Auto merge of #50966 - leodasvacas:self-in-where-clauses-is-not-object-safe, r=nikomatsakis

`Self` in where clauses may not be object safe

Needs crater, virtually certain to cause regressions.

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.

Part of #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 };
-}