1//! Traits for conversions between types.
2//!
3//! The traits in this module provide a way to convert from one type to another type.
4//! Each trait serves a different purpose:
5//!
6//! - Implement the [`AsRef`] trait for cheap reference-to-reference conversions
7//! - Implement the [`AsMut`] trait for cheap mutable-to-mutable conversions
8//! - Implement the [`From`] trait for consuming value-to-value conversions
9//! - Implement the [`Into`] trait for consuming value-to-value conversions to types
10//! outside the current crate
11//! - The [`TryFrom`] and [`TryInto`] traits behave like [`From`] and [`Into`],
12//! but should be implemented when the conversion can fail.
13//!
14//! The traits in this module are often used as trait bounds for generic functions such that to
15//! arguments of multiple types are supported. See the documentation of each trait for examples.
16//!
17//! As a library author, you should always prefer implementing [`From<T>`][`From`] or
18//! [`TryFrom<T>`][`TryFrom`] rather than [`Into<U>`][`Into`] or [`TryInto<U>`][`TryInto`],
19//! as [`From`] and [`TryFrom`] provide greater flexibility and offer
20//! equivalent [`Into`] or [`TryInto`] implementations for free, thanks to a
21//! blanket implementation in the standard library. When targeting a version prior to Rust 1.41, it
22//! may be necessary to implement [`Into`] or [`TryInto`] directly when converting to a type
23//! outside the current crate.
24//!
25//! # Generic Implementations
26//!
27//! - [`AsRef`] and [`AsMut`] auto-dereference if the inner type is a reference
28//! (but not generally for all [dereferenceable types][core::ops::Deref])
29//! - [`From`]`<U> for T` implies [`Into`]`<T> for U`
30//! - [`TryFrom`]`<U> for T` implies [`TryInto`]`<T> for U`
31//! - [`From`] and [`Into`] are reflexive, which means that all types can
32//! `into` themselves and `from` themselves
33//!
34//! See each trait for usage examples.
35
36#![stable(feature = "rust1", since = "1.0.0")]
37
38use crate::error::Error;
39use crate::fmt;
40use crate::hash::{Hash, Hasher};
41
42mod num;
43
44#[unstable(feature = "convert_float_to_int", issue = "67057")]
45pub use num::FloatToInt;
46
47/// The identity function.
48///
49/// Two things are important to note about this function:
50///
51/// - It is not always equivalent to a closure like `|x| x`, since the
52/// closure may coerce `x` into a different type.
53///
54/// - It moves the input `x` passed to the function.
55///
56/// While it might seem strange to have a function that just returns back the
57/// input, there are some interesting uses.
58///
59/// # Examples
60///
61/// Using `identity` to do nothing in a sequence of other, interesting,
62/// functions:
63///
64/// ```rust
65/// use std::convert::identity;
66///
67/// fn manipulation(x: u32) -> u32 {
68/// // Let's pretend that adding one is an interesting function.
69/// x + 1
70/// }
71///
72/// let _arr = &[identity, manipulation];
73/// ```
74///
75/// Using `identity` as a "do nothing" base case in a conditional:
76///
77/// ```rust
78/// use std::convert::identity;
79///
80/// # let condition = true;
81/// #
82/// # fn manipulation(x: u32) -> u32 { x + 1 }
83/// #
84/// let do_stuff = if condition { manipulation } else { identity };
85///
86/// // Do more interesting stuff...
87///
88/// let _results = do_stuff(42);
89/// ```
90///
91/// Using `identity` to keep the `Some` variants of an iterator of `Option<T>`:
92///
93/// ```rust
94/// use std::convert::identity;
95///
96/// let iter = [Some(1), None, Some(3)].into_iter();
97/// let filtered = iter.filter_map(identity).collect::<Vec<_>>();
98/// assert_eq!(vec![1, 3], filtered);
99/// ```
100#[stable(feature = "convert_id", since = "1.33.0")]
101#[rustc_const_stable(feature = "const_identity", since = "1.33.0")]
102#[inline(always)]
103#[rustc_diagnostic_item = "convert_identity"]
104pub const fn identity<T>(x: T) -> T {
105 x
106}
107
108/// Used to do a cheap reference-to-reference conversion.
109///
110/// This trait is similar to [`AsMut`] which is used for converting between mutable references.
111/// If you need to do a costly conversion it is better to implement [`From`] with type
112/// `&T` or write a custom function.
113///
114/// # Relation to `Borrow`
115///
116/// `AsRef` has the same signature as [`Borrow`], but [`Borrow`] is different in a few aspects:
117///
118/// - Unlike `AsRef`, [`Borrow`] has a blanket impl for any `T`, and can be used to accept either
119/// a reference or a value. (See also note on `AsRef`'s reflexibility below.)
