Transforming &[u8; 16] to [u64; 2] using transmute_copy causes Bus errors on aarch64

[newpavlov]

See this issue: RustCrypto/stream-ciphers#5

The line in question is here. Does it contain UB or is it a miscompilation? If it's the former, then I think transmute_copy documentation should be updated accordingly.

[matthewjasper]

It's definitely UB somewhere: try running this with MIRI https://play.rust-lang.org/?version=stable&mode=debug&edition=2015&gist=794f6a68cf5009fe2c9c8c1277fd42f0.

[newpavlov]

Looks like replacing transmute_copy with read_unaligned solves the problem. But is it intended for transmute_copy to use aligned load? Currently it uses ptr::read, shouldn't it be replaced with ptr::read_unaligned?

[newpavlov]

cc rust-lang/rust-memory-model#14