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coverage: Store coverage source regions as `Span` until codegen Historically, coverage spans were converted into line/column coordinates during the MIR instrumentation pass. This PR moves that conversion step into codegen, so that coverage spans spend most of their time stored as `Span` instead. In addition to being conceptually nicer, this also reduces the size of coverage mappings in MIR, because `Span` is smaller than 4x u32. --- There should be no changes to coverage output.
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.