library/core/src/ops/deref.rs - toolchain/rustc - Git at Google

 /// Used for immutable dereferencing operations, like `*v`.
 ///
 /// In addition to being used for explicit dereferencing operations with the
 /// (unary) `*` operator in immutable contexts, `Deref` is also used implicitly
 /// by the compiler in many circumstances. This mechanism is called
 /// ["`Deref` coercion"][coercion]. In mutable contexts, [`DerefMut`] is used and
 /// mutable deref coercion similarly occurs.
 ///
 /// **Warning:** Deref coercion is a powerful language feature which has
 /// far-reaching implications for every type that implements `Deref`. The
 /// compiler will silently insert calls to `Deref::deref`. For this reason, one
 /// should be careful about implementing `Deref` and only do so when deref
 /// coercion is desirable. See [below][implementing] for advice on when this is
 /// typically desirable or undesirable.
 ///
 /// Types that implement `Deref` or `DerefMut` are often called "smart
 /// pointers" and the mechanism of deref coercion has been specifically designed
 /// to facilitate the pointer-like behaviour that name suggests. Often, the
 /// purpose of a "smart pointer" type is to change the ownership semantics
 /// of a contained value (for example, [`Rc`][rc] or [`Cow`][cow]) or the
 /// storage semantics of a contained value (for example, [`Box`][box]).
 ///
 /// # Deref coercion
 ///
 /// If `T` implements `Deref<Target = U>`, and `v` is a value of type `T`, then:
 ///
 /// * In immutable contexts, `*v` (where `T` is neither a reference nor a raw
 ///   pointer) is equivalent to `*Deref::deref(&v)`.
 /// * Values of type `&T` are coerced to values of type `&U`
 /// * `T` implicitly implements all the methods of the type `U` which take the
 ///   `&self` receiver.
 ///
 /// For more details, visit [the chapter in *The Rust Programming Language*][book]
 /// as well as the reference sections on [the dereference operator][ref-deref-op],
 /// [method resolution], and [type coercions].
 ///
 /// # When to implement `Deref` or `DerefMut`
 ///
 /// The same advice applies to both deref traits. In general, deref traits
 /// **should** be implemented if:
 ///
 /// 1. a value of the type transparently behaves like a value of the target
 ///    type;
 /// 1. the implementation of the deref function is cheap; and
 /// 1. users of the type will not be surprised by any deref coercion behaviour.
 ///
 /// In general, deref traits **should not** be implemented if:
 ///
 /// 1. the deref implementations could fail unexpectedly; or
 /// 1. the type has methods that are likely to collide with methods on the
 ///    target type; or
 /// 1. committing to deref coercion as part of the public API is not desirable.
 ///
 /// Note that there's a large difference between implementing deref traits
 /// generically over many target types, and doing so only for specific target
 /// types.
 ///
 /// Generic implementations, such as for [`Box<T>`][box] (which is generic over
 /// every type and dereferences to `T`) should be careful to provide few or no
 /// methods, since the target type is unknown and therefore every method could
 /// collide with one on the target type, causing confusion for users.
