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. Only implement [`Into`] or [`TryInto`]
22 //! when a conversion to a type outside the current crate is required.
24 //! # Generic Implementations
26 //! - [`AsRef`] and [`AsMut`] auto-dereference if the inner type is a reference
27 //! - [`From`]`<U> for T` implies [`Into`]`<T> for U`
28 //! - [`TryFrom`]`<U> for T` implies [`TryInto`]`<T> for U`
29 //! - [`From`] and [`Into`] are reflexive, which means that all types can
30 //! `into` themselves and `from` themselves
32 //! See each trait for usage examples.
34 //! [`Into`]: trait.Into.html
35 //! [`From`]: trait.From.html
36 //! [`TryFrom`]: trait.TryFrom.html
37 //! [`TryInto`]: trait.TryInto.html
38 //! [`AsRef`]: trait.AsRef.html
39 //! [`AsMut`]: trait.AsMut.html
41 #![stable(feature = "rust1", since = "1.0.0")]
45 /// An identity function.
47 /// Two things are important to note about this function:
49 /// - It is not always equivalent to a closure like `|x| x` since the
50 /// closure may coerce `x` into a different type.
52 /// - It moves the input `x` passed to the function.
54 /// While it might seem strange to have a function that just returns back the
55 /// input, there are some interesting uses.
59 /// Using `identity` to do nothing among other interesting functions:
62 /// use std::convert::identity;
64 /// fn manipulation(x: u32) -> u32 {
65 /// // Let's assume that this function does something interesting.
69 /// let _arr = &[identity, manipulation];
72 /// Using `identity` to get a function that changes nothing in a conditional:
75 /// use std::convert::identity;
77 /// # let condition = true;
79 /// # fn manipulation(x: u32) -> u32 { x + 1 }
81 /// let do_stuff = if condition { manipulation } else { identity };
83 /// // do more interesting stuff..
85 /// let _results = do_stuff(42);
88 /// Using `identity` to keep the `Some` variants of an iterator of `Option<T>`:
91 /// use std::convert::identity;
93 /// let iter = vec![Some(1), None, Some(3)].into_iter();
94 /// let filtered = iter.filter_map(identity).collect::<Vec<_>>();
95 /// assert_eq!(vec![1, 3], filtered);
97 #[stable(feature = "convert_id", since = "1.33.0")]
99 pub const fn identity<T>(x: T) -> T { x }
101 /// Used to do a cheap reference-to-reference conversion.
103 /// This trait is similar to [`AsMut`] which is used for converting between mutable references.
104 /// If you need to do a costly conversion it is better to implement [`From`] with type
105 /// `&T` or write a custom function.
108 /// `AsRef` has the same signature as [`Borrow`], but `Borrow` is different in few aspects:
110 /// - Unlike `AsRef`, `Borrow` has a blanket impl for any `T`, and can be used to accept either
111 /// a reference or a value.
112 /// - `Borrow` also requires that `Hash`, `Eq` and `Ord` for borrowed value are
113 /// equivalent to those of the owned value. For this reason, if you want to
114 /// borrow only a single field of a struct you can implement `AsRef`, but not `Borrow`.
116 /// [`Borrow`]: ../../std/borrow/trait.Borrow.html
118 /// **Note: This trait must not fail**. If the conversion can fail, use a
119 /// dedicated method which returns an [`Option<T>`] or a [`Result<T, E>`].
121 /// [`Option<T>`]: ../../std/option/enum.Option.html
122 /// [`Result<T, E>`]: ../../std/result/enum.Result.html
124 /// # Generic Implementations
126 /// - `AsRef` auto-dereferences if the inner type is a reference or a mutable
127 /// reference (e.g.: `foo.as_ref()` will work the same if `foo` has type
128 /// `&mut Foo` or `&&mut Foo`)
132 /// By using trait bounds we can accept arguments of different types as long as they can be
133 /// converted a the specified type `T`.
