error[E0599]: my message
  --> $DIR/custom-on-unimplemented-diagnostic.rs:15:7
   |
LL | struct B;
   | -------- method `request` not found for this struct because it doesn't satisfy `B: ProviderExt` or `B: ProviderLt`
...
LL |     B.request();
   |       ^^^^^^^ my label
   |
note: trait bound `B: ProviderLt` was not satisfied
  --> $DIR/custom-on-unimplemented-diagnostic.rs:10:18
   |
LL | impl<T: ?Sized + ProviderLt> ProviderExt for T {}
   |                  ^^^^^^^^^^  -----------     -
   |                  |
   |                  unsatisfied trait bound introduced here
   = note: my note
note: the trait `ProviderLt` must be implemented
  --> $DIR/custom-on-unimplemented-diagnostic.rs:2:1
   |
LL | pub trait ProviderLt {}
   | ^^^^^^^^^^^^^^^^^^^^
   = help: items from traits can only be used if the trait is implemented and in scope
note: `ProviderExt` defines an item `request`, perhaps you need to implement it
  --> $DIR/custom-on-unimplemented-diagnostic.rs:4:1
   |
LL | pub trait ProviderExt {
   | ^^^^^^^^^^^^^^^^^^^^^

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0599`.