Auto merge of #115131 - frank-king:feature/unnamed-fields-lite, r=petrochenkov

Parse unnamed fields and anonymous structs or unions (no-recovery)

It is part of #114782 which implements #49804. Only parse anonymous structs or unions in struct field definition positions.

r? `@petrochenkov`

compiler/rustc_ast_passes/src/errors.rs

@@ -727,3 +727,30 @@ pub struct ConstraintOnNegativeBound {
     #[primary_span]
     pub span: Span,
 }
+
+#[derive(Diagnostic)]
+#[diag(ast_passes_invalid_unnamed_field_ty)]
+pub struct InvalidUnnamedFieldTy {
+    #[primary_span]
+    pub span: Span,
+    #[label]
+    pub ty_span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(ast_passes_invalid_unnamed_field)]
+pub struct InvalidUnnamedField {
+    #[primary_span]
+    pub span: Span,
+    #[label]
+    pub ident_span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(ast_passes_anon_struct_or_union_not_allowed)]
+pub struct AnonStructOrUnionNotAllowed {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+    pub struct_or_union: &'static str,
+}