<!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="Source to the Rust file `src/libcore/convert.rs`."><meta name="keywords" content="rust, rustlang, rust-lang"><title>convert.rs.html -- source</title><link rel="stylesheet" type="text/css" href="../../normalize1.35.0.css"><link rel="stylesheet" type="text/css" href="../../rustdoc1.35.0.css" id="mainThemeStyle"><link rel="stylesheet" type="text/css" href="../../dark1.35.0.css"><link rel="stylesheet" type="text/css" href="../../light1.35.0.css" id="themeStyle"><script src="../../storage1.35.0.js"></script><noscript><link rel="stylesheet" href="../../noscript1.35.0.css"></noscript><link rel="shortcut icon" href="../../favicon1.35.0.ico"><style type="text/css">#crate-search{background-image:url("../../down-arrow1.35.0.svg");}</style></head><body class="rustdoc source"><!--[if lte IE 8]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><div class="sidebar-menu">☰</div><a href='../../core/index.html'><img src='../../rust-logo1.35.0.png' alt='logo' width='100'></a></nav><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!"><img src="../../brush1.35.0.svg" width="18" alt="Pick another theme!"></button><div id="theme-choices"></div></div><script src="../../theme1.35.0.js"></script><nav class="sub"><form class="search-form js-only"><div class="search-container"><div><select id="crate-search"><option value="All crates">All crates</option></select><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"></div><a id="settings-menu" href="../../settings.html"><img src="../../wheel1.35.0.svg" width="18" alt="Change settings"></a></div></form></nav><section id="main" class="content"><pre class="line-numbers"><span id="1"> 1</span>
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</pre><div class="example-wrap"><pre class="rust ">
<span class="doccomment">//! Traits for conversions between types.</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! The traits in this module provide a way to convert from one type to another type.</span>
<span class="doccomment">//! Each trait serves a different purpose:</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! - Implement the [`AsRef`] trait for cheap reference-to-reference conversions</span>
<span class="doccomment">//! - Implement the [`AsMut`] trait for cheap mutable-to-mutable conversions</span>
<span class="doccomment">//! - Implement the [`From`] trait for consuming value-to-value conversions</span>
<span class="doccomment">//! - Implement the [`Into`] trait for consuming value-to-value conversions to types</span>
<span class="doccomment">//! outside the current crate</span>
<span class="doccomment">//! - The [`TryFrom`] and [`TryInto`] traits behave like [`From`] and [`Into`],</span>
<span class="doccomment">//! but should be implemented when the conversion can fail.</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! The traits in this module are often used as trait bounds for generic functions such that to</span>
<span class="doccomment">//! arguments of multiple types are supported. See the documentation of each trait for examples.</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! As a library author, you should always prefer implementing [`From<T>`][`From`] or</span>
<span class="doccomment">//! [`TryFrom<T>`][`TryFrom`] rather than [`Into<U>`][`Into`] or [`TryInto<U>`][`TryInto`],</span>
<span class="doccomment">//! as [`From`] and [`TryFrom`] provide greater flexibility and offer</span>
<span class="doccomment">//! equivalent [`Into`] or [`TryInto`] implementations for free, thanks to a</span>
<span class="doccomment">//! blanket implementation in the standard library. Only implement [`Into`] or [`TryInto`]</span>
<span class="doccomment">//! when a conversion to a type outside the current crate is required.</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! # Generic Implementations</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! - [`AsRef`] and [`AsMut`] auto-dereference if the inner type is a reference</span>
<span class="doccomment">//! - [`From`]`<U> for T` implies [`Into`]`<T> for U`</span>
<span class="doccomment">//! - [`TryFrom`]`<U> for T` implies [`TryInto`]`<T> for U`</span>
<span class="doccomment">//! - [`From`] and [`Into`] are reflexive, which means that all types can</span>
<span class="doccomment">//! `into` themselves and `from` themselves</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! See each trait for usage examples.</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! [`Into`]: trait.Into.html</span>
<span class="doccomment">//! [`From`]: trait.From.html</span>
<span class="doccomment">//! [`TryFrom`]: trait.TryFrom.html</span>
<span class="doccomment">//! [`TryInto`]: trait.TryInto.html</span>
<span class="doccomment">//! [`AsRef`]: trait.AsRef.html</span>
<span class="doccomment">//! [`AsMut`]: trait.AsMut.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">use</span> <span class="ident">fmt</span>;
<span class="doccomment">/// An identity function.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Two things are important to note about this function:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// - It is not always equivalent to a closure like `|x| x` since the</span>
<span class="doccomment">/// closure may coerce `x` into a different type.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// - It moves the input `x` passed to the function.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// While it might seem strange to have a function that just returns back the</span>
<span class="doccomment">/// input, there are some interesting uses.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Using `identity` to do nothing among other interesting functions:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```rust</span>
<span class="doccomment">/// use std::convert::identity;</span>
<span class="doccomment">///</span>
<span class="doccomment">/// fn manipulation(x: u32) -> u32 {</span>
<span class="doccomment">/// // Let's assume that this function does something interesting.</span>
<span class="doccomment">/// x + 1</span>
<span class="doccomment">/// }</span>
<span class="doccomment">///</span>
<span class="doccomment">/// let _arr = &[identity, manipulation];</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Using `identity` to get a function that changes nothing in a conditional:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```rust</span>
<span class="doccomment">/// use std::convert::identity;</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # let condition = true;</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # fn manipulation(x: u32) -> u32 { x + 1 }</span>
