08c5ffd4a3
The default value will dependend on the LLVM version in the future, so don't specify one to start with.
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.