135 /// For example: By creating a generic function that takes an `AsRef<str>` we express that we
136 /// want to accept all references that can be converted to &str as an argument.
137 /// Since both [`String`] and `&str` implement `AsRef<str>` we can accept both as input argument.
139 /// [`String`]: ../../std/string/struct.String.html
142 /// fn is_hello<T: AsRef<str>>(s: T) {
143 /// assert_eq!("hello", s.as_ref());
149 /// let s = "hello".to_string();
153 #[stable(feature = "rust1", since = "1.0.0")]
154 pub trait AsRef<T: ?Sized> {
155 /// Performs the conversion.
156 #[stable(feature = "rust1", since = "1.0.0")]
157 fn as_ref(&self) -> &T;
160 /// Used to do a cheap mutable-to-mutable reference conversion.
162 /// This trait is similar to [`AsRef`] but used for converting between mutable
163 /// references. If you need to do a costly conversion it is better to
164 /// implement [`From`] with type `&mut T` or write a custom function.
166 /// **Note: This trait must not fail**. If the conversion can fail, use a
167 /// dedicated method which returns an [`Option<T>`] or a [`Result<T, E>`].
169 /// [`Option<T>`]: ../../std/option/enum.Option.html
170 /// [`Result<T, E>`]: ../../std/result/enum.Result.html
172 /// # Generic Implementations
174 /// - `AsMut` auto-dereferences if the inner type is a mutable reference
175 /// (e.g.: `foo.as_mut()` will work the same if `foo` has type `&mut Foo`
176 /// or `&mut &mut Foo`)
180 /// Using `AsMut` as trait bound for a generic function we can accept all mutable references
181 /// that can be converted to type `&mut T`. Because [`Box<T>`] implements `AsMut<T>` we can
182 /// write a function `add_one`that takes all arguments that can be converted to `&mut u64`.
183 /// Because [`Box<T>`] implements `AsMut<T>` `add_one` accepts arguments of type
184 /// `&mut Box<u64>` as well:
186 /// fn add_one<T: AsMut<u64>>(num: &mut T) {
187 /// *num.as_mut() += 1;
190 /// let mut boxed_num = Box::new(0);
191 /// add_one(&mut boxed_num);
192 /// assert_eq!(*boxed_num, 1);
194 /// [`Box<T>`]: ../../std/boxed/struct.Box.html
196 #[stable(feature = "rust1", since = "1.0.0")]
197 pub trait AsMut<T: ?Sized> {
198 /// Performs the conversion.
199 #[stable(feature = "rust1", since = "1.0.0")]
200 fn as_mut(&mut self) -> &mut T;
203 /// A value-to-value conversion that consumes the input value. The
204 /// opposite of [`From`].
206 /// One should only implement [`Into`] if a conversion to a type outside the current crate is
207 /// required. Otherwise one should always prefer implementing [`From`] over [`Into`] because
208 /// implementing [`From`] automatically provides one with a implementation of [`Into`] thanks to
209 /// the blanket implementation in the standard library. [`From`] cannot do these type of
210 /// conversions because of Rust's orphaning rules.
212 /// **Note: This trait must not fail**. If the conversion can fail, use [`TryInto`].
214 /// # Generic Implementations
216 /// - [`From<T>`]` for U` implies `Into<U> for T`
217 /// - [`Into`]` is reflexive, which means that `Into<T> for T` is implemented
219 /// # Implementing `Into` for conversions to external types
221 /// If the destination type is not part of the current crate
222 /// then you can't implement [`From`] directly.
223 /// For example, take this code:
226 /// struct Wrapper<T>(Vec<T>);
227 /// impl<T> From<Wrapper<T>> for Vec<T> {
228 /// fn from(w: Wrapper<T>) -> Vec<T> {
233 /// This will fail to compile because we cannot implement a trait for a type
234 /// if both the trait and the type are not defined by the current crate.