<span class="doccomment">///</span>
<span class="doccomment">/// let do_stuff = if condition { manipulation } else { identity };</span>
<span class="doccomment">///</span>
<span class="doccomment">/// // do more interesting stuff..</span>
<span class="doccomment">///</span>
<span class="doccomment">/// let _results = do_stuff(42);</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Using `identity` to keep the `Some` variants of an iterator of `Option<T>`:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```rust</span>
<span class="doccomment">/// use std::convert::identity;</span>
<span class="doccomment">///</span>
<span class="doccomment">/// let iter = vec![Some(1), None, Some(3)].into_iter();</span>
<span class="doccomment">/// let filtered = iter.filter_map(identity).collect::<Vec<_>>();</span>
<span class="doccomment">/// assert_eq!(vec![1, 3], filtered);</span>
<span class="doccomment">/// ```</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"convert_id"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.33.0"</span>)]</span>
<span class="attribute">#[<span class="ident">inline</span>]</span>
<span class="kw">pub</span> <span class="kw">const</span> <span class="kw">fn</span> <span class="ident">identity</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>(<span class="ident">x</span>: <span class="ident">T</span>) <span class="op">-></span> <span class="ident">T</span> { <span class="ident">x</span> }
<span class="doccomment">/// Used to do a cheap reference-to-reference conversion.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This trait is similar to [`AsMut`] which is used for converting between mutable references.</span>
<span class="doccomment">/// If you need to do a costly conversion it is better to implement [`From`] with type</span>
<span class="doccomment">/// `&T` or write a custom function.</span>
<span class="doccomment">///</span>
<span class="doccomment">///</span>
<span class="doccomment">/// `AsRef` has the same signature as [`Borrow`], but `Borrow` is different in few aspects:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// - Unlike `AsRef`, `Borrow` has a blanket impl for any `T`, and can be used to accept either</span>
<span class="doccomment">/// a reference or a value.</span>
<span class="doccomment">/// - `Borrow` also requires that `Hash`, `Eq` and `Ord` for borrowed value are</span>
<span class="doccomment">/// equivalent to those of the owned value. For this reason, if you want to</span>
<span class="doccomment">/// borrow only a single field of a struct you can implement `AsRef`, but not `Borrow`.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`Borrow`]: ../../std/borrow/trait.Borrow.html</span>
<span class="doccomment">///</span>
<span class="doccomment">/// **Note: This trait must not fail**. If the conversion can fail, use a</span>
<span class="doccomment">/// dedicated method which returns an [`Option<T>`] or a [`Result<T, E>`].</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`Option<T>`]: ../../std/option/enum.Option.html</span>
<span class="doccomment">/// [`Result<T, E>`]: ../../std/result/enum.Result.html</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Generic Implementations</span>
<span class="doccomment">///</span>
<span class="doccomment">/// - `AsRef` auto-dereferences if the inner type is a reference or a mutable</span>
<span class="doccomment">/// reference (e.g.: `foo.as_ref()` will work the same if `foo` has type</span>
<span class="doccomment">/// `&mut Foo` or `&&mut Foo`)</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// By using trait bounds we can accept arguments of different types as long as they can be</span>
<span class="doccomment">/// converted a the specified type `T`.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// For example: By creating a generic function that takes an `AsRef<str>` we express that we</span>
<span class="doccomment">/// want to accept all references that can be converted to &str as an argument.</span>
<span class="doccomment">/// Since both [`String`] and `&str` implement `AsRef<str>` we can accept both as input argument.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`String`]: ../../std/string/struct.String.html</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// fn is_hello<T: AsRef<str>>(s: T) {</span>
<span class="doccomment">/// assert_eq!("hello", s.as_ref());</span>
<span class="doccomment">/// }</span>
<span class="doccomment">///</span>
<span class="doccomment">/// let s = "hello";</span>
<span class="doccomment">/// is_hello(s);</span>
<span class="doccomment">///</span>
<span class="doccomment">/// let s = "hello".to_string();</span>
<span class="doccomment">/// is_hello(s);</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</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">trait</span> <span class="ident">AsRef</span><span class="op"><</span><span class="ident">T</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> {
<span class="doccomment">/// Performs the conversion.</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">fn</span> <span class="ident">as_ref</span>(<span class="kw-2">&</span><span class="self">self</span>) <span class="op">-></span> <span class="kw-2">&</span><span class="ident">T</span>;
}
<span class="doccomment">/// Used to do a cheap mutable-to-mutable reference conversion.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This trait is similar to [`AsRef`] but used for converting between mutable</span>
<span class="doccomment">/// references. If you need to do a costly conversion it is better to</span>
<span class="doccomment">/// implement [`From`] with type `&mut T` or write a custom function.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// **Note: This trait must not fail**. If the conversion can fail, use a</span>
<span class="doccomment">/// dedicated method which returns an [`Option<T>`] or a [`Result<T, E>`].</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`Option<T>`]: ../../std/option/enum.Option.html</span>
<span class="doccomment">/// [`Result<T, E>`]: ../../std/result/enum.Result.html</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Generic Implementations</span>
<span class="doccomment">///</span>
<span class="doccomment">/// - `AsMut` auto-dereferences if the inner type is a mutable reference</span>
<span class="doccomment">/// (e.g.: `foo.as_mut()` will work the same if `foo` has type `&mut Foo`</span>
<span class="doccomment">/// or `&mut &mut Foo`)</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Using `AsMut` as trait bound for a generic function we can accept all mutable references</span>