120/// - [`Borrow`] also requires that [`Hash`], [`Eq`] and [`Ord`] for a borrowed value are
121/// equivalent to those of the owned value. For this reason, if you want to
122/// borrow only a single field of a struct you can implement `AsRef`, but not [`Borrow`].
123///
124/// **Note: This trait must not fail**. If the conversion can fail, use a
125/// dedicated method which returns an [`Option<T>`] or a [`Result<T, E>`].
126///
127/// # Generic Implementations
128///
129/// `AsRef` auto-dereferences if the inner type is a reference or a mutable reference
130/// (e.g.: `foo.as_ref()` will work the same if `foo` has type `&mut Foo` or `&&mut Foo`).
131///
132/// Note that due to historic reasons, the above currently does not hold generally for all
133/// [dereferenceable types], e.g. `foo.as_ref()` will *not* work the same as
134/// `Box::new(foo).as_ref()`. Instead, many smart pointers provide an `as_ref` implementation which
135/// simply returns a reference to the [pointed-to value] (but do not perform a cheap
136/// reference-to-reference conversion for that value). However, [`AsRef::as_ref`] should not be
137/// used for the sole purpose of dereferencing; instead ['`Deref` coercion'] can be used:
138///
139/// [dereferenceable types]: core::ops::Deref
140/// [pointed-to value]: core::ops::Deref::Target
141/// ['`Deref` coercion']: core::ops::Deref#deref-coercion
142///
143/// ```
144/// let x = Box::new(5i32);
145/// // Avoid this:
146/// // let y: &i32 = x.as_ref();
147/// // Better just write:
148/// let y: &i32 = &x;
149/// ```
150///
151/// Types which implement [`Deref`] should consider implementing `AsRef<T>` as follows:
152///
153/// [`Deref`]: core::ops::Deref
154///
155/// ```
156/// # use core::ops::Deref;
157/// # struct SomeType;
158/// # impl Deref for SomeType {
159/// # type Target = [u8];
160/// # fn deref(&self) -> &[u8] {
161/// # &[]
162/// # }
163/// # }
164/// impl<T> AsRef<T> for SomeType
165/// where
166/// T: ?Sized,
167/// <SomeType as Deref>::Target: AsRef<T>,
168/// {
169/// fn as_ref(&self) -> &T {
170/// self.deref().as_ref()
171/// }
172/// }
173/// ```
174///
175/// # Reflexivity
176///
177/// Ideally, `AsRef` would be reflexive, i.e. there would be an `impl<T: ?Sized> AsRef<T> for T`
178/// with [`as_ref`] simply returning its argument unchanged.
179/// Such a blanket implementation is currently *not* provided due to technical restrictions of
180/// Rust's type system (it would be overlapping with another existing blanket implementation for
181/// `&T where T: AsRef<U>` which allows `AsRef` to auto-dereference, see "Generic Implementations"
182/// above).
183///
184/// [`as_ref`]: AsRef::as_ref
185///
186/// A trivial implementation of `AsRef<T> for T` must be added explicitly for a particular type `T`
187/// where needed or desired. Note, however, that not all types from `std` contain such an
188/// implementation, and those cannot be added by external code due to orphan rules.
189///
190/// # Examples
191///
192/// By using trait bounds we can accept arguments of different types as long as they can be
193/// converted to the specified type `T`.
194///
195/// For example: By creating a generic function that takes an `AsRef<str>` we express that we
196/// want to accept all references that can be converted to [`&str`] as an argument.
197/// Since both [`String`] and [`&str`] implement `AsRef<str>` we can accept both as input argument.
198///
199/// [`&str`]: primitive@str
200/// [`Borrow`]: crate::borrow::Borrow
201/// [`Eq`]: crate::cmp::Eq
202/// [`Ord`]: crate::cmp::Ord
203/// [`String`]: ../../std/string/struct.String.html
204///
205/// ```
206/// fn is_hello<T: AsRef<str>>(s: T) {
207/// assert_eq!("hello", s.as_ref());
208/// }
209///
210/// let s = "hello";
211/// is_hello(s);
212///
213/// let s = "hello".to_string();
214/// is_hello(s);
215/// ```
216#[stable(feature = "rust1", since = "1.0.0")]
217#[cfg_attr(not(test), rustc_diagnostic_item = "AsRef")]
218pub trait AsRef<T: ?Sized> {
219 /// Converts this type into a shared reference of the (usually inferred) input type.
220 #[stable(feature = "rust1", since = "1.0.0")]
221 fn as_ref(&self) -> &T;
222}
223
224/// Used to do a cheap mutable-to-mutable reference conversion.
225///
226/// This trait is similar to [`AsRef`] but used for converting between mutable
227/// references. If you need to do a costly conversion it is better to
228/// implement [`From`] with type `&mut T` or write a custom function.
229///
230/// **Note: This trait must not fail**. If the conversion can fail, use a
231/// dedicated method which returns an [`Option<T>`] or a [`Result<T, E>`].
