Make unimplemented match variants explicit

crates/hir_ty/src/traits/chalk/mapping.rs

@@ -399,7 +399,8 @@ impl ToChalk for TypeCtor {
                 // this should not be reached, since we don't represent TypeName::Error with TypeCtor
                 unreachable!()
             }
-            _ => unimplemented!(), // FIXME
+            TypeName::Generator(_) => unimplemented!(), // FIXME
+            TypeName::GeneratorWitness(_) => unimplemented!(), // FIXME
         }
     }
 }