Commits on Jul 6, 2022

1. Revert rust-lang#94158, "Apply noundef metadata to loads of types tha… …

   …t do not permit raw init"
   
   In rust-lang#94158, we started emitting `noundef`, which means that functions
   returning uninitialized references emit IR with is both `ret void` and
   also `noundef`: https://godbolt.org/z/hbjsKKfc3
   More generally, this change makes `mem::uninitialized` dangerous in a
   way that it wasn't before. This `noundef` change was shipped in the
   past 2 stable releases, 1.61.0 and 1.62.0.
   
   This concerns me because in rust-lang#66151 we have thus far decided against
   issuing a panic for creating uninitialized references within arrays,
   on account of the breakage that shows up in crater. If this pattern
   is so widely-used that we're not willing to put in a runtime check
   for it, then it doesn't seem prudent to invite LLVM to exploit this
   UB.
   
   The pattern of creating uninit references in arrays shows up real code
   because the 0.11, 0.12, and 0.13 release series for `hyper` all use
   `mem::uninitialized` to construct arrays of `httparse::Header`, which
   contains a `&str` and `&[u8]`. There is no patch available within
   these release series, so a `cargo update` is not a viable way to
   escape the UB here. Also note that the 0.11 release series of `hyper`
   predates `MaybeUninit`, so any source-level patch for that will incur
   runtime overhead. Which would be unfortunate, but preferable to UB.
   
   I discussed this with @thomcc on the community Discord, and we think
   that it would be prudent to revert this introduction of `noundef` until
   we have a mitigation in place for the UB that this may unleash. We
   haven't been able to cook up any examples of surprising optimizations
   due to this pattern, but the whole point of `noundef` is to enable
   optimizations, and we know that there is code which uses it in a way
   which is definitely instant UB and which we have declined to inform users
   of.
   
   If possible, we would like to see `freeze` applied to the return value
   of `mem::uninitialized` as a mitigation for this problem. That may be
   able to keep old code functioning without introducing a performance hit.
   
   @rustbot labels add +T-compiler +I-compiler-nominated

   

   saethlin committed Jul 6, 2022
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