<span class="doccomment">/// that can be converted to type `&mut T`. Because [`Box<T>`] implements `AsMut<T>` we can</span>
<span class="doccomment">/// write a function `add_one`that takes all arguments that can be converted to `&mut u64`.</span>
<span class="doccomment">/// Because [`Box<T>`] implements `AsMut<T>` `add_one` accepts arguments of type</span>
<span class="doccomment">/// `&mut Box<u64>` as well:</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// fn add_one<T: AsMut<u64>>(num: &mut T) {</span>
<span class="doccomment">/// *num.as_mut() += 1;</span>
<span class="doccomment">/// }</span>
<span class="doccomment">///</span>
<span class="doccomment">/// let mut boxed_num = Box::new(0);</span>
<span class="doccomment">/// add_one(&mut boxed_num);</span>
<span class="doccomment">/// assert_eq!(*boxed_num, 1);</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// [`Box<T>`]: ../../std/boxed/struct.Box.html</span>
<span class="doccomment">///</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">trait</span> <span class="ident">AsMut</span><span class="op"><</span><span class="ident">T</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> {
<span class="doccomment">/// Performs the conversion.</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">fn</span> <span class="ident">as_mut</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>) <span class="op">-></span> <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">T</span>;
}
<span class="doccomment">/// A value-to-value conversion that consumes the input value. The</span>
<span class="doccomment">/// opposite of [`From`].</span>
<span class="doccomment">///</span>
<span class="doccomment">/// One should only implement [`Into`] if a conversion to a type outside the current crate is</span>
<span class="doccomment">/// required. Otherwise one should always prefer implementing [`From`] over [`Into`] because</span>
<span class="doccomment">/// implementing [`From`] automatically provides one with a implementation of [`Into`] thanks to</span>
<span class="doccomment">/// the blanket implementation in the standard library. [`From`] cannot do these type of</span>
<span class="doccomment">/// conversions because of Rust's orphaning rules.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// **Note: This trait must not fail**. If the conversion can fail, use [`TryInto`].</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Generic Implementations</span>
<span class="doccomment">///</span>
<span class="doccomment">/// - [`From<T>`]` for U` implies `Into<U> for T`</span>
<span class="doccomment">/// - [`Into`]` is reflexive, which means that `Into<T> for T` is implemented</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Implementing `Into` for conversions to external types</span>
<span class="doccomment">///</span>
<span class="doccomment">/// If the destination type is not part of the current crate</span>
<span class="doccomment">/// then you can't implement [`From`] directly.</span>
<span class="doccomment">/// For example, take this code:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```compile_fail</span>
<span class="doccomment">/// struct Wrapper<T>(Vec<T>);</span>
<span class="doccomment">/// impl<T> From<Wrapper<T>> for Vec<T> {</span>
<span class="doccomment">/// fn from(w: Wrapper<T>) -> Vec<T> {</span>
<span class="doccomment">/// w.0</span>
<span class="doccomment">/// }</span>
<span class="doccomment">/// }</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// This will fail to compile because we cannot implement a trait for a type</span>
<span class="doccomment">/// if both the trait and the type are not defined by the current crate.</span>
<span class="doccomment">/// This is due to Rust's orphaning rules. To bypass this, you can implement `Into` directly:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// struct Wrapper<T>(Vec<T>);</span>
<span class="doccomment">/// impl<T> Into<Vec<T>> for Wrapper<T> {</span>
<span class="doccomment">/// fn into(self) -> Vec<T> {</span>
<span class="doccomment">/// self.0</span>
<span class="doccomment">/// }</span>
<span class="doccomment">/// }</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// It is important to understand that `Into` does not provide a [`From`] implementation</span>
<span class="doccomment">/// (as [`From`] does with `Into`). Therefore, you should always try to implement [`From`]</span>
<span class="doccomment">/// and then fall back to `Into` if [`From`] can't be implemented.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Prefer using `Into` over [`From`] when specifying trait bounds on a generic function</span>
<span class="doccomment">/// to ensure that types that only implement `Into` can be used as well.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`String`] implements `Into<Vec<u8>>`:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// In order to express that we want a generic function to take all arguments that can be</span>
<span class="doccomment">/// converted to a specified type `T`, we can use a trait bound of `Into<T>`.</span>
<span class="doccomment">/// For example: The function `is_hello` takes all arguments that can be converted into a</span>
<span class="doccomment">/// `Vec<u8>`.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// fn is_hello<T: Into<Vec<u8>>>(s: T) {</span>
<span class="doccomment">/// let bytes = b"hello".to_vec();</span>
<span class="doccomment">/// assert_eq!(bytes, s.into());</span>
<span class="doccomment">/// }</span>
<span class="doccomment">///</span>
<span class="doccomment">/// let s = "hello".to_string();</span>
<span class="doccomment">/// is_hello(s);</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`TryInto`]: trait.TryInto.html</span>
<span class="doccomment">/// [`Option<T>`]: ../../std/option/enum.Option.html</span>
<span class="doccomment">/// [`Result<T, E>`]: ../../std/result/enum.Result.html</span>
<span class="doccomment">/// [`String`]: ../../std/string/struct.String.html</span>
<span class="doccomment">/// [From]: trait.From.html</span>
<span class="doccomment">/// [`into`]: trait.Into.html#tymethod.into</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">trait</span> <span class="ident">Into</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>: <span class="ident">Sized</span> {
<span class="doccomment">/// Performs the conversion.</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">fn</span> <span class="ident">into</span>(<span class="self">self</span>) <span class="op">-></span> <span class="ident">T</span>;
}
<span class="doccomment">/// Used to do value-to-value conversions while consuming the input value. It is the reciprocal of</span>