 /// `impl<T> Box<T>` has no methods (though several associated functions),
 /// partly for this reason.
 ///
 /// Specific implementations, such as for [`String`][string] (whose `Deref`
 /// implementation has `Target = str`) can have many methods, since avoiding
 /// collision is much easier. `String` and `str` both have many methods, and
 /// `String` additionally behaves as if it has every method of `str` because of
 /// deref coercion. The implementing type may also be generic while the
 /// implementation is still specific in this sense; for example, [`Vec<T>`][vec]
 /// dereferences to `[T]`, so methods of `T` are not applicable.
 ///
 /// Consider also that deref coercion means that deref traits are a much larger
 /// part of a type's public API than any other trait as it is implicitly called
 /// by the compiler. Therefore, it is advisable to consider whether this is
 /// something you are comfortable supporting as a public API.
 ///
 /// The [`AsRef`] and [`Borrow`][core::borrow::Borrow] traits have very similar
 /// signatures to `Deref`. It may be desirable to implement either or both of
 /// these, whether in addition to or rather than deref traits. See their
 /// documentation for details.
 ///
 /// # Fallibility
 ///
 /// **This trait's method should never unexpectedly fail**. Deref coercion means
 /// the compiler will often insert calls to `Deref::deref` implicitly. Failure
 /// during dereferencing can be extremely confusing when `Deref` is invoked
 /// implicitly. In the majority of uses it should be infallible, though it may
 /// be acceptable to panic if the type is misused through programmer error, for
 /// example.
 ///
 /// However, infallibility is not enforced and therefore not guaranteed.
 /// As such, `unsafe` code should not rely on infallibility in general for
 /// soundness.
 ///
 /// [book]: ../../book/ch15-02-deref.html
 /// [coercion]: #deref-coercion
 /// [implementing]: #when-to-implement-deref-or-derefmut
 /// [ref-deref-op]: ../../reference/expressions/operator-expr.html#the-dereference-operator
 /// [method resolution]: ../../reference/expressions/method-call-expr.html
 /// [type coercions]: ../../reference/type-coercions.html
 /// [box]: ../../alloc/boxed/struct.Box.html
 /// [string]: ../../alloc/string/struct.String.html
 /// [vec]: ../../alloc/vec/struct.Vec.html
 /// [rc]: ../../alloc/rc/struct.Rc.html
 /// [cow]: ../../alloc/borrow/enum.Cow.html
 ///
 /// # Examples
 ///
 /// A struct with a single field which is accessible by dereferencing the
 /// struct.
 ///
 /// ```
 /// use std::ops::Deref;
 ///
 /// struct DerefExample<T> {
 ///     value: T
 /// }
 ///
 /// impl<T> Deref for DerefExample<T> {
 ///     type Target = T;
 ///
 ///     fn deref(&self) -> &Self::Target {
 ///         &self.value
 ///     }
 /// }
 ///
 /// let x = DerefExample { value: 'a' };
 /// assert_eq!('a', *x);
 /// ```
 #[lang = "deref"]
 #[doc(alias = "*")]
 #[doc(alias = "&*")]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_diagnostic_item = "Deref"]
 pub trait Deref {
     /// The resulting type after dereferencing.
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_diagnostic_item = "deref_target"]
     #[lang = "deref_target"]
     type Target: ?Sized;

