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")]
43 /// The identity function.
45 /// Two things are important to note about this function:
47 /// - It is not always equivalent to a closure like `|x| x`, since the
48 /// closure may coerce `x` into a different type.
50 /// - It moves the input `x` passed to the function.
52 /// While it might seem strange to have a function that just returns back the
53 /// input, there are some interesting uses.
57 /// Using `identity` to do nothing in a sequence of other, interesting,
61 /// use std::convert::identity;
63 /// fn manipulation(x: u32) -> u32 {
64 /// // Let's pretend that adding one is an interesting function.
68 /// let _arr = &[identity, manipulation];
71 /// Using `identity` as a "do nothing" base case in a conditional:
74 /// use std::convert::identity;
76 /// # let condition = true;
78 /// # fn manipulation(x: u32) -> u32 { x + 1 }
80 /// let do_stuff = if condition { manipulation } else { identity };
82 /// // Do more interesting stuff...
84 /// let _results = do_stuff(42);
87 /// Using `identity` to keep the `Some` variants of an iterator of `Option<T>`:
90 /// use std::convert::identity;
92 /// let iter = vec![Some(1), None, Some(3)].into_iter();
93 /// let filtered = iter.filter_map(identity).collect::<Vec<_>>();
94 /// assert_eq!(vec![1, 3], filtered);
96 #[stable(feature = "convert_id", since = "1.33.0")]
98 pub const fn identity<T>(x: T) -> T {
102 /// Used to do a cheap reference-to-reference conversion.
104 /// This trait is similar to [`AsMut`] which is used for converting between mutable references.
105 /// If you need to do a costly conversion it is better to implement [`From`] with type
106 /// `&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 /// **Note: This trait must not fail**. If the conversion can fail, use a
117 /// dedicated method which returns an [`Option<T>`] or a [`Result<T, E>`].
119 /// # Generic Implementations
121 /// - `AsRef` auto-dereferences if the inner type is a reference or a mutable
122 /// reference (e.g.: `foo.as_ref()` will work the same if `foo` has type
123 /// `&mut Foo` or `&&mut Foo`)
127 /// By using trait bounds we can accept arguments of different types as long as they can be
128 /// converted to the specified type `T`.
130 /// For example: By creating a generic function that takes an `AsRef<str>` we express that we
131 /// want to accept all references that can be converted to [`&str`] as an argument.
132 /// Since both [`String`] and [`&str`] implement `AsRef<str>` we can accept both as input argument.
134 /// [`Option<T>`]: ../../std/option/enum.Option.html
135 /// [`Result<T, E>`]: ../../std/result/enum.Result.html
136 /// [`Borrow`]: ../../std/borrow/trait.Borrow.html
137 /// [`Hash`]: ../../std/hash/trait.Hash.html
138 /// [`Eq`]: ../../std/cmp/trait.Eq.html
139 /// [`Ord`]: ../../std/cmp/trait.Ord.html
140 /// [`&str`]: ../../std/primitive.str.html
141 /// [`String`]: ../../std/string/struct.String.html
144 /// fn is_hello<T: AsRef<str>>(s: T) {
145 /// assert_eq!("hello", s.as_ref());
151 /// let s = "hello".to_string();
154 #[stable(feature = "rust1", since = "1.0.0")]
155 pub trait AsRef<T: ?Sized> {
156 /// Performs the conversion.
157 #[stable(feature = "rust1", since = "1.0.0")]
158 fn as_ref(&self) -> &T;
161 /// Used to do a cheap mutable-to-mutable reference conversion.
163 /// This trait is similar to [`AsRef`] but used for converting between mutable
164 /// references. If you need to do a costly conversion it is better to
165 /// implement [`From`] with type `&mut T` or write a custom function.
167 /// **Note: This trait must not fail**. If the conversion can fail, use a
168 /// dedicated method which returns an [`Option<T>`] or a [`Result<T, E>`].
