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#[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#[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")]
446#[doc(search_unbox)]
447pub trait Into<T>: Sized {
448 /// Converts this type into the (usually inferred) input type.
449 #[must_use]
450 #[stable(feature = "rust1", since = "1.0.0")]
451 fn into(self) -> T;
452}
453
454/// Used to do value-to-value conversions while consuming the input value. It is the reciprocal of
455/// [`Into`].
456///
457/// One should always prefer implementing `From` over [`Into`]
458/// because implementing `From` automatically provides one with an implementation of [`Into`]
459/// thanks to the blanket implementation in the standard library.
460///
461/// Only implement [`Into`] when targeting a version prior to Rust 1.41 and converting to a type
462/// outside the current crate.
463/// `From` was not able to do these types of conversions in earlier versions because of Rust's
464/// orphaning rules.
465/// See [`Into`] for more details.
466///
467/// Prefer using [`Into`] over [`From`] when specifying trait bounds on a generic function
468/// to ensure that types that only implement [`Into`] can be used as well.
469///
470/// The `From` trait is also very useful when performing error handling. When constructing a function
471/// that is capable of failing, the return type will generally be of the form `Result<T, E>`.
472/// `From` simplifies error handling by allowing a function to return a single error type
473/// that encapsulates multiple error types. See the "Examples" section and [the book][book] for more
474/// details.
475///
476/// **Note: This trait must not fail**. The `From` trait is intended for perfect conversions.
477/// If the conversion can fail or is not perfect, use [`TryFrom`].
478///
479/// # Generic Implementations
480///
481/// - `From<T> for U` implies [`Into`]`<U> for T`
482/// - `From` is reflexive, which means that `From<T> for T` is implemented
483///
484/// # When to implement `From`
485///
486/// While there's no technical restrictions on which conversions can be done using
487/// a `From` implementation, the general expectation is that the conversions
488/// should typically be restricted as follows:
489///
490/// * The conversion is *infallible*: if the conversion can fail, use [`TryFrom`]
491/// instead; don't provide a `From` impl that panics.
492///
493/// * The conversion is *lossless*: semantically, it should not lose or discard
494/// information. For example, `i32: From<u16>` exists, where the original
495/// value can be recovered using `u16: TryFrom<i32>`. And `String: From<&str>`
496/// exists, where you can get something equivalent to the original value via
497/// `Deref`. But `From` cannot be used to convert from `u32` to `u16`, since
498/// that cannot succeed in a lossless way. (There's some wiggle room here for
499/// information not considered semantically relevant. For example,
500/// `Box<[T]>: From<Vec<T>>` exists even though it might not preserve capacity,
501/// like how two vectors can be equal despite differing capacities.)
502///
503/// * The conversion is *value-preserving*: the conceptual kind and meaning of
504/// the resulting value is the same, even though the Rust type and technical
505/// representation might be different. For example `-1_i8 as u8` is *lossless*,
506/// since `as` casting back can recover the original value, but that conversion
507/// is *not* available via `From` because `-1` and `255` are different conceptual
508/// values (despite being identical bit patterns technically). But
509/// `f32: From<i16>` *is* available because `1_i16` and `1.0_f32` are conceptually
510/// the same real number (despite having very different bit patterns technically).
511/// `String: From<char>` is available because they're both *text*, but
512/// `String: From<u32>` is *not* available, since `1` (a number) and `"1"`
513/// (text) are too different. (Converting values to text is instead covered
514/// by the [`Display`](crate::fmt::Display) trait.)
515///
516/// * The conversion is *obvious*: it's the only reasonable conversion between
517/// the two types. Otherwise it's better to have it be a named method or
518/// constructor, like how [`str::as_bytes`] is a method and how integers have
519/// methods like [`u32::from_ne_bytes`], [`u32::from_le_bytes`], and
520/// [`u32::from_be_bytes`], none of which are `From` implementations. Whereas
521/// there's only one reasonable way to wrap an [`Ipv6Addr`](crate::net::Ipv6Addr)
522/// into an [`IpAddr`](crate::net::IpAddr), thus `IpAddr: From<Ipv6Addr>` exists.