     /// Dereferences the value.
     #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_diagnostic_item = "deref_method"]
     fn deref(&self) -> &Self::Target;
 }

 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized> Deref for &T {
     type Target = T;

     #[rustc_diagnostic_item = "noop_method_deref"]
     fn deref(&self) -> &T {
         *self
     }
 }

 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized> !DerefMut for &T {}

 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized> Deref for &mut T {
     type Target = T;

     fn deref(&self) -> &T {
         *self
     }
 }

 /// Used for mutable dereferencing operations, like in `*v = 1;`.
 ///
 /// In addition to being used for explicit dereferencing operations with the
 /// (unary) `*` operator in mutable contexts, `DerefMut` is also used implicitly
 /// by the compiler in many circumstances. This mechanism is called
 /// ["mutable deref coercion"][coercion]. In immutable contexts, [`Deref`] is used.
 ///
 /// **Warning:** Deref coercion is a powerful language feature which has
 /// far-reaching implications for every type that implements `DerefMut`. The
 /// compiler will silently insert calls to `DerefMut::deref_mut`. For this
 /// reason, one should be careful about implementing `DerefMut` and only do so
 /// when mutable deref coercion is desirable. See [the `Deref` docs][implementing]
 /// for advice on when this is typically desirable or undesirable.
 ///
 /// Types that implement `DerefMut` or `Deref` are often called "smart
 /// pointers" and the mechanism of deref coercion has been specifically designed
 /// to facilitate the pointer-like behaviour that name suggests. Often, the
 /// purpose of a "smart pointer" type is to change the ownership semantics
 /// of a contained value (for example, [`Rc`][rc] or [`Cow`][cow]) or the
 /// storage semantics of a contained value (for example, [`Box`][box]).
 ///
 /// # Mutable deref coercion
 ///
 /// If `T` implements `DerefMut<Target = U>`, and `v` is a value of type `T`,
 /// then:
 ///
 /// * In mutable contexts, `*v` (where `T` is neither a reference nor a raw pointer)
 ///   is equivalent to `*DerefMut::deref_mut(&mut v)`.
 /// * Values of type `&mut T` are coerced to values of type `&mut U`
 /// * `T` implicitly implements all the (mutable) methods of the type `U`.
 ///
 /// For more details, visit [the chapter in *The Rust Programming Language*][book]
 /// as well as the reference sections on [the dereference operator][ref-deref-op],
 /// [method resolution] and [type coercions].
 ///
 /// # Fallibility
 ///
 /// **This trait's method should never unexpectedly fail**. Deref coercion means
 /// the compiler will often insert calls to `DerefMut::deref_mut` implicitly.
 /// Failure during dereferencing can be extremely confusing when `DerefMut` is
 /// invoked implicitly. In the majority of uses it should be infallible, though
 /// it may be acceptable to panic if the type is misused through programmer
 /// error, for example.
 ///
 /// However, infallibility is not enforced and therefore not guaranteed.
 /// As such, `unsafe` code should not rely on infallibility in general for
 /// soundness.
 ///
 /// [book]: ../../book/ch15-02-deref.html
 /// [coercion]: #mutable-deref-coercion
 /// [implementing]: Deref#when-to-implement-deref-or-derefmut
 /// [ref-deref-op]: ../../reference/expressions/operator-expr.html#the-dereference-operator
 /// [method resolution]: ../../reference/expressions/method-call-expr.html
 /// [type coercions]: ../../reference/type-coercions.html
 /// [box]: ../../alloc/boxed/struct.Box.html
 /// [string]: ../../alloc/string/struct.String.html
 /// [rc]: ../../alloc/rc/struct.Rc.html
 /// [cow]: ../../alloc/borrow/enum.Cow.html
 ///
 /// # Examples
 ///
 /// A struct with a single field which is modifiable by dereferencing the
 /// struct.
 ///
 /// ```
 /// use std::ops::{Deref, DerefMut};
 ///
 /// struct DerefMutExample<T> {
 ///     value: T
 /// }
 ///
 /// impl<T> Deref for DerefMutExample<T> {
 ///     type Target = T;
 ///
 ///     fn deref(&self) -> &Self::Target {
 ///         &self.value
 ///     }
 /// }
 ///
 /// impl<T> DerefMut for DerefMutExample<T> {
 ///     fn deref_mut(&mut self) -> &mut Self::Target {
 ///         &mut self.value
 ///     }
 /// }
 ///
 /// let mut x = DerefMutExample { value: 'a' };
 /// *x = 'b';
 /// assert_eq!('b', x.value);
 /// ```
 #[lang = "deref_mut"]
 #[doc(alias = "*")]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub trait DerefMut: Deref {
     /// Mutably dereferences the value.
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_diagnostic_item = "deref_mut_method"]
     fn deref_mut(&mut self) -> &mut Self::Target;
 }

 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized> DerefMut for &mut T {
     fn deref_mut(&mut self) -> &mut T {
         *self
     }
 }

 /// Indicates that a struct can be used as a method receiver, without the
 /// `arbitrary_self_types` feature. This is implemented by stdlib pointer types like `Box<T>`,
 /// `Rc<T>`, `&T`, and `Pin<P>`.
 #[lang = "receiver"]
 #[unstable(feature = "receiver_trait", issue = "none")]
 #[doc(hidden)]
 pub trait Receiver {
     // Empty.
 }

 #[unstable(feature = "receiver_trait", issue = "none")]
 impl<T: ?Sized> Receiver for &T {}