235 /// This is due to Rust's orphaning rules. To bypass this, you can implement `Into` directly:
238 /// struct Wrapper<T>(Vec<T>);
239 /// impl<T> Into<Vec<T>> for Wrapper<T> {
240 /// fn into(self) -> Vec<T> {
246 /// It is important to understand that `Into` does not provide a [`From`] implementation
247 /// (as [`From`] does with `Into`). Therefore, you should always try to implement [`From`]
248 /// and then fall back to `Into` if [`From`] can't be implemented.
250 /// Prefer using `Into` over [`From`] when specifying trait bounds on a generic function
251 /// to ensure that types that only implement `Into` can be used as well.
255 /// [`String`] implements `Into<Vec<u8>>`:
257 /// In order to express that we want a generic function to take all arguments that can be
258 /// converted to a specified type `T`, we can use a trait bound of `Into<T>`.
259 /// For example: The function `is_hello` takes all arguments that can be converted into a
263 /// fn is_hello<T: Into<Vec<u8>>>(s: T) {
264 /// let bytes = b"hello".to_vec();
265 /// assert_eq!(bytes, s.into());
268 /// let s = "hello".to_string();
272 /// [`TryInto`]: trait.TryInto.html
273 /// [`Option<T>`]: ../../std/option/enum.Option.html
274 /// [`Result<T, E>`]: ../../std/result/enum.Result.html
275 /// [`String`]: ../../std/string/struct.String.html
276 /// [From]: trait.From.html
277 /// [`into`]: trait.Into.html#tymethod.into
278 #[stable(feature = "rust1", since = "1.0.0")]
279 pub trait Into<T>: Sized {
280 /// Performs the conversion.
281 #[stable(feature = "rust1", since = "1.0.0")]
285 /// Used to do value-to-value conversions while consuming the input value. It is the reciprocal of
288 /// One should always prefer implementing [`From`] over [`Into`]
289 /// because implementing [`From`] automatically provides one with a implementation of [`Into`]
290 /// thanks to the blanket implementation in the standard library.
292 /// Only implement [`Into`] if a conversion to a type outside the current crate is required.
293 /// [`From`] cannot do these type of conversions because of Rust's orphaning rules.
294 /// See [`Into`] for more details.
296 /// Prefer using [`Into`] over using [`From`] when specifying trait bounds on a generic function.
297 /// This way, types that directly implement [`Into`] can be used as arguments as well.
299 /// The [`From`] is also very useful when performing error handling. When constructing a function
300 /// that is capable of failing, the return type will generally be of the form `Result<T, E>`.
301 /// The `From` trait simplifies error handling by allowing a function to return a single error type
302 /// that encapsulate multiple error types. See the "Examples" section and [the book][book] for more
305 /// **Note: This trait must not fail**. If the conversion can fail, use [`TryFrom`].
307 /// # Generic Implementations
309 /// - [`From<T>`]` for U` implies [`Into<U>`]` for T`
310 /// - [`From`] is reflexive, which means that `From<T> for T` is implemented
314 /// [`String`] implements `From<&str>`:
316 /// An explicit conversion from a &str to a String is done as follows:
318 /// let string = "hello".to_string();
319 /// let other_string = String::from("hello");
321 /// assert_eq!(string, other_string);
324 /// While performing error handling it is often useful to implement `From` for your own error type.
325 /// By converting underlying error types to our own custom error type that encapsulates the
326 /// underlying error type, we can return a single error type without losing information on the
327 /// underlying cause. The '?' operator automatically converts the underlying error type to our
328 /// custom error type by calling `Into<CliError>::into` which is automatically provided when
329 /// implementing `From`. The compiler then infers which implementation of `Into` should be used.
