Double mutable borrow error in a loop happens even

Question

Suppose I have several structures like in the following example, and in the next() method I need to pull the next event using a user-provided buffer, but if this event is a comment, and ignore comments flag is set to true, I need to pull the next event again:

struct Parser {
    ignore_comments: bool,
}

enum XmlEvent<'buf> {
    Comment(&'buf str),
    Other(&'buf str),
}

impl Parser {
    fn next<'buf>(&mut self, buffer: &'buf mut String) -> XmlEvent<'buf> {
        let result = loop {
            buffer.clear();

            let temp_event = self.parse_outside_tag(buffer);

            match temp_event {
                XmlEvent::Comment(_) if self.ignore_comments => {}
                _ => break temp_event,
            }
        };
        result
    }

    fn parse_outside_tag<'buf>(&mut self, _buffer: &'buf mut String) -> XmlEvent<'buf> {
        unimplemented!()
    }
}

This code, however, gives a double borrow error, even when I have #![feature(nll)] enabled:

error[E0499]: cannot borrow `*buffer` as mutable more than once at a time
  --> src/main.rs:14:13
   |
14 |             buffer.clear();
   |             ^^^^^^ second mutable borrow occurs here
15 |             
16 |             let temp_event = self.parse_outside_tag(buffer);
   |                                                     ------ first mutable borrow occurs here
   |
note: borrowed value must be valid for the lifetime 'buf as defined on the method body at 12:5...
  --> src/main.rs:12:5
   |
12 |     fn next<'buf>(&mut self, buffer: &'buf mut String) -> XmlEvent<'buf> {
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error[E0499]: cannot borrow `*buffer` as mutable more than once at a time
  --> src/main.rs:16:53
   |
16 |             let temp_event = self.parse_outside_tag(buffer);
   |                                                     ^^^^^^ mutable borrow starts here in previous iteration of loop
   |
note: borrowed value must be valid for the lifetime 'buf as defined on the method body at 12:5...
  --> src/main.rs:12:5
   |
12 |     fn next<'buf>(&mut self, buffer: &'buf mut String) -> XmlEvent<'buf> {
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to 2 previous errors

I can (approximately at least) understand why an error could happen here with the NLL feature turned off, but I don't understand why it happens with NLL.

Anyway, my end goal is to implement this without flags, so I also tried doing this (it is recursive, which is really unfortunate, but all non-recursive versions I came up with could not possibly work without NLL):

fn next<'buf>(&mut self, buffer: &'buf mut String) -> XmlEvent<'buf> {
    buffer.clear();

    {
        let temp_event = self.parse_outside_tag(buffer);

        match temp_event {
            XmlEvent::Comment(_) if self.ignore_comments => {}
            _ => return temp_event,
        }
    }

    self.next(buffer)
}

Here I tried to confine the borrow inside a lexical block, and nothing from this block leaks to the outside. However, I'm still getting an error:

error[E0499]: cannot borrow `*buffer` as mutable more than once at a time
  --> src/main.rs:23:19
   |
15 |             let temp_event = self.parse_outside_tag(buffer);
   |                                                     ------ first mutable borrow occurs here
...
23 |         self.next(buffer)
   |                   ^^^^^^ second mutable borrow occurs here
24 |     }
   |     - first borrow ends here

error: aborting due to previous error

And again, NLL does not fix it.

It has been a long time since I encountered a borrow checking error which I don't understand, so I'm hoping it is actually something simple which I'm overlooking for some reason :)

I really suspect that the root cause is somehow connected with the explicit 'buf lifetime (in particular, errors with the NLL flag turned on have these notes about it), but I can't understand what exactly is wrong here.

Answer 1

This is a limitation of the current implementation of non-lexical lifetimes This can be shown with this reduced case:

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() {}

---

This limitation prevents NLL case #3: conditional control flow across functions

In compiler developer terms, the current implementation of non-lexical lifetimes is "location insensitive". Location sensitivity was originally available but it was disabled in the name of performance.

I asked Niko Matsakis about this code:

In the context of your example: the value event only has to have the lifetime 'buf conditionally — at the return point which may or may not execute. But when we are "location insensitive", we just track the lifetime that event must have anywhere, without considering where that lifetime must hold. In this case, that means we make it hold everywhere, which is why you get a compilation failure.

One subtle thing is that the current analysis is location sensitive in one respect — where the borrow takes place. The length of the borrow is not.

The good news is that adding this concept of location sensitivity back is seen as an enhancement to the implementation of non-lexical lifetimes. The bad news:

That may or may not be before the [Rust 2018] edition.

(Note: it did not make it into the initial release of Rust 2018)

This hinges on a (even newer!) underlying implementation of non-lexical lifetimes that improves the performance. You can opt-in to this half-implemented version using -Z polonius:

rustc +nightly -Zpolonius --edition=2018 example.rs

RUSTFLAGS="-Zpolonius" cargo +nightly build

---

Because this is across functions, you can sometimes work around this by inlining the function.