 #[unstable(feature = "receiver_trait", issue = "none")]
 impl<T: ?Sized> Receiver for &mut T {}
	/// Used for immutable dereferencing operations, like `*v`.
	///
	/// In addition to being used for explicit dereferencing operations with the
	/// (unary) `*` operator in immutable contexts, `Deref` is also used implicitly
	/// by the compiler in many circumstances. This mechanism is called
	/// ["`Deref` coercion"][coercion]. In mutable contexts, [`DerefMut`] is used and
	/// mutable deref coercion similarly occurs.
	///
	/// Warning: Deref coercion is a powerful language feature which has
	/// far-reaching implications for every type that implements `Deref`. The
	/// compiler will silently insert calls to `Deref::deref`. For this reason, one
	/// should be careful about implementing `Deref` and only do so when deref
	/// coercion is desirable. See [below][implementing] for advice on when this is
	/// typically desirable or undesirable.
	///
	/// Types that implement `Deref` or `DerefMut` are often called "smart
	/// pointers" and the mechanism of deref coercion has been specifically designed
	/// to facilitate the pointer-like behaviour that name suggests. Often, the
	/// purpose of a "smart pointer" type is to change the ownership semantics
	/// of a contained value (for example, [`Rc`][rc] or [`Cow`][cow]) or the
	/// storage semantics of a contained value (for example, [`Box`][box]).
	///
	/// # Deref coercion
	///
	/// If `T` implements `Deref<Target = U>`, and `v` is a value of type `T`, then:
	///
	/// * In immutable contexts, `*v` (where `T` is neither a reference nor a raw
	/// pointer) is equivalent to `*Deref::deref(&v)`.
	/// * Values of type `&T` are coerced to values of type `&U`
	/// * `T` implicitly implements all the methods of the type `U` which take the
	/// `&self` receiver.
	///
	/// For more details, visit [the chapter in The Rust Programming Language][book]
	/// as well as the reference sections on [the dereference operator][ref-deref-op],
	/// [method resolution], and [type coercions].
	///
	/// # When to implement `Deref` or `DerefMut`
	///
	/// The same advice applies to both deref traits. In general, deref traits
	/// should be implemented if:
	///
	/// 1. a value of the type transparently behaves like a value of the target
	/// type;
	/// 1. the implementation of the deref function is cheap; and
	/// 1. users of the type will not be surprised by any deref coercion behaviour.
	///
	/// In general, deref traits should not be implemented if:
	///
	/// 1. the deref implementations could fail unexpectedly; or
	/// 1. the type has methods that are likely to collide with methods on the
	/// target type; or
	/// 1. committing to deref coercion as part of the public API is not desirable.
	///
	/// Note that there's a large difference between implementing deref traits
	/// generically over many target types, and doing so only for specific target
	/// types.
	///
	/// Generic implementations, such as for [`Box<T>`][box] (which is generic over
	/// every type and dereferences to `T`) should be careful to provide few or no
	/// methods, since the target type is unknown and therefore every method could
	/// collide with one on the target type, causing confusion for users.
