Changed the error message to more clearly explain what is allowed

src/librustc_typeck/astconv.rs

@@ -1011,7 +1011,7 @@ impl<'o, 'gcx: 'tcx, 'tcx> dyn AstConv<'gcx, 'tcx> + 'o {
 
         if regular_traits.is_empty() && auto_traits.is_empty() {
             span_err!(tcx.sess, span, E0224,
-                "at least one non-builtin trait is required for an object type");
+                "at least one trait is required for an object type");
             return tcx.types.err;
         }
 

src/test/ui/traits/trait-alias/trait-alias-only-maybe-bound.rs

@@ -11,12 +11,12 @@ trait _1 = _0;
 
 // Straight list expansion:
 type _T0 = dyn _1;
-//~^ ERROR at least one non-builtin trait is required for an object type [E0224]
+//~^ ERROR at least one trait is required for an object type [E0224]
 
 // Twice:
 trait _2 = _1 + _1;
 
 type _T1 = dyn _2;
-//~^ ERROR at least one non-builtin trait is required for an object type [E0224]
+//~^ ERROR at least one trait is required for an object type [E0224]
 
 fn main() {}

src/test/ui/traits/trait-alias/trait-alias-only-maybe-bound.stderr

@@ -1,10 +1,10 @@
-error[E0224]: at least one non-builtin trait is required for an object type
+error[E0224]: at least one trait is required for an object type
   --> $DIR/trait-alias-only-maybe-bound.rs:13:12
    |
 LL | type _T0 = dyn _1;
    |            ^^^^^^
 
-error[E0224]: at least one non-builtin trait is required for an object type
+error[E0224]: at least one trait is required for an object type
   --> $DIR/trait-alias-only-maybe-bound.rs:19:12
    |
 LL | type _T1 = dyn _2;

src/test/ui/traits/trait-object-macro-matcher.rs

@@ -8,5 +8,5 @@ fn main() {
     m!(dyn Copy + Send + 'static);
     //~^ ERROR the trait `std::marker::Copy` cannot be made into an object
     m!(dyn 'static + Send);
-    m!(dyn 'static +); //~ ERROR at least one non-builtin trait is required for an object type
+    m!(dyn 'static +); //~ ERROR at least one trait is required for an object type
 }

src/test/ui/traits/trait-object-macro-matcher.stderr

@@ -1,4 +1,4 @@
-error[E0224]: at least one non-builtin trait is required for an object type
+error[E0224]: at least one trait is required for an object type
   --> $DIR/trait-object-macro-matcher.rs:11:8
    |
 LL |     m!(dyn 'static +);

src/test/ui/traits/trait-object-vs-lifetime-2.rs

@@ -5,7 +5,7 @@
 fn g() where
     'static: 'static,
     dyn 'static +: 'static + Copy,
-    //~^ ERROR at least one non-builtin trait is required for an object type
+    //~^ ERROR at least one trait is required for an object type
 {}
 
 fn main() {}

src/test/ui/traits/trait-object-vs-lifetime-2.stderr

@@ -1,4 +1,4 @@
-error[E0224]: at least one non-builtin trait is required for an object type
+error[E0224]: at least one trait is required for an object type
   --> $DIR/trait-object-vs-lifetime-2.rs:7:5
    |
 LL |     dyn 'static +: 'static + Copy,

src/test/ui/traits/trait-object-vs-lifetime.rs

@@ -7,11 +7,11 @@ fn main() {
     // `'static` is a lifetime argument, `'static +` is a type argument
     let _: S<'static, u8>;
     let _: S<'static, dyn 'static +>;
-    //~^ at least one non-builtin trait is required for an object type
+    //~^ at least one trait is required for an object type
     let _: S<'static, 'static>;
     //~^ ERROR wrong number of lifetime arguments: expected 1, found 2
     //~| ERROR wrong number of type arguments: expected 1, found 0
     let _: S<dyn 'static +, 'static>;
     //~^ ERROR lifetime arguments must be declared prior to type arguments
-    //~| ERROR at least one non-builtin trait is required for an object type
+    //~| ERROR at least one trait is required for an object type
 }

src/test/ui/traits/trait-object-vs-lifetime.stderr

@@ -4,7 +4,7 @@ error: lifetime arguments must be declared prior to type arguments
 LL |     let _: S<dyn 'static +, 'static>;
    |                             ^^^^^^^
 
-error[E0224]: at least one non-builtin trait is required for an object type
+error[E0224]: at least one trait is required for an object type
   --> $DIR/trait-object-vs-lifetime.rs:9:23
    |
 LL |     let _: S<'static, dyn 'static +>;
@@ -22,7 +22,7 @@ error[E0107]: wrong number of type arguments: expected 1, found 0
 LL |     let _: S<'static, 'static>;
    |            ^^^^^^^^^^^^^^^^^^^ expected 1 type argument
 
-error[E0224]: at least one non-builtin trait is required for an object type
+error[E0224]: at least one trait is required for an object type
   --> $DIR/trait-object-vs-lifetime.rs:14:14
    |
 LL |     let _: S<dyn 'static +, 'static>;

src/test/ui/traits/wf-trait-object-only-maybe-bound.rs

@@ -1,7 +1,7 @@
 // Test that `dyn ?Sized` (i.e., a trait object with only a maybe buond) is not allowed.
 
 type _0 = dyn ?Sized;
-//~^ ERROR at least one non-builtin trait is required for an object type [E0224]
+//~^ ERROR at least one trait is required for an object type [E0224]
 //~| ERROR ?Trait` is not permitted in trait object types
 
 fn main() {}

src/test/ui/traits/wf-trait-object-only-maybe-bound.stderr

@@ -4,7 +4,7 @@ error: `?Trait` is not permitted in trait object types
 LL | type _0 = dyn ?Sized;
    |               ^^^^^^
 
-error[E0224]: at least one non-builtin trait is required for an object type
+error[E0224]: at least one trait is required for an object type
   --> $DIR/wf-trait-object-only-maybe-bound.rs:3:11
    |
 LL | type _0 = dyn ?Sized;