1 //! Traits for conversions between types.
3 //! The traits in this module provide a way to convert from one type to another type.
4 //! Each trait serves a different purpose:
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.
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.
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.
25 //! # Generic Implementations
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
34 //! See each trait for usage examples.
36 #![stable(feature = "rust1", since = "1.0.0")]
38 use crate::error::Error;
40 use crate::hash::{Hash, Hasher};
44 #[unstable(feature = "convert_float_to_int", issue = "67057")]
45 pub use num::FloatToInt;
47 /// The identity function.
49 /// Two things are important to note about this function:
51 /// - It is not always equivalent to a closure like `|x| x`, since the
52 /// closure may coerce `x` into a different type.
54 /// - It moves the input `x` passed to the function.
56 /// While it might seem strange to have a function that just returns back the
57 /// input, there are some interesting uses.
61 /// Using `identity` to do nothing in a sequence of other, interesting,
65 /// use std::convert::identity;
67 /// fn manipulation(x: u32) -> u32 {
68 /// // Let's pretend that adding one is an interesting function.
72 /// let _arr = &[identity, manipulation];
75 /// Using `identity` as a "do nothing" base case in a conditional:
78 /// use std::convert::identity;
80 /// # let condition = true;
82 /// # fn manipulation(x: u32) -> u32 { x + 1 }
84 /// let do_stuff = if condition { manipulation } else { identity };
86 /// // Do more interesting stuff...
88 /// let _results = do_stuff(42);
91 /// Using `identity` to keep the `Some` variants of an iterator of `Option<T>`:
94 /// use std::convert::identity;
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);
100 #[stable(feature = "convert_id", since = "1.33.0")]
101 #[rustc_const_stable(feature = "const_identity", since = "1.33.0")]
103 pub const fn identity<T>(x: T) -> T {
107 /// Used to do a cheap reference-to-reference conversion.
109 /// This trait is similar to [`AsMut`] which is used for converting between mutable references.
110 /// If you need to do a costly conversion it is better to implement [`From`] with type
111 /// `&T` or write a custom function.
113 /// # Relation to `Borrow`
115 /// `AsRef` has the same signature as [`Borrow`], but [`Borrow`] is different in a few aspects:
117 /// - Unlike `AsRef`, [`Borrow`] has a blanket impl for any `T`, and can be used to accept either
118 /// a reference or a value. (See also note on `AsRef`'s reflexibility below.)
119 /// - [`Borrow`] also requires that [`Hash`], [`Eq`] and [`Ord`] for a borrowed value are
120 /// equivalent to those of the owned value. For this reason, if you want to
121 /// borrow only a single field of a struct you can implement `AsRef`, but not [`Borrow`].
123 /// **Note: This trait must not fail**. If the conversion can fail, use a
124 /// dedicated method which returns an [`Option<T>`] or a [`Result<T, E>`].
126 /// # Generic Implementations
128 /// `AsRef` auto-dereferences if the inner type is a reference or a mutable reference
129 /// (e.g.: `foo.as_ref()` will work the same if `foo` has type `&mut Foo` or `&&mut Foo`).
131 /// Note that due to historic reasons, the above currently does not hold generally for all
132 /// [dereferenceable types], e.g. `foo.as_ref()` will *not* work the same as
133 /// `Box::new(foo).as_ref()`. Instead, many smart pointers provide an `as_ref` implementation which
134 /// simply returns a reference to the [pointed-to value] (but do not perform a cheap
135 /// reference-to-reference conversion for that value). However, [`AsRef::as_ref`] should not be
136 /// used for the sole purpose of dereferencing; instead ['`Deref` coercion'] can be used:
138 /// [dereferenceable types]: core::ops::Deref
139 /// [pointed-to value]: core::ops::Deref::Target
140 /// ['`Deref` coercion']: core::ops::Deref#more-on-deref-coercion
143 /// let x = Box::new(5i32);
145 /// // let y: &i32 = x.as_ref();
146 /// // Better just write:
147 /// let y: &i32 = &x;
150 /// Types which implement [`Deref`] should consider implementing `AsRef<T>` as follows:
152 /// [`Deref`]: core::ops::Deref
155 /// # use core::ops::Deref;
156 /// # struct SomeType;
157 /// # impl Deref for SomeType {
158 /// # type Target = [u8];
159 /// # fn deref(&self) -> &[u8] {
163 /// impl<T> AsRef<T> for SomeType
166 /// <SomeType as Deref>::Target: AsRef<T>,
168 /// fn as_ref(&self) -> &T {
169 /// self.deref().as_ref()
176 /// Ideally, `AsRef` would be reflexive, i.e. there would be an `impl<T: ?Sized> AsRef<T> for T`
177 /// with [`as_ref`] simply returning its argument unchanged.
