Add regression tests for #21174

Closes #21174.

src/test/compile-fail/issue-21174.rs

+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+trait Trait<'a> {
+    type A;
+    type B;
+}
+
+fn foo<'a, T: Trait<'a>>(value: T::A) {
+    let new: T::B = unsafe { std::mem::transmute(value) };
+//~^ ERROR: cannot transmute to or from a type that contains type parameters in its interior [E0139]
+}
+
+fn main() { }

src/test/run-pass/issue-21174.rs

+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+trait Trait<'a> {
+    type A;
+    type B;
+}
+
+fn foo<'a, T: Trait<'a>>(value: T::A) {
+    let new: T::B = unsafe { std::mem::transmute_copy(&value) };
+}
+
+fn main() { }