170 /// [`Option<T>`]: ../../std/option/enum.Option.html
171 /// [`Result<T, E>`]: ../../std/result/enum.Result.html
173 /// # Generic Implementations
175 /// - `AsMut` auto-dereferences if the inner type is a mutable reference
176 /// (e.g.: `foo.as_mut()` will work the same if `foo` has type `&mut Foo`
177 /// or `&mut &mut Foo`)
181 /// Using `AsMut` as trait bound for a generic function we can accept all mutable references
182 /// that can be converted to type `&mut T`. Because [`Box<T>`] implements `AsMut<T>` we can
183 /// write a function `add_one` that takes all arguments that can be converted to `&mut u64`.
184 /// Because [`Box<T>`] implements `AsMut<T>`, `add_one` accepts arguments of type
185 /// `&mut Box<u64>` as well:
188 /// fn add_one<T: AsMut<u64>>(num: &mut T) {
189 /// *num.as_mut() += 1;
192 /// let mut boxed_num = Box::new(0);
193 /// add_one(&mut boxed_num);
194 /// assert_eq!(*boxed_num, 1);
197 /// [`Box<T>`]: ../../std/boxed/struct.Box.html
198 #[stable(feature = "rust1", since = "1.0.0")]
199 pub trait AsMut<T: ?Sized> {
200 /// Performs the conversion.
201 #[stable(feature = "rust1", since = "1.0.0")]
202 fn as_mut(&mut self) -> &mut T;
205 /// A value-to-value conversion that consumes the input value. The
206 /// opposite of [`From`].
208 /// One should avoid implementing [`Into`] and implement [`From`] instead.
209 /// Implementing [`From`] automatically provides one with an implementation of [`Into`]
210 /// thanks to the blanket implementation in the standard library.
212 /// Prefer using [`Into`] over [`From`] when specifying trait bounds on a generic function
213 /// to ensure that types that only implement [`Into`] can be used as well.
215 /// **Note: This trait must not fail**. If the conversion can fail, use [`TryInto`].
217 /// # Generic Implementations
219 /// - [`From`]`<T> for U` implies `Into<U> for T`
220 /// - [`Into`] is reflexive, which means that `Into<T> for T` is implemented
222 /// # Implementing [`Into`] for conversions to external types in old versions of Rust
224 /// Prior to Rust 1.40, if the destination type was not part of the current crate
225 /// then you couldn't implement [`From`] directly.
226 /// For example, take this code:
229 /// struct Wrapper<T>(Vec<T>);
230 /// impl<T> From<Wrapper<T>> for Vec<T> {
231 /// fn from(w: Wrapper<T>) -> Vec<T> {
236 /// This will fail to compile in older versions of the language because Rust's orphaning rules
237 /// used to be a little bit more strict. To bypass this, you could implement [`Into`] directly:
240 /// struct Wrapper<T>(Vec<T>);
241 /// impl<T> Into<Vec<T>> for Wrapper<T> {
242 /// fn into(self) -> Vec<T> {
248 /// It is important to understand that [`Into`] does not provide a [`From`] implementation
249 /// (as [`From`] does with [`Into`]). Therefore, you should always try to implement [`From`]
250 /// and then fall back to [`Into`] if [`From`] can't be implemented.
254 /// [`String`] implements [`Into`]`<`[`Vec`]`<`[`u8`]`>>`:
256 /// In order to express that we want a generic function to take all arguments that can be
257 /// converted to a specified type `T`, we can use a trait bound of [`Into`]`<T>`.
258 /// For example: The function `is_hello` takes all arguments that can be converted into a
259 /// [`Vec`]`<`[`u8`]`>`.
