8b6b15b877
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.
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.