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Add comments regarding superfluous !Sync impls

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bstrie 2021-07-27 18:50:34 -04:00
parent 495322d776
commit 86c0ef8adc
2 changed files with 11 additions and 0 deletions
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6
library/alloc/src/rc.rs
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@ -313,6 +313,12 @@ pub struct Rc<T: ?Sized> {
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> !marker::Send for Rc<T> {}
// Note that this negative impl isn't strictly necessary for correctness,
// as `Rc` transitively contains a `Cell`, which is itself `!Sync`.
// However, given how important `Rc`'s `!Sync`-ness is,
// having an explicit negative impl is nice for documentation purposes
// and results in nicer error messages.
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> !marker::Sync for Rc<T> {}

5
library/core/src/cell.rs
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@ -240,6 +240,11 @@ pub struct Cell<T: ?Sized> {
#[stable(feature = "rust1", since = "1.0.0")]
unsafe impl<T: ?Sized> Send for Cell<T> where T: Send {}
// Note that this negative impl isn't strictly necessary for correctness,
// as `Cell` wraps `UnsafeCell`, which is itself `!Sync`.
// However, given how important `Cell`'s `!Sync`-ness is,
// having an explicit negative impl is nice for documentation purposes
// and results in nicer error messages.
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> !Sync for Cell<T> {}