<span class="doccomment">/// [`Into`].</span>
<span class="doccomment">///</span>
<span class="doccomment">/// One should always prefer implementing [`From`] over [`Into`]</span>
<span class="doccomment">/// because implementing [`From`] automatically provides one with a implementation of [`Into`]</span>
<span class="doccomment">/// thanks to the blanket implementation in the standard library.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Only implement [`Into`] if a conversion to a type outside the current crate is required.</span>
<span class="doccomment">/// [`From`] cannot do these type of conversions because of Rust's orphaning rules.</span>
<span class="doccomment">/// See [`Into`] for more details.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Prefer using [`Into`] over using [`From`] when specifying trait bounds on a generic function.</span>
<span class="doccomment">/// This way, types that directly implement [`Into`] can be used as arguments as well.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// The [`From`] is also very useful when performing error handling. When constructing a function</span>
<span class="doccomment">/// that is capable of failing, the return type will generally be of the form `Result<T, E>`.</span>
<span class="doccomment">/// The `From` trait simplifies error handling by allowing a function to return a single error type</span>
<span class="doccomment">/// that encapsulate multiple error types. See the "Examples" section and [the book][book] for more</span>
<span class="doccomment">/// details.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// **Note: This trait must not fail**. If the conversion can fail, use [`TryFrom`].</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Generic Implementations</span>
<span class="doccomment">///</span>
<span class="doccomment">/// - [`From<T>`]` for U` implies [`Into<U>`]` for T`</span>
<span class="doccomment">/// - [`From`] is reflexive, which means that `From<T> for T` is implemented</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`String`] implements `From<&str>`:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// An explicit conversion from a &str to a String is done as follows:</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// let string = "hello".to_string();</span>
<span class="doccomment">/// let other_string = String::from("hello");</span>
<span class="doccomment">///</span>
<span class="doccomment">/// assert_eq!(string, other_string);</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// While performing error handling it is often useful to implement `From` for your own error type.</span>
<span class="doccomment">/// By converting underlying error types to our own custom error type that encapsulates the</span>
<span class="doccomment">/// underlying error type, we can return a single error type without losing information on the</span>
<span class="doccomment">/// underlying cause. The '?' operator automatically converts the underlying error type to our</span>
<span class="doccomment">/// custom error type by calling `Into<CliError>::into` which is automatically provided when</span>
<span class="doccomment">/// implementing `From`. The compiler then infers which implementation of `Into` should be used.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// use std::fs;</span>
<span class="doccomment">/// use std::io;</span>
<span class="doccomment">/// use std::num;</span>
<span class="doccomment">///</span>
<span class="doccomment">/// enum CliError {</span>
<span class="doccomment">/// IoError(io::Error),</span>
<span class="doccomment">/// ParseError(num::ParseIntError),</span>
<span class="doccomment">/// }</span>
<span class="doccomment">///</span>
<span class="doccomment">/// impl From<io::Error> for CliError {</span>
<span class="doccomment">/// fn from(error: io::Error) -> Self {</span>
<span class="doccomment">/// CliError::IoError(error)</span>
<span class="doccomment">/// }</span>
<span class="doccomment">/// }</span>
<span class="doccomment">///</span>
<span class="doccomment">/// impl From<num::ParseIntError> for CliError {</span>
<span class="doccomment">/// fn from(error: num::ParseIntError) -> Self {</span>
<span class="doccomment">/// CliError::ParseError(error)</span>
<span class="doccomment">/// }</span>
<span class="doccomment">/// }</span>
<span class="doccomment">///</span>
<span class="doccomment">/// fn open_and_parse_file(file_name: &str) -> Result<i32, CliError> {</span>
<span class="doccomment">/// let mut contents = fs::read_to_string(&file_name)?;</span>
<span class="doccomment">/// let num: i32 = contents.trim().parse()?;</span>
<span class="doccomment">/// Ok(num)</span>
<span class="doccomment">/// }</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`TryFrom`]: trait.TryFrom.html</span>
<span class="doccomment">/// [`Option<T>`]: ../../std/option/enum.Option.html</span>
<span class="doccomment">/// [`Result<T, E>`]: ../../std/result/enum.Result.html</span>
<span class="doccomment">/// [`String`]: ../../std/string/struct.String.html</span>
<span class="doccomment">/// [`Into<U>`]: trait.Into.html</span>
<span class="doccomment">/// [`from`]: trait.From.html#tymethod.from</span>
<span class="doccomment">/// [book]: ../../book/ch09-00-error-handling.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="attribute">#[<span class="ident">rustc_on_unimplemented</span>(
<span class="ident">on</span>(
<span class="ident">all</span>(<span class="ident">_Self</span><span class="op">=</span><span class="string">"&str"</span>, <span class="ident">T</span><span class="op">=</span><span class="string">"std::string::String"</span>),
<span class="ident">note</span><span class="op">=</span><span class="string">"to coerce a `{T}` into a `{Self}`, use `&*` as a prefix"</span>,
)
)]</span>
<span class="kw">pub</span> <span class="kw">trait</span> <span class="ident">From</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>: <span class="ident">Sized</span> {
<span class="doccomment">/// Performs the conversion.</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">fn</span> <span class="ident">from</span>(<span class="kw">_</span>: <span class="ident">T</span>) <span class="op">-></span> <span class="self">Self</span>;
}
<span class="doccomment">/// An attempted conversion that consumes `self`, which may or may not be</span>
<span class="doccomment">/// expensive.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Library authors should usually not directly implement this trait,</span>
<span class="doccomment">/// but should prefer implementing the [`TryFrom`] trait, which offers</span>
