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Pull requests should be bors’ed with the standard priority 99 (bors r+ p=99) if and only if they a) skip or fix a flaky test or otherwise make CI more stable, and b) the flakiness being fixed is somewhat likely to be encountered in CI or as part of a bors (staging) CI run.
The rationale for these criteria is that bors splits up batches by priority. Should you choose a higher priority than all other PR’s in the queue, then bors will put the PR in a batch by itself, eschewing anything else already in the queue. This is not desirable unless that PR fixes an important issue that could conceivably be encountered in a staging build. If not, then all the other (potentially more important) PR’s in the queue were “jumped” for no reason.
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