1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311
/// 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
}
}
/// Perma-unstable marker trait. Indicates that the type has a well-behaved [`Deref`]
/// (and, if applicable, [`DerefMut`]) implementation. This is relied on for soundness
/// of deref patterns.
///
/// FIXME(deref_patterns): The precise semantics are undecided; the rough idea is that
/// successive calls to `deref`/`deref_mut` without intermediate mutation should be
/// idempotent, in the sense that they return the same value as far as pattern-matching
/// is concerned. Calls to `deref`/`deref_mut` must leave the pointer itself likewise
/// unchanged.
#[unstable(feature = "deref_pure_trait", issue = "87121")]
#[lang = "deref_pure"]
pub unsafe trait DerefPure {}
#[unstable(feature = "deref_pure_trait", issue = "87121")]
unsafe impl<T: ?Sized> DerefPure for &T {}
#[unstable(feature = "deref_pure_trait", issue = "87121")]
unsafe impl<T: ?Sized> DerefPure for &mut T {}
/// 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 {}