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
    message.
    - Defaults to equivalent of `.note`.
  - If applied to a `Span` field, creates a spanned note.
- `#[help]` or `#[help(slug)]` (_Optional_)
  - _Applied to struct or struct fields of type `Span`, `Option<()>` or `()`._
  - Adds a help subdiagnostic.
  - Value is a path to an item in `rustc_errors::fluent` for the note's
    message.
    - Defaults to equivalent of `.help`.
  - If applied to a `Span` field, creates a spanned help.
- `#[label]` or `#[label(slug)]` (_Optional_)
  - _Applied to `Span` fields._
  - Adds a label subdiagnostic.
  - Value is a path to an item in `rustc_errors::fluent` for the note's
    message.
    - Defaults to equivalent of `.label`.
- `#[warning]` or `#[warning(slug)]` (_Optional_)
  - _Applied to struct or struct fields of type `Span`, `Option<()>` or `()`._
  - Adds a warning subdiagnostic.
  - Value is a path to an item in `rustc_errors::fluent` for the note's
    message.
    - Defaults to equivalent of `.warn`.
- `#[suggestion{,_hidden,_short,_verbose}(slug, code = "...", applicability = "...")]`
  (_Optional_)
  - _Applied to `(Span, MachineApplicability)` or `Span` fields._
  - Adds a suggestion subdiagnostic.
  - 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
      `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_)
    - String which must be one of `machine-applicable`, `maybe-incorrect`,
      `has-placeholders` or `unspecified`.
- `#[subdiagnostic]`
  - _Applied to a type that implements `Subdiagnostic` (from
    `#[derive(Subdiagnostic)]`)._
  - Adds the subdiagnostic represented by the subdiagnostic struct.
- `#[primary_span]` (_Optional_)
  - _Applied to `Span` fields on `Subdiagnostic`s. Not used for `LintDiagnostic`s._
  - Indicates the primary span of the diagnostic.
- `#[skip_arg]` (_Optional_)
  - _Applied to any field._
  - Prevents the field from being provided as a diagnostic argument.

## `#[derive(Subdiagnostic)]`
It is common in the compiler to write a function that conditionally adds a
specific subdiagnostic to an error if it is applicable. Oftentimes these
subdiagnostics could be represented using a diagnostic struct even if the
overall diagnostic could not. In this circumstance, the `Subdiagnostic`
derive can be used to represent a partial diagnostic (e.g a note, label, help or
suggestion) as a struct.