178 /// Such a blanket implementation is currently *not* provided due to technical restrictions of
179 /// Rust's type system (it would be overlapping with another existing blanket implementation for
180 /// `&T where T: AsRef<U>` which allows `AsRef` to auto-dereference, see "Generic Implementations"
183 /// [`as_ref`]: AsRef::as_ref
185 /// A trivial implementation of `AsRef<T> for T` must be added explicitly for a particular type `T`
186 /// where needed or desired. Note, however, that not all types from `std` contain such an
187 /// implementation, and those cannot be added by external code due to orphan rules.
191 /// By using trait bounds we can accept arguments of different types as long as they can be
192 /// converted to the specified type `T`.
194 /// For example: By creating a generic function that takes an `AsRef<str>` we express that we
195 /// want to accept all references that can be converted to [`&str`] as an argument.
196 /// Since both [`String`] and [`&str`] implement `AsRef<str>` we can accept both as input argument.
198 /// [`&str`]: primitive@str
199 /// [`Borrow`]: crate::borrow::Borrow
200 /// [`Eq`]: crate::cmp::Eq
201 /// [`Ord`]: crate::cmp::Ord
202 /// [`String`]: ../../std/string/struct.String.html
205 /// fn is_hello<T: AsRef<str>>(s: T) {
206 /// assert_eq!("hello", s.as_ref());
212 /// let s = "hello".to_string();
215 #[stable(feature = "rust1", since = "1.0.0")]
216 #[cfg_attr(not(test), rustc_diagnostic_item = "AsRef")]
218 pub 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;
224 /// Used to do a cheap mutable-to-mutable reference conversion.
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.
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>`].
233 /// # Generic Implementations
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`).
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:
245 /// [mutably dereferenceable types]: core::ops::DerefMut
246 /// [pointed-to value]: core::ops::Deref::Target
247 /// ['`Deref` coercion']: core::ops::DerefMut#more-on-deref-coercion
250 /// let mut x = Box::new(5i32);
252 /// // let y: &mut i32 = x.as_mut();
253 /// // Better just write:
254 /// let y: &mut i32 = &mut x;
257 /// Types which implement [`DerefMut`] should consider to add an implementation of `AsMut<T>` as
260 /// [`DerefMut`]: core::ops::DerefMut
263 /// # use core::ops::{Deref, DerefMut};
264 /// # struct SomeType;
265 /// # impl Deref for SomeType {
266 /// # type Target = [u8];
267 /// # fn deref(&self) -> &[u8] {
271 /// # impl DerefMut for SomeType {
272 /// # fn deref_mut(&mut self) -> &mut [u8] {
276 /// impl<T> AsMut<T> for SomeType
278 /// <SomeType as Deref>::Target: AsMut<T>,
280 /// fn as_mut(&mut self) -> &mut T {
281 /// self.deref_mut().as_mut()
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).
295 /// [`as_mut`]: AsMut::as_mut
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.
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]>`.
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.
312 /// [dereference]: core::ops::DerefMut
313 /// [target type]: core::ops::Deref::Target
316 /// struct Document {
318 /// content: Vec<u8>,
321 /// impl<T: ?Sized> AsMut<T> for Document
323 /// Vec<u8>: AsMut<T>,
325 /// fn as_mut(&mut self) -> &mut T {
326 /// self.content.as_mut()
330 /// fn caesar<T: AsMut<[u8]>>(data: &mut T, key: u8) {
331 /// for byte in data.as_mut() {
332 /// *byte = byte.wrapping_add(key);
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 {
345 /// doit(data.as_mut());
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],
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]);
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")]
370 pub trait AsMut<T: ?Sized> {
371 /// Converts this type into a mutable reference of the (usually inferred) input type.
