Implement pointer casting.

src/test/ui/traits/trait-upcasting/replace-vptr.rs

@@ -0,0 +1,49 @@
+// run-pass
+
+#![feature(trait_upcasting)]
+#![allow(incomplete_features)]
+
+trait A {
+    fn foo_a(&self);
+}
+
+trait B {
+    fn foo_b(&self);
+}
+
+trait C: A + B {
+    fn foo_c(&self);
+}
+
+struct S(i32);
+
+impl A for S {
+    fn foo_a(&self) {
+        unreachable!();
+    }
+}
+
+impl B for S {
+    fn foo_b(&self) {
+        assert_eq!(42, self.0);
+    }
+}
+
+impl C for S {
+    fn foo_c(&self) {
+        unreachable!();
+    }
+}
+
+fn invoke_inner(b: &dyn B) {
+    b.foo_b();
+}
+
+fn invoke_outer(c: &dyn C) {
+    invoke_inner(c);
+}
+
+fn main() {
+    let s = S(42);
+    invoke_outer(&s);
+}