262 /// fn is_hello<T: Into<Vec<u8>>>(s: T) {
263 /// let bytes = b"hello".to_vec();
264 /// assert_eq!(bytes, s.into());
267 /// let s = "hello".to_string();
271 /// [`TryInto`]: trait.TryInto.html
272 /// [`Option<T>`]: ../../std/option/enum.Option.html
273 /// [`Result<T, E>`]: ../../std/result/enum.Result.html
274 /// [`String`]: ../../std/string/struct.String.html
275 /// [`From`]: trait.From.html
276 /// [`Into`]: trait.Into.html
277 /// [`Vec`]: ../../std/vec/struct.Vec.html
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:
319 /// let string = "hello".to_string();
320 /// let other_string = String::from("hello");
322 /// assert_eq!(string, other_string);
325 /// While performing error handling it is often useful to implement `From` for your own error type.
326 /// By converting underlying error types to our own custom error type that encapsulates the
327 /// underlying error type, we can return a single error type without losing information on the
328 /// underlying cause. The '?' operator automatically converts the underlying error type to our
329 /// custom error type by calling `Into<CliError>::into` which is automatically provided when
330 /// implementing `From`. The compiler then infers which implementation of `Into` should be used.
338 /// IoError(io::Error),
339 /// ParseError(num::ParseIntError),
342 /// impl From<io::Error> for CliError {
343 /// fn from(error: io::Error) -> Self {
344 /// CliError::IoError(error)
348 /// impl From<num::ParseIntError> for CliError {
349 /// fn from(error: num::ParseIntError) -> Self {
350 /// CliError::ParseError(error)
354 /// fn open_and_parse_file(file_name: &str) -> Result<i32, CliError> {
355 /// let mut contents = fs::read_to_string(&file_name)?;
356 /// let num: i32 = contents.trim().parse()?;
361 /// [`TryFrom`]: trait.TryFrom.html
362 /// [`Option<T>`]: ../../std/option/enum.Option.html
363 /// [`Result<T, E>`]: ../../std/result/enum.Result.html
364 /// [`String`]: ../../std/string/struct.String.html
365 /// [`Into`]: trait.Into.html
366 /// [`from`]: trait.From.html#tymethod.from
367 /// [book]: ../../book/ch09-00-error-handling.html
368 #[stable(feature = "rust1", since = "1.0.0")]
369 #[rustc_on_unimplemented(on(
370 all(_Self = "&str", T = "std::string::String"),
371 note = "to coerce a `{T}` into a `{Self}`, use `&*` as a prefix",
373 pub trait From<T>: Sized {
374 /// Performs the conversion.
375 #[stable(feature = "rust1", since = "1.0.0")]
376 fn from(_: T) -> Self;
379 /// An attempted conversion that consumes `self`, which may or may not be
382 /// Library authors should usually not directly implement this trait,
383 /// but should prefer implementing the [`TryFrom`] trait, which offers
384 /// greater flexibility and provides an equivalent `TryInto`
385 /// implementation for free, thanks to a blanket implementation in the
386 /// standard library. For more information on this, see the
387 /// documentation for [`Into`].
389 /// # Implementing `TryInto`
391 /// This suffers the same restrictions and reasoning as implementing
392 /// [`Into`], see there for details.
394 /// [`TryFrom`]: trait.TryFrom.html
395 /// [`Into`]: trait.Into.html
396 #[stable(feature = "try_from", since = "1.34.0")]
397 pub trait TryInto<T>: Sized {
398 /// The type returned in the event of a conversion error.
399 #[stable(feature = "try_from", since = "1.34.0")]
402 /// Performs the conversion.
403 #[stable(feature = "try_from", since = "1.34.0")]
404 fn try_into(self) -> Result<T, Self::Error>;
407 /// Simple and safe type conversions that may fail in a controlled
408 /// way under some circumstances. It is the reciprocal of [`TryInto`].
410 /// This is useful when you are doing a type conversion that may
411 /// trivially succeed but may also need special handling.
412 /// For example, there is no way to convert an [`i64`] into an [`i32`]
413 /// using the [`From`] trait, because an [`i64`] may contain a value
414 /// that an [`i32`] cannot represent and so the conversion would lose data.