372 #[stable(feature = "rust1", since = "1.0.0")]
373 fn as_mut(&mut self) -> &mut T;
376 /// A value-to-value conversion that consumes the input value. The
377 /// opposite of [`From`].
379 /// One should avoid implementing [`Into`] and implement [`From`] instead.
380 /// Implementing [`From`] automatically provides one with an implementation of [`Into`]
381 /// thanks to the blanket implementation in the standard library.
383 /// Prefer using [`Into`] over [`From`] when specifying trait bounds on a generic function
384 /// to ensure that types that only implement [`Into`] can be used as well.
386 /// **Note: This trait must not fail**. If the conversion can fail, use [`TryInto`].
388 /// # Generic Implementations
390 /// - [`From`]`<T> for U` implies `Into<U> for T`
391 /// - [`Into`] is reflexive, which means that `Into<T> for T` is implemented
393 /// # Implementing [`Into`] for conversions to external types in old versions of Rust
395 /// Prior to Rust 1.41, if the destination type was not part of the current crate
396 /// then you couldn't implement [`From`] directly.
397 /// For example, take this code:
400 /// struct Wrapper<T>(Vec<T>);
401 /// impl<T> From<Wrapper<T>> for Vec<T> {
402 /// fn from(w: Wrapper<T>) -> Vec<T> {
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:
411 /// struct Wrapper<T>(Vec<T>);
412 /// impl<T> Into<Vec<T>> for Wrapper<T> {
413 /// fn into(self) -> Vec<T> {
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.
425 /// [`String`] implements [`Into`]`<`[`Vec`]`<`[`u8`]`>>`:
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`]`>`.
433 /// fn is_hello<T: Into<Vec<u8>>>(s: T) {
434 /// let bytes = b"hello".to_vec();
435 /// assert_eq!(bytes, s.into());
438 /// let s = "hello".to_string();
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")]
447 pub trait Into<T>: Sized {
448 /// Converts this type into the (usually inferred) input type.
450 #[stable(feature = "rust1", since = "1.0.0")]
454 /// Used to do value-to-value conversions while consuming the input value. It is the reciprocal of
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.
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
465 /// See [`Into`] for more details.
467 /// Prefer using [`Into`] over using `From` when specifying trait bounds on a generic function.
468 /// This way, types that directly implement [`Into`] can be used as arguments as well.
470 /// The `From` 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 /// The `From` trait simplifies error handling by allowing a function to return a single error type
473 /// that encapsulate multiple error types. See the "Examples" section and [the book][book] for more
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`].
479 /// # Generic Implementations
481 /// - `From<T> for U` implies [`Into`]`<U> for T`
482 /// - `From` is reflexive, which means that `From<T> for T` is implemented
486 /// [`String`] implements `From<&str>`:
488 /// An explicit conversion from a `&str` to a String is done as follows:
491 /// let string = "hello".to_string();
492 /// let other_string = String::from("hello");
494 /// assert_eq!(string, other_string);
497 /// While performing error handling it is often useful to implement `From` for your own error type.
498 /// By converting underlying error types to our own custom error type that encapsulates the
499 /// underlying error type, we can return a single error type without losing information on the
500 /// underlying cause. The '?' operator automatically converts the underlying error type to our
501 /// custom error type by calling `Into<CliError>::into` which is automatically provided when
502 /// implementing `From`. The compiler then infers which implementation of `Into` should be used.
510 /// IoError(io::Error),
511 /// ParseError(num::ParseIntError),
514 /// impl From<io::Error> for CliError {
515 /// fn from(error: io::Error) -> Self {
516 /// CliError::IoError(error)
520 /// impl From<num::ParseIntError> for CliError {
521 /// fn from(error: num::ParseIntError) -> Self {
522 /// CliError::ParseError(error)
526 /// fn open_and_parse_file(file_name: &str) -> Result<i32, CliError> {
527 /// let mut contents = fs::read_to_string(&file_name)?;
528 /// let num: i32 = contents.trim().parse()?;
533 /// [`String`]: ../../std/string/struct.String.html
534 /// [`from`]: From::from
535 /// [book]: ../../book/ch09-00-error-handling.html
536 #[rustc_diagnostic_item = "From"]
537 #[stable(feature = "rust1", since = "1.0.0")]
538 #[rustc_on_unimplemented(on(
539 all(_Self = "&str", T = "std::string::String"),
540 note = "to coerce a `{T}` into a `{Self}`, use `&*` as a prefix",
543 pub trait From<T>: Sized {
544 /// Converts to this type from the input type.
