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cg_llvm: Use `LLVMDIBuilderCreateGlobalVariableExpression` - Part of rust-lang/rust#134001 - Follow-up to rust-lang/rust#146763 --- This PR dismantles the somewhat complicated `LLVMRustDIBuilderCreateStaticVariable` function, and replaces it with equivalent calls to `LLVMDIBuilderCreateGlobalVariableExpression` and `LLVMGlobalSetMetadata`. A key difference is that the new code does not replicate the attempted downcast of `InitVal`. As far as I can tell, those downcasts were actually dead, because `llvm::ConstantInt` and `llvm::ConstantFP` are not subclasses of `llvm::GlobalVariable`. I tried replacing those code paths with fatal errors, and was unable to induce failure in any of the relevant test suites I ran. I have also confirmed that if the calls to `create_static_variable` are commented out, debuginfo tests will fail, demonstrating some amount of relevant test coverage. The new `DIBuilder` methods have been added via an extension trait, not as inherent methods, to avoid impeding rust-lang/rust#142897.
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.