Diagnostic and subdiagnostic structs

rustc has two diagnostic derives that can be used to create simple diagnostics, which are recommended to be used when they are applicable: #[derive(SessionDiagnostic)] and #[derive(SessionSubdiagnostic)].

Diagnostics created with the derive macros can be translated into different languages and each has a slug that uniquely identifies the diagnostic.

#[derive(SessionDiagnostic)]

Instead of using the DiagnosticBuilder API to create and emit diagnostics, the SessionDiagnostic derive can be used. #[derive(SessionDiagnostic)] is only applicable for simple diagnostics that don't require much logic in deciding whether or not to add additional subdiagnostics.

Consider the definition of the "field already declared" diagnostic shown below:

#[derive(SessionDiagnostic)]
#[error(typeck::field_already_declared, code = "E0124")]
pub struct FieldAlreadyDeclared {
    pub field_name: Ident,
    #[primary_span]
    #[label]
    pub span: Span,
    #[label = "previous-decl-label"]
    pub prev_span: Span,
}

SessionDiagnostic can only be applied to structs. Every SessionDiagnostic has to have one attribute applied to the struct itself: either #[error(..)] for defining errors, or #[warning(..)] for defining warnings.

If an error has an error code (e.g. "E0624"), then that can be specified using the code sub-attribute. Specifying a code isn't mandatory, but if you are porting a diagnostic that uses DiagnosticBuilder to use SessionDiagnostic then you should keep the code if there was one.

Both #[error(..)] and #[warning(..)] must provide a slug as the first positional argument (a path to an item in rustc_errors::fluent::*). A slug uniquely identifies the diagnostic and is also how the compiler knows what error message to emit (in the default locale of the compiler, or in the locale requested by the user). See translation documentation to learn more about how translatable error messages are written and how slug items are generated.

In our example, the Fluent message for the "field already declared" diagnostic looks like this:

typeck-field-already-declared =
    field `{$field_name}` is already declared
    .label = field already declared
    .previous-decl-label = `{$field_name}` first declared here

typeck-field-already-declared is the slug from our example and is followed by the diagnostic message.

Every field of the SessionDiagnostic which does not have an annotation is available in Fluent messages as a variable, like field_name in the example above. Fields can be annotated #[skip_arg] if this is undesired.

Using the #[primary_span] attribute on a field (that has type Span) indicates the primary span of the diagnostic which will have the main message of the diagnostic.

Diagnostics are more than just their primary message, they often include labels, notes, help messages and suggestions, all of which can also be specified on a SessionDiagnostic.

#[label], #[help] and #[note] can all be applied to fields which have the 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 typeck-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 SessionDiagnostic derive:

  • Any attribute applied to an Option<T> and 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] 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] subdiagnostics.

Suggestions can be emitted using one of four field attributes:

  • #[suggestion(message = "...", code = "...", applicability = "...")]
  • #[suggestion_hidden(message = "...", code = "...", applicability = "...")]
  • #[suggestion_short(message = "...", code = "...", applicability = "...")]
  • #[suggestion_verbose(message = "...", code = "...", applicability = "...")]

Suggestions must be applied on either a Span field or a (Span, MachineApplicability) field. Similarly to other field attributes, message specifies the Fluent attribute with the message and defaults to .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 SessionDiagnostic derive will generate an implementation of SessionDiagnostic that looks like the following:

impl SessionDiagnostic for FieldAlreadyDeclared {
    fn into_diagnostic(self, sess: &'_ rustc_session::Session) -> DiagnosticBuilder<'_> {
        let mut diag = sess.struct_err_with_code(
            rustc_errors::DiagnosticMessage::fluent("typeck-field-already-declared"),
            rustc_errors::DiagnosticId::Error("E0124")
        );
        diag.set_span(self.span);
        diag.span_label(
            self.span,
            rustc_errors::DiagnosticMessage::fluent_attr("typeck-field-already-declared", "label")
        );
        diag.span_label(
            self.prev_span,
            rustc_errors::DiagnosticMessage::fluent_attr("typeck-field-already-declared", "previous-decl-label")
        );
        diag
    }
}

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

tcx.sess.emit_err(FieldAlreadyDeclared {
    field_name: f.ident,
    span: f.span,
    prev_span,
});

Reference

#[derive(SessionDiagnostic)] supports the following attributes:

  • #[error(slug, code = "...")] or #[warning(slug, code = "...")]
    • Applied to struct.
    • Mandatory
    • Defines the struct to be representing an error or a warning.
    • Slug (Mandatory)
      • Uniquely identifies the diagnostic and corresponds to its Fluent message, mandatory.
      • A path to an item in rustc_errors::fluent. Always in a module starting with a Fluent resource name (which is typically the name of the crate that the diagnostic is from), e.g. rustc_errors::fluent::typeck::field_already_declared (rustc_errors::fluent is implicit in the attribute, so just typeck::field_already_declared).
      • See translation documentation.
    • code = "..." (Optional)
      • Specifies the error code.
  • #[note] or #[note = "..."] (Optional)
    • Applied to struct or Span/() fields.
    • Adds a note subdiagnostic.
    • Value is the Fluent attribute (relative to the Fluent message specified by slug) for the note's message
      • Defaults to note.
    • If applied to a Span field, creates a spanned note.
  • #[help] or #[help = "..."] (Optional)
    • Applied to struct or Span/() fields.
    • Adds a help subdiagnostic.
    • Value is the Fluent attribute (relative to the Fluent message specified by slug) for the help's message.
      • Defaults to help.
    • If applied to a Span field, creates a spanned help.
  • #[label] or #[label = "..."] (Optional)
    • Applied to Span fields.
    • Adds a label subdiagnostic.
    • Value is the Fluent attribute (relative to the Fluent message specified by slug) for the label's message.
      • Defaults to label.
  • #[suggestion{,_hidden,_short,_verbose}(message = "...", code = "...", applicability = "...")] (Optional)
    • Applied to (Span, MachineApplicability) or Span fields.
    • Adds a suggestion subdiagnostic.
    • message = "..." (Mandatory)
      • Value is the Fluent attribute (relative to the Fluent message specified by slug) for the suggestion's message.
      • Defaults to suggestion.
    • code = "..." (Mandatory)
      • Value is a format string indicating the code to be suggested as a replacement.
    • applicability = "..." (Optional)
      • String which must be one of machine-applicable, maybe-incorrect, has-placeholders or unspecified.
  • #[subdiagnostic]
    • Applied to a type that implements AddToDiagnostic (from #[derive(SessionSubdiagnostic)]).
    • Adds the subdiagnostic represented by the subdiagnostic struct.
  • #[primary_span] (Optional)
    • Applied to Span fields.
    • 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(SessionSubdiagnostic)]