547 #[stable(feature = "rust1", since = "1.0.0")]
548 fn from(value: T) -> Self;
551 /// An attempted conversion that consumes `self`, which may or may not be
554 /// Library authors should usually not directly implement this trait,
555 /// but should prefer implementing the [`TryFrom`] trait, which offers
556 /// greater flexibility and provides an equivalent `TryInto`
557 /// implementation for free, thanks to a blanket implementation in the
558 /// standard library. For more information on this, see the
559 /// documentation for [`Into`].
561 /// # Implementing `TryInto`
563 /// This suffers the same restrictions and reasoning as implementing
564 /// [`Into`], see there for details.
565 #[rustc_diagnostic_item = "TryInto"]
566 #[stable(feature = "try_from", since = "1.34.0")]
568 pub trait TryInto<T>: Sized {
569 /// The type returned in the event of a conversion error.
570 #[stable(feature = "try_from", since = "1.34.0")]
573 /// Performs the conversion.
574 #[stable(feature = "try_from", since = "1.34.0")]
575 fn try_into(self) -> Result<T, Self::Error>;
578 /// Simple and safe type conversions that may fail in a controlled
579 /// way under some circumstances. It is the reciprocal of [`TryInto`].
581 /// This is useful when you are doing a type conversion that may
582 /// trivially succeed but may also need special handling.
583 /// For example, there is no way to convert an [`i64`] into an [`i32`]
584 /// using the [`From`] trait, because an [`i64`] may contain a value
585 /// that an [`i32`] cannot represent and so the conversion would lose data.
586 /// This might be handled by truncating the [`i64`] to an [`i32`] (essentially
587 /// giving the [`i64`]'s value modulo [`i32::MAX`]) or by simply returning
588 /// [`i32::MAX`], or by some other method. The [`From`] trait is intended
589 /// for perfect conversions, so the `TryFrom` trait informs the
590 /// programmer when a type conversion could go bad and lets them
591 /// decide how to handle it.
593 /// # Generic Implementations
595 /// - `TryFrom<T> for U` implies [`TryInto`]`<U> for T`
596 /// - [`try_from`] is reflexive, which means that `TryFrom<T> for T`
597 /// is implemented and cannot fail -- the associated `Error` type for
598 /// calling `T::try_from()` on a value of type `T` is [`Infallible`].
599 /// When the [`!`] type is stabilized [`Infallible`] and [`!`] will be
602 /// `TryFrom<T>` can be implemented as follows:
605 /// struct GreaterThanZero(i32);
607 /// impl TryFrom<i32> for GreaterThanZero {
608 /// type Error = &'static str;
610 /// fn try_from(value: i32) -> Result<Self, Self::Error> {
612 /// Err("GreaterThanZero only accepts values greater than zero!")
614 /// Ok(GreaterThanZero(value))
622 /// As described, [`i32`] implements `TryFrom<`[`i64`]`>`:
625 /// let big_number = 1_000_000_000_000i64;
626 /// // Silently truncates `big_number`, requires detecting
627 /// // and handling the truncation after the fact.
628 /// let smaller_number = big_number as i32;
629 /// assert_eq!(smaller_number, -727379968);
631 /// // Returns an error because `big_number` is too big to
632 /// // fit in an `i32`.
633 /// let try_smaller_number = i32::try_from(big_number);
634 /// assert!(try_smaller_number.is_err());
636 /// // Returns `Ok(3)`.
637 /// let try_successful_smaller_number = i32::try_from(3);
638 /// assert!(try_successful_smaller_number.is_ok());
641 /// [`try_from`]: TryFrom::try_from
642 #[rustc_diagnostic_item = "TryFrom"]
643 #[stable(feature = "try_from", since = "1.34.0")]
645 pub trait TryFrom<T>: Sized {
646 /// The type returned in the event of a conversion error.