	/// `impl<T> Box<T>` has no methods (though several associated functions),
	/// partly for this reason.
	///
	/// Specific implementations, such as for [`String`][string] (whose `Deref`
	/// implementation has `Target = str`) can have many methods, since avoiding
	/// collision is much easier. `String` and `str` both have many methods, and
	/// `String` additionally behaves as if it has every method of `str` because of
	/// deref coercion. The implementing type may also be generic while the
	/// implementation is still specific in this sense; for example, [`Vec<T>`][vec]
	/// dereferences to `[T]`, so methods of `T` are not applicable.
	///
	/// Consider also that deref coercion means that deref traits are a much larger
	/// part of a type's public API than any other trait as it is implicitly called
	/// by the compiler. Therefore, it is advisable to consider whether this is
	/// something you are comfortable supporting as a public API.
	///
	/// The [`AsRef`] and [`Borrow`][core::borrow::Borrow] traits have very similar
	/// signatures to `Deref`. It may be desirable to implement either or both of
	/// these, whether in addition to or rather than deref traits. See their
	/// documentation for details.
	///
	/// # Fallibility
	///
	/// This trait's method should never unexpectedly fail. Deref coercion means
	/// the compiler will often insert calls to `Deref::deref` implicitly. Failure
	/// during dereferencing can be extremely confusing when `Deref` is invoked
	/// implicitly. In the majority of uses it should be infallible, though it may
	/// be acceptable to panic if the type is misused through programmer error, for
	/// example.
	///
	/// However, infallibility is not enforced and therefore not guaranteed.
	/// As such, `unsafe` code should not rely on infallibility in general for
	/// soundness.
	///
	/// [book]: ../../book/ch15-02-deref.html
	/// [coercion]: #deref-coercion
	/// [implementing]: #when-to-implement-deref-or-derefmut
	/// [ref-deref-op]: ../../reference/expressions/operator-expr.html#the-dereference-operator
	/// [method resolution]: ../../reference/expressions/method-call-expr.html
	/// [type coercions]: ../../reference/type-coercions.html
	/// [box]: ../../alloc/boxed/struct.Box.html
	/// [string]: ../../alloc/string/struct.String.html
	/// [vec]: ../../alloc/vec/struct.Vec.html
	/// [rc]: ../../alloc/rc/struct.Rc.html
	/// [cow]: ../../alloc/borrow/enum.Cow.html
	///
	/// # Examples
	///
	/// A struct with a single field which is accessible by dereferencing the
	/// struct.
	///
	/// ```
	/// use std::ops::Deref;
	///
	/// struct DerefExample<T> {
	/// value: T
	/// }
	///
	/// impl<T> Deref for DerefExample<T> {
	/// type Target = T;
	///
	/// fn deref(&self) -> &Self::Target {
	/// &self.value
	/// }
	/// }
	///
	/// let x = DerefExample { value: 'a' };
	/// assert_eq!('a', *x);
	/// ```
	#[lang = "deref"]
	#[doc(alias = "*")]
	#[doc(alias = "&*")]
	#[stable(feature = "rust1", since = "1.0.0")]
	#[rustc_diagnostic_item = "Deref"]
	pub trait Deref {
	/// The resulting type after dereferencing.
	#[stable(feature = "rust1", since = "1.0.0")]
	#[rustc_diagnostic_item = "deref_target"]
	#[lang = "deref_target"]
	type Target: ?Sized;