415 /// This might be handled by truncating the [`i64`] to an [`i32`] (essentially
416 /// giving the [`i64`]'s value modulo [`i32::MAX`]) or by simply returning
417 /// [`i32::MAX`], or by some other method. The [`From`] trait is intended
418 /// for perfect conversions, so the `TryFrom` trait informs the
419 /// programmer when a type conversion could go bad and lets them
420 /// decide how to handle it.
422 /// # Generic Implementations
424 /// - `TryFrom<T> for U` implies [`TryInto`]`<U> for T`
425 /// - [`try_from`] is reflexive, which means that `TryFrom<T> for T`
426 /// is implemented and cannot fail -- the associated `Error` type for
427 /// calling `T::try_from()` on a value of type `T` is [`!`].
429 /// `TryFrom<T>` can be implemented as follows:
432 /// use std::convert::TryFrom;
434 /// struct GreaterThanZero(i32);
436 /// impl TryFrom<i32> for GreaterThanZero {
437 /// type Error = &'static str;
439 /// fn try_from(value: i32) -> Result<Self, Self::Error> {
441 /// Err("GreaterThanZero only accepts value superior than zero!")
443 /// Ok(GreaterThanZero(value))
451 /// As described, [`i32`] implements `TryFrom<`[`i64`]`>`:
454 /// use std::convert::TryFrom;
456 /// let big_number = 1_000_000_000_000i64;
457 /// // Silently truncates `big_number`, requires detecting
458 /// // and handling the truncation after the fact.
459 /// let smaller_number = big_number as i32;
460 /// assert_eq!(smaller_number, -727379968);
462 /// // Returns an error because `big_number` is too big to
463 /// // fit in an `i32`.
464 /// let try_smaller_number = i32::try_from(big_number);
465 /// assert!(try_smaller_number.is_err());
467 /// // Returns `Ok(3)`.
468 /// let try_successful_smaller_number = i32::try_from(3);
469 /// assert!(try_successful_smaller_number.is_ok());
472 /// [`try_from`]: trait.TryFrom.html#tymethod.try_from
473 /// [`TryInto`]: trait.TryInto.html
474 /// [`i32::MAX`]: ../../std/i32/constant.MAX.html
475 /// [`!`]: ../../std/primitive.never.html
476 #[stable(feature = "try_from", since = "1.34.0")]
477 pub trait TryFrom<T>: Sized {
478 /// The type returned in the event of a conversion error.
479 #[stable(feature = "try_from", since = "1.34.0")]
482 /// Performs the conversion.
483 #[stable(feature = "try_from", since = "1.34.0")]
484 fn try_from(value: T) -> Result<Self, Self::Error>;
487 ////////////////////////////////////////////////////////////////////////////////
489 ////////////////////////////////////////////////////////////////////////////////
492 #[stable(feature = "rust1", since = "1.0.0")]
493 impl<T: ?Sized, U: ?Sized> AsRef<U> for &T
497 fn as_ref(&self) -> &U {
498 <T as AsRef<U>>::as_ref(*self)
502 // As lifts over &mut
503 #[stable(feature = "rust1", since = "1.0.0")]
504 impl<T: ?Sized, U: ?Sized> AsRef<U> for &mut T
508 fn as_ref(&self) -> &U {
509 <T as AsRef<U>>::as_ref(*self)
513 // FIXME (#45742): replace the above impls for &/&mut with the following more general one:
514 // // As lifts over Deref
515 // impl<D: ?Sized + Deref<Target: AsRef<U>>, U: ?Sized> AsRef<U> for D {
516 // fn as_ref(&self) -> &U {
517 // self.deref().as_ref()
521 // AsMut lifts over &mut
522 #[stable(feature = "rust1", since = "1.0.0")]
523 impl<T: ?Sized, U: ?Sized> AsMut<U> for &mut T
527 fn as_mut(&mut self) -> &mut U {
532 // FIXME (#45742): replace the above impl for &mut with the following more general one:
533 // // AsMut lifts over DerefMut
534 // impl<D: ?Sized + Deref<Target: AsMut<U>>, U: ?Sized> AsMut<U> for D {
535 // fn as_mut(&mut self) -> &mut U {
536 // self.deref_mut().as_mut()
541 #[stable(feature = "rust1", since = "1.0.0")]
542 impl<T, U> Into<U> for T
551 // From (and thus Into) is reflexive
552 #[stable(feature = "rust1", since = "1.0.0")]
553 impl<T> From<T> for T {
559 /// **Stability note:** This impl does not yet exist, but we are
560 /// "reserving space" to add it in the future. See
561 /// [rust-lang/rust#64715][#64715] for details.
