NLL: Better Diagnostic When "Later" means "A Future Loop Iteration"

[davidtwco]

Part of #52663.

r? @pnkfelix

[pnkfelix]

This is officially awesome. Check out those diagnostics, especially for a case like

rust/src/test/ui/issue-52126-assign-op-invariance.rs

Lines 42 to 58 in 1863cb7

	let mut acc = Counter{map: HashMap::new()};
	for line in vec!["123456789".to_string(), "12345678".to_string()] {
	let v: Vec<&str> = line.split_whitespace().collect();
	//~^ ERROR `line` does not live long enough
	println!("accumulator before add_assign {:?}", acc.map);
	let mut map = HashMap::new();
	for str_ref in v {
	let e = map.entry(str_ref);
	println!("entry: {:?}", e);
	let count = e.or_insert(0);
	*count += 1;
	}
	let cnt2 = Counter{map};
	acc += cnt2;
	println!("accumulator after add_assign {:?}", acc.map);
	// line gets dropped here but references are kept in acc.map
	}

which previously under AST-borrowck printed:

rust/src/test/ui/issue-52126-assign-op-invariance.stderr

Lines 1 to 10 in 1863cb7

	error[E0597]: `line` does not live long enough
	--> $DIR/issue-52126-assign-op-invariance.rs:44:28
	|
	LL | let v: Vec<&str> = line.split_whitespace().collect();
	| ^^^^ borrowed value does not live long enough
	...
	LL | }
	| - `line` dropped here while still borrowed
	LL | }
	| - borrowed value needs to live until here

but with this PR prints:

rust/src/test/ui/issue-52126-assign-op-invariance.nll.stderr

Lines 1 to 11 in 1863cb7

	error[E0597]: `line` does not live long enough
	--> $DIR/issue-52126-assign-op-invariance.rs:44:28
	|
	LL | let v: Vec<&str> = line.split_whitespace().collect();
	| ^^^^ borrowed value does not live long enough
	LL | //~^ ERROR `line` does not live long enough
	LL | println!("accumulator before add_assign {:?}", acc.map);
	| ------- borrow used here in later iteration of loop
	...
	LL | }
	| - `line` dropped here while still borrowed

[pnkfelix]

@bors r+

[bors]

📌 Commit 1863cb7 has been approved by pnkfelix

[bors]

⌛ Testing commit 1863cb7 with merge e415b5e...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: pnkfelix
Pushing e415b5e to master...