type `Span`. Applying any of these attributes will create the corresponding
subdiagnostic with that `Span`. These attributes will look for their
diagnostic message in a Fluent attribute attached to the primary Fluent
message. In our example, `#[label]` will look for
`hir_analysis_field_already_declared.label` (which has the message "field already
declared"). If there is more than one subdiagnostic of the same type, then
these attributes can also take a value that is the attribute name to look for
(e.g. `previous_decl_label` in our example).

Other types have special behavior when used in a `Diagnostic` derive:

- Any attribute applied to an `Option<T>` will only emit a
  subdiagnostic if the option is `Some(..)`.
- Any attribute applied to a `Vec<T>` will be repeated for each element of the
  vector.

`#[help]`, `#[warning]` and `#[note]` can also be applied to the struct itself, in which case
they work exactly like when applied to fields except the subdiagnostic won't
have a `Span`. These attributes can also be applied to fields of type `()` for
the same effect, which when combined with the `Option` type can be used to
represent optional `#[note]`/`#[help]`/`#[warning]` subdiagnostics.

Suggestions can be emitted using one of four field attributes:

- `#[suggestion(slug, code = "...", applicability = "...")]`
- `#[suggestion_hidden(slug, code = "...", applicability = "...")]`
- `#[suggestion_short(slug, code = "...", applicability = "...")]`
- `#[suggestion_verbose(slug, code = "...", applicability = "...")]`

Suggestions must be applied on either a `Span` field or a `(Span,
MachineApplicability)` field. Similarly to other field attributes, the slug
specifies the Fluent attribute with the message and defaults to the equivalent
of `.suggestion`. `code` specifies the code that should be suggested as a
replacement and is a format string (e.g. `{field_name}` would be replaced by
the value of the `field_name` field of the struct), not a Fluent identifier.
`applicability` can be used to specify the applicability in the attribute, it
cannot be used when the field's type contains an `Applicability`.

In the end, the `Diagnostic` derive will generate an implementation of
`Diagnostic` that looks like the following:

```rust,ignore
impl<'a, G: EmissionGuarantee> Diagnostic<'a> for FieldAlreadyDeclared {
    fn into_diag(self, dcx: &'a DiagCtxt, level: Level) -> Diag<'a, G> {
        let mut diag = Diag::new(dcx, level, fluent::hir_analysis_field_already_declared);
        diag.set_span(self.span);
        diag.span_label(
            self.span,
            fluent::hir_analysis_label
        );
        diag.span_label(
            self.prev_span,
            fluent::hir_analysis_previous_decl_label
        );
        diag
    }
}
```

Now that we've defined our diagnostic, how do we [use it][use]? It's quite
straightforward, just create an instance of the struct and pass it to
`emit_err` (or `emit_warning`):

```rust,ignore
tcx.dcx().emit_err(FieldAlreadyDeclared {
    field_name: f.ident,
    span: f.span,
    prev_span,
});
```

### Reference
`#[derive(Diagnostic)]` and `#[derive(LintDiagnostic)]` support the
following attributes:

- `#[diag(slug, code = "...")]`
  - _Applied to struct or enum variant._
  - _Mandatory_
  - Defines the text and error code to be associated with the diagnostic.
  - 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).
  - `code = "..."` (_Optional_)
    - Specifies the error code.
- `#[note]` or `#[note(slug)]` (_Optional_)
  - _Applied to struct or struct fields of type `Span`, `Option<()>` or `()`._
  - Adds a note subdiagnostic.
  - Value is a path to an item in `rustc_errors::fluent` for the note's