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/ui/issue-50781.rs

@@ -0,0 +1,29 @@
+// 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.
+
+#![deny(where_clauses_object_safety)]
+
+trait Trait {}
+
+trait X {
+    fn foo(&self) where Self: Trait; //~ ERROR the trait `X` cannot be made into an object
+    //~^ WARN this was previously accepted by the compiler but is being phased out
+}
+
+impl X for () {
+    fn foo(&self) {}
+}
+
+impl Trait for dyn X {}
+
+pub fn main() {
+    // Check that this does not segfault.
+    <X as X>::foo(&());
+}

src/test/ui/issue-50781.stderr

@@ -0,0 +1,17 @@
+error: the trait `X` cannot be made into an object
+  --> $DIR/issue-50781.rs:16:5
+   |
+LL |     fn foo(&self) where Self: Trait; //~ ERROR the trait `X` cannot be made into an object
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: lint level defined here
+  --> $DIR/issue-50781.rs:11:9
+   |
+LL | #![deny(where_clauses_object_safety)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #51443 <https://github.com/rust-lang/rust/issues/51443>
+   = note: method `foo` references the `Self` type in where clauses
+
+error: aborting due to previous error
+