Update RUSTFLAGS warning to suggest CARGO_BUILD_RUSTFLAGS#2252
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Modify the warning message when RUSTFLAGS is set to recommend using CARGO_BUILD_RUSTFLAGS instead, which the CLI can merge with remapping.
dmkozh
approved these changes
Oct 13, 2025
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What
Update the warning message when RUSTFLAGS is set to suggest using CARGO_BUILD_RUSTFLAGS instead, which the CLI can merge with remapping.
Why
When users set RUSTFLAGS, dependency path remapping is disabled because the CLI cannot merge remapping with RUSTFLAGS. However, CARGO_BUILD_RUSTFLAGS supports merging, so the warning should guide users to use that instead to maintain reproducible builds.
Came across this myself where I wanted to set RUSTFLAGS but the warning warned me not to. I just happened to remember that we added support for merging with the CARGO_BUILD_RUSTFLAGS, but that's some serioues tribal knowledge. The CLI should help someone move forward with using custom flags.
Known limitations
N/A