Self in where clauses may not be object safe

This is virtually certain to cause regressions, needs crater.

In #50781 it was discovered that our object safety rules are not sound because we allow `Self` in where clauses without restrain. This PR is a direct fix to the rules so that we disallow methods with unsound where clauses.

This currently uses hard error to measure impact, but we will want to downgrade it to a future compat error.

Fixes #50781.

r? @nikomatsakis

src/test/run-pass/issue-23435.rs

@@ -1,37 +0,0 @@
-// 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.
-
-// Test that we do not ICE when a default method implementation has
-// requirements (in this case, `Self : Baz`) that do not hold for some
-// specific impl (in this case, `Foo : Bar`). This causes problems
-// only when building a vtable, because that goes along and
-// instantiates all the methods, even those that could not otherwise
-// be called.
-
-// pretty-expanded FIXME #23616
-
-struct Foo {
-    x: i32
-}
-
-trait Bar {
-    fn bar(&self) where Self : Baz { self.baz(); }
-}
-
-trait Baz {
-    fn baz(&self);
-}
-
-impl Bar for Foo {
-}
-
-fn main() {
-    let x: &Bar = &Foo { x: 22 };
-}

src/test/ui/issue-50781.rs

@@ -0,0 +1,27 @@
+// Copyright 2018 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 {}
+
+trait X {
+    fn foo(&self) where Self: Trait;
+}
+
+impl X for () {
+    fn foo(&self) {}
+}
+
+impl Trait for dyn X {}
+//~^ ERROR the trait `X` cannot be made into an object
+
+pub fn main() {
+    // Check that this does not segfault.
+    <X as X>::foo(&());
+}

src/test/ui/issue-50781.stderr

@@ -0,0 +1,11 @@
+error[E0038]: the trait `X` cannot be made into an object
+  --> $DIR/issue-50781.rs:21:6
+   |
+LL | impl Trait for dyn X {}
+   |      ^^^^^ the trait `X` cannot be made into an object
+   |
+   = note: method `foo` references the `Self` type in its arguments or return type
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0038`.