5b5964f569
Only crate root def-ids don't have a parent, and in majority of cases the argument of `DefIdTree::parent` cannot be a crate root. So we now panic by default in `parent` and introduce a new non-panicing function `opt_parent` for cases where the argument can be a crate root. Same applies to `local_parent`/`opt_local_parent`.
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.