Auto merge of #45298 - toidiu:ak-44493-infer-predicate, r=nikomatsakis

Ak 44493 infer predicate

 **WIP**  Implements #44493

Things to do:

- [x] add feature gate and appropriate tests (see [forge](https://forge.rust-lang.org/feature-guide.html) for some details)
- [x] add a unit testing system similar to `#[rustc_variance]`
  - [x] to see how, maybe `rg rustc_variance` and take some notes
- [ ] add more tests:
- [x] we need to decide how to handle `struct Foo<'a, T> { x: &'a T::Item }`
- [x] handle explicit predicates on types
- [ ] handle explicit predicates on `dyn Trait` (this could be put off to a follow-up PR)
- [ ] handle explicit predicates on projections (this could be put off to a follow-up PR)

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