add diagnostic for dangling impl

crates/syntax/src/validation.rs

@@ -315,11 +315,22 @@ fn validate_path_keywords(segment: ast::PathSegment, errors: &mut Vec<SyntaxErro
 }
 
 fn validate_trait_object_ref_ty(ty: ast::RefType, errors: &mut Vec<SyntaxError>) {
-    if let Some(ast::Type::DynTraitType(ty)) = ty.ty() {
+    match ty.ty() {
+        Some(ast::Type::DynTraitType(ty)) => {
             if let Some(err) = validate_trait_object_ty(ty) {
                 errors.push(err);
             }
         }
+        Some(ast::Type::ImplTraitType(ty)) => {
+            if ty.type_bound_list().map_or(0, |tbl| tbl.bounds().count()) == 0 {
+                errors.push(SyntaxError::new(
+                    "At least one trait must be specified",
+                    ty.syntax().text_range(),
+                ));
+            }
+        }
+        _ => {}
+    }
 }
 
 fn validate_trait_object_ptr_ty(ty: ast::PtrType, errors: &mut Vec<SyntaxError>) {

crates/syntax/test_data/parser/validation/dangling_impl.rast

@@ -0,0 +1,25 @@
+SOURCE_FILE@0..17
+  FN@0..17
+    FN_KW@0..2 "fn"
+    WHITESPACE@2..3 " "
+    NAME@3..4
+      IDENT@3..4 "f"
+    PARAM_LIST@4..14
+      L_PAREN@4..5 "("
+      PARAM@5..13
+        WILDCARD_PAT@5..6
+          UNDERSCORE@5..6 "_"
+        COLON@6..7 ":"
+        WHITESPACE@7..8 " "
+        REF_TYPE@8..13
+          AMP@8..9 "&"
+          IMPL_TRAIT_TYPE@9..13
+            IMPL_KW@9..13 "impl"
+            TYPE_BOUND_LIST@13..13
+      R_PAREN@13..14 ")"
+    WHITESPACE@14..15 " "
+    BLOCK_EXPR@15..17
+      STMT_LIST@15..17
+        L_CURLY@15..16 "{"
+        R_CURLY@16..17 "}"
+error 9..13: At least one trait must be specified

crates/syntax/test_data/parser/validation/dangling_impl.rs

@@ -0,0 +1 @@
+fn f(_: &impl) {}