NLL regression: lifetime 'static required

[timvermeulen]

NLL causes compilation to fail here:

fn f(s: &str) {
    g(s, |x| h(x));
}

fn g<T, F>(_: T, _: F)
where F: Fn(&mut (&(T, T), T)) {}

fn h<T>(_: &mut (&(T, T), T)) {}

error[E0621]: explicit lifetime required in the type of `s`
 --> src/lib.rs:2:5
  |
1 | fn f(s: &str) {
  |         ---- help: add explicit lifetime `'static` to the type of `s`: `&'static str`
2 |     g(s, |x| h(x));
  |     ^^^^^^^^^^^^^^ lifetime `'static` required

error: aborting due to previous error

For more information about this error, try `rustc --explain E0621`.

Oddly, it does compile if I replace g(s, |x| h(x)) by g(s, h). Alternatively, if I give &str an explicit lifetime 'a, the compiler says that s escapes the function body:

warning[E0521]: borrowed data escapes outside of function
 --> src/lib.rs:2:5
  |
1 | fn f<'a>(s: &'a str) {
  |          - `s` is a reference that is only valid in the function body
2 |     g(s, |x| h(x));
  |     ^^^^^^^^^^^^^^ `s` escapes the function body here
  |
  = warning: this error has been downgraded to a warning for backwards compatibility with previous releases
  = warning: this represents potential undefined behavior in your code and this warning will become a hard error in the future

[matthewjasper]

relevant code (cc @nikomatsakis):

rust/src/librustc_mir/borrow_check/nll/type_check/free_region_relations.rs

Lines 160 to 165 in 8ae730a

	// In case we find more than one, reduce to one for
	// convenience. This is to prevent us from generating more
	// complex constraints, but it will cause spurious errors.
	let post_dom = relation
	.mutual_immediate_postdominator(external_parents)
	.cloned();