compiler-builtins: remap RUSTC_DEBUGINFO_MAP paths via -ffile-prefix-map

[paradoxicalguy]

libcompiler_builtins.rlib leaks the local build path into DW_AT_comp_dir for C-compiled units (compiler-rt), even with rust-lang/rust's rust.remap-debuginfo = true.
existing -ffile-prefix-map={root}=. only covers the compiler-rt source root, not comp_dir.

fix: iterate RUSTC_DEBUGINFO_MAP and apply each mapping via -ffile-prefix-map, mirroring rustc_llvm's build.rs.

before: DW_AT_comp_dir ("/rustc/08f62523b1c3262cc2f0288a3369b760ab4bda45/library/compiler-builtins/compiler-builtins")
after: DW_AT_comp_dir ("/rustc/08f62523b1c3262cc2f0288a3369b760ab4bda45")

see discussion

r? @Urgau
cc @tgross35

[rustbot]

Failed to set assignee to Urgau: invalid assignee

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[rustbot]

Thanks for the pull request, and welcome! The Rust Project is excited to review your changes, and you should hear from @Urgau (or someone else) some time within the next two weeks.

[Mark-Simulacrum]

I am not a fan of this living outside of rust-lang/rust. That usually leads to confusion and pain when we want to change the interface.

Can you say more about what information is missing in Cargo to allow reproducible artifacts from compiler-builtins? If root.display() above is not enough, what would be enough?

Somewhat relatedly, if rust-lang/rust's bootstrap feels this should be threaded down, why can't it use CFLAGS / CXXFLAGS to do so, rather than a custom env variable?

View changes since the review

[paradoxicalguy]

root.display() only covers the compiler-rt source root, it doesn't cover this crate's own directory (CARGO_MANIFEST_DIR), which is what DW_AT_comp_dir was leaking.

on CFLAGS/CXXFLAGS, i don't know why bootstrap uses a custom env var instead of that. i'll check with @Urgau , who was explaining the RUSTFLAGS-forwarding issue (rust-lang/cargo#4423) that led to RUSTC_DEBUGINFO_MAP existing in the first place. not sure if the same limitation applies to CFLAGS/CXXFLAGS.

happy to move this into rust-lang/rust directly if that's preferred. wasn't sure which repo was the right home for it given compiler-builtins is developed separately.

[tgross35]

Sounds like this may be something to update elsewhere, rust-lang/rust#156570 (comment)

[paradoxicalguy]

closing this
fixed via bootstrap