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Auto merge of #78446 - RalfJung:box, r=Amanieu

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fix Box::into_unique

https://github.com/rust-lang/rust/pull/77187/ broke Stacked Borrows pointer tagging around `Box::into_unique` (this is caused by `Box` being a special case in the type system, which box-internal code needs to account for). This PR fixes that.

r? `@Amanieu` Cc `@TimDiekmann`

Fixes https://github.com/rust-lang/rust/issues/78419.
This commit is contained in:
bors 2020-10-29 12:08:16 +00:00
commit a53fb30e3b
1 changed files with 2 additions and 7 deletions
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9
library/alloc/src/boxed.rs
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@ -724,15 +724,10 @@ impl<T: ?Sized, A: AllocRef> Box<T, A> {
// Box is recognized as a "unique pointer" by Stacked Borrows, but internally it is a // Box is recognized as a "unique pointer" by Stacked Borrows, but internally it is a
// raw pointer for the type system. Turning it directly into a raw pointer would not be // raw pointer for the type system. Turning it directly into a raw pointer would not be
// recognized as "releasing" the unique pointer to permit aliased raw accesses, // recognized as "releasing" the unique pointer to permit aliased raw accesses,
// so all raw pointer methods have to leak the box. Turning *that* to a raw pointer // so all raw pointer methods have to go through `Box::leak`. Turning *that* to a raw pointer
// behaves correctly. // behaves correctly.
let b = mem::ManuallyDrop::new(b);
// The box is unitiliazed later when moving out the allocator. The pointer is stored
// beforehand.
let ptr = b.0;
let alloc = unsafe { ptr::read(&b.1) }; let alloc = unsafe { ptr::read(&b.1) };
(ptr, alloc) (Unique::from(Box::leak(b)), alloc)
} }
/// Returns a reference to the underlying allocator. /// Returns a reference to the underlying allocator.