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coverage: Remove or migrate all unstable values of `-Cinstrument-coverage` (This PR was substantially overhauled from its original version, which migrated all of the existing unstable values intact.) This PR takes the three nightly-only values that are currently accepted by `-Cinstrument-coverage`, completely removes two of them (`except-unused-functions` and `except-unused-generics`), and migrates the third (`branch`) over to a newly-introduced unstable flag `-Zcoverage-options`. I have a few motivations for wanting to do this: - It's unclear whether anyone actually uses the `except-unused-*` values, so this serves as an opportunity to either remove them, or prompt existing users to object to their removal. - After #117199, the stable values of `-Cinstrument-coverage` treat it as a boolean-valued flag, so having nightly-only extra values feels out-of-place. - Nightly-only values also require extra ad-hoc code to make sure they aren't accidentally exposed to stable users. - The new system allows multiple different settings to be toggled independently, which isn't possible in the current single-value system. - The new system makes it easier to introduce new behaviour behind an unstable toggle, and then gather nightly-user feedback before possibly making it the default behaviour for all users. - The new system also gives us a convenient place to put relatively-narrow options that won't ever be the default, but that nightly users might still want access to. - It's likely that we will eventually want to give stable users more fine-grained control over coverage instrumentation. The new flag serves as a prototype of what that stable UI might eventually look like. The `branch` option is a placeholder that currently does nothing. It will be used by #122322 to opt into branch coverage instrumentation. --- I see `-Zcoverage-options` as something that will exist more-or-less indefinitely, though individual sub-options might come and go as appropriate. I think there will always be some demand for nightly-only toggles, so I don't see `-Zcoverage-options` itself ever being stable, though we might eventually stabilize something similar to it. |
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