Prohibit type parameter shadowing with a clunky hammer.

This is a [breaking-change].

Change

    impl<T> Foo<T> {
        fn bar<T>(...

to (for example)

    impl<T> Foo<T> {
        fn bar<U>(...

Per RFC 459.

Closes #19390.

src/test/compile-fail/shadowed-type-parameter.rs

@@ -0,0 +1,36 @@
+// Copyright 2013 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.
+
+// Test that shadowed lifetimes generate an error.
+
+struct Foo<T>;
+
+impl<T> Foo<T> {
+    fn shadow_in_method<T>(&self) {}
+    //~^ ERROR type parameter `T` shadows another type parameter
+
+    fn not_shadow_in_item<U>(&self) {
+        struct Bar<T, U>; // not a shadow, separate item
+        fn foo<T, U>() {} // same
+    }
+}
+
+trait<T> Bar<T> {
+    fn shadow_in_required<T>(&self);
+    //~^ ERROR type parameter `T` shadows another type parameter
+
+    fn shadow_in_provided<T>(&self) {}
+    //~^ ERROR type parameter `T` shadows another type parameter
+
+    fn not_shadow_in_required<U>(&self);
+    fn not_shadow_in_provided<U>(&self) {}
+}
+
+fn main() {}