<span class="doccomment">/// greater flexibility and provides an equivalent `TryInto`</span>
<span class="doccomment">/// implementation for free, thanks to a blanket implementation in the</span>
<span class="doccomment">/// standard library. For more information on this, see the</span>
<span class="doccomment">/// documentation for [`Into`].</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Implementing `TryInto`</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This suffers the same restrictions and reasoning as implementing</span>
<span class="doccomment">/// [`Into`], see there for details.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`TryFrom`]: trait.TryFrom.html</span>
<span class="doccomment">/// [`Into`]: trait.Into.html</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"try_from"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.34.0"</span>)]</span>
<span class="kw">pub</span> <span class="kw">trait</span> <span class="ident">TryInto</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>: <span class="ident">Sized</span> {
<span class="doccomment">/// The type returned in the event of a conversion error.</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"try_from"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.34.0"</span>)]</span>
<span class="kw">type</span> <span class="ident">Error</span>;
<span class="doccomment">/// Performs the conversion.</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"try_from"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.34.0"</span>)]</span>
<span class="kw">fn</span> <span class="ident">try_into</span>(<span class="self">self</span>) <span class="op">-></span> <span class="prelude-ty">Result</span><span class="op"><</span><span class="ident">T</span>, <span class="self">Self</span>::<span class="ident">Error</span><span class="op">></span>;
}
<span class="doccomment">/// Simple and safe type conversions that may fail in a controlled</span>
<span class="doccomment">/// way under some circumstances. It is the reciprocal of [`TryInto`].</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This is useful when you are doing a type conversion that may</span>
<span class="doccomment">/// trivially succeed but may also need special handling.</span>
<span class="doccomment">/// For example, there is no way to convert an `i64` into an `i32`</span>
<span class="doccomment">/// using the [`From`] trait, because an `i64` may contain a value</span>
<span class="doccomment">/// that an `i32` cannot represent and so the conversion would lose data.</span>
<span class="doccomment">/// This might be handled by truncating the `i64` to an `i32` (essentially</span>
<span class="doccomment">/// giving the `i64`'s value modulo `i32::MAX`) or by simply returning</span>
<span class="doccomment">/// `i32::MAX`, or by some other method. The `From` trait is intended</span>
<span class="doccomment">/// for perfect conversions, so the `TryFrom` trait informs the</span>
<span class="doccomment">/// programmer when a type conversion could go bad and lets them</span>
<span class="doccomment">/// decide how to handle it.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Generic Implementations</span>
<span class="doccomment">///</span>
<span class="doccomment">/// - `TryFrom<T> for U` implies [`TryInto<U>`]` for T`</span>
<span class="doccomment">/// - [`try_from`] is reflexive, which means that `TryFrom<T> for T`</span>
<span class="doccomment">/// is implemented and cannot fail -- the associated `Error` type for</span>
<span class="doccomment">/// calling `T::try_from()` on a value of type `T` is `Infallible`.</span>
<span class="doccomment">/// When the `!` type is stablized `Infallible` and `!` will be</span>
<span class="doccomment">/// equivalent.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// `TryFrom<T>` can be implemented as follows:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// use std::convert::TryFrom;</span>
<span class="doccomment">///</span>
<span class="doccomment">/// struct SuperiorThanZero(i32);</span>
<span class="doccomment">///</span>
<span class="doccomment">/// impl TryFrom<i32> for SuperiorThanZero {</span>
<span class="doccomment">/// type Error = &'static str;</span>
<span class="doccomment">///</span>
<span class="doccomment">/// fn try_from(value: i32) -> Result<Self, Self::Error> {</span>
<span class="doccomment">/// if value < 0 {</span>
<span class="doccomment">/// Err("SuperiorThanZero only accepts value superior than zero!")</span>
<span class="doccomment">/// } else {</span>
<span class="doccomment">/// Ok(SuperiorThanZero(value))</span>
<span class="doccomment">/// }</span>
<span class="doccomment">/// }</span>
<span class="doccomment">/// }</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// As described, [`i32`] implements `TryFrom<i64>`:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// use std::convert::TryFrom;</span>
<span class="doccomment">///</span>
<span class="doccomment">/// let big_number = 1_000_000_000_000i64;</span>
<span class="doccomment">/// // Silently truncates `big_number`, requires detecting</span>
<span class="doccomment">/// // and handling the truncation after the fact.</span>
<span class="doccomment">/// let smaller_number = big_number as i32;</span>
<span class="doccomment">/// assert_eq!(smaller_number, -727379968);</span>
<span class="doccomment">///</span>
<span class="doccomment">/// // Returns an error because `big_number` is too big to</span>
<span class="doccomment">/// // fit in an `i32`.</span>
<span class="doccomment">/// let try_smaller_number = i32::try_from(big_number);</span>
<span class="doccomment">/// assert!(try_smaller_number.is_err());</span>
<span class="doccomment">///</span>
<span class="doccomment">/// // Returns `Ok(3)`.</span>
<span class="doccomment">/// let try_successful_smaller_number = i32::try_from(3);</span>
<span class="doccomment">/// assert!(try_successful_smaller_number.is_ok());</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`try_from`]: trait.TryFrom.html#tymethod.try_from</span>
<span class="doccomment">/// [`TryInto`]: trait.TryInto.html</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"try_from"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.34.0"</span>)]</span>
<span class="kw">pub</span> <span class="kw">trait</span> <span class="ident">TryFrom</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>: <span class="ident">Sized</span> {
<span class="doccomment">/// The type returned in the event of a conversion error.</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"try_from"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.34.0"</span>)]</span>
