Struct wasmtime_environ::__core::mem::ManuallyDrop
1.20.0 · source · pub struct ManuallyDrop<T>where
T: ?Sized,{ /* private fields */ }
Expand description
A wrapper to inhibit the compiler from automatically calling T
’s destructor.
This wrapper is 0-cost.
ManuallyDrop<T>
is guaranteed to have the same layout and bit validity as
T
, and is subject to the same layout optimizations as T
. As a consequence,
it has no effect on the assumptions that the compiler makes about its
contents. For example, initializing a ManuallyDrop<&mut T>
with mem::zeroed
is undefined behavior. If you need to handle uninitialized data, use
MaybeUninit<T>
instead.
Note that accessing the value inside a ManuallyDrop<T>
is safe.
This means that a ManuallyDrop<T>
whose content has been dropped must not
be exposed through a public safe API.
Correspondingly, ManuallyDrop::drop
is unsafe.
§ManuallyDrop
and drop order.
Rust has a well-defined drop order of values. To make sure that fields or locals are dropped in a specific order, reorder the declarations such that the implicit drop order is the correct one.
It is possible to use ManuallyDrop
to control the drop order, but this
requires unsafe code and is hard to do correctly in the presence of
unwinding.
For example, if you want to make sure that a specific field is dropped after the others, make it the last field of a struct:
struct Context;
struct Widget {
children: Vec<Widget>,
// `context` will be dropped after `children`.
// Rust guarantees that fields are dropped in the order of declaration.
context: Context,
}
Implementations§
source§impl<T> ManuallyDrop<T>
impl<T> ManuallyDrop<T>
1.20.0 (const: 1.32.0) · sourcepub const fn new(value: T) -> ManuallyDrop<T>
pub const fn new(value: T) -> ManuallyDrop<T>
Wrap a value to be manually dropped.
§Examples
use std::mem::ManuallyDrop;
let mut x = ManuallyDrop::new(String::from("Hello World!"));
x.truncate(5); // You can still safely operate on the value
assert_eq!(*x, "Hello");
// But `Drop` will not be run here
1.20.0 (const: 1.32.0) · sourcepub const fn into_inner(slot: ManuallyDrop<T>) -> T
pub const fn into_inner(slot: ManuallyDrop<T>) -> T
Extracts the value from the ManuallyDrop
container.
This allows the value to be dropped again.
§Examples
use std::mem::ManuallyDrop;
let x = ManuallyDrop::new(Box::new(()));
let _: Box<()> = ManuallyDrop::into_inner(x); // This drops the `Box`.
1.42.0 · sourcepub unsafe fn take(slot: &mut ManuallyDrop<T>) -> T
pub unsafe fn take(slot: &mut ManuallyDrop<T>) -> T
Takes the value from the ManuallyDrop<T>
container out.
This method is primarily intended for moving out values in drop.
Instead of using ManuallyDrop::drop
to manually drop the value,
you can use this method to take the value and use it however desired.
Whenever possible, it is preferable to use into_inner
instead, which prevents duplicating the content of the ManuallyDrop<T>
.
§Safety
This function semantically moves out the contained value without preventing further usage,
leaving the state of this container unchanged.
It is your responsibility to ensure that this ManuallyDrop
is not used again.
source§impl<T> ManuallyDrop<T>where
T: ?Sized,
impl<T> ManuallyDrop<T>where
T: ?Sized,
1.20.0 · sourcepub unsafe fn drop(slot: &mut ManuallyDrop<T>)
pub unsafe fn drop(slot: &mut ManuallyDrop<T>)
Manually drops the contained value.
This is exactly equivalent to calling ptr::drop_in_place
with a
pointer to the contained value. As such, unless the contained value is a
packed struct, the destructor will be called in-place without moving the
value, and thus can be used to safely drop pinned data.
If you have ownership of the value, you can use ManuallyDrop::into_inner
instead.
§Safety
This function runs the destructor of the contained value. Other than changes made by
the destructor itself, the memory is left unchanged, and so as far as the compiler is
concerned still holds a bit-pattern which is valid for the type T
.
However, this “zombie” value should not be exposed to safe code, and this function
should not be called more than once. To use a value after it’s been dropped, or drop
a value multiple times, can cause Undefined Behavior (depending on what drop
does).
