Deduplicate codegen backends in bootstrap config

[kn1g78]

Deduplicate rust.codegen-backends and target.<triple>.codegen-backends during bootstrap config parsing while preserving first occurrence order.

This avoids carrying redundant backend entries into later bootstrap planning.

test:

$env:CARGO_NET_OFFLINE='false'; $env:CARGO_TARGET_DIR='target-codex-test'; $cargo = Join-Path $env:USERPROFILE '.cargo\bin\cargo.exe'; & $cargo test --manifest-path src/bootstrap/Cargo.toml deduplicates_codegen_backends

result:

running 1 test
test core::config::tests::deduplicates_codegen_backends ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 255 filtered out; finished in 0.00s

[rustbot]

This PR modifies src/bootstrap/src/core/config.

If appropriate, please update CONFIG_CHANGE_HISTORY in src/bootstrap/src/utils/change_tracker.rs.

[rustbot]

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

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Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: bootstrap
• bootstrap expanded to 6 candidates
• Random selection from Mark-Simulacrum, clubby789, jieyouxu

[jieyouxu]

I don't understand what you mean by

This avoids carrying redundant backend entries into later bootstrap planning.

Can you elaborate? Also the testing section in PR description is surprising, why does that go through cargo test, and not... bootstrap?

@rustbot author

View changes since this review

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[kn1g78]

Thanks, that sentence was unclear. I meant that duplicate entries in codegen-backends can currently remain in the parsed config, so enabled_codegen_backends may return the same backend more than once. This patch normalizes the parsed list so later bootstrap code sees each backend once, while preserving first occurrence order.

You're right about the testing section. I reran it through bootstrap:

python x.py test bootstrap --test-args deduplicates_codegen_backends

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 255 filtered out
Build completed successfully in 0:06:30

@rustbot ready

[Kobzol]

We treat this array as a set anyway, except for caring which of the backends was specified as the first one. If anything, specifying the same backend multiple times should be an error, not a silent skip.

[jieyouxu]

Yeah, let's instead reject trying to specify the same cg backend multiple times instead

View changes since this review

[kn1g78]

i finished it, thank u @rustbot ready