<span class="kw">type</span> <span class="ident">Error</span>;
<span class="doccomment">/// Performs the conversion.</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"try_from"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.34.0"</span>)]</span>
<span class="kw">fn</span> <span class="ident">try_from</span>(<span class="ident">value</span>: <span class="ident">T</span>) <span class="op">-></span> <span class="prelude-ty">Result</span><span class="op"><</span><span class="self">Self</span>, <span class="self">Self</span>::<span class="ident">Error</span><span class="op">></span>;
}
<span class="comment">////////////////////////////////////////////////////////////////////////////////</span>
<span class="comment">// GENERIC IMPLS</span>
<span class="comment">////////////////////////////////////////////////////////////////////////////////</span>
<span class="comment">// As lifts over &</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">impl</span><span class="op"><</span><span class="ident">T</span>: <span class="question-mark">?</span><span class="ident">Sized</span>, <span class="ident">U</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> <span class="ident">AsRef</span><span class="op"><</span><span class="ident">U</span><span class="op">></span> <span class="kw">for</span> <span class="kw-2">&</span><span class="ident">T</span> <span class="kw">where</span> <span class="ident">T</span>: <span class="ident">AsRef</span><span class="op"><</span><span class="ident">U</span><span class="op">></span>
{
<span class="kw">fn</span> <span class="ident">as_ref</span>(<span class="kw-2">&</span><span class="self">self</span>) <span class="op">-></span> <span class="kw-2">&</span><span class="ident">U</span> {
<span class="op"><</span><span class="ident">T</span> <span class="kw">as</span> <span class="ident">AsRef</span><span class="op"><</span><span class="ident">U</span><span class="op">>></span>::<span class="ident">as_ref</span>(<span class="kw-2">*</span><span class="self">self</span>)
}
}
<span class="comment">// As lifts over &mut</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">impl</span><span class="op"><</span><span class="ident">T</span>: <span class="question-mark">?</span><span class="ident">Sized</span>, <span class="ident">U</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> <span class="ident">AsRef</span><span class="op"><</span><span class="ident">U</span><span class="op">></span> <span class="kw">for</span> <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">T</span> <span class="kw">where</span> <span class="ident">T</span>: <span class="ident">AsRef</span><span class="op"><</span><span class="ident">U</span><span class="op">></span>
{
<span class="kw">fn</span> <span class="ident">as_ref</span>(<span class="kw-2">&</span><span class="self">self</span>) <span class="op">-></span> <span class="kw-2">&</span><span class="ident">U</span> {
<span class="op"><</span><span class="ident">T</span> <span class="kw">as</span> <span class="ident">AsRef</span><span class="op"><</span><span class="ident">U</span><span class="op">>></span>::<span class="ident">as_ref</span>(<span class="kw-2">*</span><span class="self">self</span>)
}
}
<span class="comment">// FIXME (#45742): replace the above impls for &/&mut with the following more general one:</span>
<span class="comment">// // As lifts over Deref</span>
<span class="comment">// impl<D: ?Sized + Deref, U: ?Sized> AsRef<U> for D where D::Target: AsRef<U> {</span>
<span class="comment">// fn as_ref(&self) -> &U {</span>
<span class="comment">// self.deref().as_ref()</span>
<span class="comment">// }</span>
<span class="comment">// }</span>
<span class="comment">// AsMut lifts over &mut</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">impl</span><span class="op"><</span><span class="ident">T</span>: <span class="question-mark">?</span><span class="ident">Sized</span>, <span class="ident">U</span>: <span class="question-mark">?</span><span class="ident">Sized</span><span class="op">></span> <span class="ident">AsMut</span><span class="op"><</span><span class="ident">U</span><span class="op">></span> <span class="kw">for</span> <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">T</span> <span class="kw">where</span> <span class="ident">T</span>: <span class="ident">AsMut</span><span class="op"><</span><span class="ident">U</span><span class="op">></span>
{
<span class="kw">fn</span> <span class="ident">as_mut</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>) <span class="op">-></span> <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">U</span> {
(<span class="kw-2">*</span><span class="self">self</span>).<span class="ident">as_mut</span>()
}
}
<span class="comment">// FIXME (#45742): replace the above impl for &mut with the following more general one:</span>
<span class="comment">// // AsMut lifts over DerefMut</span>
<span class="comment">// impl<D: ?Sized + Deref, U: ?Sized> AsMut<U> for D where D::Target: AsMut<U> {</span>
<span class="comment">// fn as_mut(&mut self) -> &mut U {</span>
<span class="comment">// self.deref_mut().as_mut()</span>
<span class="comment">// }</span>
<span class="comment">// }</span>
<span class="comment">// From implies Into</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">impl</span><span class="op"><</span><span class="ident">T</span>, <span class="ident">U</span><span class="op">></span> <span class="ident">Into</span><span class="op"><</span><span class="ident">U</span><span class="op">></span> <span class="kw">for</span> <span class="ident">T</span> <span class="kw">where</span> <span class="ident">U</span>: <span class="ident">From</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>
{
<span class="kw">fn</span> <span class="ident">into</span>(<span class="self">self</span>) <span class="op">-></span> <span class="ident">U</span> {
<span class="ident">U</span>::<span class="ident">from</span>(<span class="self">self</span>)
}
}
<span class="comment">// From (and thus Into) is reflexive</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">impl</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="ident">From</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="kw">for</span> <span class="ident">T</span> {
<span class="kw">fn</span> <span class="ident">from</span>(<span class="ident">t</span>: <span class="ident">T</span>) <span class="op">-></span> <span class="ident">T</span> { <span class="ident">t</span> }
}