523///
524/// # Examples
525///
526/// [`String`] implements `From<&str>`:
527///
528/// An explicit conversion from a `&str` to a String is done as follows:
529///
530/// ```
531/// let string = "hello".to_string();
532/// let other_string = String::from("hello");
533///
534/// assert_eq!(string, other_string);
535/// ```
536///
537/// While performing error handling it is often useful to implement `From` for your own error type.
538/// By converting underlying error types to our own custom error type that encapsulates the
539/// underlying error type, we can return a single error type without losing information on the
540/// underlying cause. The '?' operator automatically converts the underlying error type to our
541/// custom error type with `From::from`.
542///
543/// ```
544/// use std::fs;
545/// use std::io;
546/// use std::num;
547///
548/// enum CliError {
549/// IoError(io::Error),
550/// ParseError(num::ParseIntError),
551/// }
552///
553/// impl From<io::Error> for CliError {
554/// fn from(error: io::Error) -> Self {
555/// CliError::IoError(error)
556/// }
557/// }
558///
559/// impl From<num::ParseIntError> for CliError {
560/// fn from(error: num::ParseIntError) -> Self {
561/// CliError::ParseError(error)
562/// }
563/// }
564///
565/// fn open_and_parse_file(file_name: &str) -> Result<i32, CliError> {
566/// let mut contents = fs::read_to_string(&file_name)?;
567/// let num: i32 = contents.trim().parse()?;
568/// Ok(num)
569/// }
570/// ```
571///
572/// [`String`]: ../../std/string/struct.String.html
573/// [`from`]: From::from
574/// [book]: ../../book/ch09-00-error-handling.html
575#[rustc_diagnostic_item = "From"]
576#[stable(feature = "rust1", since = "1.0.0")]
577#[rustc_on_unimplemented(on(
578 all(Self = "&str", T = "alloc::string::String"),
579 note = "to coerce a `{T}` into a `{Self}`, use `&*` as a prefix",
580))]
581#[doc(search_unbox)]
582pub trait From<T>: Sized {
583 /// Converts to this type from the input type.
584 #[rustc_diagnostic_item = "from_fn"]
585 #[must_use]
586 #[stable(feature = "rust1", since = "1.0.0")]
587 fn from(value: T) -> Self;
588}
589
590/// An attempted conversion that consumes `self`, which may or may not be
591/// expensive.
592///
593/// Library authors should usually not directly implement this trait,
594/// but should prefer implementing the [`TryFrom`] trait, which offers
595/// greater flexibility and provides an equivalent `TryInto`
596/// implementation for free, thanks to a blanket implementation in the
597/// standard library. For more information on this, see the
598/// documentation for [`Into`].
599///
600/// Prefer using [`TryInto`] over [`TryFrom`] when specifying trait bounds on a generic function
601/// to ensure that types that only implement [`TryInto`] can be used as well.
602///
603/// # Implementing `TryInto`
604///
605/// This suffers the same restrictions and reasoning as implementing
606/// [`Into`], see there for details.
607#[rustc_diagnostic_item = "TryInto"]
608#[stable(feature = "try_from", since = "1.34.0")]
609pub trait TryInto<T>: Sized {
610 /// The type returned in the event of a conversion error.
611 #[stable(feature = "try_from", since = "1.34.0")]
612 type Error;
613
614 /// Performs the conversion.
615 #[stable(feature = "try_from", since = "1.34.0")]
616 fn try_into(self) -> Result<T, Self::Error>;
617}
618
619/// Simple and safe type conversions that may fail in a controlled
620/// way under some circumstances. It is the reciprocal of [`TryInto`].
621///
622/// This is useful when you are doing a type conversion that may
623/// trivially succeed but may also need special handling.
624/// For example, there is no way to convert an [`i64`] into an [`i32`]
625/// using the [`From`] trait, because an [`i64`] may contain a value
626/// that an [`i32`] cannot represent and so the conversion would lose data.
