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Emit a warning around thin &CStr interior mutability breaks #118513
Description
From @chorman0773 https://rust-lang.zulipchat.com/#narrow/stream/219381-t-libs/topic/CStr.20as.20thin.20pointer/near/405432566
Passing an &UnsafeCell<CStr> to size_of_val is currently sound because it just returns the length parameter of the fat pointer. After making CStr thin however, size_of_val will need to call strlen on the data. This is not ok in a &UnsafeCell because another context could be writing the data, e.g. temporarily overwriting the \0.
This seems like something we may be able to emit a warning for?
Thin cstr: #59905
@rustbot label +T-libs +T-compiler +A-diagnostics
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tgross35 commentedon Dec 1, 2023
Quick search shows this is hopefully not very common https://github.com/search?q=language%3Arust+%2F%26.*UnsafeCell%3CCStr%3E%2F&type=code
chorman0773 commentedon Dec 1, 2023
Incidentally, I wonder if
&UnsafeCell<ThinCStr>itself is sound. I wonder if T-opsem should weigh in on that question. A write could happen and change the size of the memory the reference refers to, and the tag won't be correctly sized - maybe it will be shorter than the memory the reference refers to.&mut ThinCStrmight also have similar issues.jmillikin commentedon Dec 2, 2023
Related: my RFC 3536 for
!Sizedthin pointers had to introduce a new?Traitto prevent such types from being used withsize_of_val(). In general the C concept of DSTs (pointer + computed value size) seems to be incompatible with the Rust semantics ofT: ?Sizedandsize_of_val().