Trait DerefMut

pub trait DerefMut: Deref {
    // Required method
    fn deref_mut(&mut self) -> &mut Self::Target;
}

Available on crate feature mtls only.

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”. 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 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 behavior that name suggests. Often, the purpose of a “smart pointer” type is to change the ownership semantics of a contained value (for example, Rc or Cow) or the storage semantics of a contained value (for example, 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 as well as the reference sections on the dereference operator, 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.

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);

Required Methods

fn deref_mut(&mut self) -> &mut Self::Target

Mutably dereferences the value.

Implementors

impl DerefMut for InlinableString

impl DerefMut for Connection

impl DerefMut for NamedFile

impl DerefMut for String

impl DerefMut for OsString

impl DerefMut for PathBuf

impl DerefMut for BytesMut

impl DerefMut for InlineString

impl DerefMut for ClientConnection

impl DerefMut for ServerConnection

impl DerefMut for DocumentMut

impl<'a> DerefMut for IoSliceMut<'a>

impl<'a> DerefMut for MaybeUninitSlice<'a>

impl<'a, 'f> DerefMut for VaList<'a, 'f>

where 'f: 'a,

impl<'a, R, T> DerefMut for lock_api::mutex::MappedMutexGuard<'a, R, T>

where R: RawMutex + 'a, T: 'a + ?Sized,

impl<'a, R, T> DerefMut for lock_api::mutex::MutexGuard<'a, R, T>

where R: RawMutex + 'a, T: 'a + ?Sized,

impl<'a, R, T> DerefMut for lock_api::rwlock::MappedRwLockWriteGuard<'a, R, T>

where R: RawRwLock + 'a, T: 'a + ?Sized,

impl<'a, R, T> DerefMut for lock_api::rwlock::RwLockWriteGuard<'a, R, T>

where R: RawRwLock + 'a, T: 'a + ?Sized,

impl<'a, T> DerefMut for SpinMutexGuard<'a, T>

where T: ?Sized,

impl<'a, T> DerefMut for spin::mutex::MutexGuard<'a, T>

where T: ?Sized,

impl<'a, T> DerefMut for tokio::sync::mutex::MappedMutexGuard<'a, T>

where T: ?Sized,

impl<'c> DerefMut for rocket::local::asynchronous::LocalRequest<'c>

impl<'c> DerefMut for rocket::local::blocking::LocalRequest<'c>

impl<'rwlock, T, R> DerefMut for spin::rwlock::RwLockWriteGuard<'rwlock, T, R>

where T: ?Sized,

impl<'v> DerefMut for Errors<'v>

impl<A> DerefMut for SmallVec<A>

where A: Array,

impl<B, T> DerefMut for Ref<B, [T]>

where B: ByteSliceMut, T: FromBytes + AsBytes,

impl<B, T> DerefMut for Ref<B, T>

where B: ByteSliceMut, T: FromBytes + AsBytes,

impl<I> DerefMut for Pear<I>

where I: Input,

impl<L, R> DerefMut for Either<L, R>

where L: DerefMut, R: DerefMut<Target = <L as Deref>::Target>,

impl<Ptr> DerefMut for Pin<Ptr>

where Ptr: DerefMut, <Ptr as Deref>::Target: Unpin,

impl<T> !DerefMut for &T

where T: ?Sized,

impl<T> DerefMut for &mut T

where T: ?Sized,

impl<T> DerefMut for Capped<T>

impl<T> DerefMut for Form<T>

impl<T> DerefMut for Lenient<T>

impl<T> DerefMut for Strict<T>

impl<T> DerefMut for Json<T>

Available on crate feature json only.

impl<T> DerefMut for MsgPack<T>

Available on crate feature msgpack only.

impl<T> DerefMut for ThinBox<T>

where T: ?Sized,

impl<T> DerefMut for ManuallyDrop<T>

where T: ?Sized,

impl<T> DerefMut for RefMut<'_, T>

where T: ?Sized,

impl<T> DerefMut for AssertUnwindSafe<T>

impl<T> DerefMut for std::sync::mutex::MappedMutexGuard<'_, T>

where T: ?Sized,

impl<T> DerefMut for std::sync::mutex::MutexGuard<'_, T>

where T: ?Sized,

impl<T> DerefMut for std::sync::rwlock::MappedRwLockWriteGuard<'_, T>

where T: ?Sized,

impl<T> DerefMut for std::sync::rwlock::RwLockWriteGuard<'_, T>

where T: ?Sized,

impl<T> DerefMut for futures_util::lock::mutex::MutexGuard<'_, T>

where T: ?Sized,

impl<T> DerefMut for futures_util::lock::mutex::OwnedMutexGuard<T>

where T: ?Sized,

impl<T> DerefMut for ConnectionCommon<T>

impl<T> DerefMut for tokio::sync::mutex::MutexGuard<'_, T>

where T: ?Sized,

impl<T> DerefMut for tokio::sync::mutex::OwnedMutexGuard<T>

where T: ?Sized,

impl<T> DerefMut for OwnedRwLockWriteGuard<T>

where T: ?Sized,

impl<T> DerefMut for tokio::sync::rwlock::write_guard::RwLockWriteGuard<'_, T>

where T: ?Sized,

impl<T> DerefMut for RwLockMappedWriteGuard<'_, T>

where T: ?Sized,

impl<T> DerefMut for Unalign<T>

where T: Unaligned,

impl<T, A> DerefMut for Box<T, A>

where A: Allocator, T: ?Sized,

impl<T, A> DerefMut for PeekMut<'_, T, A>

where T: Ord, A: Allocator,

impl<T, A> DerefMut for Vec<T, A>

where A: Allocator,

impl<T, A> DerefMut for UniqueRc<T, A>

where A: Allocator, T: ?Sized,

impl<T, F> DerefMut for once_cell::sync::Lazy<T, F>

where F: FnOnce() -> T,

impl<T, F> DerefMut for once_cell::unsync::Lazy<T, F>

where F: FnOnce() -> T,

impl<T, F, S> DerefMut for ScopeGuard<T, F, S>

where F: FnOnce(T), S: Strategy,

impl<T, N> DerefMut for GenericArray<T, N>

where N: ArrayLength<T>,

impl<T, U> DerefMut for futures_util::lock::mutex::MappedMutexGuard<'_, T, U>

where T: ?Sized, U: ?Sized,

impl<T, U> DerefMut for OwnedMappedMutexGuard<T, U>

where T: ?Sized, U: ?Sized,

impl<T, U> DerefMut for OwnedRwLockMappedWriteGuard<T, U>

where T: ?Sized, U: ?Sized,