627/// This might be handled by truncating the [`i64`] to an [`i32`] or by
628/// simply returning [`i32::MAX`], or by some other method. The [`From`]
629/// trait is intended for perfect conversions, so the `TryFrom` trait
630/// informs the programmer when a type conversion could go bad and lets
631/// them decide how to handle it.
632///
633/// # Generic Implementations
634///
635/// - `TryFrom<T> for U` implies [`TryInto`]`<U> for T`
636/// - [`try_from`] is reflexive, which means that `TryFrom<T> for T`
637/// is implemented and cannot fail -- the associated `Error` type for
638/// calling `T::try_from()` on a value of type `T` is [`Infallible`].
639/// When the [`!`] type is stabilized [`Infallible`] and [`!`] will be
640/// equivalent.
641///
642/// Prefer using [`TryInto`] over [`TryFrom`] when specifying trait bounds on a generic function
643/// to ensure that types that only implement [`TryInto`] can be used as well.
644///
645/// `TryFrom<T>` can be implemented as follows:
646///
647/// ```
648/// struct GreaterThanZero(i32);
649///
650/// impl TryFrom<i32> for GreaterThanZero {
651/// type Error = &'static str;
652///
653/// fn try_from(value: i32) -> Result<Self, Self::Error> {
654/// if value <= 0 {
655/// Err("GreaterThanZero only accepts values greater than zero!")
656/// } else {
657/// Ok(GreaterThanZero(value))
658/// }
659/// }
660/// }
661/// ```
662///
663/// # Examples
664///
665/// As described, [`i32`] implements `TryFrom<`[`i64`]`>`:
666///
667/// ```
668/// let big_number = 1_000_000_000_000i64;
669/// // Silently truncates `big_number`, requires detecting
670/// // and handling the truncation after the fact.
671/// let smaller_number = big_number as i32;
672/// assert_eq!(smaller_number, -727379968);
673///
674/// // Returns an error because `big_number` is too big to
675/// // fit in an `i32`.
676/// let try_smaller_number = i32::try_from(big_number);
677/// assert!(try_smaller_number.is_err());
678///
679/// // Returns `Ok(3)`.
680/// let try_successful_smaller_number = i32::try_from(3);
681/// assert!(try_successful_smaller_number.is_ok());
682/// ```
683///
684/// [`try_from`]: TryFrom::try_from
685#[rustc_diagnostic_item = "TryFrom"]
686#[stable(feature = "try_from", since = "1.34.0")]
687pub trait TryFrom<T>: Sized {
688 /// The type returned in the event of a conversion error.
689 #[stable(feature = "try_from", since = "1.34.0")]
690 type Error;
691
692 /// Performs the conversion.
693 #[stable(feature = "try_from", since = "1.34.0")]
694 #[rustc_diagnostic_item = "try_from_fn"]
695 fn try_from(value: T) -> Result<Self, Self::Error>;
696}
697
698////////////////////////////////////////////////////////////////////////////////
699// GENERIC IMPLS
700////////////////////////////////////////////////////////////////////////////////
701
702// As lifts over &
703#[stable(feature = "rust1", since = "1.0.0")]
704impl<T: ?Sized, U: ?Sized> AsRef<U> for &T
705where
706 T: AsRef<U>,
707{
708 #[inline]
709 fn as_ref(&self) -> &U {
710 <T as AsRef<U>>::as_ref(*self)
711 }
712}
713
714// As lifts over &mut
715#[stable(feature = "rust1", since = "1.0.0")]
716impl<T: ?Sized, U: ?Sized> AsRef<U> for &mut T
717where
718 T: AsRef<U>,
719{
720 #[inline]
721 fn as_ref(&self) -> &U {
722 <T as AsRef<U>>::as_ref(*self)
723 }
724}
725
726// FIXME (#45742): replace the above impls for &/&mut with the following more general one:
727// // As lifts over Deref
728// impl<D: ?Sized + Deref<Target: AsRef<U>>, U: ?Sized> AsRef<U> for D {
729// fn as_ref(&self) -> &U {
730// self.deref().as_ref()
731// }
732// }
733
734// AsMut lifts over &mut
735#[stable(feature = "rust1", since = "1.0.0")]
736impl<T: ?Sized, U: ?Sized> AsMut<U> for &mut T
737where
738 T: AsMut<U>,
739{
740 #[inline]
741 fn as_mut(&mut self) -> &mut U {
742 (*self).as_mut()
743 }
744}
745
746// FIXME (#45742): replace the above impl for &mut with the following more general one:
747// // AsMut lifts over DerefMut
748// impl<D: ?Sized + Deref<Target: AsMut<U>>, U: ?Sized> AsMut<U> for D {
749// fn as_mut(&mut self) -> &mut U {
750// self.deref_mut().as_mut()
751// }
752// }
753
754// From implies Into
755#[stable(feature = "rust1", since = "1.0.0")]
756impl<T, U> Into<U> for T
757where
758 U: From<T>,
759{
760 /// Calls `U::from(self)`.
