I did something like this in https://chromium-review.googlesource.com/c/520384/ which may or may not pass CQ. I using git diff --name-only and used that as the input to run-webkit-tests, but that's actually not quite right. If you pass in a full path to run-webkit-tests it'll run that test even if it wouldn't if you passed in its directory path. If that could be fixed, it'd probably be fairly straightforward.

In https://chromium-review.googlesource.com/c/520384/ there's unsurprisingly a way just using the modified files can easily go wrong, interfaces/fullscreen.idl was updated and therefore fullscreen/interfaces.html would fail.

If doing this, we'd need to experiment with different heuristics to get average import delay down by enough to justify the complexity. One way could be to also grep for the filenames of any modified files, and run any tests with matches as well.

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robertma@, what do you think, would this be sound, to often save a cycle of bot? It ought to roughly halve the fastest import time, but do you think it's worth the complexity? If not, please WontFix.

Marking this as wontfix because (IMHO) the complexity significantly outweighs the potential benefits:

1. The cron bot (wpt-importer) isn't a builder so it doesn't compile code at all. It'd take some effort to add a compilation step to the bot, which in turn will introduce >10min of delay (compiling asynchronously is possible but much more complicated).
2. Nowadays all bots run layout tests on Swarming. It's easy to increase the number of shards (and we've already done this a few times) when running layout tests on some bot becomes a bottleneck.