Improve the rustc-side clippy development experience

[Aaron1011]

When working on #75573, I ended up needing to modify Clippy (both a lint implementation and some UI tests). This process left a lot to be desired, and could be a significant roadblock to new contributors. I came up with some ideas for improving the process:

• Run Clippy tests on the PR builder (if we have the CI budget). My PR was initially approved, then unapproved due to a (correct) suspicion that Clippy UI tests would be broken. However, this could easily be missed on other PRs, leading to wasted merge queue time and failed rollups. By running these tests on the PR builder, we could catch Clippy changes prior to PR approval.
• Allow running Clippy tests in-tree from stage1. Currently, running ./x.py test src/tools/clippy requires a full compiler bootstrap (e.g. building a stage2 compiler). This takes a significant amount of time, and interacts badly with incremental compilation. For contributors with lower-end machines, this may make the contribution process incredibly frustrating. Attempting to build Clippy against a stage1 compiler (which requires running ./x.py test src/tools/clippy --stage 0 for some reason) works, but the tests fail with a confusing error about LLVM being missing.
• Integrate Clippy UI tests with ./x.py test --bless. Currently, blessing Clippy UI tests requires manually running scripts from inside src/tools/clippy. It would be nice if ./x.py test --bless worked on all UI tests, allowing Clippy to be largely treated as 'just another directory'.
• Document the interaction between -D warnings and Clippy lints. It seems as though denying a built-in lint with -D warnings prevents Clippy lints from being run. Since Clippy UI tests are run with -D warnings, adding a new builtin Rust lint can end up preventing Clippy UI tests from testing Clippy lints. Ideally, we would allow both sets of lints to run - if this is not possible, we should document the workaround (adding #![allow(problematic_rustc_lint)] to the affected Clippy UI tests).

[jyn514]

Note that at some point test --stage 0 src/tools/clippy broke: #78717

[RalfJung]

Last time I tried blessing clippy tests, that did not work, so I ended up manually adjusting stderr files. Maybe that's because I didn't try stage 2. Note that ./x.py test --bless (which is listed as 'working' in the list above) nowadays runs stage 1, so the OP text likely needs adjustments.

@rust-lang/clippy this issue has remained open for more than half a year. The clippy-subtree-merger put the burden of keeping clippy working onto all rustc contributors, so having sub-par tooling like this is now a much bigger problem than it was back when only clippy devs had to use that tooling.

[jyn514]

Last time I tried blessing clippy tests, that did not work, so I ended up manually adjusting stderr files. Maybe that's because I didn't try stage 2.

What was the error? That should work, if not it's a bug.

[flip1995]

./x.py test --bless src/tools/clippy works. ./x.py test --bless does not run the Clippy tests, so it also doesn't update them.

[RalfJung]

I think ./x.py test --bless src/tools/clippy is what I tried and got tons of strange linking errors. I figured this was expected so I didn't report a bug, sorry.

[RalfJung]

Ah, turns out what I actually tried is ./x.py test src/tools/clippy --bless --stage 0, since I did not want to wait for two full rustc builds. (Stage 1 clippy uses the stage 1 rustc libs, so those have to be built first.)

[flip1995]

Run Clippy tests on the PR builder (if we have the CI budget)

This is already done, if something in the src/tools/clippy is touched by the PR. IIRC the decision was made that running it on every PR is not necessary, since most PRs won't cause problems with it.

Allow running Clippy tests in-tree from stage1

That would be great. Unfortunately most Clippy folks don't really know how the bootstrapping works exactly and therefore fixing this would be really hard for us to do.

Document the interaction between -D warnings and Clippy lints.

Changes to rustc should not change Clippy tests, except for line number updates. The only exception I can think of is if a lint is being uplifted from Clippy to rustc. Fixing the issue where a new rust error/lint suppresses Clippy lints should be fixed by either adding #[allow(problematic_lint)] or by fixing the issue in the test, if it doesn't change the semantics of the test.

Ah, turns out what I actually tried is ./x.py test src/tools/clippy --bless --stage 0

Yes, Clippy on --stage 0 doesn't work (yet). I don't know how to fix that.

[jyn514]

That would be great. Unfortunately most Clippy folks don't really know how the bootstrapping works exactly and therefore fixing this would be really hard for us to do.

The error that @RalfJung linked is not a bootstrapping issue. Clippy's fork of compile test needs to be fixed not to reuse the same target directory for tests as for building clippy itself.