Allow nesting subdiagnostics

compiler/rustc_macros/src/diagnostics/subdiagnostic.rs

@@ -71,6 +71,7 @@ pub(crate) fn into_tokens(self, mut structure: Structure<'_>) -> TokenStream {
                     span_field: None,
                     applicability: None,
                     has_suggestion_parts: false,
+                    has_subdiagnostic: false,
                     is_enum,
                 };
                 builder.into_tokens().unwrap_or_else(|v| v.to_compile_error())
@@ -133,6 +134,10 @@ struct SubdiagnosticDeriveVariantBuilder<'parent, 'a> {
     /// during finalization if still `false`.
     has_suggestion_parts: bool,
 
+    /// Set to true when a `#[subdiagnostic]` field is encountered, used to suppress the error
+    /// emitted when no subdiagnostic kinds are specified on the variant itself.
+    has_subdiagnostic: bool,
+
     /// Set to true when this variant is an enum variant rather than just the body of a struct.
     is_enum: bool,
 }
@@ -373,6 +378,13 @@ fn generate_field_code_inner_path(
 
                 Ok(quote! {})
             }
+            "subdiagnostic" => {
+                let f = &self.parent.f;
+                let diag = &self.parent.diag;
+                let binding = &info.binding;
+                self.has_subdiagnostic = true;
+                Ok(quote! { #binding.add_to_diag_with(#diag, #f); })
+            }
             _ => {
                 let mut span_attrs = vec![];
                 if kind_stats.has_multipart_suggestion {
@@ -480,18 +492,6 @@ fn generate_field_code_inner_list(
 
     pub(crate) fn into_tokens(&mut self) -> Result<TokenStream, DiagnosticDeriveError> {
         let kind_slugs = self.identify_kind()?;
-        if kind_slugs.is_empty() {
-            if self.is_enum {
-                // It's okay for a variant to not be a subdiagnostic at all..
-                return Ok(quote! {});
-            } else {
-                // ..but structs should always be _something_.
-                throw_span_err!(
-                    self.variant.ast().ident.span().unwrap(),
-                    "subdiagnostic kind not specified"
-                );
-            }
-        };
 
         let kind_stats: KindsStatistics =
             kind_slugs.iter().map(|(kind, _slug, _no_span)| kind).collect();
@@ -510,6 +510,19 @@ pub(crate) fn into_tokens(&mut self) -> Result<TokenStream, DiagnosticDeriveErro
             .map(|binding| self.generate_field_attr_code(binding, kind_stats))
             .collect();
 
+        if kind_slugs.is_empty() {
+            if self.is_enum {
+                // It's okay for a variant to not be a subdiagnostic at all..
+                return Ok(quote! {});
+            } else if !self.has_subdiagnostic {
+                // ..but structs should always be _something_.
+                throw_span_err!(
+                    self.variant.ast().ident.span().unwrap(),
+                    "subdiagnostic kind not specified"
+                );
+            }
+        };
+
         let span_field = self.span_field.value_ref();
 
         let diag = &self.parent.diag;

compiler/rustc_macros/src/lib.rs

@@ -144,6 +144,7 @@ pub fn extension(attr: TokenStream, input: TokenStream) -> TokenStream {
         help,
         note,
         warning,
+        subdiagnostic,
         suggestion,
         suggestion_short,
         suggestion_hidden,

tests/ui-fulldeps/session-diagnostic/subdiagnostic-derive.rs

@@ -827,3 +827,13 @@ struct PrimarySpanOnVec {
     //~| NOTE there must be exactly one primary span
     sub: Vec<Span>,
 }
+
+#[derive(Subdiagnostic)]
+struct NestedParent {
+    #[subdiagnostic]
+    single_sub: A,
+    #[subdiagnostic]
+    option_sub: Option<A>,
+    #[subdiagnostic]
+    vec_sub: Vec<A>,
+}