incremental compilation.
This is symmetric to PR #67020, which handled the case where the LLVM module's
*imports* changed. This commit builds upon the infrastructure added there; the
export map is just the inverse of the import map, so we can build the export map
at the same time that we load the serialized import map.
Fix#69798
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.
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