[nll] compilation error from mozjs-0.1.10

[nikomatsakis]

In #47596, @SimonSapin reported several NLL errors in dependencies of the servo crate. @lqd later minimized one of those errors into this example:

#![feature(nll)]

struct AutoGCRooter {
    stackTop: *mut *mut AutoGCRooter,
}

impl AutoGCRooter {
    unsafe fn add_to_root_stack(&mut self) {
        *self.stackTop = self;
    }
}

fn main() {}

which yields:

error[E0506]: cannot assign to `*self.stackTop` because it is borrowed
 --> src/main.rs:9:9
  |
9 |         *self.stackTop = self;
  |         ^^^^^^^^^^^^^^^^^----
  |         |                |
  |         |                borrow of `*self.stackTop` occurs here
  |         assignment to borrowed `*self.stackTop` occurs here

error: aborting due to previous error

Removing feature(nll) makes the code work, so probably this is an NLL bug.

[Aaron1011]

I'd like to take a look at this.

[Aaron1011]

Explicitly casting self to *mut AutoGCRooter fixes the issue: https://play.rust-lang.org/?gist=15c060db3de0a3212c3ab2b2a3d32b30&version=nightly

#![feature(nll)]

struct AutoGCRooter {
    stackTop: *mut *mut AutoGCRooter,
}

impl AutoGCRooter {
    unsafe fn add_to_root_stack(&mut self) {
        *self.stackTop = self as *mut AutoGCRooter;
    }
}

fn main() {}

My guess is that there's an issue with the automatic coercion from &mut self to *mut self. I'll investigate further.

[Aaron1011]

It turns out you can remove one layer of indirection and still get the same error:

#![feature(nll)]

struct AutoGCRooter {
    stackTop: *mut AutoGCRooter,
}

impl AutoGCRooter {
    unsafe fn add_to_root_stack(&mut self) {
        self.stackTop = self;
    }
}

fn main() {}