337 /// IoError(io::Error),
338 /// ParseError(num::ParseIntError),
341 /// impl From<io::Error> for CliError {
342 /// fn from(error: io::Error) -> Self {
343 /// CliError::IoError(error)
347 /// impl From<num::ParseIntError> for CliError {
348 /// fn from(error: num::ParseIntError) -> Self {
349 /// CliError::ParseError(error)
353 /// fn open_and_parse_file(file_name: &str) -> Result<i32, CliError> {
354 /// let mut contents = fs::read_to_string(&file_name)?;
355 /// let num: i32 = contents.trim().parse()?;
360 /// [`TryFrom`]: trait.TryFrom.html
361 /// [`Option<T>`]: ../../std/option/enum.Option.html
362 /// [`Result<T, E>`]: ../../std/result/enum.Result.html
363 /// [`String`]: ../../std/string/struct.String.html
364 /// [`Into<U>`]: trait.Into.html
365 /// [`from`]: trait.From.html#tymethod.from
366 /// [book]: ../../book/ch09-00-error-handling.html
367 #[stable(feature = "rust1", since = "1.0.0")]
368 #[rustc_on_unimplemented(
370 all(_Self="&str", T="std::string::String"),
371 note="to coerce a `{T}` into a `{Self}`, use `&*` as a prefix",
374 pub trait From<T>: Sized {
375 /// Performs the conversion.
376 #[stable(feature = "rust1", since = "1.0.0")]
377 fn from(_: T) -> Self;
380 /// An attempted conversion that consumes `self`, which may or may not be
383 /// Library authors should usually not directly implement this trait,
384 /// but should prefer implementing the [`TryFrom`] trait, which offers
385 /// greater flexibility and provides an equivalent `TryInto`
386 /// implementation for free, thanks to a blanket implementation in the
387 /// standard library. For more information on this, see the
388 /// documentation for [`Into`].
390 /// # Implementing `TryInto`
392 /// This suffers the same restrictions and reasoning as implementing
393 /// [`Into`], see there for details.
395 /// [`TryFrom`]: trait.TryFrom.html
396 /// [`Into`]: trait.Into.html
397 #[stable(feature = "try_from", since = "1.34.0")]
398 pub trait TryInto<T>: Sized {
399 /// The type returned in the event of a conversion error.
400 #[stable(feature = "try_from", since = "1.34.0")]
403 /// Performs the conversion.
404 #[stable(feature = "try_from", since = "1.34.0")]
405 fn try_into(self) -> Result<T, Self::Error>;
408 /// Simple and safe type conversions that may fail in a controlled
409 /// way under some circumstances. It is the reciprocal of [`TryInto`].
411 /// This is useful when you are doing a type conversion that may
412 /// trivially succeed but may also need special handling.
413 /// For example, there is no way to convert an `i64` into an `i32`
414 /// using the [`From`] trait, because an `i64` may contain a value
415 /// that an `i32` cannot represent and so the conversion would lose data.
416 /// This might be handled by truncating the `i64` to an `i32` (essentially
417 /// giving the `i64`'s value modulo `i32::MAX`) or by simply returning
418 /// `i32::MAX`, or by some other method. The `From` trait is intended
419 /// for perfect conversions, so the `TryFrom` trait informs the
420 /// programmer when a type conversion could go bad and lets them
421 /// decide how to handle it.
423 /// # Generic Implementations
425 /// - `TryFrom<T> for U` implies [`TryInto<U>`]` for T`
426 /// - [`try_from`] is reflexive, which means that `TryFrom<T> for T`
427 /// is implemented and cannot fail -- the associated `Error` type for
428 /// calling `T::try_from()` on a value of type `T` is `Infallible`.
429 /// When the `!` type is stablized `Infallible` and `!` will be
434 /// As described, [`i32`] implements `TryFrom<i64>`:
437 /// use std::convert::TryFrom;
439 /// let big_number = 1_000_000_000_000i64;
440 /// // Silently truncates `big_number`, requires detecting
441 /// // and handling the truncation after the fact.
442 /// let smaller_number = big_number as i32;
443 /// assert_eq!(smaller_number, -727379968);
445 /// // Returns an error because `big_number` is too big to
446 /// // fit in an `i32`.
