ebc70e2e9e
Change bounds on `TryFrom` blanket impl to use `Into` instead of `From` This is from this [comment](https://github.com/rust-lang/rust/issues/33417#issuecomment-447111156) I made. This will expand the impls available for `TryFrom` and `TryInto`, without losing anything in the process.
499 lines
15 KiB
Rust
499 lines
15 KiB
Rust
//! Traits for conversions between types.
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//!
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//! The traits in this module provide a general way to talk about conversions
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//! from one type to another. They follow the standard Rust conventions of
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//! `as`/`into`/`from`.
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//!
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//! Like many traits, these are often used as bounds for generic functions, to
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//! support arguments of multiple types.
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//!
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//! - Implement the `As*` traits for reference-to-reference conversions
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//! - Implement the [`Into`] trait when you want to consume the value in the conversion
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//! - The [`From`] trait is the most flexible, useful for value _and_ reference conversions
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//! - The [`TryFrom`] and [`TryInto`] traits behave like [`From`] and [`Into`], but allow for the
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//! conversion to fail
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//!
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//! As a library author, you should prefer implementing [`From<T>`][`From`] or
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//! [`TryFrom<T>`][`TryFrom`] rather than [`Into<U>`][`Into`] or [`TryInto<U>`][`TryInto`],
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//! as [`From`] and [`TryFrom`] provide greater flexibility and offer
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//! equivalent [`Into`] or [`TryInto`] implementations for free, thanks to a
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//! blanket implementation in the standard library.
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//!
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//! # Generic Implementations
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//!
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//! - [`AsRef`] and [`AsMut`] auto-dereference if the inner type is a reference
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//! - [`From`]`<U> for T` implies [`Into`]`<T> for U`
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//! - [`TryFrom`]`<U> for T` implies [`TryInto`]`<T> for U`
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//! - [`From`] and [`Into`] are reflexive, which means that all types can
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//! `into` themselves and `from` themselves
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//!
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//! See each trait for usage examples.
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//!
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//! [`Into`]: trait.Into.html
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//! [`From`]: trait.From.html
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//! [`TryFrom`]: trait.TryFrom.html
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//! [`TryInto`]: trait.TryInto.html
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//! [`AsRef`]: trait.AsRef.html
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//! [`AsMut`]: trait.AsMut.html
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#![stable(feature = "rust1", since = "1.0.0")]
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/// An identity function.
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///
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/// Two things are important to note about this function:
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///
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/// - It is not always equivalent to a closure like `|x| x` since the
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/// closure may coerce `x` into a different type.
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///
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/// - It moves the input `x` passed to the function.
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///
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/// While it might seem strange to have a function that just returns back the
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/// input, there are some interesting uses.
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///
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/// # Examples
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///
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/// Using `identity` to do nothing among other interesting functions:
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///
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/// ```rust
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/// use std::convert::identity;
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///
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/// fn manipulation(x: u32) -> u32 {
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/// // Let's assume that this function does something interesting.
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/// x + 1
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/// }
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///
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/// let _arr = &[identity, manipulation];
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/// ```
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///
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/// Using `identity` to get a function that changes nothing in a conditional:
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///
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/// ```rust
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/// use std::convert::identity;
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///
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/// # let condition = true;
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///
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/// # fn manipulation(x: u32) -> u32 { x + 1 }
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///
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/// let do_stuff = if condition { manipulation } else { identity };
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///
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/// // do more interesting stuff..
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///
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/// let _results = do_stuff(42);
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/// ```
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///
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/// Using `identity` to keep the `Some` variants of an iterator of `Option<T>`:
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///
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/// ```rust
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/// use std::convert::identity;
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///
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/// let iter = vec![Some(1), None, Some(3)].into_iter();
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/// let filtered = iter.filter_map(identity).collect::<Vec<_>>();
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/// assert_eq!(vec![1, 3], filtered);
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/// ```
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#[stable(feature = "convert_id", since = "1.33.0")]
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#[inline]
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pub const fn identity<T>(x: T) -> T { x }
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/// A cheap reference-to-reference conversion. Used to convert a value to a
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/// reference value within generic code.
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///
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/// `AsRef` is very similar to, but serves a slightly different purpose than,
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/// [`Borrow`].
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///
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/// `AsRef` is to be used when wishing to convert to a reference of another
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/// type.
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/// `Borrow` is more related to the notion of taking the reference. It is
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/// useful when wishing to abstract over the type of reference
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/// (`&T`, `&mut T`) or allow both the referenced and owned type to be treated
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/// in the same manner.