232///
233/// # Generic Implementations
234///
235/// `AsMut` auto-dereferences if the inner type is a mutable reference
236/// (e.g.: `foo.as_mut()` will work the same if `foo` has type `&mut Foo` or `&mut &mut Foo`).
237///
238/// Note that due to historic reasons, the above currently does not hold generally for all
239/// [mutably dereferenceable types], e.g. `foo.as_mut()` will *not* work the same as
240/// `Box::new(foo).as_mut()`. Instead, many smart pointers provide an `as_mut` implementation which
241/// simply returns a reference to the [pointed-to value] (but do not perform a cheap
242/// reference-to-reference conversion for that value). However, [`AsMut::as_mut`] should not be
243/// used for the sole purpose of mutable dereferencing; instead ['`Deref` coercion'] can be used:
244///
245/// [mutably dereferenceable types]: core::ops::DerefMut
246/// [pointed-to value]: core::ops::Deref::Target
247/// ['`Deref` coercion']: core::ops::DerefMut#mutable-deref-coercion
248///
249/// ```
250/// let mut x = Box::new(5i32);
251/// // Avoid this:
252/// // let y: &mut i32 = x.as_mut();
253/// // Better just write:
254/// let y: &mut i32 = &mut x;
255/// ```
256///
257/// Types which implement [`DerefMut`] should consider to add an implementation of `AsMut<T>` as
258/// follows:
259///
260/// [`DerefMut`]: core::ops::DerefMut
261///
262/// ```
263/// # use core::ops::{Deref, DerefMut};
264/// # struct SomeType;
265/// # impl Deref for SomeType {
266/// # type Target = [u8];
267/// # fn deref(&self) -> &[u8] {
268/// # &[]
269/// # }
270/// # }
271/// # impl DerefMut for SomeType {
272/// # fn deref_mut(&mut self) -> &mut [u8] {
273/// # &mut []
274/// # }
275/// # }
276/// impl<T> AsMut<T> for SomeType
277/// where
278/// <SomeType as Deref>::Target: AsMut<T>,
279/// {
280/// fn as_mut(&mut self) -> &mut T {
281/// self.deref_mut().as_mut()
282/// }
283/// }
284/// ```
285///
286/// # Reflexivity
287///
288/// Ideally, `AsMut` would be reflexive, i.e. there would be an `impl<T: ?Sized> AsMut<T> for T`
289/// with [`as_mut`] simply returning its argument unchanged.
290/// Such a blanket implementation is currently *not* provided due to technical restrictions of
291/// Rust's type system (it would be overlapping with another existing blanket implementation for
292/// `&mut T where T: AsMut<U>` which allows `AsMut` to auto-dereference, see "Generic
293/// Implementations" above).
294///
295/// [`as_mut`]: AsMut::as_mut
296///
297/// A trivial implementation of `AsMut<T> for T` must be added explicitly for a particular type `T`
298/// where needed or desired. Note, however, that not all types from `std` contain such an
299/// implementation, and those cannot be added by external code due to orphan rules.
300///
301/// # Examples
302///
303/// Using `AsMut` as trait bound for a generic function, we can accept all mutable references that
304/// can be converted to type `&mut T`. Unlike [dereference], which has a single [target type],
305/// there can be multiple implementations of `AsMut` for a type. In particular, `Vec<T>` implements
306/// both `AsMut<Vec<T>>` and `AsMut<[T]>`.
307///
308/// In the following, the example functions `caesar` and `null_terminate` provide a generic
309/// interface which work with any type that can be converted by cheap mutable-to-mutable conversion
310/// into a byte slice (`[u8]`) or byte vector (`Vec<u8>`), respectively.
311///
312/// [dereference]: core::ops::DerefMut
313/// [target type]: core::ops::Deref::Target
314///
315/// ```
316/// struct Document {
317/// info: String,
318/// content: Vec<u8>,
319/// }
320///
321/// impl<T: ?Sized> AsMut<T> for Document
322/// where
323/// Vec<u8>: AsMut<T>,
324/// {
325/// fn as_mut(&mut self) -> &mut T {
326/// self.content.as_mut()
327/// }
328/// }
329///
330/// fn caesar<T: AsMut<[u8]>>(data: &mut T, key: u8) {
331/// for byte in data.as_mut() {
332/// *byte = byte.wrapping_add(key);
333/// }
334/// }
335///
336/// fn null_terminate<T: AsMut<Vec<u8>>>(data: &mut T) {
337/// // Using a non-generic inner function, which contains most of the
338/// // functionality, helps to minimize monomorphization overhead.
