[estebank]

Point at returned expression(s) when an equality projection fails on the return type. Point at each component of an method chain to indicate where expected assoc type diverged. Close the gap in output quality between the current and next solver.

error[E0271]: expected `Map<IterMut<'_, Vec<u8>>, fn(&mut Vec<u8>) {foo}>` to be an iterator that yields `i32`, but it yields `()`
  --> $DIR/issue-106991.rs:8:13
   |
LL | fn bar() -> impl Iterator<Item = i32> {
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^ expected `i32`, found `()`
...
LL |     x.iter_mut().map(foo)
   |     --------------------- `Map<std::slice::IterMut<'_, Vec<u8>>, for<'a> fn(&'a mut Vec<u8>) {foo}>` returned here
   |
note: the method call chain might not have had the expected associated types
  --> $DIR/issue-106991.rs:12:18
   |
LL |     let mut x: Vec<Vec<u8>> = vec![vec![0, 2, 1], vec![5, 4, 3]];
   |                               ---------------------------------- this expression has type `Vec<Vec<u8>>`
LL |     x.iter_mut().map(foo)
   |       ---------- ^^^^^^^^ `Iterator::Item` changed to `()` here
   |       |
   |       `Iterator::Item` is `&mut Vec<u8>` here

Fix #106991. Drive by fix of incorrectly pointing at argument when error meant to point at return type (because logic didn't account for methods with receivers).

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This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

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