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Split an item bounds and an item's super predicates This is the moral equivalent of #107614, but instead for predicates this applies to **item bounds**. This PR splits out the item bounds (i.e. *all* predicates that are assumed to hold for the alias) from the item *super predicates*, which are the subset of item bounds which share the same self type as the alias. ## Why? Much like #107614, there are places in the compiler where we *only* care about super-predicates, and considering predicates that possibly don't have anything to do with the alias is problematic. This includes things like closure signature inference (which is at its core searching for `Self: Fn(..)` style bounds), but also lints like `#[must_use]`, error reporting for aliases, computing type outlives predicates. Even in cases where considering all of the `item_bounds` doesn't lead to bugs, unnecessarily considering irrelevant bounds does lead to a regression (#121121) due to doing extra work in the solver. ## Example 1 - Trait Aliases This is best explored via an example: ``` type TAIT<T> = impl TraitAlias<T>; trait TraitAlias<T> = A + B where T: C; ``` The item bounds list for `Tait<T>` will include: * `Tait<T>: A` * `Tait<T>: B` * `T: C` While `item_super_predicates` query will include just the first two predicates. Side-note: You may wonder why `T: C` is included in the item bounds for `TAIT`? This is because when we elaborate `TraitAlias<T>`, we will also elaborate all the predicates on the trait. ## Example 2 - Associated Type Bounds ``` type TAIT<T> = impl Iterator<Item: A>; ``` The `item_bounds` list for `TAIT<T>` will include: * `Tait<T>: Iterator` * `<Tait<T> as Iterator>::Item: A` But the `item_super_predicates` will just include the first bound, since that's the only bound that is relevant to the *alias* itself. ## So what This leads to some diagnostics duplication just like #107614, but none of it will be user-facing. We only see it in the UI test suite because we explicitly disable diagnostic deduplication. Regarding naming, I went with `super_predicates` kind of arbitrarily; this can easily be changed, but I'd consider better names as long as we don't block this PR in perpetuity. |
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| README.md | ||
For high-level intro to how type checking works in rustc, see the type checking chapter of the rustc dev guide.