#[rustc_on_unimplemented(message = "message", label = "label", note = "note")]
trait MyIterator<A> {
    fn next(&mut self) -> A;
}
```

Would generate the following output:

```text
error[E0277]: message
  --> <file>:10:19
   |
10 |     iterate_chars(&[1, 2, 3][..]);
   |     ------------- ^^^^^^^^^^^^^^ label
   |     |
   |     required by a bound introduced by this call
   |
   = help: the trait `MyIterator<char>` is not implemented for `&[{integer}]`
   = note: note
note: required by a bound in `iterate_chars`
```

The functionality discussed so far is also available with
[`#[diagnostic::on_unimplemented]`](https://doc.rust-lang.org/nightly/reference/attributes/diagnostics.html#the-diagnosticon_unimplemented-attribute).
If you can, you should use that instead.

### Filtering

To allow more targeted error messages, it is possible to filter the
application of these fields with `on`.

You can filter on the following boolean flags:
 - `crate_local`: whether the code causing the trait bound to not be
   fulfilled is part of the user's crate. This is used to avoid suggesting
   code changes that would require modifying a dependency.
 - `direct`: whether this is an user-specified rather than derived obligation.
 - `from_desugaring`: whether we are in some kind of desugaring, like `?`
   or a `try` block for example. This flag can also be matched on, see below.

You can match on the following names and values, using `name = "value"`:
 - `cause`: Match against one variant of the `ObligationCauseCode`
   enum. Only `"MainFunctionType"` is supported.
 - `from_desugaring`: Match against a particular variant of the `DesugaringKind`
   enum. The desugaring is identified by its variant name, for example
   `"QuestionMark"` for `?` desugaring or `"TryBlock"` for `try` blocks.
 - `Self` and any generic arguments of the trait, like `Self = "alloc::string::String"`
   or `Rhs="i32"`.
   
The compiler can provide several values to match on, for example:
  - the self_ty, pretty printed with and without type arguments resolved.
  - `"{integral}"`, if self_ty is an integral of which the type is known.
  - `"[]"`, `"[{ty}]"`, `"[{ty}; _]"`, `"[{ty}; $N]"` when applicable.
  - references to said slices and arrays.
  - `"fn"`, `"unsafe fn"` or `"#[target_feature] fn"` when self is a function.
  - `"{integer}"` and `"{float}"` if the type is a number but we haven't inferred it yet.
  - combinations of the above, like `"[{integral}; _]"`.