339/// fn doit(data: &mut Vec<u8>) {
340/// let len = data.len();
341/// if len == 0 || data[len-1] != 0 {
342/// data.push(0);
343/// }
344/// }
345/// doit(data.as_mut());
346/// }
347///
348/// fn main() {
349/// let mut v: Vec<u8> = vec![1, 2, 3];
350/// caesar(&mut v, 5);
351/// assert_eq!(v, [6, 7, 8]);
352/// null_terminate(&mut v);
353/// assert_eq!(v, [6, 7, 8, 0]);
354/// let mut doc = Document {
355/// info: String::from("Example"),
356/// content: vec![17, 19, 8],
357/// };
358/// caesar(&mut doc, 1);
359/// assert_eq!(doc.content, [18, 20, 9]);
360/// null_terminate(&mut doc);
361/// assert_eq!(doc.content, [18, 20, 9, 0]);
362/// }
363/// ```
364///
365/// Note, however, that APIs don't need to be generic. In many cases taking a `&mut [u8]` or
366/// `&mut Vec<u8>`, for example, is the better choice (callers need to pass the correct type then).
367#[stable(feature = "rust1", since = "1.0.0")]
368#[cfg_attr(not(test), rustc_diagnostic_item = "AsMut")]
369pub trait AsMut<T: ?Sized> {
370 /// Converts this type into a mutable reference of the (usually inferred) input type.
371 #[stable(feature = "rust1", since = "1.0.0")]
372 fn as_mut(&mut self) -> &mut T;
373}
374
375/// A value-to-value conversion that consumes the input value. The
376/// opposite of [`From`].
377///
378/// One should avoid implementing [`Into`] and implement [`From`] instead.
379/// Implementing [`From`] automatically provides one with an implementation of [`Into`]
380/// thanks to the blanket implementation in the standard library.
381///
382/// Prefer using [`Into`] over [`From`] when specifying trait bounds on a generic function
383/// to ensure that types that only implement [`Into`] can be used as well.
384///
385/// **Note: This trait must not fail**. If the conversion can fail, use [`TryInto`].
386///
387/// # Generic Implementations
388///
389/// - [`From`]`<T> for U` implies `Into<U> for T`
390/// - [`Into`] is reflexive, which means that `Into<T> for T` is implemented
391///
392/// # Implementing [`Into`] for conversions to external types in old versions of Rust
393///
394/// Prior to Rust 1.41, if the destination type was not part of the current crate
395/// then you couldn't implement [`From`] directly.
396/// For example, take this code:
397///
398/// ```
399/// # #![allow(non_local_definitions)]
400/// struct Wrapper<T>(Vec<T>);
401/// impl<T> From<Wrapper<T>> for Vec<T> {
402/// fn from(w: Wrapper<T>) -> Vec<T> {
403/// w.0
404/// }
405/// }
406/// ```
407/// This will fail to compile in older versions of the language because Rust's orphaning rules
408/// used to be a little bit more strict. To bypass this, you could implement [`Into`] directly:
409///
410/// ```
411/// struct Wrapper<T>(Vec<T>);
412/// impl<T> Into<Vec<T>> for Wrapper<T> {
413/// fn into(self) -> Vec<T> {
414/// self.0
415/// }
416/// }
417/// ```
418///
419/// It is important to understand that [`Into`] does not provide a [`From`] implementation
420/// (as [`From`] does with [`Into`]). Therefore, you should always try to implement [`From`]
421/// and then fall back to [`Into`] if [`From`] can't be implemented.
422///
423/// # Examples
424///
425/// [`String`] implements [`Into`]`<`[`Vec`]`<`[`u8`]`>>`:
426///
427/// In order to express that we want a generic function to take all arguments that can be
428/// converted to a specified type `T`, we can use a trait bound of [`Into`]`<T>`.
429/// For example: The function `is_hello` takes all arguments that can be converted into a
430/// [`Vec`]`<`[`u8`]`>`.
431///
432/// ```
433/// fn is_hello<T: Into<Vec<u8>>>(s: T) {
434/// let bytes = b"hello".to_vec();
435/// assert_eq!(bytes, s.into());
436/// }
437///
438/// let s = "hello".to_string();
439/// is_hello(s);
440/// ```
441///
442/// [`String`]: ../../std/string/struct.String.html
443/// [`Vec`]: ../../std/vec/struct.Vec.html
444#[rustc_diagnostic_item = "Into"]
445#[stable(feature = "rust1", since = "1.0.0")]
446pub trait Into<T>: Sized {
447 /// Converts this type into the (usually inferred) input type.
448 #[must_use]
449 #[stable(feature = "rust1", since = "1.0.0")]
450 fn into(self) -> T;
451}
452
453/// Used to do value-to-value conversions while consuming the input value. It is the reciprocal of
454/// [`Into`].
455///
456/// One should always prefer implementing `From` over [`Into`]
457/// because implementing `From` automatically provides one with an implementation of [`Into`]
458/// thanks to the blanket implementation in the standard library.
