macros: support doc comments in diag derives

Documentation comments shouldn't affect the diagnostic derive in any
way, but explicit support has to be added for ignoring the `doc`
attribute.

Signed-off-by: David Wood <david.wood@huawei.com>

compiler/rustc_macros/src/diagnostics/diagnostic_builder.rs

@@ -5,7 +5,7 @@ use crate::diagnostics::error::{
     DiagnosticDeriveError,
 };
 use crate::diagnostics::utils::{
-    build_field_mapping, report_error_if_not_applied_to_span, report_type_error,
+    build_field_mapping, is_doc_comment, report_error_if_not_applied_to_span, report_type_error,
     should_generate_set_arg, type_is_unit, type_matches_path, FieldInfo, FieldInnerTy, FieldMap,
     HasFieldMap, SetOnce, SpannedOption, SubdiagnosticKind,
 };
@@ -152,8 +152,12 @@ impl<'a> DiagnosticDeriveVariantBuilder<'a> {
     fn parse_subdiag_attribute(
         &self,
         attr: &Attribute,
-    ) -> Result<(SubdiagnosticKind, Path), DiagnosticDeriveError> {
+    ) -> Result<Option<(SubdiagnosticKind, Path)>, DiagnosticDeriveError> {
-        let (subdiag, slug) = SubdiagnosticKind::from_attr(attr, self)?;
+        let Some((subdiag, slug)) = SubdiagnosticKind::from_attr(attr, self)? else {
+            // Some attributes aren't errors - like documentation comments - but also aren't
+            // subdiagnostics.
+            return Ok(None);
+        };
 
         if let SubdiagnosticKind::MultipartSuggestion { .. } = subdiag {
             let meta = attr.parse_meta()?;
@@ -170,7 +174,7 @@ impl<'a> DiagnosticDeriveVariantBuilder<'a> {
             SubdiagnosticKind::MultipartSuggestion { .. } => unreachable!(),
         });
 
-        Ok((subdiag, slug))
+        Ok(Some((subdiag, slug)))
     }
 
     /// Establishes state in the `DiagnosticDeriveBuilder` resulting from the struct
@@ -182,6 +186,11 @@ impl<'a> DiagnosticDeriveVariantBuilder<'a> {
     ) -> Result<TokenStream, DiagnosticDeriveError> {
         let diag = &self.parent.diag;
 
+        // Always allow documentation comments.
+        if is_doc_comment(attr) {
+            return Ok(quote! {});
+        }
+
         let name = attr.path.segments.last().unwrap().ident.to_string();
         let name = name.as_str();
         let meta = attr.parse_meta()?;
@@ -250,7 +259,11 @@ impl<'a> DiagnosticDeriveVariantBuilder<'a> {
             return Ok(tokens);
         }
 
-        let (subdiag, slug) = self.parse_subdiag_attribute(attr)?;
+        let Some((subdiag, slug)) = self.parse_subdiag_attribute(attr)? else {
+            // Some attributes aren't errors - like documentation comments - but also aren't
+            // subdiagnostics.
+            return Ok(quote! {});
+        };
         let fn_ident = format_ident!("{}", subdiag);
         match subdiag {
             SubdiagnosticKind::Note | SubdiagnosticKind::Help | SubdiagnosticKind::Warn => {
@@ -291,6 +304,11 @@ impl<'a> DiagnosticDeriveVariantBuilder<'a> {
             .attrs
             .iter()
             .map(move |attr| {
+                // Always allow documentation comments.
+                if is_doc_comment(attr) {
+                    return quote! {};
+                }
+
                 let name = attr.path.segments.last().unwrap().ident.to_string();
                 let needs_clone =
                     name == "primary_span" && matches!(inner_ty, FieldInnerTy::Vec(_));
@@ -397,8 +415,11 @@ impl<'a> DiagnosticDeriveVariantBuilder<'a> {
             _ => (),
         }
 
-        let (subdiag, slug) = self.parse_subdiag_attribute(attr)?;
+        let Some((subdiag, slug)) = self.parse_subdiag_attribute(attr)? else {
-
+            // Some attributes aren't errors - like documentation comments - but also aren't
+            // subdiagnostics.
+            return Ok(quote! {});
+        };
         let fn_ident = format_ident!("{}", subdiag);
         match subdiag {
             SubdiagnosticKind::Label => {

compiler/rustc_macros/src/diagnostics/subdiagnostic.rs

@@ -5,9 +5,9 @@ use crate::diagnostics::error::{
     DiagnosticDeriveError,
 };
 use crate::diagnostics::utils::{
-    build_field_mapping, new_code_ident, report_error_if_not_applied_to_applicability,
+    build_field_mapping, is_doc_comment, new_code_ident,
-    report_error_if_not_applied_to_span, FieldInfo, FieldInnerTy, FieldMap, HasFieldMap, SetOnce,
+    report_error_if_not_applied_to_applicability, report_error_if_not_applied_to_span, FieldInfo,
-    SpannedOption, SubdiagnosticKind,
+    FieldInnerTy, FieldMap, HasFieldMap, SetOnce, SpannedOption, SubdiagnosticKind,
 };
 use proc_macro2::TokenStream;
 use quote::{format_ident, quote};
@@ -43,6 +43,11 @@ impl SubdiagnosticDeriveBuilder {
 