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///
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/// The key difference between the two traits is the intention:
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///
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/// - Use `AsRef` when the goal is to simply convert into a reference
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/// - Use `Borrow` when the goal is related to writing code that is agnostic to
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/// the type of borrow and whether it is a reference or value
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///
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/// See [the book][book] for a more detailed comparison.
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///
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/// [book]: ../../book/first-edition/borrow-and-asref.html
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/// [`Borrow`]: ../../std/borrow/trait.Borrow.html
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///
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/// **Note: this trait must not fail**. If the conversion can fail, use a
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/// dedicated method which returns an [`Option<T>`] or a [`Result<T, E>`].
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///
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/// [`Option<T>`]: ../../std/option/enum.Option.html
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/// [`Result<T, E>`]: ../../std/result/enum.Result.html
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///
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/// # Generic Implementations
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///
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/// - `AsRef` auto-dereferences if the inner type is a reference or a mutable
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/// reference (e.g.: `foo.as_ref()` will work the same if `foo` has type
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/// `&mut Foo` or `&&mut Foo`)
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///
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/// # Examples
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///
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/// Both [`String`] and `&str` implement `AsRef<str>`:
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///
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/// [`String`]: ../../std/string/struct.String.html
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///
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/// ```
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/// fn is_hello<T: AsRef<str>>(s: T) {
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/// assert_eq!("hello", s.as_ref());
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/// }
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///
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/// let s = "hello";
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/// is_hello(s);
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///
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/// let s = "hello".to_string();
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/// is_hello(s);
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/// ```
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///
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#[stable(feature = "rust1", since = "1.0.0")]
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pub trait AsRef<T: ?Sized> {
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/// Performs the conversion.
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#[stable(feature = "rust1", since = "1.0.0")]
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fn as_ref(&self) -> &T;
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}
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/// A cheap, mutable reference-to-mutable reference conversion.
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///
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/// This trait is similar to `AsRef` but used for converting between mutable
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/// references.
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///
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/// **Note: this trait must not fail**. If the conversion can fail, use a
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/// dedicated method which returns an [`Option<T>`] or a [`Result<T, E>`].
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///
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/// [`Option<T>`]: ../../std/option/enum.Option.html
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/// [`Result<T, E>`]: ../../std/result/enum.Result.html
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///
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/// # Generic Implementations
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///
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/// - `AsMut` auto-dereferences if the inner type is a mutable reference
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/// (e.g.: `foo.as_mut()` will work the same if `foo` has type `&mut Foo`
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/// or `&mut &mut Foo`)
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///
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/// # Examples
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///
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/// [`Box<T>`] implements `AsMut<T>`:
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///
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/// [`Box<T>`]: ../../std/boxed/struct.Box.html
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///
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/// ```
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/// fn add_one<T: AsMut<u64>>(num: &mut T) {
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/// *num.as_mut() += 1;
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/// }
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///
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/// let mut boxed_num = Box::new(0);
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/// add_one(&mut boxed_num);
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/// assert_eq!(*boxed_num, 1);
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/// ```
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///
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///
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#[stable(feature = "rust1", since = "1.0.0")]
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pub trait AsMut<T: ?Sized> {
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/// Performs the conversion.
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#[stable(feature = "rust1", since = "1.0.0")]
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fn as_mut(&mut self) -> &mut T;
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}
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/// A conversion that consumes `self`, which may or may not be expensive. The
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/// reciprocal of [`From`][From].
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///
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/// **Note: this trait must not fail**. If the conversion can fail, use
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/// [`TryInto`] or a dedicated method which returns an [`Option<T>`] or a
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/// [`Result<T, E>`].
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///
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/// Library authors should not directly implement this trait, but should prefer
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/// implementing the [`From`][From] trait, which offers greater flexibility and
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/// provides an equivalent `Into` implementation for free, thanks to a blanket
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/// implementation in the standard library.
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///
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/// # Generic Implementations
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///
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/// - [`From<T>`][From]` for U` implies `Into<U> for T`
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/// - [`into`] is reflexive, which means that `Into<T> for T` is implemented
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///
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/// # Implementing `Into`
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///
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/// There is one exception to implementing `Into`, and it's kind of esoteric.
