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compiler-builtins: remap RUSTC_DEBUGINFO_MAP paths via -ffile-prefix-map #1236

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compiler-builtins: remap RUSTC_DEBUGINFO_MAP paths via -ffile-prefix-map #1236

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17 changes: 16 additions & 1 deletion compiler-builtins/build.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -538,7 +538,22 @@ mod c {
// Support deterministic builds by remapping the __FILE__ prefix if the
// compiler supports it. This fixes the nondeterminism caused by the
// use of that macro in lib/builtins/int_util.h in compiler-rt.
build.flag_if_supported(&format!("-ffile-prefix-map={}=.", root.display()));

// `RUSTC_DEBUGINFO_MAP` is a bootstrap-specific env var (not a general
// Cargo/rustc mechanism) containing the `--remap-path-prefix` mappings
// bootstrap passes to rustc via its rustc shim. Bootstrap uses this
// instead of RUSTFLAGS because Cargo doesn't always forward RUSTFLAGS to
// build scripts and proc-macros (rust-lang/cargo#4423).
//
// Apply each mapping via -ffile-prefix-map, mirroring rustc_llvm's
// build.rs for the same env var.
if let Some(maps) = env::var_os("RUSTC_DEBUGINFO_MAP")

@Mark-Simulacrum Mark-Simulacrum Jul 19, 2026 •
edited by rustbot
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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?

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@paradoxicalguy paradoxicalguy Jul 19, 2026

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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.

&& let Some(maps_str) = maps.to_str()
{
for map in maps_str.split('\t') {
build.flag_if_supported(&format!("-ffile-prefix-map={map}"));
}
}

// Include out-of-line atomics for aarch64, which are all generated by supplying different
// sets of flags to the same source file.
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