Auto merge of #51215 - eddyb:visit-for-a-lifetime, r=nikomatsakis

rustc: don't visit lifetime parameters through visit_lifetime.

Ideally we'd also not use the `Lifetime` struct for parameters, but I'll leave that to @varkor (#48149).
Fixes #51185 (discovered while auditing all the `visit_lifetime` implementations).
r? @nikomatsakis

src/test/compile-fail/underscore-lifetime-binders.rs

@@ -23,7 +23,6 @@ impl<'a> Meh<'a> for u8 {}
 
 fn meh() -> Box<for<'_> Meh<'_>> //~ ERROR invalid lifetime parameter name: `'_`
 //~^ ERROR missing lifetime specifier
-//~^^ ERROR missing lifetime specifier
 {
   Box::new(5u8)
 }

src/test/run-pass/issue-51185.rs

@@ -0,0 +1,17 @@
+// 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.
+
+fn foo() -> impl Into<for<'a> fn(&'a ())> {
+    (|_| {}) as for<'a> fn(&'a ())
+}
+
+fn main() {
+    foo().into()(&());
+}