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, 1 insertions, 1 deletions

diff --git a/tests/ui/modules/issue-107649.stderr b/tests/ui/modules/issue-107649.stderr
index 0d203c1aacb..d5405c6576a 100644
--- a/tests/ui/modules/issue-107649.stderr
+++ b/tests/ui/modules/issue-107649.stderr
@@ -4,7 +4,7 @@ error[E0277]: `Dummy` doesn't implement `Debug`
 105 |     dbg!(lib::Dummy);
     |     ^^^^^^^^^^^^^^^^ `Dummy` cannot be formatted using `{:?}`
     |
-    = help: the trait `Debug` is not implemented for `Dummy`
+    = help: the trait `Debug` is not implemented for `Dummy`, which is required by `&Dummy: Debug`
     = note: add `#[derive(Debug)]` to `Dummy` or manually `impl Debug for Dummy`
     = note: this error originates in the macro `$crate::format_args_nl` which comes from the expansion of the macro `dbg` (in Nightly builds, run with -Z macro-backtrace for more info)
 help: consider annotating `Dummy` with `#[derive(Debug)]`