<span class="comment">// TryFrom implies TryInto</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"try_from"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.34.0"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span>, <span class="ident">U</span><span class="op">></span> <span class="ident">TryInto</span><span class="op"><</span><span class="ident">U</span><span class="op">></span> <span class="kw">for</span> <span class="ident">T</span> <span class="kw">where</span> <span class="ident">U</span>: <span class="ident">TryFrom</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>
{
<span class="kw">type</span> <span class="ident">Error</span> <span class="op">=</span> <span class="ident">U</span>::<span class="ident">Error</span>;
<span class="kw">fn</span> <span class="ident">try_into</span>(<span class="self">self</span>) <span class="op">-></span> <span class="prelude-ty">Result</span><span class="op"><</span><span class="ident">U</span>, <span class="ident">U</span>::<span class="ident">Error</span><span class="op">></span> {
<span class="ident">U</span>::<span class="ident">try_from</span>(<span class="self">self</span>)
}
}
<span class="comment">// Infallible conversions are semantically equivalent to fallible conversions</span>
<span class="comment">// with an uninhabited error type.</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"try_from"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.34.0"</span>)]</span>
<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span>, <span class="ident">U</span><span class="op">></span> <span class="ident">TryFrom</span><span class="op"><</span><span class="ident">U</span><span class="op">></span> <span class="kw">for</span> <span class="ident">T</span> <span class="kw">where</span> <span class="ident">U</span>: <span class="ident">Into</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="kw">type</span> <span class="ident">Error</span> <span class="op">=</span> <span class="ident">Infallible</span>;
<span class="kw">fn</span> <span class="ident">try_from</span>(<span class="ident">value</span>: <span class="ident">U</span>) <span class="op">-></span> <span class="prelude-ty">Result</span><span class="op"><</span><span class="self">Self</span>, <span class="self">Self</span>::<span class="ident">Error</span><span class="op">></span> {
<span class="prelude-val">Ok</span>(<span class="ident">U</span>::<span class="ident">into</span>(<span class="ident">value</span>))
}
}
<span class="comment">////////////////////////////////////////////////////////////////////////////////</span>
<span class="comment">// CONCRETE IMPLS</span>
<span class="comment">////////////////////////////////////////////////////////////////////////////////</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">impl</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="ident">AsRef</span><span class="op"><</span>[<span class="ident">T</span>]<span class="op">></span> <span class="kw">for</span> [<span class="ident">T</span>] {
<span class="kw">fn</span> <span class="ident">as_ref</span>(<span class="kw-2">&</span><span class="self">self</span>) <span class="op">-></span> <span class="kw-2">&</span>[<span class="ident">T</span>] {
<span class="self">self</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">impl</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="ident">AsMut</span><span class="op"><</span>[<span class="ident">T</span>]<span class="op">></span> <span class="kw">for</span> [<span class="ident">T</span>] {
<span class="kw">fn</span> <span class="ident">as_mut</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>) <span class="op">-></span> <span class="kw-2">&</span><span class="kw-2">mut</span> [<span class="ident">T</span>] {
<span class="self">self</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">impl</span> <span class="ident">AsRef</span><span class="op"><</span><span class="ident">str</span><span class="op">></span> <span class="kw">for</span> <span class="ident">str</span> {
<span class="attribute">#[<span class="ident">inline</span>]</span>
<span class="kw">fn</span> <span class="ident">as_ref</span>(<span class="kw-2">&</span><span class="self">self</span>) <span class="op">-></span> <span class="kw-2">&</span><span class="ident">str</span> {
<span class="self">self</span>
}
}
<span class="comment">////////////////////////////////////////////////////////////////////////////////</span>
<span class="comment">// THE NO-ERROR ERROR TYPE</span>
<span class="comment">////////////////////////////////////////////////////////////////////////////////</span>
<span class="doccomment">/// The error type for errors that can never happen.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Since this enum has no variant, a value of this type can never actually exist.</span>
<span class="doccomment">/// This can be useful for generic APIs that use [`Result`] and parameterize the error type,</span>
<span class="doccomment">/// to indicate that the result is always [`Ok`].</span>
<span class="doccomment">///</span>
<span class="doccomment">/// For example, the [`TryFrom`] trait (conversion that returns a [`Result`])</span>
<span class="doccomment">/// has a blanket implementation for all types where a reverse [`Into`] implementation exists.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```ignore (illustrates std code, duplicating the impl in a doctest would be an error)</span>
<span class="doccomment">/// impl<T, U> TryFrom<U> for T where U: Into<T> {</span>
<span class="doccomment">/// type Error = Infallible;</span>
<span class="doccomment">///</span>
<span class="doccomment">/// fn try_from(value: U) -> Result<Self, Infallible> {</span>
<span class="doccomment">/// Ok(U::into(value)) // Never returns `Err`</span>
<span class="doccomment">/// }</span>
<span class="doccomment">/// }</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Future compatibility</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This enum has the same role as [the `!` “never” type][never],</span>
<span class="doccomment">/// which is unstable in this version of Rust.</span>
<span class="doccomment">/// When `!` is stabilized, we plan to make `Infallible` a type alias to it:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```ignore (illustrates future std change)</span>
<span class="doccomment">/// pub type Infallible = !;</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// … and eventually deprecate `Infallible`.</span>
<span class="doccomment">///</span>
<span class="doccomment">///</span>
<span class="doccomment">/// However there is one case where `!` syntax can be used</span>
<span class="doccomment">/// before `!` is stabilized as a full-fleged type: in the position of a function’s return type.</span>
