Update error message for E0172

src/librustc_typeck/astconv.rs

@@ -1075,8 +1075,10 @@ impl<'o, 'gcx: 'tcx, 'tcx> AstConv<'gcx, 'tcx>+'o {
                         Ok((trait_ref, projection_bounds))
                     }
                     _ => {
-                        span_err!(self.tcx().sess, ty.span, E0172,
-                                  "expected a reference to a trait");
+                        struct_span_err!(self.tcx().sess, ty.span, E0172,
+                                  "expected a reference to a trait")
+                            .span_label(ty.span, &format!("expected a trait"))
+                            .emit();
                         Err(ErrorReported)
                     }
                 }

src/test/compile-fail/E0172.rs

@@ -8,7 +8,9 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-fn foo(bar: i32+std::fmt::Display) {} //~ ERROR E0172
+fn foo(bar: i32+std::fmt::Display) {}
+    //~^ ERROR E0172
+    //~| NOTE expected a trait
 
 fn main() {
 }