ExpectedReturnTypeLabel::Other { span, expected } => {
                diag.set_arg("expected", expected);
                diag.span_label(span, rustc_errors::fluent::hir_analysis_expected_return_type)
            }
        }
    }
}
```

Once defined, a subdiagnostic can be used by passing it to the `subdiagnostic`
function ([example][subdiag_use_1] and [example][subdiag_use_2]) on a
diagnostic or by assigning it to a `#[subdiagnostic]`-annotated field of a
diagnostic struct.

### Reference
`#[derive(Subdiagnostic)]` supports the following attributes:

- `#[label(slug)]`, `#[help(slug)]`, `#[warning(slug)]` or `#[note(slug)]`
  - _Applied to struct or enum variant. Mutually exclusive with struct/enum variant attributes._
  - _Mandatory_
  - Defines the type to be representing a label, help or note.
  - Slug (_Mandatory_)
    - Uniquely identifies the diagnostic and corresponds to its Fluent message,
      mandatory.
    - A path to an item in `rustc_errors::fluent`, e.g.
      `rustc_errors::fluent::hir_analysis_field_already_declared`
      (`rustc_errors::fluent` is implicit in the attribute, so just
      `hir_analysis_field_already_declared`).
    - See [translation documentation](./translation.md).
- `#[suggestion{,_hidden,_short,_verbose}(slug, code = "...", applicability = "...")]`
  - _Applied to struct or enum variant. Mutually exclusive with struct/enum variant attributes._
  - _Mandatory_
  - Defines the type to be representing a suggestion.
  - Slug (_Mandatory_)
    - A path to an item in `rustc_errors::fluent`, e.g.
      `rustc_errors::fluent::hir_analysis_field_already_declared`
      (`rustc_errors::fluent` is implicit in the attribute, so just
      `hir_analysis::field_already_declared`). Fluent attributes for all messages
      exist as top-level items in that module (so `hir_analysis_message.attr` is just
      `hir_analysis::attr`).
    - See [translation documentation](./translation.md).
    - Defaults to `rustc_errors::fluent::_subdiag::suggestion` (or
    - `.suggestion` in Fluent).
  - `code = "..."`/`code("...", ...)` (_Mandatory_)
    - One or multiple format strings indicating the code to be suggested as a
      replacement. Multiple values signify multiple possible replacements.
  - `applicability = "..."` (_Optional_)
    - _Mutually exclusive with `#[applicability]` on a field._
    - Value is the applicability of the suggestion.
    - String which must be one of:
      - `machine-applicable`
      - `maybe-incorrect`
      - `has-placeholders`
      - `unspecified`
- `#[multipart_suggestion{,_hidden,_short,_verbose}(slug, applicability = "...")]`
  - _Applied to struct or enum variant. Mutually exclusive with struct/enum variant attributes._
  - _Mandatory_
  - Defines the type to be representing a multipart suggestion.
  - Slug (_Mandatory_): see `#[suggestion]`
  - `applicability = "..."` (_Optional_): see `#[suggestion]`
- `#[primary_span]` (_Mandatory_ for labels and suggestions; _optional_ otherwise; not applicable
to multipart suggestions)
  - _Applied to `Span` fields._
  - Indicates the primary span of the subdiagnostic.
- `#[suggestion_part(code = "...")]` (_Mandatory_; only applicable to multipart suggestions)
  - _Applied to `Span` fields._
  - Indicates the span to be one part of the multipart suggestion.
  - `code = "..."` (_Mandatory_)
    - Value is a format string indicating the code to be suggested as a
      replacement.
- `#[applicability]` (_Optional_; only applicable to (simple and multipart) suggestions)
  - _Applied to `Applicability` fields._
  - Indicates the applicability of the suggestion.
- `#[skip_arg]` (_Optional_)
  - _Applied to any field._
  - Prevents the field from being provided as a diagnostic argument.