report an error if we see an unexpected lifetime in impl Trait

But leave closure substs alone.

src/test/ui/impl-trait/region-escape-via-bound-contravariant.rs

@@ -0,0 +1,35 @@
+// Copyright 2016 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.
+
+// In contrast to `region-escape-via-bound-invariant`, in this case we
+// *can* return a value of type `&'x u32`, even though `'x` does not
+// appear in the bounds. This is because `&` is contravariant, and so
+// we are *actually* returning a `&'y u32`.
+//
+// See https://github.com/rust-lang/rust/issues/46541 for more details.
+
+// run-pass
+
+#![allow(dead_code)]
+#![feature(conservative_impl_trait)]
+#![feature(in_band_lifetimes)]
+#![feature(nll)]
+
+trait Trait<'a> { }
+
+impl Trait<'b> for &'a u32 { }
+
+fn foo(x: &'x u32) -> impl Trait<'y>
+where 'x: 'y
+{
+    x
+}
+
+fn main() { }

src/test/ui/impl-trait/region-escape-via-bound.rs

@@ -0,0 +1,34 @@
+// Copyright 2016 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 allow the region `'x` to escape in the impl
+// trait **even though** `'y` escapes, which outlives `'x`.
+//
+// See https://github.com/rust-lang/rust/issues/46541 for more details.
+
+#![allow(dead_code)]
+#![feature(conservative_impl_trait)]
+#![feature(in_band_lifetimes)]
+#![feature(nll)]
+
+use std::cell::Cell;
+
+trait Trait<'a> { }
+
+impl Trait<'b> for Cell<&'a u32> { }
+
+fn foo(x: Cell<&'x u32>) -> impl Trait<'y>
+    //~^ ERROR hidden type for `impl Trait` captures lifetime that does not appear in bounds [E0909]
+where 'x: 'y
+{
+    x
+}
+
+fn main() { }

src/test/ui/impl-trait/region-escape-via-bound.stderr

@@ -0,0 +1,20 @@
+error[E0909]: hidden type for `impl Trait` captures lifetime that does not appear in bounds
+  --> $DIR/region-escape-via-bound.rs:27:29
+   |
+LL | fn foo(x: Cell<&'x u32>) -> impl Trait<'y>
+   |                             ^^^^^^^^^^^^^^
+   |
+note: hidden type `std::cell::Cell<&'x u32>` captures the lifetime 'x as defined on the function body at 27:1
+  --> $DIR/region-escape-via-bound.rs:27:1
+   |
+LL | / fn foo(x: Cell<&'x u32>) -> impl Trait<'y>
+LL | |     //~^ ERROR hidden type for `impl Trait` captures lifetime that does not appear in bounds [E0909]
+LL | | where 'x: 'y
+LL | | {
+LL | |     x
+LL | | }
+   | |_^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0909`.