add is_trait(DefId) helper to TyCtxt

Co-authored-by: Tyler Mandry <tmandry@gmail.com>

src/librustc/ty/util.rs

@@ -518,10 +518,25 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         result
     }
 
+    /// True if `def_id` refers to a closure (e.g., `|x| x * 2`). Note
+    /// that closures have a def-id, but the closure *expression* also
+    /// has a `HirId` that is located within the context where the
+    /// closure appears (and, sadly, a corresponding `NodeId`, since
+    /// those are not yet phased out). The parent of the closure's
+    /// def-id will also be the context where it appears.
     pub fn is_closure(self, def_id: DefId) -> bool {
         self.def_key(def_id).disambiguated_data.data == DefPathData::ClosureExpr
     }
 
+    /// True if `def_id` refers to a trait (e.g., `trait Foo { ... }`).
+    pub fn is_trait(self, def_id: DefId) -> bool {
+        if let DefPathData::Trait(_) = self.def_key(def_id).disambiguated_data.data {
+            true
+        } else {
+            false
+        }
+    }
+
     /// True if this def-id refers to the implicit constructor for
     /// a tuple struct like `struct Foo(u32)`.
     pub fn is_struct_constructor(self, def_id: DefId) -> bool {