Provide more context on derived obligation error primary label

Expand the primary span of E0277 when the immediate unmet bound is not what the user wrote:

```
error[E0277]: the trait bound `i32: Bar` is not satisfied
 --> f100.rs:6:6
  |
6 |     <i32 as Foo>::foo();
  |      ^^^ the trait `Bar` is not implemented for `i32`, which is required by `i32: Foo`
  |
help: this trait has no implementations, consider adding one
 --> f100.rs:2:1
  |
2 | trait Bar {}
  | ^^^^^^^^^
note: required for `i32` to implement `Foo`
 --> f100.rs:3:14
  |
3 | impl<T: Bar> Foo for T {}
  |         ---  ^^^     ^
  |         |
  |         unsatisfied trait bound introduced here
```

Fix #40120.

1 files changed, 2 insertions, 2 deletions

diff --git a/tests/ui/function-pointer/unsized-ret.stderr b/tests/ui/function-pointer/unsized-ret.stderr
index 66116273ff4..81d603f4b20 100644
--- a/tests/ui/function-pointer/unsized-ret.stderr
+++ b/tests/ui/function-pointer/unsized-ret.stderr
@@ -4,7 +4,7 @@ error[E0277]: the size for values of type `str` cannot be known at compilation t
 LL |     foo::<fn() -> str, _>(None, ());
    |           ^^^^^^^^^^^ doesn't have a size known at compile-time
    |
-   = help: within `fn() -> str`, the trait `Sized` is not implemented for `str`
+   = help: within `fn() -> str`, the trait `Sized` is not implemented for `str`, which is required by `fn() -> str: Fn<_>`
    = note: required because it appears within the type `fn() -> str`
 note: required by a bound in `foo`
   --> $DIR/unsized-ret.rs:5:11
@@ -18,7 +18,7 @@ error[E0277]: the size for values of type `(dyn std::fmt::Display + 'a)` cannot
 LL |     foo::<for<'a> fn(&'a ()) -> (dyn std::fmt::Display + 'a), _>(None, (&(),));
    |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
    |
-   = help: within `for<'a> fn(&'a ()) -> (dyn std::fmt::Display + 'a)`, the trait `for<'a> Sized` is not implemented for `(dyn std::fmt::Display + 'a)`
+   = help: within `for<'a> fn(&'a ()) -> (dyn std::fmt::Display + 'a)`, the trait `for<'a> Sized` is not implemented for `(dyn std::fmt::Display + 'a)`, which is required by `for<'a> fn(&'a ()) -> (dyn std::fmt::Display + 'a): Fn<_>`
    = note: required because it appears within the type `for<'a> fn(&'a ()) -> (dyn std::fmt::Display + 'a)`
 note: required by a bound in `foo`
   --> $DIR/unsized-ret.rs:5:11