647 #[stable(feature = "try_from", since = "1.34.0")]
650 /// Performs the conversion.
651 #[stable(feature = "try_from", since = "1.34.0")]
652 fn try_from(value: T) -> Result<Self, Self::Error>;
655 ////////////////////////////////////////////////////////////////////////////////
657 ////////////////////////////////////////////////////////////////////////////////
660 #[stable(feature = "rust1", since = "1.0.0")]
661 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
662 impl<T: ?Sized, U: ?Sized> const AsRef<U> for &T
667 fn as_ref(&self) -> &U {
668 <T as AsRef<U>>::as_ref(*self)
672 // As lifts over &mut
673 #[stable(feature = "rust1", since = "1.0.0")]
674 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
675 impl<T: ?Sized, U: ?Sized> const AsRef<U> for &mut T
680 fn as_ref(&self) -> &U {
681 <T as AsRef<U>>::as_ref(*self)
685 // FIXME (#45742): replace the above impls for &/&mut with the following more general one:
686 // // As lifts over Deref
687 // impl<D: ?Sized + Deref<Target: AsRef<U>>, U: ?Sized> AsRef<U> for D {
688 // fn as_ref(&self) -> &U {
689 // self.deref().as_ref()
693 // AsMut lifts over &mut
694 #[stable(feature = "rust1", since = "1.0.0")]
695 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
696 impl<T: ?Sized, U: ?Sized> const AsMut<U> for &mut T
701 fn as_mut(&mut self) -> &mut U {
706 // FIXME (#45742): replace the above impl for &mut with the following more general one:
707 // // AsMut lifts over DerefMut
708 // impl<D: ?Sized + Deref<Target: AsMut<U>>, U: ?Sized> AsMut<U> for D {
709 // fn as_mut(&mut self) -> &mut U {
710 // self.deref_mut().as_mut()
715 #[stable(feature = "rust1", since = "1.0.0")]
716 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
717 impl<T, U> const Into<U> for T
721 /// Calls `U::from(self)`.
723 /// That is, this conversion is whatever the implementation of
724 /// <code>[From]<T> for U</code> chooses to do.
730 // From (and thus Into) is reflexive
731 #[stable(feature = "rust1", since = "1.0.0")]
732 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
733 impl<T> const From<T> for T {
734 /// Returns the argument unchanged.
741 /// **Stability note:** This impl does not yet exist, but we are
742 /// "reserving space" to add it in the future. See
743 /// [rust-lang/rust#64715][#64715] for details.
745 /// [#64715]: https://github.com/rust-lang/rust/issues/64715
746 #[stable(feature = "convert_infallible", since = "1.34.0")]
747 #[allow(unused_attributes)] // FIXME(#58633): do a principled fix instead.
748 #[rustc_reservation_impl = "permitting this impl would forbid us from adding \
749 `impl<T> From<!> for T` later; see rust-lang/rust#64715 for details"]
750 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
751 impl<T> const From<!> for T {
757 // TryFrom implies TryInto
758 #[stable(feature = "try_from", since = "1.34.0")]
759 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
760 impl<T, U> const TryInto<U> for T
762 U: ~const TryFrom<T>,
764 type Error = U::Error;
766 fn try_into(self) -> Result<U, U::Error> {
771 // Infallible conversions are semantically equivalent to fallible conversions
772 // with an uninhabited error type.
773 #[stable(feature = "try_from", since = "1.34.0")]
774 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
775 impl<T, U> const TryFrom<U> for T
779 type Error = Infallible;
781 fn try_from(value: U) -> Result<Self, Self::Error> {
786 ////////////////////////////////////////////////////////////////////////////////
788 ////////////////////////////////////////////////////////////////////////////////
790 #[stable(feature = "rust1", since = "1.0.0")]
791 impl<T> AsRef<[T]> for [T] {
793 fn as_ref(&self) -> &[T] {
798 #[stable(feature = "rust1", since = "1.0.0")]
799 impl<T> AsMut<[T]> for [T] {
801 fn as_mut(&mut self) -> &mut [T] {
806 #[stable(feature = "rust1", since = "1.0.0")]
807 impl AsRef<str> for str {
809 fn as_ref(&self) -> &str {
814 #[stable(feature = "as_mut_str_for_str", since = "1.51.0")]
815 impl AsMut<str> for str {
817 fn as_mut(&mut self) -> &mut str {
822 ////////////////////////////////////////////////////////////////////////////////
823 // THE NO-ERROR ERROR TYPE
824 ////////////////////////////////////////////////////////////////////////////////
826 /// The error type for errors that can never happen.
