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Sometimes, a backend may need to emit warnings, errors, or otherwise need to know the span of the current item in a basic block. So, add a set_span method to give the backend that information.
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.