Expand const-if-const trait bounds correctly

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -1567,8 +1567,18 @@ impl<'a> State<'a> {
 
             match bound {
                 GenericBound::Trait(tref, modifier) => {
-                    if modifier == &TraitBoundModifier::Maybe {
-                        self.word("?");
+                    match modifier {
+                        TraitBoundModifier::None => {}
+                        TraitBoundModifier::Maybe => {
+                            self.word("?");
+                        }
+                        TraitBoundModifier::MaybeConst => {
+                            self.word_space("~const");
+                        }
+                        TraitBoundModifier::MaybeConstMaybe => {
+                            self.word_space("~const");
+                            self.word("?");
+                        }
                     }
                     self.print_poly_trait_ref(tref);
                 }

tests/ui/macros/stringify.rs

@@ -626,7 +626,7 @@ fn test_item() {
         stringify_item!(
             impl ~const Struct {}
         ),
-        "impl Struct {}", // FIXME
+        "impl ~const Struct {}",
     );
 
     // ItemKind::MacCall
@@ -838,7 +838,7 @@ fn test_ty() {
     assert_eq!(stringify_ty!(dyn Send + 'a), "dyn Send + 'a");
     assert_eq!(stringify_ty!(dyn 'a + Send), "dyn 'a + Send");
     assert_eq!(stringify_ty!(dyn ?Sized), "dyn ?Sized");
-    assert_eq!(stringify_ty!(dyn ~const Clone), "dyn Clone"); // FIXME
+    assert_eq!(stringify_ty!(dyn ~const Clone), "dyn ~const Clone");
     assert_eq!(stringify_ty!(dyn for<'a> Send), "dyn for<'a> Send");
 
     // TyKind::ImplTrait
@@ -846,7 +846,7 @@ fn test_ty() {
     assert_eq!(stringify_ty!(impl Send + 'a), "impl Send + 'a");
     assert_eq!(stringify_ty!(impl 'a + Send), "impl 'a + Send");
     assert_eq!(stringify_ty!(impl ?Sized), "impl ?Sized");
-    assert_eq!(stringify_ty!(impl ~const Clone), "impl Clone"); // FIXME
+    assert_eq!(stringify_ty!(impl ~const Clone), "impl ~const Clone");
     assert_eq!(stringify_ty!(impl for<'a> Send), "impl for<'a> Send");
 
     // TyKind::Paren

tests/ui/unpretty/ast-const-trait-bound.rs

@@ -0,0 +1,4 @@
+// compile-flags: -Zunpretty=normal
+// check-pass
+
+fn foo() where T: ~const Bar {}

tests/ui/unpretty/ast-const-trait-bound.stdout

@@ -0,0 +1,4 @@
+// compile-flags: -Zunpretty=normal
+// check-pass
+
+fn foo() where T: ~const Bar {}