828 /// Since this enum has no variant, a value of this type can never actually exist.
829 /// This can be useful for generic APIs that use [`Result`] and parameterize the error type,
830 /// to indicate that the result is always [`Ok`].
832 /// For example, the [`TryFrom`] trait (conversion that returns a [`Result`])
833 /// has a blanket implementation for all types where a reverse [`Into`] implementation exists.
835 /// ```ignore (illustrates std code, duplicating the impl in a doctest would be an error)
836 /// impl<T, U> TryFrom<U> for T where U: Into<T> {
837 /// type Error = Infallible;
839 /// fn try_from(value: U) -> Result<Self, Infallible> {
840 /// Ok(U::into(value)) // Never returns `Err`
845 /// # Future compatibility
847 /// This enum has the same role as [the `!` “never” type][never],
848 /// which is unstable in this version of Rust.
849 /// When `!` is stabilized, we plan to make `Infallible` a type alias to it:
851 /// ```ignore (illustrates future std change)
852 /// pub type Infallible = !;
855 /// … and eventually deprecate `Infallible`.
857 /// However there is one case where `!` syntax can be used
858 /// before `!` is stabilized as a full-fledged type: in the position of a function’s return type.
859 /// Specifically, it is possible to have implementations for two different function pointer types:
863 /// impl MyTrait for fn() -> ! {}
864 /// impl MyTrait for fn() -> std::convert::Infallible {}
867 /// With `Infallible` being an enum, this code is valid.
868 /// However when `Infallible` becomes an alias for the never type,
869 /// the two `impl`s will start to overlap
870 /// and therefore will be disallowed by the language’s trait coherence rules.
871 #[stable(feature = "convert_infallible", since = "1.34.0")]
873 pub enum Infallible {}
875 #[stable(feature = "convert_infallible", since = "1.34.0")]
876 #[rustc_const_unstable(feature = "const_clone", issue = "91805")]
877 impl const Clone for Infallible {
878 fn clone(&self) -> Infallible {
883 #[stable(feature = "convert_infallible", since = "1.34.0")]
884 impl fmt::Debug for Infallible {
885 fn fmt(&self, _: &mut fmt::Formatter<'_>) -> fmt::Result {
890 #[stable(feature = "convert_infallible", since = "1.34.0")]
891 impl fmt::Display for Infallible {
892 fn fmt(&self, _: &mut fmt::Formatter<'_>) -> fmt::Result {
897 #[stable(feature = "str_parse_error2", since = "1.8.0")]
898 impl Error for Infallible {
899 fn description(&self) -> &str {
904 #[stable(feature = "convert_infallible", since = "1.34.0")]
905 impl PartialEq for Infallible {
906 fn eq(&self, _: &Infallible) -> bool {
911 #[stable(feature = "convert_infallible", since = "1.34.0")]
912 impl Eq for Infallible {}
914 #[stable(feature = "convert_infallible", since = "1.34.0")]
915 impl PartialOrd for Infallible {
916 fn partial_cmp(&self, _other: &Self) -> Option<crate::cmp::Ordering> {
921 #[stable(feature = "convert_infallible", since = "1.34.0")]
922 impl Ord for Infallible {
923 fn cmp(&self, _other: &Self) -> crate::cmp::Ordering {
928 #[stable(feature = "convert_infallible", since = "1.34.0")]
929 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
930 impl const From<!> for Infallible {
931 fn from(x: !) -> Self {
936 #[stable(feature = "convert_infallible_hash", since = "1.44.0")]
937 impl Hash for Infallible {
938 fn hash<H: Hasher>(&self, _: &mut H) {