add test comparing free region to bound region

suggested by arielb1

src/test/ui/nll/closure-requirements/propagate-approximated-shorter-to-static-comparing-against-free.rs

+// 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 a case where we setup relationships like `'x: 'a` or `'a: 'x`,
+// where `'x` is bound in closure type but `'a` is free. This forces
+// us to approximate `'x` one way or the other.
+
+// compile-flags:-Znll -Zborrowck=mir -Zverbose
+
+#![feature(rustc_attrs)]
+
+use std::cell::Cell;
+
+fn foo<'a, F>(_cell: Cell<&'a u32>, _f: F)
+where
+    F: for<'x> FnOnce(Cell<&'a u32>, Cell<&'x u32>),
+{
+}
+
+#[rustc_regions]
+fn case1() {
+    let a = 0;
+    let cell = Cell::new(&a);
+    foo(cell, |cell_a, cell_x| {
+        //~^ WARNING not reporting region error due to -Znll
+        cell_a.set(cell_x.get()); // forces 'x: 'a, error in closure
+        //~^ ERROR free region `'_#2r` does not outlive free region `'_#1r`
+    })
+}
+
+#[rustc_regions]
+fn case2() {
+    let a = 0;
+    let cell = Cell::new(&a);
+
+    // As you can see in the stderr output, this closure propoagates a
+    // requirement that `'a: 'static'.
+    //
+    // FIXME(#45827) However, because of shortcomings in the MIR type
+    // checker, this does not result in errors later on (yet).
+    foo(cell, |cell_a, cell_x| {
+        cell_x.set(cell_a.get()); // forces 'a: 'x, implies 'a = 'static -> borrow error
+    })
+}
+
+fn main() { }

src/test/ui/nll/closure-requirements/propagate-approximated-shorter-to-static-comparing-against-free.stderr

+warning: not reporting region error due to -Znll
+  --> $DIR/propagate-approximated-shorter-to-static-comparing-against-free.rs:31:5
+   |
+31 |     foo(cell, |cell_a, cell_x| {
+   |     ^^^
+
+error: free region `'_#2r` does not outlive free region `'_#1r`
+  --> $DIR/propagate-approximated-shorter-to-static-comparing-against-free.rs:33:9
+   |
+33 |         cell_a.set(cell_x.get()); // forces 'x: 'a, error in closure
+   |         ^^^^^^
+
+note: External requirements
+  --> $DIR/propagate-approximated-shorter-to-static-comparing-against-free.rs:31:15
+   |
+31 |       foo(cell, |cell_a, cell_x| {
+   |  _______________^
+32 | |         //~^ WARNING not reporting region error due to -Znll
+33 | |         cell_a.set(cell_x.get()); // forces 'x: 'a, error in closure
+34 | |         //~^ ERROR free region `'_#2r` does not outlive free region `'_#1r`
+35 | |     })
+   | |_____^
+   |
+   = note: defining type: DefId(0/1:12 ~ propagate_approximated_shorter_to_static_comparing_against_free[317d]::case1[0]::{{closure}}[0]) with closure substs [
+               i32,
+               for<'r> extern "rust-call" fn((std::cell::Cell<&'_#1r u32>, std::cell::Cell<&ReLateBound(DebruijnIndex { depth: 1 }, BrNamed(crate0:DefIndex(0:0), 'r)) u32>))
+           ]
+   = note: number of external vids: 2
+
+note: No external requirements
+  --> $DIR/propagate-approximated-shorter-to-static-comparing-against-free.rs:28:1
+   |
+28 | / fn case1() {
+29 | |     let a = 0;
+30 | |     let cell = Cell::new(&a);
+31 | |     foo(cell, |cell_a, cell_x| {
+...  |
+35 | |     })
+36 | | }
+   | |_^
+   |
+   = note: defining type: DefId(0/0:5 ~ propagate_approximated_shorter_to_static_comparing_against_free[317d]::case1[0]) with substs []
+
+note: External requirements
+  --> $DIR/propagate-approximated-shorter-to-static-comparing-against-free.rs:48:15
+   |
+48 |       foo(cell, |cell_a, cell_x| {
+   |  _______________^
+49 | |         cell_x.set(cell_a.get()); // forces 'a: 'x, implies 'a = 'static -> borrow error
+50 | |     })
+   | |_____^
+   |
+   = note: defining type: DefId(0/1:13 ~ propagate_approximated_shorter_to_static_comparing_against_free[317d]::case2[0]::{{closure}}[0]) with closure substs [
+               i32,
+               for<'r> extern "rust-call" fn((std::cell::Cell<&'_#1r u32>, std::cell::Cell<&ReLateBound(DebruijnIndex { depth: 1 }, BrNamed(crate0:DefIndex(0:0), 'r)) u32>))
+           ]
+   = note: number of external vids: 2
+   = note: where '_#1r: '_#0r
+
+note: No external requirements
+  --> $DIR/propagate-approximated-shorter-to-static-comparing-against-free.rs:39:1
+   |
+39 | / fn case2() {
+40 | |     let a = 0;
+41 | |     let cell = Cell::new(&a);
+42 | |
+...  |
+50 | |     })
+51 | | }
+   | |_^
+   |
+   = note: defining type: DefId(0/0:6 ~ propagate_approximated_shorter_to_static_comparing_against_free[317d]::case2[0]) with substs []
+
+error: aborting due to previous error
+