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Box::into_unique: do the reborrow-to-raw *after* destroying the Box #61086

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May 24, 2019
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Box::into_unique: do the reborrow-to-raw *after* destroying the Box #61086

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Box::into_unique: do the reborrow-to-raw *after* destroying the Box
RalfJung May 23, 2019
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RalfJung May 23, 2019
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11 changes: 6 additions & 5 deletions src/liballoc/boxed.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -253,15 +253,16 @@ impl<T: ?Sized> Box<T> {
#[unstable(feature = "ptr_internals", issue = "0", reason = "use into_raw_non_null instead")]
#[inline]
#[doc(hidden)]
pub fn into_unique(mut b: Box<T>) -> Unique<T> {
pub fn into_unique(b: Box<T>) -> Unique<T> {
let mut unique = b.0;
mem::forget(b);
// Box is kind-of a library type, but recognized as a "unique pointer" by
// Stacked Borrows. This function here corresponds to "reborrowing to
// a raw pointer", but there is no actual reborrow here -- so
// without some care, the pointer we are returning here still carries
// the `Uniq` tag. We round-trip through a mutable reference to avoid that.
let unique = unsafe { b.0.as_mut() as *mut T };
mem::forget(b);
unsafe { Unique::new_unchecked(unique) }
// the tag of `b`, with `Unique` permission.
// We round-trip through a mutable reference to avoid that.
unsafe { Unique::new_unchecked(unique.as_mut() as *mut T) }
}

/// Consumes and leaks the `Box`, returning a mutable reference,
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