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transmute
calculates incorrect size of some generic types involving pointers to T: ?Sized
.
#101084
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Category: This is a bug.
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zachs18
changed the title
Aug 27, 2022
transmute
does not correctly calculate size of some types.transmute
does not correctly calculate size of some generic types involving pointers to T: ?Sized
.
zachs18
changed the title
Aug 28, 2022
transmute
does not correctly calculate size of some generic types involving pointers to T: ?Sized
.transmute
calculates incorrect size of some generic types involving pointers to T: ?Sized
.
ChrisDenton
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Area: Lints (warnings about flaws in source code) such as unused_mut.
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Jan 26, 2024
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…li-obk transmute size check: properly account for alignment Fixes another place where ZST alignment was ignored when checking whether something is a newtype. I wonder how many more of these there are... Fixes rust-lang#101084
matthiaskrgr
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…li-obk transmute size check: properly account for alignment Fixes another place where ZST alignment was ignored when checking whether something is a newtype. I wonder how many more of these there are... Fixes rust-lang#101084
tiif
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transmute size check: properly account for alignment Fixes another place where ZST alignment was ignored when checking whether something is a newtype. I wonder how many more of these there are... Fixes rust-lang/rust#101084
flip1995
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transmute size check: properly account for alignment Fixes another place where ZST alignment was ignored when checking whether something is a newtype. I wonder how many more of these there are... Fixes rust-lang/rust#101084
lnicola
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transmute size check: properly account for alignment Fixes another place where ZST alignment was ignored when checking whether something is a newtype. I wonder how many more of these there are... Fixes rust-lang/rust#101084
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Labels
C-bug
Category: This is a bug.
T-compiler
Relevant to the compiler team, which will review and decide on the PR/issue.
T-opsem
Relevant to the opsem team
I tried this code: playground link
I expected to see this happen: Compiler error because
*mut T
andPtrAndOverAlignZST<T>
are not (known to be) the same size.Instead, this happened: The above code compiles (and runs), but ICEs miri.
Meta
rustc --version --verbose
:rustc +nightly --version --verbose
:Backtrace
No backtrace for `rustc`, since no error occurs.
AFICT, rustc currently only supports transmuting a subset of Sized types that it conservatively can guarantee are the same size. For generic struct types with ?Sized parameters, I think rustc can only guarantee the size if the whole struct's size does not depend on the parameters, or if the struct's only non-zst field is a (transitive transparent wrapper of a) pointer type depending on a generic parameter (
rustc_middle::ty::layout::SizeSkeleton
only hasKnown(usize)
andPointer { .. }
variants, so can only express completely known and only-a-pointer sizes). I think this size-calculation ignoring all ZSTs (not just 1-ZSTs) when there is a pointer field can cause transmute to (incorrectly?) allow transmuting between differently-sized types.rustc_middle::ty::layout::SizeSkeleton
probably needs to be made aware of alignment, and then needs to changecompute
to account for alignment when checking fields.See also: #101081
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