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Fix stacked borrows when dropping Error (#81)

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Tim Diekmann 2022-07-07 21:01:42 +02:00 committed by GitHub
parent 64f88d7279
commit dba523177e
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1 changed files with 10 additions and 2 deletions
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12
src/error.rs
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@ -518,7 +518,11 @@ struct ErrorVTable {
unsafe fn object_drop<E>(e: Box<ErrorImpl<()>>) { unsafe fn object_drop<E>(e: Box<ErrorImpl<()>>) {
// Cast back to ErrorImpl<E> so that the allocator receives the correct // Cast back to ErrorImpl<E> so that the allocator receives the correct
// Layout to deallocate the Box's memory. // Layout to deallocate the Box's memory.
let unerased = mem::transmute::<Box<ErrorImpl<()>>, Box<ErrorImpl<E>>>(e); // Note: This must not use `mem::transmute` because it tries to reborrow the `Unique`
// contained in `Box`, which must not be done. In practice this probably won't make any
// difference by now, but technically it's unsound.
// see: https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md
let unerased: Box<ErrorImpl<E>> = Box::from_raw(Box::into_raw(e).cast());
drop(unerased); drop(unerased);
} }
@ -528,7 +532,11 @@ unsafe fn object_drop_front<E>(e: Box<ErrorImpl<()>>, target: TypeId) {
// without dropping E itself. This is used by downcast after doing a // without dropping E itself. This is used by downcast after doing a
// ptr::read to take ownership of the E. // ptr::read to take ownership of the E.
let _ = target; let _ = target;
let unerased = mem::transmute::<Box<ErrorImpl<()>>, Box<ErrorImpl<ManuallyDrop<E>>>>(e); // Note: This must not use `mem::transmute` because it tries to reborrow the `Unique`
// contained in `Box`, which must not be done. In practice this probably won't make any
// difference by now, but technically it's unsound.
// see: https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.m
let unerased: Box<ErrorImpl<ManuallyDrop<E>>> = Box::from_raw(Box::into_raw(e).cast());
drop(unerased); drop(unerased);
} }