MIRI reports UB on <Vec<u8> as ByteVecExt>::try_extend_zeroed

[Joeoc2001]

Running the test suite in this repository through MIRI reports that <Vec<u8> as ByteVecExt>::try_extend_zeroed results in Undefined Behaviour.

I believe that this is because the end of as_mut_ptr_end is open and therefore is not pointing to anything borrowed. I'll open a pull request in a minute with a fix which both adds MIRI to CI and which passes all tests with MIRI.

[teoxoy]

Hmm, good catch!

So this is because as_mut_ptr_end is actually a method on slices and the Vec will Deref to a slice first.
The PR fixes it via Vec's own as_mut_ptr impl and adding the length.

I doubt this specific code is UB in practice though; backports and/or an immediate release might not be necessary.

[Joeoc2001]

To be honest, I just want MIRI to pass on my project, and this was a snag. Like you say, I doubt the compiler is about to abuse this UB in any weird ways.