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Add comments regarding superfluous !Sync impls #87530

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Nov 9, 2021
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Add comments regarding superfluous !Sync impls #87530

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6 changes: 6 additions & 0 deletions library/alloc/src/rc.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -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> {}

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5 changes: 5 additions & 0 deletions library/core/src/cell.rs
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Expand Up @@ -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> {}

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