Sometimes clippy gets outdated by months, and its current support setup
means that each Servo component need to opt into it by depending on
the plugins crate manually, and not all components do that.
Revert several changes that broke tests
This is based on #15158 by @aneeshusa, with additional reverts.
This reverts #15064, which is causing many tests not to run, and #15129 and #15155 which landed while tests were not running and may have caused some new failures in iframe tests.
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Also remove the obsolete `--cpu` and `--gpu` renderer flags,
which also are no longer used.
Update tests and wptrunner to not pass these flags.
wptrunner changes have been upstreamed in:
- w3c/wptrunner#224
- w3c/wptrunner#226
Index the session past correctly when discarding.
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Oops, indexed from the wrong end when discarding documents in #14312.
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- [X] `./mach test-tidy` does not report any errors
- [X] These changes do not require tests because we're not testing document discarding
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