459///
460/// Only implement [`Into`] when targeting a version prior to Rust 1.41 and converting to a type
461/// outside the current crate.
462/// `From` was not able to do these types of conversions in earlier versions because of Rust's
463/// orphaning rules.
464/// See [`Into`] for more details.
465///
466/// Prefer using [`Into`] over using `From` when specifying trait bounds on a generic function.
467/// This way, types that directly implement [`Into`] can be used as arguments as well.
468///
469/// The `From` trait is also very useful when performing error handling. When constructing a function
470/// that is capable of failing, the return type will generally be of the form `Result<T, E>`.
471/// `From` simplifies error handling by allowing a function to return a single error type
472/// that encapsulates multiple error types. See the "Examples" section and [the book][book] for more
473/// details.
474///
475/// **Note: This trait must not fail**. The `From` trait is intended for perfect conversions.
476/// If the conversion can fail or is not perfect, use [`TryFrom`].
477///
478/// # Generic Implementations
479///
480/// - `From<T> for U` implies [`Into`]`<U> for T`
481/// - `From` is reflexive, which means that `From<T> for T` is implemented
482///
483/// # When to implement `From`
484///
485/// While there's no technical restrictions on which conversions can be done using
486/// a `From` implementation, the general expectation is that the conversions
487/// should typically be restricted as follows:
488///
489/// * The conversion is *infallible*: if the conversion can fail, use [`TryFrom`]
490/// instead; don't provide a `From` impl that panics.
491///
492/// * The conversion is *lossless*: semantically, it should not lose or discard
493/// information. For example, `i32: From<u16>` exists, where the original
494/// value can be recovered using `u16: TryFrom<i32>`. And `String: From<&str>`
495/// exists, where you can get something equivalent to the original value via
496/// `Deref`. But `From` cannot be used to convert from `u32` to `u16`, since
497/// that cannot succeed in a lossless way. (There's some wiggle room here for
498/// information not considered semantically relevant. For example,
499/// `Box<[T]>: From<Vec<T>>` exists even though it might not preserve capacity,
500/// like how two vectors can be equal despite differing capacities.)
501///
502/// * The conversion is *value-preserving*: the conceptual kind and meaning of
503/// the resulting value is the same, even though the Rust type and technical
504/// representation might be different. For example `-1_i8 as u8` is *lossless*,
505/// since `as` casting back can recover the original value, but that conversion
506/// is *not* available via `From` because `-1` and `255` are different conceptual
507/// values (despite being identical bit patterns technically). But
508/// `f32: From<i16>` *is* available because `1_i16` and `1.0_f32` are conceptually
509/// the same real number (despite having very different bit patterns technically).
510/// `String: From<char>` is available because they're both *text*, but
511/// `String: From<u32>` is *not* available, since `1` (a number) and `"1"`
512/// (text) are too different. (Converting values to text is instead covered
513/// by the [`Display`](crate::fmt::Display) trait.)
514///
515/// * The conversion is *obvious*: it's the only reasonable conversion between
516/// the two types. Otherwise it's better to have it be a named method or
517/// constructor, like how [`str::as_bytes`] is a method and how integers have
518/// methods like [`u32::from_ne_bytes`], [`u32::from_le_bytes`], and
519/// [`u32::from_be_bytes`], none of which are `From` implementations. Whereas
520/// there's only one reasonable way to wrap an [`Ipv6Addr`](crate::net::Ipv6Addr)
521/// into an [`IpAddr`](crate::net::IpAddr), thus `IpAddr: From<Ipv6Addr>` exists.
522///
523/// # Examples
524///
525/// [`String`] implements `From<&str>`:
526///
527/// An explicit conversion from a `&str` to a String is done as follows:
528///
529/// ```
530/// let string = "hello".to_string();
531/// let other_string = String::from("hello");
532///
533/// assert_eq!(string, other_string);
534/// ```
535///
536/// While performing error handling it is often useful to implement `From` for your own error type.
537/// By converting underlying error types to our own custom error type that encapsulates the
538/// underlying error type, we can return a single error type without losing information on the
539/// underlying cause. The '?' operator automatically converts the underlying error type to our
540/// custom error type with `From::from`.
