Arguments`.
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//! This structure can then be passed to the `write` and `format` functions
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//! inside this module in order to process the format string.
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//! The goal of this macro is to even further prevent intermediate allocations
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//! when dealing formatting strings.
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//!
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//! For example, a logging library could use the standard formatting syntax, but
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//! it would internally pass around this structure until it has been determined
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//! where output should go to.
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//!
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//! ## Syntax
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//!
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//! The syntax for the formatting language used is drawn from other languages,
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//! so it should not be too alien. Arguments are formatted with python-like
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//! syntax, meaning that arguments are surrounded by `{}` instead of the C-like
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//! `%`. The actual grammar for the formatting syntax is:
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//!
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//! ```text
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//! format_string := <text> [ format <text> ] *
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//! format := '{' [ argument ] [ ':' format_spec ] '}'
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//! argument := integer | identifier
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//!
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//! format_spec := [[fill]align][sign]['#'][0][width]['.' precision][type]
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//! fill := character
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//! align := '<' | '^' | '>'
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//! sign := '+' | '-'
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//! width := count
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//! precision := count | '*'
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//! type := identifier | ''
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//! count := parameter | integer
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//! parameter := integer '$'
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//! ```
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//!
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//! ## Formatting Parameters
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//!
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//! Each argument being formatted can be transformed by a number of formatting
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//! parameters (corresponding to `format_spec` in the syntax above). These
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//! parameters affect the string representation of what's being formatted. This
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//! syntax draws heavily from Python's, so it may seem a bit familiar.
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//!
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//! ### Fill/Alignment
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//!
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//! The fill character is provided normally in conjunction with the `width`
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//! parameter. This indicates that if the value being formatted is smaller than
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//! `width` some extra characters will be printed around it. The extra
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//! characters are specified by `fill`, and the alignment can be one of two
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//! options:
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//!
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//! * `<` - the argument is left-aligned in `width` columns
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//! * `^` - the argument is center-aligned in `width` columns
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//! * `>` - the argument is right-aligned in `width` columns
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//!
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//! ### Sign/#/0
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//!
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//! These can all be interpreted as flags for a particular formatter.
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//!
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//! * '+' - This is intended for numeric types and indicates that the sign
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//! should always be printed. Positive signs are never printed by
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//! default, and the negative sign is only printed by default for the
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//! `Signed` trait. This flag indicates that the correct sign (+ or -)
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//! should always be printed.
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//! * '-' - Currently not used
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//! * '#' - This flag is indicates that the "alternate" form of printing should
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//! be used. By default, this only applies to the integer formatting
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//! traits and performs like:
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//! * `x` - precedes the argument with a "0x"
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//! * `X` - precedes the argument with a "0x"
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//! * `t` - precedes the argument with a "0b"
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//! * `o` - precedes the argument with a "0o"
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//! * '0' - This is used to indicate for integer formats that the padding should
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//! both be done with a `0` character as well as be sign-aware. A format
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//! like `{:08d}` would yield `00000001` for the integer `1`, while the
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//! same format would yield `-0000001` for the integer `-1`. Notice that
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//! the negative version has one fewer zero than the positive version.
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//!
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//! ### Width
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//!
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//! This is a parameter for the "minimum width" that the format should take up.
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//! If the value's string does not fill up this many characters, then the
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//! padding specified by fill/alignment will be used to take up the required
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//! space.
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//!
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//! The default fill/alignment for non-numerics is a space and left-aligned. The
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//! defaults for numeric formatters is also a space but with right-alignment. If
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//! the '0' flag is specified for numerics, then the implicit fill character is
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//! '0'.
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//!
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//! The value for the width can also be provided as a `uint` in the list of
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//! parameters by using the `2$` syntax indicating that the second argument is a
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//! `uint` specifying the width.
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//!
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//! ### Precision
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//!
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//! For non-numeric types, this can be considered a "maximum width". If the
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//! resulting string is longer than this width, then it is truncated down to
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//! this many characters and only those are emitted.
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//!
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//! For integral types, this has no meaning currently.
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//!
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//! For floating-point types, this indicates how many digits after the decimal
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//! point should be printed.
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//!
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//! ## Escaping
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//!
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//! The literal characters `{` and `}` may be included in a string by preceding
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//! them with the same character. For example, the `{` character is escaped with
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//! `{{` and the `}` character is escaped with `}}`.
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#![experimental]
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use io::Writer;
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use io;
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use result::Result::{Ok, Err};
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use string;
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use vec::Vec;
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pub use core::fmt::{Formatter, Result, FormatWriter, rt};
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pub use core::fmt::{Show, Octal, Binary};
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pub use core::fmt::{LowerHex, UpperHex, Pointer};
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pub use core::fmt::{LowerExp, UpperExp};
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pub use core::fmt::Error;
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pub use core::fmt::{Argument, Arguments, write, radix, Radix, RadixFmt};
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#[doc(hidden)]
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pub use core::fmt::{argument, argumentuint};
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/// The format function takes a precompiled format string and a list of
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/// arguments, to return the resulting formatted string.
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///
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/// # Arguments
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///
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/// * args - a structure of arguments generated via the `format_args!` macro.
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///
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/// # Example
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///
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/// ```rust
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/// use std::fmt;
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///
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/// let s = fmt::format(format_args!("Hello, {}!", "world"));
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/// assert_eq!(s, "Hello, world!".to_string());
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/// ```
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#[experimental = "this is an implementation detail of format! and should not \
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be called directly"]
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pub fn format(args: Arguments) -> string::String {
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let mut output = Vec::new();
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let _ = write!(&mut output as &mut Writer, "{}", args);
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string::String::from_utf8(output).unwrap()
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}
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impl<'a> Writer for Formatter<'a> {
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fn write(&mut self, b: &[u8]) -> io::IoResult<()> {
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match (*self).write(b) {
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Ok(()) => Ok(()),
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Err(Error) => Err(io::standard_error(io::OtherIoError))
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}
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}
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}
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