Struct wasmtime_environ::__core::mem::ManuallyDrop
1.20.0 · source · [−]#[repr(transparent)]pub struct ManuallyDrop<T> where
T: ?Sized, { /* private fields */ }
Expand description
A wrapper to inhibit compiler from automatically calling T
’s destructor.
This wrapper is 0-cost.
ManuallyDrop<T>
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
sourceimpl<T> ManuallyDrop<T>
impl<T> ManuallyDrop<T>
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
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.
sourceimpl<T> ManuallyDrop<T> where
T: ?Sized,
impl<T> ManuallyDrop<T> where
T: ?Sized,
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
sourceimpl<T> Clone for ManuallyDrop<T> where
T: Clone + ?Sized,
impl<T> Clone for ManuallyDrop<T> where
T: Clone + ?Sized,
sourcefn clone(&self) -> ManuallyDrop<T>
fn clone(&self) -> ManuallyDrop<T>
Returns a copy of the value. Read more
1.0.0 · sourcefn clone_from(&mut self, source: &Self)
fn clone_from(&mut self, source: &Self)
Performs copy-assignment from source
. Read more
sourceimpl<T> Debug for ManuallyDrop<T> where
T: Debug + ?Sized,
impl<T> Debug for ManuallyDrop<T> where
T: Debug + ?Sized,
sourceimpl<T> Default for ManuallyDrop<T> where
T: Default + ?Sized,
impl<T> Default for ManuallyDrop<T> where
T: Default + ?Sized,
sourcefn default() -> ManuallyDrop<T>
fn default() -> ManuallyDrop<T>
Returns the “default value” for a type. Read more
const: unstable · sourceimpl<T> Deref for ManuallyDrop<T> where
T: ?Sized,
impl<T> Deref for ManuallyDrop<T> where
T: ?Sized,
const: unstable · sourceimpl<T> DerefMut for ManuallyDrop<T> where
T: ?Sized,
impl<T> DerefMut for ManuallyDrop<T> where
T: ?Sized,
sourceimpl<T> Hash for ManuallyDrop<T> where
T: Hash + ?Sized,
impl<T> Hash for ManuallyDrop<T> where
T: Hash + ?Sized,
sourceimpl<T> Ord for ManuallyDrop<T> where
T: Ord + ?Sized,
impl<T> Ord for ManuallyDrop<T> where
T: Ord + ?Sized,
sourceimpl<T> PartialEq<ManuallyDrop<T>> for ManuallyDrop<T> where
T: PartialEq<T> + ?Sized,
impl<T> PartialEq<ManuallyDrop<T>> for ManuallyDrop<T> where
T: PartialEq<T> + ?Sized,
sourcefn eq(&self, other: &ManuallyDrop<T>) -> bool
fn eq(&self, other: &ManuallyDrop<T>) -> bool
This method tests for self
and other
values to be equal, and is used
by ==
. Read more
sourcefn ne(&self, other: &ManuallyDrop<T>) -> bool
fn ne(&self, other: &ManuallyDrop<T>) -> bool
This method tests for !=
.
sourceimpl<T> PartialOrd<ManuallyDrop<T>> for ManuallyDrop<T> where
T: PartialOrd<T> + ?Sized,
impl<T> PartialOrd<ManuallyDrop<T>> for ManuallyDrop<T> where
T: PartialOrd<T> + ?Sized,
sourcefn partial_cmp(&self, other: &ManuallyDrop<T>) -> Option<Ordering>
fn partial_cmp(&self, other: &ManuallyDrop<T>) -> Option<Ordering>
This method returns an ordering between self
and other
values if one exists. Read more
1.0.0 · sourcefn lt(&self, other: &Rhs) -> bool
fn lt(&self, other: &Rhs) -> bool
This method tests less than (for self
and other
) and is used by the <
operator. Read more
1.0.0 · sourcefn le(&self, other: &Rhs) -> bool
fn le(&self, other: &Rhs) -> bool
This method tests less than or equal to (for self
and other
) and is used by the <=
operator. Read more
impl<T> Copy for ManuallyDrop<T> where
T: Copy + ?Sized,
impl<T> Eq for ManuallyDrop<T> where
T: Eq + ?Sized,
impl<T> StructuralEq for ManuallyDrop<T> where
T: ?Sized,
impl<T> StructuralPartialEq for ManuallyDrop<T> where
T: ?Sized,
Auto Trait Implementations
impl<T: ?Sized> RefUnwindSafe for ManuallyDrop<T> where
T: RefUnwindSafe,
impl<T: ?Sized> Send for ManuallyDrop<T> where
T: Send,
impl<T: ?Sized> Sync for ManuallyDrop<T> where
T: Sync,
impl<T: ?Sized> Unpin for ManuallyDrop<T> where
T: Unpin,
impl<T: ?Sized> UnwindSafe for ManuallyDrop<T> where
T: UnwindSafe,
Blanket Implementations
sourceimpl<T> BorrowMut<T> for T where
T: ?Sized,
impl<T> BorrowMut<T> for T where
T: ?Sized,
const: unstable · sourcefn borrow_mut(&mut self) -> &mut T
fn borrow_mut(&mut self) -> &mut T
Mutably borrows from an owned value. Read more
sourceimpl<Q, K> Equivalent<K> for Q where
Q: Eq + ?Sized,
K: Borrow<Q> + ?Sized,
impl<Q, K> Equivalent<K> for Q where
Q: Eq + ?Sized,
K: Borrow<Q> + ?Sized,
sourcefn equivalent(&self, key: &K) -> bool
fn equivalent(&self, key: &K) -> bool
Compare self to key
and return true
if they are equal.
sourceimpl<T> ToOwned for T where
T: Clone,
impl<T> ToOwned for T where
T: Clone,
type Owned = T
type Owned = T
The resulting type after obtaining ownership.
sourcefn clone_into(&self, target: &mut T)
fn clone_into(&self, target: &mut T)
toowned_clone_into
)Uses borrowed data to replace owned data, usually by cloning. Read more