<span class="doccomment">/// Specifically, it is possible implementations for two different function pointer types:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// trait MyTrait {}</span>
<span class="doccomment">/// impl MyTrait for fn() -> ! {}</span>
<span class="doccomment">/// impl MyTrait for fn() -> std::convert::Infallible {}</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// With `Infallible` being an enum, this code is valid.</span>
<span class="doccomment">/// However when `Infallible` becomes an alias for the never type,</span>
<span class="doccomment">/// the two `impl`s will start to overlap</span>
<span class="doccomment">/// and therefore will be disallowed by the language’s trait coherence rules.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`Ok`]: ../result/enum.Result.html#variant.Ok</span>
<span class="doccomment">/// [`Result`]: ../result/enum.Result.html</span>
<span class="doccomment">/// [`TryFrom`]: trait.TryFrom.html</span>
<span class="doccomment">/// [`Into`]: trait.Into.html</span>
<span class="doccomment">/// [never]: ../../std/primitive.never.html</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"convert_infallible"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.34.0"</span>)]</span>
<span class="attribute">#[<span class="ident">derive</span>(<span class="ident">Copy</span>)]</span>
<span class="kw">pub</span> <span class="kw">enum</span> <span class="ident">Infallible</span> {}
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"convert_infallible"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.34.0"</span>)]</span>
<span class="kw">impl</span> <span class="ident">Clone</span> <span class="kw">for</span> <span class="ident">Infallible</span> {
<span class="kw">fn</span> <span class="ident">clone</span>(<span class="kw-2">&</span><span class="self">self</span>) <span class="op">-></span> <span class="ident">Infallible</span> {
<span class="kw">match</span> <span class="kw-2">*</span><span class="self">self</span> {}
}
}
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"convert_infallible"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.34.0"</span>)]</span>
<span class="kw">impl</span> <span class="ident">fmt</span>::<span class="ident">Debug</span> <span class="kw">for</span> <span class="ident">Infallible</span> {
<span class="kw">fn</span> <span class="ident">fmt</span>(<span class="kw-2">&</span><span class="self">self</span>, <span class="kw">_</span>: <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">fmt</span>::<span class="ident">Formatter</span><span class="op"><</span><span class="lifetime">'_</span><span class="op">></span>) <span class="op">-></span> <span class="ident">fmt</span>::<span class="prelude-ty">Result</span> {
<span class="kw">match</span> <span class="kw-2">*</span><span class="self">self</span> {}
}
}
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"convert_infallible"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.34.0"</span>)]</span>
<span class="kw">impl</span> <span class="ident">fmt</span>::<span class="ident">Display</span> <span class="kw">for</span> <span class="ident">Infallible</span> {
<span class="kw">fn</span> <span class="ident">fmt</span>(<span class="kw-2">&</span><span class="self">self</span>, <span class="kw">_</span>: <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">fmt</span>::<span class="ident">Formatter</span><span class="op"><</span><span class="lifetime">'_</span><span class="op">></span>) <span class="op">-></span> <span class="ident">fmt</span>::<span class="prelude-ty">Result</span> {
<span class="kw">match</span> <span class="kw-2">*</span><span class="self">self</span> {}
}
}
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"convert_infallible"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.34.0"</span>)]</span>
<span class="kw">impl</span> <span class="ident">PartialEq</span> <span class="kw">for</span> <span class="ident">Infallible</span> {
<span class="kw">fn</span> <span class="ident">eq</span>(<span class="kw-2">&</span><span class="self">self</span>, <span class="kw">_</span>: <span class="kw-2">&</span><span class="ident">Infallible</span>) <span class="op">-></span> <span class="ident">bool</span> {
<span class="kw">match</span> <span class="kw-2">*</span><span class="self">self</span> {}
}
}
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"convert_infallible"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.34.0"</span>)]</span>
<span class="kw">impl</span> <span class="ident">Eq</span> <span class="kw">for</span> <span class="ident">Infallible</span> {}
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"convert_infallible"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.34.0"</span>)]</span>
<span class="kw">impl</span> <span class="ident">PartialOrd</span> <span class="kw">for</span> <span class="ident">Infallible</span> {
<span class="kw">fn</span> <span class="ident">partial_cmp</span>(<span class="kw-2">&</span><span class="self">self</span>, <span class="ident">_other</span>: <span class="kw-2">&</span><span class="self">Self</span>) <span class="op">-></span> <span class="prelude-ty">Option</span><span class="op"><</span><span class="kw">crate</span>::<span class="ident">cmp</span>::<span class="ident">Ordering</span><span class="op">></span> {
<span class="kw">match</span> <span class="kw-2">*</span><span class="self">self</span> {}
}
}
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"convert_infallible"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.34.0"</span>)]</span>
<span class="kw">impl</span> <span class="ident">Ord</span> <span class="kw">for</span> <span class="ident">Infallible</span> {
<span class="kw">fn</span> <span class="ident">cmp</span>(<span class="kw-2">&</span><span class="self">self</span>, <span class="ident">_other</span>: <span class="kw-2">&</span><span class="self">Self</span>) <span class="op">-></span> <span class="kw">crate</span>::<span class="ident">cmp</span>::<span class="ident">Ordering</span> {
<span class="kw">match</span> <span class="kw-2">*</span><span class="self">self</span> {}
}
}
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"convert_infallible"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.34.0"</span>)]</span>
<span class="kw">impl</span> <span class="ident">From</span><span class="op"><</span><span class="op">!</span><span class="op">></span> <span class="kw">for</span> <span class="ident">Infallible</span> {
<span class="kw">fn</span> <span class="ident">from</span>(<span class="ident">x</span>: <span class="op">!</span>) <span class="op">-></span> <span class="self">Self</span> {
<span class="ident">x</span>
}
}
</pre></div>
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