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/compile-fail/issue-43431.rs

@@ -11,7 +11,7 @@
 #![feature(fn_traits)]
 
 trait CallSingle<A, B> {
-    fn call(&self, a: A) -> B where Self: Fn(A) -> B;
+    fn call(&self, a: A) -> B where Self: Sized, Self: Fn(A) -> B;
 }
 
 impl<A, B, F: Fn(A) -> B> CallSingle<A, B> for F {

src/test/compile-fail/wf-trait-fn-where-clause.rs

@@ -17,7 +17,7 @@
 struct Bar<T:Eq+?Sized> { value: Box<T> }
 
 trait Foo {
-    fn bar(&self) where Bar<Self>: Copy;
+    fn bar(&self) where Self: Sized, Bar<Self>: Copy;
         //~^ ERROR E0277
         //
         // Here, Eq ought to be implemented.

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 +1,29 @@
+// 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
+