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 SessionSubdiagnostic derive can be used to represent a partial diagnostic (e.g a note, label, help or suggestion) as a struct.

Consider the definition of the "expected return type" label shown below:


#![allow(unused)]
fn main() {
#[derive(SessionSubdiagnostic)]
pub enum ExpectedReturnTypeLabel<'tcx> {
    #[label(typeck::expected_default_return_type)]
    Unit {
        #[primary_span]
        span: Span,
    },
    #[label(typeck::expected_return_type)]
    Other {
        #[primary_span]
        span: Span,
        expected: Ty<'tcx>,
    },
}
}

Unlike SessionDiagnostic, SessionSubdiagnostic can be applied to structs or enums. Attributes that are placed on the type for structs are placed on each variants for enums (or vice versa). Each SessionSubdiagnostic should have one attribute applied to the struct or each variant, one of:

  • #[label(..)] for defining a label
  • #[note(..)] for defining a note
  • #[help(..)] for defining a help
  • #[suggestion{,_hidden,_short,_verbose}(..)] for defining a suggestion

All of the above must provide a slug as the first positional argument (a path to an item in rustc_errors::fluent::*). A slug uniquely identifies the diagnostic and is also how the compiler knows what error message to emit (in the default locale of the compiler, or in the locale requested by the user). See translation documentation to learn more about how translatable error messages are written and how slug items are generated.

In our example, the Fluent message for the "expected return type" label looks like this:

typeck-expected-default-return-type = expected `()` because of default return type

typeck-expected-return-type = expected `{$expected}` because of return type

Using the #[primary_span] attribute on a field (with type Span) will denote the primary span of the subdiagnostic. A primary span is only necessary for a label or suggestion, which can not be spanless.

Every field of the type/variant which does not have an annotation is available in Fluent messages as a variable. Fields can be annotated #[skip_arg] if this is undesired.

Like SessionDiagnostic, SessionSubdiagnostic supports Option<T> and Vec<T> fields.

Suggestions can be emitted using one of four attributes on the type/variant:

  • #[suggestion(message = "...", code = "...", applicability = "...")]
  • #[suggestion_hidden(message = "...", code = "...", applicability = "...")]
  • #[suggestion_short(message = "...", code = "...", applicability = "...")]
  • #[suggestion_verbose(message = "...", code = "...", applicability = "...")]

Suggestions require #[primary_span] be set on a field and can have the following sub-attributes:

  • message specifies the Fluent attribute with the message and defaults to .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.

Applicabilities can also be specified as a field (of type Applicability) using the #[applicability] attribute.

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


#![allow(unused)]
fn main() {
impl<'tcx> AddToDiagnostic for ExpectedReturnTypeLabel<'tcx> {
    fn add_to_diagnostic(self, diag: &mut rustc_errors::Diagnostic) {
        use rustc_errors::{Applicability, IntoDiagnosticArg};
        match self {
            ExpectedReturnTypeLabel::Unit { span } => {
                diag.span_label(span, DiagnosticMessage::fluent("typeck-expected-default-return-type"))
            }
            ExpectedReturnTypeLabel::Other { span, expected } => {
                diag.set_arg("expected", expected);
                diag.span_label(span, DiagnosticMessage::fluent("typeck-expected-return-type"))
            }

        }
    }
}
}

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

Reference

#[derive(SessionSubdiagnostic)] supports the following attributes:

  • #[label(slug)], #[help(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. Always in a module starting with a Fluent resource name (which is typically the name of the crate that the diagnostic is from), e.g. rustc_errors::fluent::typeck::field_already_declared (rustc_errors::fluent is implicit in the attribute, so just typeck::field_already_declared).
      • See translation documentation.
  • #[suggestion{,_hidden,_short,_verbose}(message = "...", code = "...", applicability = "...")]
    • Applied to struct or enum variant. Mutually exclusive with struct/enum variant attributes.
    • Mandatory
    • Defines the type to be representing a suggestion.
    • message = "..." (Mandatory)
      • Value is the Fluent attribute (relative to the Fluent message specified by slug) for the suggestion's message.
      • Defaults to suggestion.
    • code = "..." (Mandatory)
      • Value is a format string indicating the code to be suggested as a replacement.
    • 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
  • #[primary_span] (Mandatory for labels and suggestions; optional otherwise)
    • Applied to Span fields.
    • Indicates the primary span of the subdiagnostic.
  • #[applicability] (Optional; only applicable to 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.