use subdiagnostic for message

Author: compiler-errors

compiler/rustc_error_messages/locales/en-US/expand.ftl

@@ -0,0 +1,5 @@
+expand-explain-doc-comment-outer =
+    outer doc comments expand to `#[doc = "..."]`, which is what this macro attempted to match
+
+expand-explain-doc-comment-inner =
+    inner doc comments expand to `#![doc = "..."]`, which is what this macro attempted to match

compiler/rustc_error_messages/src/lib.rs

@@ -33,11 +33,12 @@ pub use unic_langid::{langid, LanguageIdentifier};
 fluent_messages! {
     borrowck => "../locales/en-US/borrowck.ftl",
     builtin_macros => "../locales/en-US/builtin_macros.ftl",
+    const_eval => "../locales/en-US/const_eval.ftl",
+    expand => "../locales/en-US/expand.ftl",
     lint => "../locales/en-US/lint.ftl",
     parser => "../locales/en-US/parser.ftl",
     privacy => "../locales/en-US/privacy.ftl",
     typeck => "../locales/en-US/typeck.ftl",
-    const_eval => "../locales/en-US/const_eval.ftl",
 }
 
 pub use fluent_generated::{self as fluent, DEFAULT_LOCALE_RESOURCES};

compiler/rustc_expand/src/mbe/macro_rules.rs

@@ -594,16 +594,30 @@ pub fn compile_declarative_macro(
     (mk_syn_ext(expander), rule_spans)
 }
 
+#[derive(SessionSubdiagnostic)]
+enum ExplainDocComment {
+    #[label(expand::explain_doc_comment_inner)]
+    Inner {
+        #[primary_span]
+        span: Span,
+    },
+    #[label(expand::explain_doc_comment_outer)]
+    Outer {
+        #[primary_span]
+        span: Span,
+    },
+}
+
 fn annotate_doc_comment(
     err: &mut DiagnosticBuilder<'_, ErrorGuaranteed>,
     sm: &SourceMap,
     span: Span,
 ) {
     if let Ok(src) = sm.span_to_snippet(span) {
         if src.starts_with("///") || src.starts_with("/**") {
-            err.span_label(span, "outer doc comments expand to `#[doc = \"...\"]`, which is what this macro attempted to match");
+            err.subdiagnostic(ExplainDocComment::Outer { span });
         } else if src.starts_with("//!") || src.starts_with("/*!") {
-            err.span_label(span, "inner doc comments expand to `#![doc = \"...\"]`, which is what this macro attempted to match");
+            err.subdiagnostic(ExplainDocComment::Inner { span });
         }
     }
 }