Skip running clippy without default features

[jcgruenhage]

Problem

Running clippy with cargo hack --feature-powerset in CI isn't particularly fast. This PR follows-up on #8912 to improve the speed of our clippy runs.

Parallelism as suggested in #9901 was tested, but didn't show consistent enough improvements to be worth it. It actually increased the amount of work done, as there's less cache hits when clippy runs are spread out over multiple target directories. Additionally, parallelism makes it so caching needs to be thought about more actively and copying around target directories to enable parallelism eats the rest of the performance gains from parallel execution.

After some discussion, the decision was to instead cut down on the number of jobs that are running further. The easiest way to do this is to not run clippy without default features. The list of default features is empty for all crates, and I haven't found anything using cfg(feature = "default") either, so this is likely not going to change anything except speeding the runs up.

Summary of changes

Reduce the amount of feature combinations tried by cargo hack (as suggested in #8912 (review)) by never disabling default features.

Alternatives

• We can split things out into different jobs which reduces the time until everything is finished by running more things in parallel. This does however decreases the amount of cache hits and increases the amount of time spent on overhead tasks like repo cloning and restoring caches by doing those multiple times instead of once.
• We could replace cargo hack [...] clippy with cargo clippy [...]; cargo clippy --features testing. I'm not 100% sure how this compares to the change here in the PR, but it does seem to run a bit faster. That likely means it's doing less work, but without understanding what exactly we loose by that I'd rather not do that for now. I'd appreciate input on this though.

[github-actions]

7095 tests run: 6798 passed, 0 failed, 297 skipped (full report)

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Flaky tests (4)

Postgres 16

• test_pgdata_import_smoke[None-1024-RelBlockSize.MULTIPLE_RELATION_SEGMENTS]: release-arm64
• test_pageserver_chaos[None]: release-arm64
• test_physical_replication_config_mismatch_too_many_known_xids: release-arm64

Postgres 14

• test_lr_with_slow_safekeeper: release-arm64

Code coverage* (full report)

• functions: 31.2% (8395 of 26865 functions)
• lines: 48.0% (66648 of 138939 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
6a2af57 at 2024-12-19T11:50:22.625Z :recycle:}