541///
542/// ```
543/// use std::fs;
544/// use std::io;
545/// use std::num;
546///
547/// enum CliError {
548/// IoError(io::Error),
549/// ParseError(num::ParseIntError),
550/// }
551///
552/// impl From<io::Error> for CliError {
553/// fn from(error: io::Error) -> Self {
554/// CliError::IoError(error)
555/// }
556/// }
557///
558/// impl From<num::ParseIntError> for CliError {
559/// fn from(error: num::ParseIntError) -> Self {
560/// CliError::ParseError(error)
561/// }
562/// }
563///
564/// fn open_and_parse_file(file_name: &str) -> Result<i32, CliError> {
565/// let mut contents = fs::read_to_string(&file_name)?;
566/// let num: i32 = contents.trim().parse()?;
567/// Ok(num)
568/// }
569/// ```
570///
571/// [`String`]: ../../std/string/struct.String.html
572/// [`from`]: From::from
573/// [book]: ../../book/ch09-00-error-handling.html
574#[rustc_diagnostic_item = "From"]
575#[stable(feature = "rust1", since = "1.0.0")]
576#[rustc_on_unimplemented(on(
577 all(_Self = "&str", T = "alloc::string::String"),
578 note = "to coerce a `{T}` into a `{Self}`, use `&*` as a prefix",
579))]
580pub trait From<T>: Sized {
581 /// Converts to this type from the input type.
582 #[rustc_diagnostic_item = "from_fn"]
583 #[must_use]
584 #[stable(feature = "rust1", since = "1.0.0")]
585 fn from(value: T) -> Self;
586}
587
588/// An attempted conversion that consumes `self`, which may or may not be
589/// expensive.
590///
591/// Library authors should usually not directly implement this trait,
592/// but should prefer implementing the [`TryFrom`] trait, which offers
593/// greater flexibility and provides an equivalent `TryInto`
594/// implementation for free, thanks to a blanket implementation in the
595/// standard library. For more information on this, see the
596/// documentation for [`Into`].
597///
598/// # Implementing `TryInto`
599///
600/// This suffers the same restrictions and reasoning as implementing
601/// [`Into`], see there for details.
602#[rustc_diagnostic_item = "TryInto"]
603#[stable(feature = "try_from", since = "1.34.0")]
604pub trait TryInto<T>: Sized {
605 /// The type returned in the event of a conversion error.
606 #[stable(feature = "try_from", since = "1.34.0")]
607 type Error;
608
609 /// Performs the conversion.
610 #[stable(feature = "try_from", since = "1.34.0")]
611 fn try_into(self) -> Result<T, Self::Error>;
612}
613
614/// Simple and safe type conversions that may fail in a controlled
615/// way under some circumstances. It is the reciprocal of [`TryInto`].
616///
617/// This is useful when you are doing a type conversion that may
618/// trivially succeed but may also need special handling.
619/// For example, there is no way to convert an [`i64`] into an [`i32`]
620/// using the [`From`] trait, because an [`i64`] may contain a value
621/// that an [`i32`] cannot represent and so the conversion would lose data.
622/// This might be handled by truncating the [`i64`] to an [`i32`] or by
623/// simply returning [`i32::MAX`], or by some other method. The [`From`]
624/// trait is intended for perfect conversions, so the `TryFrom` trait
625/// informs the programmer when a type conversion could go bad and lets
626/// them decide how to handle it.
627///
628/// # Generic Implementations
629///
630/// - `TryFrom<T> for U` implies [`TryInto`]`<U> for T`
631/// - [`try_from`] is reflexive, which means that `TryFrom<T> for T`
632/// is implemented and cannot fail -- the associated `Error` type for
633/// calling `T::try_from()` on a value of type `T` is [`Infallible`].
634/// When the [`!`] type is stabilized [`Infallible`] and [`!`] will be
635/// equivalent.
636///
637/// `TryFrom<T>` can be implemented as follows:
638///
639/// ```
640/// struct GreaterThanZero(i32);
641///
642/// impl TryFrom<i32> for GreaterThanZero {
643/// type Error = &'static str;
644///
645/// fn try_from(value: i32) -> Result<Self, Self::Error> {
646/// if value <= 0 {
647/// Err("GreaterThanZero only accepts values greater than zero!")
648/// } else {
649/// Ok(GreaterThanZero(value))
650/// }
651/// }
652/// }
653/// ```
654///
655/// # Examples
656///
657/// As described, [`i32`] implements `TryFrom<`[`i64`]`>`:
658///
659/// ```
660/// let big_number = 1_000_000_000_000i64;
661/// // Silently truncates `big_number`, requires detecting
662/// // and handling the truncation after the fact.
663/// let smaller_number = big_number as i32;
664/// assert_eq!(smaller_number, -727379968);
665///
666/// // Returns an error because `big_number` is too big to
667/// // fit in an `i32`.
668/// let try_smaller_number = i32::try_from(big_number);
669/// assert!(try_smaller_number.is_err());
670///
671/// // Returns `Ok(3)`.
672/// let try_successful_smaller_number = i32::try_from(3);
673/// assert!(try_successful_smaller_number.is_ok());
674/// ```
675///
676/// [`try_from`]: TryFrom::try_from
677#[rustc_diagnostic_item = "TryFrom"]
678#[stable(feature = "try_from", since = "1.34.0")]
679pub trait TryFrom<T>: Sized {
680 /// The type returned in the event of a conversion error.
681 #[stable(feature = "try_from", since = "1.34.0")]
682 type Error;
683
684 /// Performs the conversion.
