Auto merge of #24527 - nikomatsakis:issue-24085, r=nikomatsakis

Expand the "givens" set to cover transitive relations.  The givens array
stores relationships like `'c <= '0` (where `'c` is a free region and
`'0` is an inference variable) that are derived from closure
arguments. These are (rather hackily) ignored for purposes of inference,
preventing spurious errors. The current code did not handle transitive
cases like `'c <= '0` and `'0 <= '1`. Fixes #24085.

r? @pnkfelix 
cc @bkoropoff

*But* I am not sure whether this fix will have a compile-time hit. I'd like to push to try branch observe cycle times.

src/test/run-pass/issue-24085.rs

@@ -0,0 +1,27 @@
+// 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.
+
+// Regression test for #24085. Errors were occuring in region
+// inference due to the requirement that `'a:b'`, which was getting
+// incorrectly translated in connection with the closure below.
+
+#[derive(Copy,Clone)]
+struct Path<'a:'b, 'b> {
+    x: &'a i32,
+    tail: Option<&'b Path<'a, 'b>>
+}
+
+#[allow(dead_code, unconditional_recursion)]
+fn foo<'a,'b,F>(p: Path<'a, 'b>, mut f: F)
+                where F: for<'c> FnMut(Path<'a, 'c>) {
+    foo(p, |x| f(x))
+}
+
+fn main() { }