Point at definition of item with type parameters that couldn't be inferred

[estebank]

When calling a function with type parameters that are unconstrained by the call expression, we mention the type parameter that couldn't be inferred, but the user previously didn't see where the name was coming from. By pointing at the function itself, including the type parameter, we give better context on what they should have done, specially when the inference machinery fails to provide any other context.

error[E0282]: type annotations needed
  --> $DIR/unresolved_type_param.rs:9:5
   |
LL |     bar().await;
   |     ^^^ cannot infer type of the type parameter `T` declared on the function `bar`
   |
note: type must be known for this
  --> $DIR/unresolved_type_param.rs:6:1
   |
LL | async fn bar<T>() -> () {}
   | ^^^^^^^^^^^^^^^^^^^^^^^
help: consider specifying the generic argument
   |
LL |     bar::<T>().await;
   |        +++++

We should further refine when we present this info to make sure we don't provide redundant notes in some cases. Also, there are cases (like those in issue 94969) that don't provide any new information with this change.

[rustbot]

Some changes occurred in need_type_info.rs

cc @lcnr

[rustbot]

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