	/// Dereferences the value.
	#[must_use]
	#[stable(feature = "rust1", since = "1.0.0")]
	#[rustc_diagnostic_item = "deref_method"]
	fn deref(&self) -> &Self::Target;
	}

	#[stable(feature = "rust1", since = "1.0.0")]
	impl<T: ?Sized> Deref for &T {
	type Target = T;

	#[rustc_diagnostic_item = "noop_method_deref"]
	fn deref(&self) -> &T {
	*self
	}
	}

	#[stable(feature = "rust1", since = "1.0.0")]
	impl<T: ?Sized> !DerefMut for &T {}

	#[stable(feature = "rust1", since = "1.0.0")]
	impl<T: ?Sized> Deref for &mut T {
	type Target = T;

	fn deref(&self) -> &T {
	*self
	}
	}

	/// Used for mutable dereferencing operations, like in `*v = 1;`.
	///
	/// In addition to being used for explicit dereferencing operations with the
	/// (unary) `*` operator in mutable contexts, `DerefMut` is also used implicitly
	/// by the compiler in many circumstances. This mechanism is called
	/// ["mutable deref coercion"][coercion]. In immutable contexts, [`Deref`] is used.
	///
	/// Warning: Deref coercion is a powerful language feature which has
	/// far-reaching implications for every type that implements `DerefMut`. The
	/// compiler will silently insert calls to `DerefMut::deref_mut`. For this
	/// reason, one should be careful about implementing `DerefMut` and only do so
	/// when mutable deref coercion is desirable. See [the `Deref` docs][implementing]
	/// for advice on when this is typically desirable or undesirable.
	///
	/// Types that implement `DerefMut` or `Deref` are often called "smart
	/// pointers" and the mechanism of deref coercion has been specifically designed
	/// to facilitate the pointer-like behaviour that name suggests. Often, the
	/// purpose of a "smart pointer" type is to change the ownership semantics
	/// of a contained value (for example, [`Rc`][rc] or [`Cow`][cow]) or the
	/// storage semantics of a contained value (for example, [`Box`][box]).
	///
	/// # Mutable deref coercion
	///
	/// If `T` implements `DerefMut<Target = U>`, and `v` is a value of type `T`,
	/// then:
	///
	/// * In mutable contexts, `*v` (where `T` is neither a reference nor a raw pointer)
	/// is equivalent to `*DerefMut::deref_mut(&mut v)`.
	/// * Values of type `&mut T` are coerced to values of type `&mut U`
	/// * `T` implicitly implements all the (mutable) methods of the type `U`.
	///
	/// For more details, visit [the chapter in The Rust Programming Language][book]
	/// as well as the reference sections on [the dereference operator][ref-deref-op],
	/// [method resolution] and [type coercions].
	///
	/// # Fallibility
	///
	/// This trait's method should never unexpectedly fail. Deref coercion means
	/// the compiler will often insert calls to `DerefMut::deref_mut` implicitly.
	/// Failure during dereferencing can be extremely confusing when `DerefMut` is
	/// invoked implicitly. In the majority of uses it should be infallible, though
	/// it may be acceptable to panic if the type is misused through programmer
	/// error, for example.
	///
	/// However, infallibility is not enforced and therefore not guaranteed.
	/// As such, `unsafe` code should not rely on infallibility in general for
	/// soundness.
	///
	/// [book]: ../../book/ch15-02-deref.html
	/// [coercion]: #mutable-deref-coercion
	/// [implementing]: Deref#when-to-implement-deref-or-derefmut
	/// [ref-deref-op]: ../../reference/expressions/operator-expr.html#the-dereference-operator
	/// [method resolution]: ../../reference/expressions/method-call-expr.html
	/// [type coercions]: ../../reference/type-coercions.html
	/// [box]: ../../alloc/boxed/struct.Box.html
	/// [string]: ../../alloc/string/struct.String.html
	/// [rc]: ../../alloc/rc/struct.Rc.html
	/// [cow]: ../../alloc/borrow/enum.Cow.html
	///
	/// # Examples
	///
	/// A struct with a single field which is modifiable by dereferencing the
	/// struct.
	///
	/// ```
	/// use std::ops::{Deref, DerefMut};
	///
	/// struct DerefMutExample<T> {
	/// value: T
	/// }
	///
	/// impl<T> Deref for DerefMutExample<T> {
	/// type Target = T;
	///
	/// fn deref(&self) -> &Self::Target {
	/// &self.value
	/// }
	/// }
	///
	/// impl<T> DerefMut for DerefMutExample<T> {
	/// fn deref_mut(&mut self) -> &mut Self::Target {
	/// &mut self.value
	/// }
	/// }
	///
	/// let mut x = DerefMutExample { value: 'a' };
	/// *x = 'b';
	/// assert_eq!('b', x.value);
	/// ```
	#[lang = "deref_mut"]
	#[doc(alias = "*")]
	#[stable(feature = "rust1", since = "1.0.0")]
	pub trait DerefMut: Deref {
	/// Mutably dereferences the value.
	#[stable(feature = "rust1", since = "1.0.0")]
	#[rustc_diagnostic_item = "deref_mut_method"]
	fn deref_mut(&mut self) -> &mut Self::Target;
	}

	#[stable(feature = "rust1", since = "1.0.0")]
	impl<T: ?Sized> DerefMut for &mut T {
	fn deref_mut(&mut self) -> &mut T {
	*self
	}
	}

	/// Indicates that a struct can be used as a method receiver, without the
	/// `arbitrary_self_types` feature. This is implemented by stdlib pointer types like `Box<T>`,
	/// `Rc<T>`, `&T`, and `Pin<P>`.
	#[lang = "receiver"]
	#[unstable(feature = "receiver_trait", issue = "none")]
	#[doc(hidden)]
	pub trait Receiver {
	// Empty.
	}

	#[unstable(feature = "receiver_trait", issue = "none")]
	impl<T: ?Sized> Receiver for &T {}

	#[unstable(feature = "receiver_trait", issue = "none")]
	impl<T: ?Sized> Receiver for &mut T {}