761 ///
762 /// That is, this conversion is whatever the implementation of
763 /// <code>[From]&lt;T&gt; for U</code> chooses to do.
764 #[inline]
765 #[track_caller]
766 fn into(self) -> U {
767 U::from(self)
768 }
769}
770
771// From (and thus Into) is reflexive
772#[stable(feature = "rust1", since = "1.0.0")]
773impl<T> From<T> for T {
774 /// Returns the argument unchanged.
775 #[inline(always)]
776 fn from(t: T) -> T {
777 t
778 }
779}
780
781/// **Stability note:** This impl does not yet exist, but we are
782/// "reserving space" to add it in the future. See
783/// [rust-lang/rust#64715][#64715] for details.
784///
785/// [#64715]: https://github.com/rust-lang/rust/issues/64715
786#[stable(feature = "convert_infallible", since = "1.34.0")]
787#[rustc_reservation_impl = "permitting this impl would forbid us from adding \
788 `impl<T> From<!> for T` later; see rust-lang/rust#64715 for details"]
789impl<T> From<!> for T {
790 fn from(t: !) -> T {
791 t
792 }
793}
794
795// TryFrom implies TryInto
796#[stable(feature = "try_from", since = "1.34.0")]
797impl<T, U> TryInto<U> for T
798where
799 U: TryFrom<T>,
800{
801 type Error = U::Error;
802
803 #[inline]
804 fn try_into(self) -> Result<U, U::Error> {
805 U::try_from(self)
806 }
807}
808
809// Infallible conversions are semantically equivalent to fallible conversions
810// with an uninhabited error type.
811#[stable(feature = "try_from", since = "1.34.0")]
812impl<T, U> TryFrom<U> for T
813where
814 U: Into<T>,
815{
816 type Error = Infallible;
817
818 #[inline]
819 fn try_from(value: U) -> Result<Self, Self::Error> {
820 Ok(U::into(self:value))
821 }
822}
823
824////////////////////////////////////////////////////////////////////////////////
825// CONCRETE IMPLS
826////////////////////////////////////////////////////////////////////////////////
827
828#[stable(feature = "rust1", since = "1.0.0")]
829impl<T> AsRef<[T]> for [T] {
830 #[inline(always)]
831 fn as_ref(&self) -> &[T] {
832 self
833 }
834}
835
836#[stable(feature = "rust1", since = "1.0.0")]
837impl<T> AsMut<[T]> for [T] {
838 #[inline(always)]
839 fn as_mut(&mut self) -> &mut [T] {
840 self
841 }
842}
843
844#[stable(feature = "rust1", since = "1.0.0")]
845impl AsRef<str> for str {
846 #[inline(always)]
847 fn as_ref(&self) -> &str {
848 self
849 }
850}
851
852#[stable(feature = "as_mut_str_for_str", since = "1.51.0")]
853impl AsMut<str> for str {
854 #[inline(always)]
855 fn as_mut(&mut self) -> &mut str {
856 self
857 }
858}
859
860////////////////////////////////////////////////////////////////////////////////
861// THE NO-ERROR ERROR TYPE
862////////////////////////////////////////////////////////////////////////////////
863
864/// The error type for errors that can never happen.
