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Introduce debuginfo to statements in MIR The PR introduces support for debug information within dead statements. Currently, only the reference statement is supported, which is sufficient to fix rust-lang/rust#128081. I don't modify Stable MIR, as I don't think we need debug information when using it. This PR represents the debug information for the dead reference statement via `#dbg_value`. For example, `let _foo_b = &foo.b` becomes `#dbg_value(ptr %foo, !22, !DIExpression(DW_OP_plus_uconst, 4, DW_OP_stack_value), !26)`. You can see this here: https://rust.godbolt.org/z/d43js6adv. The general principle for handling debug information is to never provide less debug information than the optimized LLVM IR. The current rules for dropping debug information in this PR are: - If the LLVM IR cannot represent a reference address, it's replaced with poison or simply dropped. For example, see: https://rust.godbolt.org/z/shGqPec8W. I'm using poison in all such cases now. - All debuginfos is dropped when merging multiple successor BBs. An example is available here: https://rust.godbolt.org/z/TE1q3Wq6M. I doesn't drop debuginfos in `MatchBranchSimplification`, because LLVM also pick one branch for it.
Please read the rustc-dev-guide chapter on Backend Agnostic Codegen.