I've been experimenting with a version of Miri that can execute foreign functions. It interprets the LLVM bytecode that is produced during a crate's build process. We're hoping our results can assist with the Krabcake project.

I ran your test cases in this tool, and it found an instance of undefined behavior related to transmute in the test tests::large.

error: Undefined Behavior: constructing invalid value: encountered an unaligned reference (required 8 byte alignment but found 1)
   --> src/e8.rs:93:43
    |
93  |             let bitslice_ref: &[u64; 4] = std::mem::transmute(&E8_BITSLICE_ROUNDCONSTANT[roundnumber+0]);
    |                                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value: encountered an unaligned reference (required 8 byte alignment but found 1)
    |

Though the lengths of each slice within E8_BITSLICE_ROUNDCONSTANT seem to be compatible for this transmutation, it's still considered undefined behavior, since the underlying reference must also be aligned to the size of the type. I think slice::align_to might be what you're looking for here.