Do not call TyCtxt::type_of() on a trait

Author: samueltardieu

clippy_lints/src/methods/useless_asref.rs

@@ -79,7 +79,8 @@ pub(super) fn check(cx: &LateContext<'_>, expr: &hir::Expr<'_>, call_name: &str,
                 applicability,
             );
         }
-    } else if let Some(impl_id) = cx.tcx.opt_parent(def_id)
+    } else if cx.tcx.trait_of_item(def_id).is_none()
+        && let Some(impl_id) = cx.tcx.opt_parent(def_id)
         && let Some(adt) = cx.tcx.type_of(impl_id).instantiate_identity().ty_adt_def()
         && (cx.tcx.lang_items().option_type() == Some(adt.did()) || cx.tcx.is_diagnostic_item(sym::Result, adt.did()))
     {

tests/ui/useless_asref.fixed

@@ -248,6 +248,16 @@ impl Issue12357 {
     }
 }
 
+fn issue_14828() {
+    pub trait T {
+        fn as_ref(&self) {}
+    }
+
+    impl T for () {}
+
+    ().as_ref();
+}
+
 fn main() {
     not_ok();
     ok();

tests/ui/useless_asref.rs

@@ -248,6 +248,16 @@ impl Issue12357 {
     }
 }
 
+fn issue_14828() {
+    pub trait T {
+        fn as_ref(&self) {}
+    }
+
+    impl T for () {}
+
+    ().as_ref();
+}
+
 fn main() {
     not_ok();
     ok();