This is normally prevented by the type system, but users of ManuallyDrop
must
uphold those guarantees without assistance from the compiler.
Trait Implementations§
source§impl<T> AsBytes for ManuallyDrop<T>
impl<T> AsBytes for ManuallyDrop<T>
1.20.0 · source§impl<T> Clone for ManuallyDrop<T>
impl<T> Clone for ManuallyDrop<T>
source§fn clone(&self) -> ManuallyDrop<T>
fn clone(&self) -> ManuallyDrop<T>
1.0.0 · source§fn clone_from(&mut self, source: &Self)
fn clone_from(&mut self, source: &Self)
source
. Read more1.20.0 · source§impl<T> Debug for ManuallyDrop<T>
impl<T> Debug for ManuallyDrop<T>
1.20.0 · source§impl<T> Default for ManuallyDrop<T>
impl<T> Default for ManuallyDrop<T>
source§fn default() -> ManuallyDrop<T>
fn default() -> ManuallyDrop<T>
1.20.0 · source§impl<T> Deref for ManuallyDrop<T>where
T: ?Sized,
impl<T> Deref for ManuallyDrop<T>where
T: ?Sized,
1.20.0 · source§impl<T> DerefMut for ManuallyDrop<T>where
T: ?Sized,
impl<T> DerefMut for ManuallyDrop<T>where
T: ?Sized,
source§impl<T> FromZeroes for ManuallyDrop<T>where
T: FromZeroes + ?Sized,
impl<T> FromZeroes for ManuallyDrop<T>where
T: FromZeroes + ?Sized,
1.20.0 · source§impl<T> Hash for ManuallyDrop<T>
impl<T> Hash for ManuallyDrop<T>
1.20.0 · source§impl<T> Ord for ManuallyDrop<T>
impl<T> Ord for ManuallyDrop<T>
1.20.0 · source§impl<T> PartialEq for ManuallyDrop<T>
impl<T> PartialEq for ManuallyDrop<T>
1.20.0 · source§impl<T> PartialOrd for ManuallyDrop<T>where
T: PartialOrd + ?Sized,
impl<T> PartialOrd for ManuallyDrop<T>where
T: PartialOrd + ?Sized,
impl<T> Copy for ManuallyDrop<T>
impl<T> DerefPure for ManuallyDrop<T>where
T: ?Sized,
impl<T> Eq for ManuallyDrop<T>
impl<T> FromBytes for ManuallyDrop<T>
impl<T> StructuralPartialEq for ManuallyDrop<T>where
T: ?Sized,
impl<T> Unaligned for ManuallyDrop<T>
Auto Trait Implementations§
impl<T> Freeze for ManuallyDrop<T>
impl<T> RefUnwindSafe for ManuallyDrop<T>where
T: RefUnwindSafe + ?Sized,
impl<T> Send for ManuallyDrop<T>
impl<T> Sync for ManuallyDrop<T>
impl<T> Unpin for ManuallyDrop<T>
impl<T> UnwindSafe for ManuallyDrop<T>where
T: UnwindSafe + ?Sized,
Blanket Implementations§
source§impl<T> BorrowMut<T> for Twhere
T: ?Sized,
impl<T> BorrowMut<T> for Twhere
T: ?Sized,
source§fn borrow_mut(&mut self) -> &mut T
fn borrow_mut(&mut self) -> &mut T
source§impl<T> CloneToUninit for Twhere
T: Clone,
impl<T> CloneToUninit for Twhere
T: Clone,
source§unsafe fn clone_to_uninit(&self, dst: *mut T)
unsafe fn clone_to_uninit(&self, dst: *mut T)
clone_to_uninit
)source§impl<Q, K> Comparable<K> for Q
impl<Q, K> Comparable<K> for Q
source§impl<Q, K> Equivalent<K> for Q
impl<Q, K> Equivalent<K> for Q
source§impl<Q, K> Equivalent<K> for Q
impl<Q, K> Equivalent<K> for Q
source§fn equivalent(&self, key: &K) -> bool
fn equivalent(&self, key: &K) -> bool
key
and return true
if they are equal.