563 /// [#64715]: https://github.com/rust-lang/rust/issues/64715
564 #[stable(feature = "convert_infallible", since = "1.34.0")]
565 #[rustc_reservation_impl = "permitting this impl would forbid us from adding \
566 `impl<T> From<!> for T` later; see rust-lang/rust#64715 for details"]
567 impl<T> From<!> for T {
573 // TryFrom implies TryInto
574 #[stable(feature = "try_from", since = "1.34.0")]
575 impl<T, U> TryInto<U> for T
579 type Error = U::Error;
581 fn try_into(self) -> Result<U, U::Error> {
586 // Infallible conversions are semantically equivalent to fallible conversions
587 // with an uninhabited error type.
588 #[stable(feature = "try_from", since = "1.34.0")]
589 impl<T, U> TryFrom<U> for T
593 type Error = Infallible;
595 fn try_from(value: U) -> Result<Self, Self::Error> {
600 ////////////////////////////////////////////////////////////////////////////////
602 ////////////////////////////////////////////////////////////////////////////////
604 #[stable(feature = "rust1", since = "1.0.0")]
605 impl<T> AsRef<[T]> for [T] {
606 fn as_ref(&self) -> &[T] {
611 #[stable(feature = "rust1", since = "1.0.0")]
612 impl<T> AsMut<[T]> for [T] {
613 fn as_mut(&mut self) -> &mut [T] {
618 #[stable(feature = "rust1", since = "1.0.0")]
619 impl AsRef<str> for str {
621 fn as_ref(&self) -> &str {
626 ////////////////////////////////////////////////////////////////////////////////
627 // THE NO-ERROR ERROR TYPE
628 ////////////////////////////////////////////////////////////////////////////////
630 /// A type alias for [the `!` “never” type][never].
632 /// `Infallible` represents types of errors that can never happen since `!` has no valid values.
633 /// This can be useful for generic APIs that use [`Result`] and parameterize the error type,
634 /// to indicate that the result is always [`Ok`].
636 /// For example, the [`TryFrom`] trait (conversion that returns a [`Result`])
637 /// has a blanket implementation for all types where a reverse [`Into`] implementation exists.
639 /// ```ignore (illustrates std code, duplicating the impl in a doctest would be an error)
640 /// impl<T, U> TryFrom<U> for T where U: Into<T> {
641 /// type Error = Infallible;
643 /// fn try_from(value: U) -> Result<Self, Infallible> {
644 /// Ok(U::into(value)) // Never returns `Err`
649 /// # Eventual deprecation
651 /// Previously, `Infallible` was defined as `enum Infallible {}`.
652 /// Now that it is merely a type alias to `!`, we will eventually deprecate `Infallible`.
654 /// [`Ok`]: ../result/enum.Result.html#variant.Ok
655 /// [`Result`]: ../result/enum.Result.html
656 /// [`TryFrom`]: trait.TryFrom.html
657 /// [`Into`]: trait.Into.html
658 /// [never]: ../../std/primitive.never.html
659 #[stable(feature = "convert_infallible", since = "1.34.0")]
660 pub type Infallible = !;