447 /// let try_smaller_number = i32::try_from(big_number);
448 /// assert!(try_smaller_number.is_err());
450 /// // Returns `Ok(3)`.
451 /// let try_successful_smaller_number = i32::try_from(3);
452 /// assert!(try_successful_smaller_number.is_ok());
455 /// [`try_from`]: trait.TryFrom.html#tymethod.try_from
456 /// [`TryInto`]: trait.TryInto.html
457 #[stable(feature = "try_from", since = "1.34.0")]
458 pub trait TryFrom<T>: Sized {
459 /// The type returned in the event of a conversion error.
460 #[stable(feature = "try_from", since = "1.34.0")]
463 /// Performs the conversion.
464 #[stable(feature = "try_from", since = "1.34.0")]
465 fn try_from(value: T) -> Result<Self, Self::Error>;
468 ////////////////////////////////////////////////////////////////////////////////
470 ////////////////////////////////////////////////////////////////////////////////
473 #[stable(feature = "rust1", since = "1.0.0")]
474 impl<T: ?Sized, U: ?Sized> AsRef<U> for &T where T: AsRef<U>
476 fn as_ref(&self) -> &U {
477 <T as AsRef<U>>::as_ref(*self)
481 // As lifts over &mut
482 #[stable(feature = "rust1", since = "1.0.0")]
483 impl<T: ?Sized, U: ?Sized> AsRef<U> for &mut T where T: AsRef<U>
485 fn as_ref(&self) -> &U {
486 <T as AsRef<U>>::as_ref(*self)
490 // FIXME (#45742): replace the above impls for &/&mut with the following more general one:
491 // // As lifts over Deref
492 // impl<D: ?Sized + Deref, U: ?Sized> AsRef<U> for D where D::Target: AsRef<U> {
493 // fn as_ref(&self) -> &U {
494 // self.deref().as_ref()
498 // AsMut lifts over &mut
499 #[stable(feature = "rust1", since = "1.0.0")]
500 impl<T: ?Sized, U: ?Sized> AsMut<U> for &mut T where T: AsMut<U>
502 fn as_mut(&mut self) -> &mut U {
507 // FIXME (#45742): replace the above impl for &mut with the following more general one:
508 // // AsMut lifts over DerefMut
509 // impl<D: ?Sized + Deref, U: ?Sized> AsMut<U> for D where D::Target: AsMut<U> {
510 // fn as_mut(&mut self) -> &mut U {
511 // self.deref_mut().as_mut()
516 #[stable(feature = "rust1", since = "1.0.0")]
517 impl<T, U> Into<U> for T where U: From<T>
524 // From (and thus Into) is reflexive
525 #[stable(feature = "rust1", since = "1.0.0")]
526 impl<T> From<T> for T {
527 fn from(t: T) -> T { t }
531 // TryFrom implies TryInto
532 #[stable(feature = "try_from", since = "1.34.0")]
533 impl<T, U> TryInto<U> for T where U: TryFrom<T>
535 type Error = U::Error;
537 fn try_into(self) -> Result<U, U::Error> {
542 // Infallible conversions are semantically equivalent to fallible conversions
543 // with an uninhabited error type.
544 #[stable(feature = "try_from", since = "1.34.0")]
545 impl<T, U> TryFrom<U> for T where U: Into<T> {
546 type Error = Infallible;
548 fn try_from(value: U) -> Result<Self, Self::Error> {
553 ////////////////////////////////////////////////////////////////////////////////
555 ////////////////////////////////////////////////////////////////////////////////
557 #[stable(feature = "rust1", since = "1.0.0")]
558 impl<T> AsRef<[T]> for [T] {
559 fn as_ref(&self) -> &[T] {
564 #[stable(feature = "rust1", since = "1.0.0")]
565 impl<T> AsMut<[T]> for [T] {
566 fn as_mut(&mut self) -> &mut [T] {
571 #[stable(feature = "rust1", since = "1.0.0")]
572 impl AsRef<str> for str {
574 fn as_ref(&self) -> &str {
579 ////////////////////////////////////////////////////////////////////////////////
580 // THE NO-ERROR ERROR TYPE
581 ////////////////////////////////////////////////////////////////////////////////
583 /// The error type for errors that can never happen.
