Deduplicate bounds on associated types when deriving

[ecstatic-morse]

polonius uses a trait with a few associated types to define which newtyped indexes represent CFG points, loans, etc. rustc makes no effort to deduplicate bounds when deriving common traits on structures whose fields mention associated types, resulting in very large, redundant where clauses like this one. I find this mildly annoying when reading documentation.

This PR adds deduplication of simple paths (no generic arguments or qualified self types) to the derive machinery. As a result, the derived impl I linked above has only a single bound for each associated type, as one might expect.

[rust-highfive]

r? @wesleywiser

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[ecstatic-morse]

I didn't see a test for derived impls on declarations referencing associated types. Am I missing something obvious? AFAIK, there's no way to test that deduplication is actually occurring in a UI test, but we should have something to ensure that I haven't broken anything.

[wesleywiser]

I didn't see a test for derived impls on declarations referencing associated types. Am I missing something obvious? AFAIK, there's no way to test that deduplication is actually occurring in a UI test, but we should have something to ensure that I haven't broken anything.

I found derive-assoc-type-not-impl.rs which seems slightly related. It would be good to have more comprehensive tests for derives with associated types.

Since you noticed the issue via rustdoc, perhaps a rustdoc test would be able to show the deduplication is occurring?

[bors]

☔ The latest upstream changes (presumably #91359) made this pull request unmergeable. Please resolve the merge conflicts.

[JohnCSimon]

Ping from Triage:
@ecstatic-morse this has sat idle for a month with an approval... can you please post the status of this PR? Thanks.

[Dylan-DPC]

Closing this as inactive