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/// If the destination type is not part of the current crate, and it uses a
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/// generic variable, then you can't implement `From` directly. For example,
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/// take this crate:
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///
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/// ```compile_fail
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/// struct Wrapper<T>(Vec<T>);
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/// impl<T> From<Wrapper<T>> for Vec<T> {
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/// fn from(w: Wrapper<T>) -> Vec<T> {
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/// w.0
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/// }
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/// }
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/// ```
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///
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/// To fix this, you can implement `Into` directly:
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///
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/// ```
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/// struct Wrapper<T>(Vec<T>);
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/// impl<T> Into<Vec<T>> for Wrapper<T> {
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/// fn into(self) -> Vec<T> {
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/// self.0
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/// }
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/// }
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/// ```
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///
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/// This won't always allow the conversion: for example, `try!` and `?`
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/// always use `From`. However, in most cases, people use `Into` to do the
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/// conversions, and this will allow that.
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///
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/// In almost all cases, you should try to implement `From`, then fall back
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/// to `Into` if `From` can't be implemented.
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///
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/// # Examples
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///
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/// [`String`] implements `Into<Vec<u8>>`:
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///
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/// ```
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/// fn is_hello<T: Into<Vec<u8>>>(s: T) {
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/// let bytes = b"hello".to_vec();
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/// assert_eq!(bytes, s.into());
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/// }
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///
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/// let s = "hello".to_string();
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/// is_hello(s);
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/// ```
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///
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/// [`TryInto`]: trait.TryInto.html
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/// [`Option<T>`]: ../../std/option/enum.Option.html
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/// [`Result<T, E>`]: ../../std/result/enum.Result.html
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/// [`String`]: ../../std/string/struct.String.html
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/// [From]: trait.From.html
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/// [`into`]: trait.Into.html#tymethod.into
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#[stable(feature = "rust1", since = "1.0.0")]
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pub trait Into<T>: Sized {
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/// Performs the conversion.
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#[stable(feature = "rust1", since = "1.0.0")]
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fn into(self) -> T;
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}
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/// Simple and safe type conversions in to `Self`. It is the reciprocal of
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/// `Into`.
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///
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/// This trait is useful when performing error handling as described by
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/// [the book][book] and is closely related to the `?` operator.
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///
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/// When constructing a function that is capable of failing the return type
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/// will generally be of the form `Result<T, E>`.
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///
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/// The `From` trait allows for simplification of error handling by providing a
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/// means of returning a single error type that encapsulates numerous possible
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/// erroneous situations.
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///
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/// This trait is not limited to error handling, rather the general case for
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/// this trait would be in any type conversions to have an explicit definition
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/// of how they are performed.
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///
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/// **Note: this trait must not fail**. If the conversion can fail, use
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/// [`TryFrom`] or a dedicated method which returns an [`Option<T>`] or a
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/// [`Result<T, E>`].
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///
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/// # Generic Implementations
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///
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/// - `From<T> for U` implies [`Into<U>`]` for T`
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/// - [`from`] is reflexive, which means that `From<T> for T` is implemented
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///
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/// # Examples
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///
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/// [`String`] implements `From<&str>`:
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///
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/// ```
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/// let string = "hello".to_string();
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/// let other_string = String::from("hello");
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///
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/// assert_eq!(string, other_string);
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/// ```
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///
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/// An example usage for error handling:
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///
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/// ```
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/// use std::fs;
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/// use std::io;
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/// use std::num;
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///
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/// enum CliError {
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/// IoError(io::Error),
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/// ParseError(num::ParseIntError),
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/// }
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///
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/// impl From<io::Error> for CliError {
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/// fn from(error: io::Error) -> Self {
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/// CliError::IoError(error)
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/// }
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/// }
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///
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/// impl From<num::ParseIntError> for CliError {
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/// fn from(error: num::ParseIntError) -> Self {
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/// CliError::ParseError(error)
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/// }
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/// }
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///
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/// fn open_and_parse_file(file_name: &str) -> Result<i32, CliError> {
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/// let mut contents = fs::read_to_string(&file_name)?;
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/// let num: i32 = contents.trim().parse()?;
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/// Ok(num)
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/// }
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/// ```
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///
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/// [`TryFrom`]: trait.TryFrom.html
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/// [`Option<T>`]: ../../std/option/enum.Option.html
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/// [`Result<T, E>`]: ../../std/result/enum.Result.html
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/// [`String`]: ../../std/string/struct.String.html
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/// [`Into<U>`]: trait.Into.html
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/// [`from`]: trait.From.html#tymethod.from
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/// [book]: ../../book/first-edition/error-handling.html
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#[stable(feature = "rust1", since = "1.0.0")]
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pub trait From<T>: Sized {
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/// Performs the conversion.
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#[stable(feature = "rust1", since = "1.0.0")]
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fn from(_: T) -> Self;
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}
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/// An attempted conversion that consumes `self`, which may or may not be
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/// expensive.
