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Because `PassMode::Cast` is by far the largest variant, but is relatively rare. This requires making `PassMode` not impl `Copy`, and `Clone` is no longer necessary. This causes lots of sigil adjusting, but nothing very notable.
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.