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/librustc/traits/object_safety.rs

@@ -288,6 +288,17 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
             return Some(MethodViolationCode::Generic);
         }
 
+        if self.predicates_of(method.def_id).predicates.into_iter()
+                // A trait object can't claim to live more than the concrete type,
+                // so outlives predicates will always hold.
+                .filter(|p| p.to_opt_type_outlives().is_none())
+                .collect::<Vec<_>>()
+                // Do a shallow visit so that `contains_illegal_self_type_reference`
+                // may apply it's custom visiting.
+                .visit_tys_shallow(|t| self.contains_illegal_self_type_reference(trait_def_id, t)) {
+            return Some(MethodViolationCode::ReferencesSelf);
+        }
+
         None
     }
 

src/librustc/ty/fold.rs

@@ -136,6 +136,20 @@ pub trait TypeFoldable<'tcx>: fmt::Debug + Clone {
     fn has_late_bound_regions(&self) -> bool {
         self.has_type_flags(TypeFlags::HAS_RE_LATE_BOUND)
     }
+
+    /// A visitor that does not recurse into types, works like `fn walk_shallow` in `Ty`.
+    fn visit_tys_shallow(&self, visit: impl FnMut(Ty<'tcx>) -> bool) -> bool {
+
+        pub struct Visitor<F>(F);
+
+        impl<'tcx, F: FnMut(Ty<'tcx>) -> bool> TypeVisitor<'tcx> for Visitor<F> {
+            fn visit_ty(&mut self, ty: Ty<'tcx>) -> bool {
+                self.0(ty)
+            }
+        }
+
+        self.visit_with(&mut Visitor(visit))
+    }
 }
 
 /// The TypeFolder trait defines the actual *folding*. There is a

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`.