685 #[stable(feature = "try_from", since = "1.34.0")]
686 #[rustc_diagnostic_item = "try_from_fn"]
687 fn try_from(value: T) -> Result<Self, Self::Error>;
688}
689
690////////////////////////////////////////////////////////////////////////////////
691// GENERIC IMPLS
692////////////////////////////////////////////////////////////////////////////////
693
694// As lifts over &
695#[stable(feature = "rust1", since = "1.0.0")]
696impl<T: ?Sized, U: ?Sized> AsRef<U> for &T
697where
698 T: AsRef<U>,
699{
700 #[inline]
701 fn as_ref(&self) -> &U {
702 <T as AsRef<U>>::as_ref(*self)
703 }
704}
705
706// As lifts over &mut
707#[stable(feature = "rust1", since = "1.0.0")]
708impl<T: ?Sized, U: ?Sized> AsRef<U> for &mut T
709where
710 T: AsRef<U>,
711{
712 #[inline]
713 fn as_ref(&self) -> &U {
714 <T as AsRef<U>>::as_ref(*self)
715 }
716}
717
718// FIXME (#45742): replace the above impls for &/&mut with the following more general one:
719// // As lifts over Deref
720// impl<D: ?Sized + Deref<Target: AsRef<U>>, U: ?Sized> AsRef<U> for D {
721// fn as_ref(&self) -> &U {
722// self.deref().as_ref()
723// }
724// }
725
726// AsMut lifts over &mut
727#[stable(feature = "rust1", since = "1.0.0")]
728impl<T: ?Sized, U: ?Sized> AsMut<U> for &mut T
729where
730 T: AsMut<U>,
731{
732 #[inline]
733 fn as_mut(&mut self) -> &mut U {
734 (*self).as_mut()
735 }
736}
737
738// FIXME (#45742): replace the above impl for &mut with the following more general one:
739// // AsMut lifts over DerefMut
740// impl<D: ?Sized + Deref<Target: AsMut<U>>, U: ?Sized> AsMut<U> for D {
741// fn as_mut(&mut self) -> &mut U {
742// self.deref_mut().as_mut()
743// }
744// }
745
746// From implies Into
747#[stable(feature = "rust1", since = "1.0.0")]
748impl<T, U> Into<U> for T
749where
750 U: From<T>,
751{
752 /// Calls `U::from(self)`.
753 ///
754 /// That is, this conversion is whatever the implementation of
755 /// <code>[From]&lt;T&gt; for U</code> chooses to do.
756 #[inline]
757 #[track_caller]
758 fn into(self) -> U {
759 U::from(self)
760 }
761}
762
763// From (and thus Into) is reflexive
764#[stable(feature = "rust1", since = "1.0.0")]
765impl<T> From<T> for T {
766 /// Returns the argument unchanged.
767 #[inline(always)]
768 fn from(t: T) -> T {
769 t
770 }
771}
772
773/// **Stability note:** This impl does not yet exist, but we are
774/// "reserving space" to add it in the future. See
775/// [rust-lang/rust#64715][#64715] for details.
776///
777/// [#64715]: https://github.com/rust-lang/rust/issues/64715
778#[stable(feature = "convert_infallible", since = "1.34.0")]
779#[allow(unused_attributes)] // FIXME(#58633): do a principled fix instead.
780#[rustc_reservation_impl = "permitting this impl would forbid us from adding \
781 `impl<T> From<!> for T` later; see rust-lang/rust#64715 for details"]
782impl<T> From<!> for T {
783 fn from(t: !) -> T {
784 t
785 }
786}
787
788// TryFrom implies TryInto
789#[stable(feature = "try_from", since = "1.34.0")]
790impl<T, U> TryInto<U> for T
791where
792 U: TryFrom<T>,
793{
794 type Error = U::Error;
795
796 #[inline]
797 fn try_into(self) -> Result<U, U::Error> {
798 U::try_from(self)
799 }
800}
801
802// Infallible conversions are semantically equivalent to fallible conversions
803// with an uninhabited error type.
