97f8f4a3da
fix: infer for-loop item type with `IntoIterator` and `Iterator` Part of #13299 We've been inferring the type of the yielded values in for-loop as `<T as IntoIterator>::Item`. We infer the correct type most of the time when we normalize the projection type, but it turns out not always. We should infer the type as `<<T as IntoIterator>::IntoIter as Iterator>::Item`. When one specifies `IntoIter` assoc type of `IntoIterator` but not `Item` in generic bounds, we fail to normalize `<T as IntoIterator>::Item` (even though `IntoIter` is defined like so: `type IntoIter: Iterator<Item = Self::Item>` - rustc does *not* normalize projections based on other projection's bound I believe; see [this playground](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=e88e19385094cb98fadbf647b4c2082e)). Note that this doesn't fully fix # 13299 - given the following code, chalk can normalize `<I as IntoIterator>::IntoIter` to `S`, but cannot normalize `<S as Iterator>::Item` to `i32`. ```rust struct S; impl Iterator for S { type Item = i32; /* ... */ } fn f<I: IntoIterator<IntoIter = S>>(it: I) { for elem in it {} //^^^^{unknown} } ``` This is because chalk finds multiple answers that satisfy the query `AliasEq(<S as Iterator>::Item = ?X`: `?X = i32` and `?X = <I as IntoIterator>::Item` - which are supposed to be the same type due to the aforementioned bound on `IntoIter` but chalk is unable to figure it out.
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