2026/08/12 -- hrsraiden -- bootstrap: warn when download-ci-llvm is unavailable with assertions

[HRSRaiden]

Fixes #160919.

When llvm.download-ci-llvm = true and llvm.assertions = true are set in config.toml, but no assertion-enabled alt CI LLVM binary exists for the host triple, bootstrap previously built LLVM from source silently without any notification.

This PR adds a WARNING and HELP message when download-ci-llvm = true is explicitly requested on an unsupported target.

Verification

• cargo test --manifest-path src/bootstrap/Cargo.toml download_ci_llvm_unsupported_assertions (1 passed, 0.00s)
• python x.py check (Passed)
• python x.py fmt --check (Passed)

r? @jieyouxu
@rustbot label +A-bootstrap

[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.

Please see the contribution instructions for more information. Namely, in order to ensure the minimum review times lag, PR authors and assigned reviewers should ensure that the review label (S-waiting-on-review and S-waiting-on-author) stays updated, invoking these commands when appropriate:

• @rustbot author: the review is finished, PR author should check the comments and take action accordingly
• @rustbot review: the author is ready for a review, this PR will be queued again in the reviewer's queue

[rustbot]

⚠️ Warning ⚠️

• There are issue links (such as #123) in the commit messages of the following commits.
  Please move them to the PR description, to avoid spamming the issues with references to the commit, and so this bot can automatically canonicalize them to avoid issues with subtree.
  • 3857f5c

[rustbot]

Error: Unknown labels: A-bootstrap

Please file an issue on GitHub at triagebot if there's a problem with this bot, or reach out on #triagebot on Zulip.

[jieyouxu]

Please consult the LLM usage policy. Was this change created with an LLM?

View changes since this review

[rustbot]

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

[HRSRaiden]

Please consult the LLM usage policy. Was this change created with an LLM?

View changes since this review

Yes, I used an LLM assistant to help explore the bootstrap codebase, trace the logic in parse_download_ci_llvm and suggest a fix.
Afterward, I made the changes and have confirmed the behaviour locally (python x.py check, cargo test, and python x.py fmt --check)
Since this is my first contribution, I used an LLM to simply the codebase for me and understand it. Let me know if I missed something or need any more change.

[jieyouxu]

Please consult the LLM usage policy. Was this change created with an LLM?
View changes since this review

Yes, I used an LLM assistant to help explore the bootstrap codebase, trace the logic in parse_download_ci_llvm and suggest a fix. Afterward, I made the changes and have confirmed the behaviour locally (python x.py check, cargo test, and python x.py fmt --check) Since this is my first contribution, I used an LLM to simply the codebase for me and understand it. Let me know if I missed something or need any more change.

Thank you for the interest in contributing. Please consult the usage policy, where current rules for this repos are that code changes created originally by LLMs generally need to be pre-arranged with a consenting reviewer, and you need to understand the rationale for why the change is correct. You can visit #llm-reviews for examples of such pre-arrangements.

LLMs typically give you these

### Verification
- `cargo test --manifest-path src/bootstrap/Cargo.toml download_ci_llvm_unsupported_assertions` (1 passed, 0.00s)
- `python x.py check` (Passed)
- `python x.py fmt --check` (Passed)

which immediately tells the reviewer (me) that the verification isn't verifying the actual change.

I'm going to close this PR as per the LLM usage policy and rust-lang/compiler-team#893.

[Kobzol]

This should be implemented in a different way, in a centralized place, once we actually have that centralized place for LLVM CI handling (#160916).