fcps.md

FCPs

Here we track all the FCPs of the team. These reflect consensus decisions and are thus a useful starting point for figuring out what is (and is not) guaranteed. Note however that only the official documentation (the Reference and standard library docs) are actually normative stable guarantees.

• Stack-allocated 8-aligned objects are insufficiently aligned by MSCV on x86-32. That is indeed UB when such pointers flow to Rust.
• The validity invariant of integers, floats, bool, thin raw pointers, and char is as documented here.
• The validity invariant of function pointers is as documented here.
• Typed copies reset all padding bytes to uninitialized memory.
• Layout cannot change at runtime.
• Unsafe code gets to assume that size == stride in arrays.
• The * projection (value-to-place conversion) itself is never UB; instead, inbounds/alignment checks get deferred to later during place evaluation.
• _ patterns in match do not read from the place, they only require place construction itself to succeed without UB.
• Atomic loads can work on read-only memory, but only under some conditions.
• Zero-sized memory accesses and offsets are NOPs
• The validity invariant of slice wide pointer metadata are defined here.
• Int-to-pointer transmutation is questionable.
• Redundant StorageDead/StorageLive are legal.
• size_of_val is safe to call on slice-tailed unsized types with a dynamic slice length of 0.
• Validity of references does not depend on the contents of memory
• Unions are always valid
• References to uninhabited types are uninhabited

Also see this automatically maintained list, which however might not be fully accurate.