804#[stable(feature = "try_from", since = "1.34.0")]
805impl<T, U> TryFrom<U> for T
806where
807 U: Into<T>,
808{
809 type Error = Infallible;
810
811 #[inline]
812 fn try_from(value: U) -> Result<Self, Self::Error> {
813 Ok(U::into(self:value))
814 }
815}
816
817////////////////////////////////////////////////////////////////////////////////
818// CONCRETE IMPLS
819////////////////////////////////////////////////////////////////////////////////
820
821#[stable(feature = "rust1", since = "1.0.0")]
822impl<T> AsRef<[T]> for [T] {
823 #[inline(always)]
824 fn as_ref(&self) -> &[T] {
825 self
826 }
827}
828
829#[stable(feature = "rust1", since = "1.0.0")]
830impl<T> AsMut<[T]> for [T] {
831 #[inline(always)]
832 fn as_mut(&mut self) -> &mut [T] {
833 self
834 }
835}
836
837#[stable(feature = "rust1", since = "1.0.0")]
838impl AsRef<str> for str {
839 #[inline(always)]
840 fn as_ref(&self) -> &str {
841 self
842 }
843}
844
845#[stable(feature = "as_mut_str_for_str", since = "1.51.0")]
846impl AsMut<str> for str {
847 #[inline(always)]
848 fn as_mut(&mut self) -> &mut str {
849 self
850 }
851}
852
853////////////////////////////////////////////////////////////////////////////////
854// THE NO-ERROR ERROR TYPE
855////////////////////////////////////////////////////////////////////////////////
856
857/// The error type for errors that can never happen.
858///
859/// Since this enum has no variant, a value of this type can never actually exist.
860/// This can be useful for generic APIs that use [`Result`] and parameterize the error type,
861/// to indicate that the result is always [`Ok`].
862///
863/// For example, the [`TryFrom`] trait (conversion that returns a [`Result`])
864/// has a blanket implementation for all types where a reverse [`Into`] implementation exists.
865///
866/// ```ignore (illustrates std code, duplicating the impl in a doctest would be an error)
867/// impl<T, U> TryFrom<U> for T where U: Into<T> {
868/// type Error = Infallible;
869///
870/// fn try_from(value: U) -> Result<Self, Infallible> {
871/// Ok(U::into(value)) // Never returns `Err`
872/// }
873/// }
874/// ```
875///
876/// # Future compatibility
877///
878/// This enum has the same role as [the `!` “never” type][never],
879/// which is unstable in this version of Rust.
880/// When `!` is stabilized, we plan to make `Infallible` a type alias to it:
881///
882/// ```ignore (illustrates future std change)
883/// pub type Infallible = !;
884/// ```
885///
886/// … and eventually deprecate `Infallible`.
887///
888/// However there is one case where `!` syntax can be used
889/// before `!` is stabilized as a full-fledged type: in the position of a function’s return type.
890/// Specifically, it is possible to have implementations for two different function pointer types:
891///
892/// ```
893/// trait MyTrait {}
894/// impl MyTrait for fn() -> ! {}
895/// impl MyTrait for fn() -> std::convert::Infallible {}
896/// ```
897///
898/// With `Infallible` being an enum, this code is valid.
899/// However when `Infallible` becomes an alias for the never type,
900/// the two `impl`s will start to overlap
901/// and therefore will be disallowed by the language’s trait coherence rules.
902#[stable(feature = "convert_infallible", since = "1.34.0")]
903#[derive(Copy)]
904pub enum Infallible {}
905
906#[stable(feature = "convert_infallible", since = "1.34.0")]
907impl Clone for Infallible {
908 fn clone(&self) -> Infallible {
909 match *self {}
910 }
911}
912
913#[stable(feature = "convert_infallible", since = "1.34.0")]
914impl fmt::Debug for Infallible {
915 fn fmt(&self, _: &mut fmt::Formatter<'_>) -> fmt::Result {
916 match *self {}
917 }
918}
919
920#[stable(feature = "convert_infallible", since = "1.34.0")]
921impl fmt::Display for Infallible {
922 fn fmt(&self, _: &mut fmt::Formatter<'_>) -> fmt::Result {
923 match *self {}
924 }
925}
926
927#[stable(feature = "str_parse_error2", since = "1.8.0")]
928impl Error for Infallible {
929 fn description(&self) -> &str {
930 match *self {}
931 }
932}
933
934#[stable(feature = "convert_infallible", since = "1.34.0")]
935impl PartialEq for Infallible {
936 fn eq(&self, _: &Infallible) -> bool {
937 match *self {}
938 }
939}
940
941#[stable(feature = "convert_infallible", since = "1.34.0")]
942impl Eq for Infallible {}
943
944#[stable(feature = "convert_infallible", since = "1.34.0")]
945impl PartialOrd for Infallible {
946 fn partial_cmp(&self, _other: &Self) -> Option<crate::cmp::Ordering> {
947 match *self {}
948 }
949}
950
951#[stable(feature = "convert_infallible", since = "1.34.0")]
952impl Ord for Infallible {
953 fn cmp(&self, _other: &Self) -> crate::cmp::Ordering {
954 match *self {}
955 }
956}
957
958#[stable(feature = "convert_infallible", since = "1.34.0")]
959impl From<!> for Infallible {
960 #[inline]
961 fn from(x: !) -> Self {
962 x
963 }
964}
965
966#[stable(feature = "convert_infallible_hash", since = "1.44.0")]
967impl Hash for Infallible {
968 fn hash<H: Hasher>(&self, _: &mut H) {
969 match *self {}
970 }
971}
972