Add feature gate tests.

compiler/rustc_feature/src/active.rs

@@ -685,7 +685,7 @@ declare_features! (
 
     /// Allows upcasting trait objects via supertraits.
     /// Trait upcasting is casting, e.g., `dyn Foo -> dyn Bar` where `Foo: Bar`.
-    (active, trait_upcasting, "1.56.0", Some(65991), None),
+    (incomplete, trait_upcasting, "1.56.0", Some(65991), None),
 
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates

src/test/ui/feature-gates/feature-gate-trait_upcasting.rs

@@ -0,0 +1,13 @@
+trait Foo {}
+
+trait Bar: Foo {}
+
+impl Foo for () {}
+
+impl Bar for () {}
+
+fn main() {
+    let bar: &dyn Bar = &();
+    let foo: &dyn Foo = bar;
+    //~^ ERROR trait upcasting is experimental [E0658]
+}

src/test/ui/feature-gates/feature-gate-trait_upcasting.stderr

@@ -0,0 +1,12 @@
+error[E0658]: trait upcasting is experimental
+  --> $DIR/feature-gate-trait_upcasting.rs:11:25
+   |
+LL |     let foo: &dyn Foo = bar;
+   |                         ^^^
+   |
+   = note: see issue #65991 <https://github.com/rust-lang/rust/issues/65991> for more information
+   = help: add `#![feature(trait_upcasting)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/issues/issue-11515.rs

@@ -6,5 +6,5 @@ struct Test {
 
 fn main() {
     let closure: Box<dyn Fn() + 'static> = Box::new(|| ());
-    let test = box Test { func: closure }; //~ ERROR mismatched types
+    let test = box Test { func: closure }; //~ ERROR trait upcasting is experimental [E0658]
 }

src/test/ui/issues/issue-11515.stderr

@@ -1,12 +1,12 @@
-error[E0308]: mismatched types
+error[E0658]: trait upcasting is experimental
   --> $DIR/issue-11515.rs:9:33
    |
 LL |     let test = box Test { func: closure };
-   |                                 ^^^^^^^ expected trait `FnMut`, found trait `Fn`
+   |                                 ^^^^^^^
    |
-   = note: expected struct `Box<(dyn FnMut() + 'static)>`
-              found struct `Box<(dyn Fn() + 'static)>`
+   = note: see issue #65991 <https://github.com/rust-lang/rust/issues/65991> for more information
+   = help: add `#![feature(trait_upcasting)]` to the crate attributes to enable
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0308`.
+For more information about this error, try `rustc --explain E0658`.