Add diagnostic for missing ambiguity error for impl trait

crates/syntax/src/validation.rs

@@ -363,11 +363,40 @@ fn validate_trait_object_ty(ty: ast::DynTraitType) -> Option<SyntaxError> {
 }
 
 fn validate_impl_object_ty(ty: ast::ImplTraitType, errors: &mut Vec<SyntaxError>) {
-    if ty.type_bound_list().map_or(0, |tbl| tbl.bounds().count()) == 0 {
+    let Some(bound_list) = ty.type_bound_list() else {
         errors.push(SyntaxError::new(
             "At least one trait must be specified",
             ty.syntax().text_range(),
         ));
+        return;
+    };
+
+    let bounds: Vec<_> = bound_list.bounds().collect();
+
+    if !bounds.iter().any(|b| !matches!(b.kind(), ast::TypeBoundKind::Lifetime(_))) {
+        errors.push(SyntaxError::new(
+            "At least one trait must be specified",
+            ty.syntax().text_range(),
+        ));
+        return;
+    }
+
+    if bounds.len() == 1 {
+        return;
+    }
+
+    let Some(preceding_token) = ty
+        .impl_token()
+        .and_then(|token| token.prev_token())
+        .and_then(|prev| algo::skip_trivia_token(prev, Direction::Prev))
+    else {
+        return;
+    };
+
+    if !matches!(preceding_token.kind(), T!['('] | T![<] | T![=])
+        && matches!(preceding_token.kind(), T![&])
+    {
+        errors.push(SyntaxError::new("ambiguous `+` in a type", ty.syntax().text_range()));
     }
 }
 

crates/syntax/test_data/parser/validation/0261_dangling_impl_undeclared_lifetime.rast

@@ -0,0 +1,38 @@
+[email protected]
+  [email protected]
+    [email protected] "fn"
+    [email protected] " "
+    [email protected]
+      [email protected] "f"
+    [email protected]
+      [email protected] "("
+      [email protected]
+        [email protected]
+          [email protected] "_"
+        [email protected] ":"
+        [email protected] " "
+        [email protected]
+          [email protected] "&"
+          [email protected]
+            [email protected] "impl"
+            [email protected] " "
+            [email protected]
+              [email protected]
+                [email protected]
+                  [email protected] "'a"
+              [email protected] " "
+              [email protected] "+"
+              [email protected] " "
+              [email protected]
+                [email protected]
+                  [email protected]
+                    [email protected]
+                      [email protected]
+                        [email protected] "Sized"
+      [email protected] ")"
+    [email protected] " "
+    [email protected]
+      [email protected]
+        [email protected] "{"
+        [email protected] "}"
+error 9..24: ambiguous `+` in a type

crates/syntax/test_data/parser/validation/0261_dangling_impl_undeclared_lifetime.rs

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

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

@@ -0,0 +1,29 @@
+[email protected]
+  [email protected]
+    [email protected] "fn"
+    [email protected] " "
+    [email protected]
+      [email protected] "f"
+    [email protected]
+      [email protected] "("
+      [email protected]
+        [email protected]
+          [email protected] "_"
+        [email protected] ":"
+        [email protected] " "
+        [email protected]
+          [email protected] "&"
+          [email protected]
+            [email protected] "impl"
+            [email protected] " "
+            [email protected]
+              [email protected]
+                [email protected]
+                  [email protected] "'a"
+      [email protected] ")"
+    [email protected] " "
+    [email protected]
+      [email protected]
+        [email protected] "{"
+        [email protected] "}"
+error 9..16: At least one trait must be specified

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

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