865///
866/// Since this enum has no variant, a value of this type can never actually exist.
867/// This can be useful for generic APIs that use [`Result`] and parameterize the error type,
868/// to indicate that the result is always [`Ok`].
869///
870/// For example, the [`TryFrom`] trait (conversion that returns a [`Result`])
871/// has a blanket implementation for all types where a reverse [`Into`] implementation exists.
872///
873/// ```ignore (illustrates std code, duplicating the impl in a doctest would be an error)
874/// impl<T, U> TryFrom<U> for T where U: Into<T> {
875/// type Error = Infallible;
876///
877/// fn try_from(value: U) -> Result<Self, Infallible> {
878/// Ok(U::into(value)) // Never returns `Err`
879/// }
880/// }
881/// ```
882///
883/// # Future compatibility
884///
885/// This enum has the same role as [the `!` “never” type][never],
886/// which is unstable in this version of Rust.
887/// When `!` is stabilized, we plan to make `Infallible` a type alias to it:
888///
889/// ```ignore (illustrates future std change)
890/// pub type Infallible = !;
891/// ```
892///
893/// … and eventually deprecate `Infallible`.
894///
895/// However there is one case where `!` syntax can be used
896/// before `!` is stabilized as a full-fledged type: in the position of a function’s return type.
897/// Specifically, it is possible to have implementations for two different function pointer types:
898///
899/// ```
900/// trait MyTrait {}
901/// impl MyTrait for fn() -> ! {}
902/// impl MyTrait for fn() -> std::convert::Infallible {}
903/// ```
904///
905/// With `Infallible` being an enum, this code is valid.
906/// However when `Infallible` becomes an alias for the never type,
907/// the two `impl`s will start to overlap
908/// and therefore will be disallowed by the language’s trait coherence rules.
909#[stable(feature = "convert_infallible", since = "1.34.0")]
910#[derive(Copy)]
911pub enum Infallible {}
912
913#[stable(feature = "convert_infallible", since = "1.34.0")]
914impl Clone for Infallible {
915 fn clone(&self) -> Infallible {
916 match *self {}
917 }
918}
919
920#[stable(feature = "convert_infallible", since = "1.34.0")]
921impl fmt::Debug for Infallible {
922 fn fmt(&self, _: &mut fmt::Formatter<'_>) -> fmt::Result {
923 match *self {}
924 }
925}
926
927#[stable(feature = "convert_infallible", since = "1.34.0")]
928impl fmt::Display for Infallible {
929 fn fmt(&self, _: &mut fmt::Formatter<'_>) -> fmt::Result {
930 match *self {}
931 }
932}
933
934#[stable(feature = "str_parse_error2", since = "1.8.0")]
935impl Error for Infallible {
936 fn description(&self) -> &str {
937 match *self {}
938 }
939}
940
941#[stable(feature = "convert_infallible", since = "1.34.0")]
942impl PartialEq for Infallible {
943 fn eq(&self, _: &Infallible) -> bool {
944 match *self {}
945 }
946}
947
948#[stable(feature = "convert_infallible", since = "1.34.0")]
949impl Eq for Infallible {}
950
951#[stable(feature = "convert_infallible", since = "1.34.0")]
952impl PartialOrd for Infallible {
953 fn partial_cmp(&self, _other: &Self) -> Option<crate::cmp::Ordering> {
954 match *self {}
955 }
956}
957
958#[stable(feature = "convert_infallible", since = "1.34.0")]
959impl Ord for Infallible {
960 fn cmp(&self, _other: &Self) -> crate::cmp::Ordering {
961 match *self {}
962 }
963}
964
965#[stable(feature = "convert_infallible", since = "1.34.0")]
966impl From<!> for Infallible {
967 #[inline]
968 fn from(x: !) -> Self {
969 x
970 }
971}
972
973#[stable(feature = "convert_infallible_hash", since = "1.44.0")]
974impl Hash for Infallible {
975 fn hash<H: Hasher>(&self, _: &mut H) {
976 match *self {}
977 }
978}
979