Backport "Make CheckUnused not slow." to LTS #21101
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It is used for every single tree in `CheckUnused`, so this is worth it. [Cherry-picked 69664f7]
It is not efficient when the results are always used exactly once. [Cherry-picked 0bd4b15]
[Cherry-picked 3188177]
`Tree`s have structural equality. Even if `==` should be able to exit quickly either because of `eq` or an early difference, sets systematically call `hashCode`, which is going to recurse into the entire structure. [Cherry-picked a55ee4d]
It is pointless to sort a list before converting it into a Set. [Cherry-picked 701d69f]
[Cherry-picked 803dff7]
[Cherry-picked 0c1f090]
Instead of dealing with entire `tpd.Import`s at the end of the scope, we eagerly flatten them into individual `ImportSelector`s. We store them along with some data, including a mutable flag for whether a selector has been used. This allows to dramatically simplify `isInImport`, as well as more aggressively cache the resolution of selectors. We also get rid of the `IdentityHashMap`. The algorithm is still O(n*m) where n is the number of imports in a scope, and m the number of references found in that scope. It is not entirely clear to me whether the previous logic was already O(n*m) or worse (it may have included an additional p factor for the number of possible selections from a given qualifier). Regardless, it is already quite a bit faster than before, thanks to smaller constant factors. [Cherry-picked 975df4a]
That test does not rely on any information dependent on the import selectors. It only relies on information at the use site. Eagerly checking it means we do not put as many symbols into the `usedInScope` set, which is good because it is one of the complexity factors of the unused-imports analysis. [Cherry-picked 8553bfc]
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No regressions detected in the community build up to lts-20279. |
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Backports #20321 to the LTS branch.
PR submitted by the release tooling.
[skip ci]