             if matches!(ast.data, syn::Data::Enum(..)) {
                 for attr in &ast.attrs {
+                    // Always allow documentation comments.
+                    if is_doc_comment(attr) {
+                        continue;
+                    }
+
                     span_err(
                         attr.span().unwrap(),
                         "unsupported type attribute for subdiagnostic enum",
@@ -173,7 +178,11 @@ impl<'parent, 'a> SubdiagnosticDeriveVariantBuilder<'parent, 'a> {
         let mut kind_slugs = vec![];
 
         for attr in self.variant.ast().attrs {
-            let (kind, slug) = SubdiagnosticKind::from_attr(attr, self)?;
+            let Some((kind, slug)) = SubdiagnosticKind::from_attr(attr, self)? else {
+                // Some attributes aren't errors - like documentation comments - but also aren't
+                // subdiagnostics.
+                continue;
+            };
 
             let Some(slug) = slug else {
                 let name = attr.path.segments.last().unwrap().ident.to_string();
@@ -227,6 +236,11 @@ impl<'parent, 'a> SubdiagnosticDeriveVariantBuilder<'parent, 'a> {
         ast.attrs
             .iter()
             .map(|attr| {
+                // Always allow documentation comments.
+                if is_doc_comment(attr) {
+                    return quote! {};
+                }
+
                 let info = FieldInfo {
                     binding,
                     ty: inner_ty.inner_type().unwrap_or(&ast.ty),

compiler/rustc_macros/src/diagnostics/utils.rs

@@ -477,7 +477,12 @@ impl SubdiagnosticKind {
     pub(super) fn from_attr(
         attr: &Attribute,
         fields: &impl HasFieldMap,
-    ) -> Result<(SubdiagnosticKind, Option<Path>), DiagnosticDeriveError> {
+    ) -> Result<Option<(SubdiagnosticKind, Option<Path>)>, DiagnosticDeriveError> {
+        // Always allow documentation comments.
+        if is_doc_comment(attr) {
+            return Ok(None);
+        }
+
         let span = attr.span().unwrap();
 
         let name = attr.path.segments.last().unwrap().ident.to_string();
@@ -526,7 +531,9 @@ impl SubdiagnosticKind {
                     | SubdiagnosticKind::Note
                     | SubdiagnosticKind::Help
                     | SubdiagnosticKind::Warn
-                    | SubdiagnosticKind::MultipartSuggestion { .. } => return Ok((kind, None)),
+                    | SubdiagnosticKind::MultipartSuggestion { .. } => {
+                        return Ok(Some((kind, None)));
+                    }
                     SubdiagnosticKind::Suggestion { .. } => {
                         throw_span_err!(span, "suggestion without `code = \"...\"`")
                     }
@@ -626,7 +633,7 @@ impl SubdiagnosticKind {
             | SubdiagnosticKind::MultipartSuggestion { .. } => {}
         }
 
-        Ok((kind, slug))
+        Ok(Some((kind, slug)))
     }
 }
 
@@ -654,3 +661,7 @@ impl quote::IdentFragment for SubdiagnosticKind {
 pub(super) fn should_generate_set_arg(field: &Field) -> bool {
     field.attrs.is_empty()
 }
+
+pub(super) fn is_doc_comment(attr: &Attribute) -> bool {
+    attr.path.segments.last().unwrap().ident.to_string() == "doc"
+}

src/test/ui-fulldeps/session-diagnostic/diagnostic-derive.rs

@@ -749,3 +749,12 @@ struct SubdiagnosticEagerSuggestion {
     #[subdiagnostic(eager)]
     sub: SubdiagnosticWithSuggestion,
 }
+
+/// with a doc comment on the type..
+#[derive(Diagnostic)]
+#[diag(compiletest::example, code = "E0123")]
+struct WithDocComment {
+    /// ..and the field
+    #[primary_span]
+    span: Span,
+}

src/test/ui-fulldeps/session-diagnostic/subdiagnostic-derive.rs

@@ -641,3 +641,24 @@ struct BJ {
     span: Span,
     r#type: String,
 }
+
+/// with a doc comment on the type..
+#[derive(Subdiagnostic)]
+#[label(parser::add_paren)]
+struct BK {
+    /// ..and the field
+    #[primary_span]
+    span: Span,
+}
+
+/// with a doc comment on the type..
+#[derive(Subdiagnostic)]
+enum BL {
+    /// ..and the variant..
+    #[label(parser::add_paren)]
+    Foo {
+        /// ..and the field
+        #[primary_span]
+        span: Span,
+    }
+}