Precice capturing does not normalize aliases/projections (e.g. Self::Assoc)

[QuineDot]

I tried this code:

pub trait Trait<'a, T> {
    type Item;
    fn method(&self) -> Option<impl Iterator<Item = Self::Item>>;
}

impl<'a, 's, T, U> Trait<'a, T> for &'s U
where
    U: std::ops::Deref<Target: Sized>,
{
    type Item = U::Target;
    #[allow(refining_impl_trait)]
    fn method(&self) -> Option<impl Iterator<Item = Self::Item> + use<T, U>> {
        None::<std::vec::IntoIter<U::Target>>
    }
}

I expected to see this happen (in increasing order of preference):

• Underline Self specifically as the origin of lifetime capturing (not the entire opaque)
• Suggest the following fix

  -    fn method(&self) -> Option<impl Iterator<Item = Self::Item> + use<T, U>> {
  +    fn method(&self) -> Option<impl Iterator<Item = U::Target> + use<T, U>> {

• Compile successfully

Further explanation: It's natural to copy the bounds from the trait definition. Moreover one may, say, add precise captures during the course of development by editing an opaque that is already present (which may use aliases such as Self). It's easy to miss that any use of Self can pull in other parameters. In the example it pulls in not only the lifetime in the implementing type, but also the one in the trait:

impl Iterator<Item = <&'s U as Trait<'a, T>>::Item>

It would be a breaking change to make Self::Item capture the lifetimes, so normalizing shouldn't be a future compatibility concern for the example at least.

If normalization/successful compilation isn't possible for whatever reason, this can be considered purely a diagnostic issue.

---

Instead, this happened:

error: `impl Trait` captures lifetime parameter, but it is not mentioned in `use<...>` precise captures list
  --> src/lib.rs:13:32
   |
6  | impl<'a, 's, T, U> Trait<'a, T> for &'s U
   |      -- this lifetime parameter is captured
...
13 |     fn method(&self) -> Option<impl Iterator<Item = Self::Item> + use<T, U>> {
   |                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ lifetime captured due to being mentioned in the bounds of the `impl Trait`

error: `impl Trait` captures lifetime parameter, but it is not mentioned in `use<...>` precise captures list
  --> src/lib.rs:13:32
   |
6  | impl<'a, 's, T, U> Trait<'a, T> for &'s U
   |          -- this lifetime parameter is captured
...
13 |     fn method(&self) -> Option<impl Iterator<Item = Self::Item> + use<T, U>> {
   |                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ lifetime captured due to being mentioned in the bounds of the `impl Trait`

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[compiler-errors]

Yeah this is purely a diagnostics issue. It's not possible to eagerly normalize in bound computation.