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Coverage has two FFI functions for computing the hash of a byte string. One takes a ptr/len pair, and the other takes a NUL-terminated C string. But on closer inspection, the C string version is unnecessary. The calling-side code converts a Rust `&str` into a C string, and the C++ code then immediately turns it back into a ptr/len string before actually hashing it.
The codegen crate contains the code to convert from MIR into LLVM IR,
and then from LLVM IR into machine code. In general it contains code
that runs towards the end of the compilation process.
For more information about how codegen works, see the rustc dev guide.