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safe transmute: require that src referent is smaller than dst

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The source referent absolutely must be smaller than the destination
referent of a ref-to-ref transmute; the excess bytes referenced
cannot arise from thin air, even if those bytes are uninitialized.
This commit is contained in:
Jack Wrenn 2024-03-13 00:11:36 +00:00
parent a165f1f650
commit 216df4a8e6
8 changed files with 122 additions and 5 deletions
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5
compiler/rustc_transmute/src/maybe_transmutable/mod.rs
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@ -266,6 +266,11 @@ where
src_min_align: src_ref.min_align(),
dst_min_align: dst_ref.min_align(),
})
} else if dst_ref.size() > src_ref.size() {
Answer::No(Reason::DstRefIsTooBig {
src: src_ref,
dst: dst_ref,
})
} else {
// ...such that `src` is transmutable into `dst`, if
// `src_ref` is transmutability into `dst_ref`.