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coverage: Infer `instances_used` from `pgo_func_name_var_map` In obscure circumstances involving macro-expanded spans, we would sometimes emit a covfun record for a function with no physical coverage counters, and therefore no corresponding entry in the “PGO names” section of the binary. The absence of that name entry causes `llvm-cov` to fail with the cryptic error message: ```text malformed instrumentation profile data: function name is empty ``` We can eliminate this mismatch by removing `instances_used` entirely, and instead inferring its contents from the keys of `pgo_func_name_var_map`. This makes it impossible for a "used" function to lack a PGO name entry. --- This is an attempt to eliminate the cause of rust-lang/rust#141577 when re-landing changes like rust-lang/rust#144298 in the future. I haven't been able to reproduce the underlying issue in an in-tree test, because the only known repro involves a non-trivial derive proc-macro that relies on `syn` and `proc-macro2`. But I have manually verified in a separate branch that this change would have prevented the reoccurrence of https://github.com/rust-lang/rust/issues/141577#issuecomment-3120667286.
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.