First, add a unit test to src/test/compile-fail, in the same manner as the other tests in that directory e.g. E0001.rs.

Then the description will need to be updated from the following, though I'll defer to @jonathandturner as to what the exact wording should be:

error[E0513]: no type for local variable 322
   --> src/gelf.rs:103:41
    |
103 |             mem::transmute_copy::<&[u8; hdr_sz], GelfChunkHeader>(&b[..hdr_sz]);
    |                                         ^^^^^^