Borrowing an immutable reference of a mutable reference through a function call in a loop is not accepted

[shepmaster]

This code appears to be sound because event should either leave the loop or be thrown away before the next iteration:

#![feature(nll)]

fn next<'buf>(buffer: &'buf mut String) -> &'buf str {
    loop {
        let event = parse(buffer);

        if true {
            return event;
        }
    }
}

fn parse<'buf>(_buffer: &'buf mut String) -> &'buf str {
    unimplemented!()
}

fn main() {}

The current (1.28.0-nightly 2018-05-25 990d8aa) implementation still marks this as an error:

error[E0499]: cannot borrow `*buffer` as mutable more than once at a time
 --> src/main.rs:5:27
  |
5 |         let event = parse(buffer);
  |                           ^^^^^^ mutable borrow starts here in previous iteration of loop
  |
note: borrowed value must be valid for the lifetime 'buf as defined on the function body at 3:1...
 --> src/main.rs:3:1
  |
3 | fn next<'buf>(buffer: &'buf mut String) -> &'buf str {
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

@nikomatsakis said:

this should throw an error (for now) but this error should eventually go away. This is precisely a case where "Location sensitivity" is needed, but we removed that feature in the name of performance — once polonius support lands, though, this code would be accepted. That may or may not be before the edition.

[nikomatsakis]

Tagging as NLL-deferred. This is a polonius problem. =)

[PvdBerg1998]

Recently bumped into this problem. Would love to see this fixed!

[let4be]

Any update on this?...

[Legend-of-iPhoenix]

It's worth noting for people coming across this issue in the future that inlining the function being called (in this case, parse) generally resolves this error- see https://stackoverflow.com/a/50570026

Not sure what to do when the function being called is sizable/too messy to inline, though :(