585 /// Since this enum has no variant, a value of this type can never actually exist.
586 /// This can be useful for generic APIs that use [`Result`] and parameterize the error type,
587 /// to indicate that the result is always [`Ok`].
589 /// For example, the [`TryFrom`] trait (conversion that returns a [`Result`])
590 /// has a blanket implementation for all types where a reverse [`Into`] implementation exists.
592 /// ```ignore (illustrates std code, duplicating the impl in a doctest would be an error)
593 /// impl<T, U> TryFrom<U> for T where U: Into<T> {
594 /// type Error = Infallible;
596 /// fn try_from(value: U) -> Result<Self, Infallible> {
597 /// Ok(U::into(value)) // Never returns `Err`
602 /// # Future compatibility
604 /// This enum has the same role as [the `!` “never” type][never],
605 /// which is unstable in this version of Rust.
606 /// When `!` is stabilized, we plan to make `Infallible` a type alias to it:
608 /// ```ignore (illustrates future std change)
609 /// pub type Infallible = !;
612 /// … and eventually deprecate `Infallible`.
615 /// However there is one case where `!` syntax can be used
616 /// before `!` is stabilized as a full-fleged type: in the position of a function’s return type.
617 /// Specifically, it is possible implementations for two different function pointer types:
621 /// impl MyTrait for fn() -> ! {}
622 /// impl MyTrait for fn() -> std::convert::Infallible {}
625 /// With `Infallible` being an enum, this code is valid.
626 /// However when `Infallible` becomes an alias for the never type,
627 /// the two `impl`s will start to overlap
628 /// and therefore will be disallowed by the language’s trait coherence rules.
630 /// [`Ok`]: ../result/enum.Result.html#variant.Ok
631 /// [`Result`]: ../result/enum.Result.html
632 /// [`TryFrom`]: trait.TryFrom.html
633 /// [`Into`]: trait.Into.html
634 /// [never]: ../../std/primitive.never.html
635 #[stable(feature = "convert_infallible", since = "1.34.0")]
637 pub enum Infallible {}
639 #[stable(feature = "convert_infallible", since = "1.34.0")]
640 impl Clone for Infallible {
641 fn clone(&self) -> Infallible {
646 #[stable(feature = "convert_infallible", since = "1.34.0")]
647 impl fmt::Debug for Infallible {
648 fn fmt(&self, _: &mut fmt::Formatter<'_>) -> fmt::Result {
653 #[stable(feature = "convert_infallible", since = "1.34.0")]
654 impl fmt::Display for Infallible {
655 fn fmt(&self, _: &mut fmt::Formatter<'_>) -> fmt::Result {
660 #[stable(feature = "convert_infallible", since = "1.34.0")]
661 impl PartialEq for Infallible {
662 fn eq(&self, _: &Infallible) -> bool {
667 #[stable(feature = "convert_infallible", since = "1.34.0")]
668 impl Eq for Infallible {}
670 #[stable(feature = "convert_infallible", since = "1.34.0")]
671 impl PartialOrd for Infallible {
672 fn partial_cmp(&self, _other: &Self) -> Option<crate::cmp::Ordering> {
677 #[stable(feature = "convert_infallible", since = "1.34.0")]
678 impl Ord for Infallible {
679 fn cmp(&self, _other: &Self) -> crate::cmp::Ordering {
684 #[stable(feature = "convert_infallible", since = "1.34.0")]
685 impl From<!> for Infallible {
686 fn from(x: !) -> Self {