add diagnostic for dangling dyn

Author: Shourya742

src/tools/rust-analyzer/crates/syntax/src/validation.rs

@@ -340,17 +340,25 @@ fn validate_trait_object_fn_ptr_ret_ty(ty: ast::FnPtrType, errors: &mut Vec<Synt
 
 fn validate_trait_object_ty(ty: ast::DynTraitType) -> Option<SyntaxError> {
     let tbl = ty.type_bound_list()?;
-
-    if tbl.bounds().count() > 1 {
-        let dyn_token = ty.dyn_token()?;
-        let potential_parenthesis =
-            algo::skip_trivia_token(dyn_token.prev_token()?, Direction::Prev)?;
-        let kind = potential_parenthesis.kind();
-        if !matches!(kind, T!['('] | T![<] | T![=]) {
-            return Some(SyntaxError::new("ambiguous `+` in a type", ty.syntax().text_range()));
+    let bounds_count = tbl.bounds().count();
+
+    match bounds_count {
+        0 => Some(SyntaxError::new(
+            "At least one trait is required for an object type",
+            ty.syntax().text_range(),
+        )),
+        _ if bounds_count > 1 => {
+            let dyn_token = ty.dyn_token()?;
+            let preceding_token =
+                algo::skip_trivia_token(dyn_token.prev_token()?, Direction::Prev)?;
+
+            if !matches!(preceding_token.kind(), T!['('] | T![<] | T![=]) {
+                return Some(SyntaxError::new("ambiguous `+` in a type", ty.syntax().text_range()));
+            }
+            None
         }
+        _ => None,
     }
-    None
 }
 
 fn validate_macro_rules(mac: ast::MacroRules, errors: &mut Vec<SyntaxError>) {

src/tools/rust-analyzer/crates/syntax/test_data/parser/validation/0224_dangling_dyn.rast

@@ -0,0 +1,25 @@
+SOURCE_FILE@0..16
+  FN@0..16
+    FN_KW@0..2 "fn"
+    WHITESPACE@2..3 " "
+    NAME@3..4
+      IDENT@3..4 "f"
+    PARAM_LIST@4..13
+      L_PAREN@4..5 "("
+      PARAM@5..12
+        WILDCARD_PAT@5..6
+          UNDERSCORE@5..6 "_"
+        COLON@6..7 ":"
+        WHITESPACE@7..8 " "
+        REF_TYPE@8..12
+          AMP@8..9 "&"
+          DYN_TRAIT_TYPE@9..12
+            DYN_KW@9..12 "dyn"
+            TYPE_BOUND_LIST@12..12
+      R_PAREN@12..13 ")"
+    WHITESPACE@13..14 " "
+    BLOCK_EXPR@14..16
+      STMT_LIST@14..16
+        L_CURLY@14..15 "{"
+        R_CURLY@15..16 "}"
+error 9..12: At least one trait is required for an object type

src/tools/rust-analyzer/crates/syntax/test_data/parser/validation/0224_dangling_dyn.rs

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