check the self type is well-formed

This fixes `issue-28848.rs` -- it also handles another case that the
AST region checker gets wrong (`wf-self-type.rs`).  I don't actually
think that this is the *right way* to be enforcing this constraint --
I think we should probably do it more generally, perhaps by editing
`predicates_of` for the impl itself. The chalk-style implied bounds
setup ought to fix this.

src/librustc_mir/borrow_check/nll/type_check/mod.rs

@@ -1033,6 +1033,8 @@ fn relate_type_and_user_type(
                     assert!(!impl_self_ty.has_infer_types());
 
                     self.eq_types(self_ty, impl_self_ty, locations, category)?;
+
+                    self.prove_predicate(ty::Predicate::WellFormed(impl_self_ty), locations, category);
                 }
 
                 // Prove the predicates coming along with `def_id`.

src/test/ui/issues/issue-28848.rs

@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// ignore-compare-mode-nll
-
 struct Foo<'a, 'b: 'a>(&'a &'b ());
 
 impl<'a, 'b> Foo<'a, 'b> {

src/test/ui/issues/issue-28848.stderr

 error[E0478]: lifetime bound not satisfied
-  --> $DIR/issue-28848.rs:22:5
+  --> $DIR/issue-28848.rs:20:5
    |
 LL |     Foo::<'a, 'b>::xmute(u) //~ ERROR lifetime bound not satisfied
    |     ^^^^^^^^^^^^^^^^^^^^
    |
-note: lifetime parameter instantiated with the lifetime 'b as defined on the function body at 21:16
-  --> $DIR/issue-28848.rs:21:16
+note: lifetime parameter instantiated with the lifetime 'b as defined on the function body at 19:16
+  --> $DIR/issue-28848.rs:19:16
    |
 LL | pub fn foo<'a, 'b>(u: &'b ()) -> &'a () {
    |                ^^
-note: but lifetime parameter must outlive the lifetime 'a as defined on the function body at 21:12
-  --> $DIR/issue-28848.rs:21:12
+note: but lifetime parameter must outlive the lifetime 'a as defined on the function body at 19:12
+  --> $DIR/issue-28848.rs:19:12
    |
 LL | pub fn foo<'a, 'b>(u: &'b ()) -> &'a () {
    |            ^^

src/test/ui/nll/user-annotations/wf-self-type.rs

+// Copyright 2017 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.
+
+#![feature(nll)]
+
+struct Foo<'a, 'b: 'a>(&'a &'b ());
+
+impl<'a, 'b> Foo<'a, 'b> {
+    fn xmute(a: &'b ()) -> &'a () {
+        unreachable!()
+    }
+}
+
+pub fn foo<'a, 'b>(u: &'b ()) -> &'a () {
+    Foo::xmute(u) //~ ERROR unsatisfied lifetime constraints
+}
+
+fn main() {}

src/test/ui/nll/user-annotations/wf-self-type.stderr

+error: unsatisfied lifetime constraints
+  --> $DIR/wf-self-type.rs:22:5
+   |
+LL | pub fn foo<'a, 'b>(u: &'b ()) -> &'a () {
+   |            --  -- lifetime `'b` defined here
+   |            |
+   |            lifetime `'a` defined here
+LL |     Foo::xmute(u) //~ ERROR unsatisfied lifetime constraints
+   |     ^^^^^^^^^^^^^ returning this value requires that `'b` must outlive `'a`
+
+error: aborting due to previous error
+