rollup merge of #20263: crhino/lifetime-elision-help

Fixes #19707.

In terms of output, it currently uses the form `argument #1`, `argument #2`, etc. If anyone has any better suggestions I would be glad to consider them.

src/test/compile-fail/issue-19707.rs

@@ -0,0 +1,20 @@
+// Copyright 2014 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(unboxed_closures)]
+#![allow(dead_code)]
+
+type foo = fn(&u8, &u8) -> &u8; //~ ERROR missing lifetime specifier
+//~^ HELP the signature does not say whether it is borrowed from argument 1 or argument 2
+
+fn bar<F: Fn(&u8, &u8) -> &u8>(f: &F) {} //~ ERROR missing lifetime specifier
+//~^ HELP the signature does not say whether it is borrowed from argument 1 or argument 2
+
+fn main() {}