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///
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/// Library authors should not directly implement this trait, but should prefer
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/// implementing the [`TryFrom`] trait, which offers greater flexibility and
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/// provides an equivalent `TryInto` implementation for free, thanks to a
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/// blanket implementation in the standard library. For more information on this,
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/// see the documentation for [`Into`].
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///
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/// [`TryFrom`]: trait.TryFrom.html
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/// [`Into`]: trait.Into.html
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#[unstable(feature = "try_from", issue = "33417")]
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pub trait TryInto<T>: Sized {
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/// The type returned in the event of a conversion error.
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type Error;
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/// Performs the conversion.
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fn try_into(self) -> Result<T, Self::Error>;
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}
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/// Attempt to construct `Self` via a conversion.
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#[unstable(feature = "try_from", issue = "33417")]
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pub trait TryFrom<T>: Sized {
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/// The type returned in the event of a conversion error.
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type Error;
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/// Performs the conversion.
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fn try_from(value: T) -> Result<Self, Self::Error>;
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}
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////////////////////////////////////////////////////////////////////////////////
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// GENERIC IMPLS
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////////////////////////////////////////////////////////////////////////////////
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// As lifts over &
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#[stable(feature = "rust1", since = "1.0.0")]
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impl<T: ?Sized, U: ?Sized> AsRef<U> for &T where T: AsRef<U>
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{
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fn as_ref(&self) -> &U {
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<T as AsRef<U>>::as_ref(*self)
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}
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}
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// As lifts over &mut
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#[stable(feature = "rust1", since = "1.0.0")]
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impl<T: ?Sized, U: ?Sized> AsRef<U> for &mut T where T: AsRef<U>
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{
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fn as_ref(&self) -> &U {
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<T as AsRef<U>>::as_ref(*self)
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}
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}
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// FIXME (#45742): replace the above impls for &/&mut with the following more general one:
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// // As lifts over Deref
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// impl<D: ?Sized + Deref, U: ?Sized> AsRef<U> for D where D::Target: AsRef<U> {
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// fn as_ref(&self) -> &U {
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// self.deref().as_ref()
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// }
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// }
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// AsMut lifts over &mut
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#[stable(feature = "rust1", since = "1.0.0")]
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impl<T: ?Sized, U: ?Sized> AsMut<U> for &mut T where T: AsMut<U>
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{
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fn as_mut(&mut self) -> &mut U {
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(*self).as_mut()
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}
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}
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// FIXME (#45742): replace the above impl for &mut with the following more general one:
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// // AsMut lifts over DerefMut
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// impl<D: ?Sized + Deref, U: ?Sized> AsMut<U> for D where D::Target: AsMut<U> {
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// fn as_mut(&mut self) -> &mut U {
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// self.deref_mut().as_mut()
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// }
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// }
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// From implies Into
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#[stable(feature = "rust1", since = "1.0.0")]
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impl<T, U> Into<U> for T where U: From<T>
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{
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fn into(self) -> U {
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U::from(self)
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}
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}
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// From (and thus Into) is reflexive
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#[stable(feature = "rust1", since = "1.0.0")]
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impl<T> From<T> for T {
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fn from(t: T) -> T { t }
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}
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// TryFrom implies TryInto
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#[unstable(feature = "try_from", issue = "33417")]
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impl<T, U> TryInto<U> for T where U: TryFrom<T>
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{
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type Error = U::Error;
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fn try_into(self) -> Result<U, U::Error> {
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U::try_from(self)
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}
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}
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// Infallible conversions are semantically equivalent to fallible conversions
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// with an uninhabited error type.
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#[unstable(feature = "try_from", issue = "33417")]
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impl<T, U> TryFrom<U> for T where U: Into<T> {
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type Error = !;
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fn try_from(value: U) -> Result<Self, Self::Error> {
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Ok(U::into(value))
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}
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}
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////////////////////////////////////////////////////////////////////////////////
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// CONCRETE IMPLS
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////////////////////////////////////////////////////////////////////////////////
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#[stable(feature = "rust1", since = "1.0.0")]
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impl<T> AsRef<[T]> for [T] {
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fn as_ref(&self) -> &[T] {
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self
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}
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|
}
|
|
|
|
#[stable(feature = "rust1", since = "1.0.0")]
|
|
impl<T> AsMut<[T]> for [T] {
|
|
fn as_mut(&mut self) -> &mut [T] {
|
|
self
|
|
}
|
|
}
|
|
|
|
#[stable(feature = "rust1", since = "1.0.0")]
|
|
impl AsRef<str> for str {
|
|
#[inline]
|
|
fn as_ref(&self) -> &str {
|
|
self
|
|
}
|
|
}
|