ReSkolemized can leak into error messages

[arielb1]

STR

fn foo(f: for<'a,'b> fn(&'a ()) -> &'a &'b ()) {
    bar(f);
}

fn bar(f: for<'a,'b> fn(&'b ()) -> &'a &'b ()) {
}

fn main() {}

Actual Results

<anon>:2:9: 2:10 error: mismatched types:
 expected `fn(&'b ()) -> &'a &'b ()`,
    found `fn(&'a ()) -> &'a &'b ()`
(expected concrete lifetime,
    found bound lifetime parameter 'a) [E0308]
<anon>:2     bar(f);
                 ^
<anon>:2:9: 2:10 help: see the detailed explanation for E0308
note: expected concrete lifetime is lifetime ReInfer(ReSkolemized(0, BrNamed(DefId { krate: 0, node: 31 }, "\'b"(64))))
error: aborting due to previous error
playpen: application terminated with error code 101

Expected Results

No leak of rustc internals into error messages.

[Stebalien]

Lines 172-174 of src/librustc/util/ppaux.rs. /cc @nikomatsakis

[jroesch]

cc me

[ghost]

Gonna close this as a dupe of #13998 if you don't mind.