Erroneous borrow error from 2018 edition's NLL migration mode.

[nrc]

I have:

pub fn infer_defaults(&mut self, project_dir: &Path) -> CargoResult<()> {
    ....
    match &mut self.target_dir {
            Inferrable::Specified(Some(ref mut path)) if path.is_relative() => {
                *path = project_dir.join(&path);
            }
            _ => {},
        }
    ...
}

(from https://github.com/rust-lang-nursery/rls/blob/master/src/config.rs#L238-L277)

And I get the following warning:

warning[E0502]: cannot borrow `_` as mutable because it is also borrowed as immutable
   --> src/config.rs:259:40
    |
257 |         match &mut self.target_dir {
    |               -------------------- immutable borrow occurs here
258 |             // We require an absolute path, so adjust a relative one if it's passed.
259 |             Inferrable::Specified(Some(ref mut path)) if path.is_relative() => {
    |                                        ^^^^^^^^^^^^ mutable borrow occurs here
...
262 |             _ => {},
    |             - borrow later used here
    |
    = warning: This error has been downgraded to a warning for backwards compatibility with previous releases.
            It represents potential unsoundness in your code.
            This warning will become a hard error in the future.

The warning is the same without the &mut.

It seems to me this code is fine and there should not be a warning.

cc @nikomatsakis @pnkfelix

[pnkfelix]

Smaller example (playground):

fn main() {
    let mut stuff = ("left", "right");
    match stuff {
        (ref mut left, _) if *left == "left" => { }
        _ => {}
    }
}

which emits the diagnostic:

warning[E0502]: cannot borrow `stuff.0` as mutable because it is also borrowed as immutable
 --> src/main.rs:4:10
  |
3 |     match stuff {
  |           ----- immutable borrow occurs here
4 |         (ref mut left, _) if *left == "left" => { }
  |          ^^^^^^^^^^^^ mutable borrow occurs here
5 |         _ => {}
  |         - borrow later used here
  |
  = warning: This error has been downgraded to a warning for backwards compatibility with previous releases.
          It represents potential unsoundness in your code.
          This warning will become a hard error in the future.

[pnkfelix]

Here's the (very?) good news: this is not an NLL bug, per se.

In particular, when we opt into NLL explicitly, the example works (playground):

#![feature(nll)]

fn main() {
    let mut stuff = ("left", "right");
    match stuff {
        (ref mut left, _) if *left == "left" => { }
        _ => {}
    }
}

There is a bug in how I linked the migration mode into the 2018 edition (PR #52681). Basically, its not toggling on enough switches, and one of the ones I missed is causing something to break with the new way we handle match.

[pnkfelix]

This issue strikes me as an argument that we should turn on something analogous to compare-mode=nll in compiletest, except specialized to just the 2018 edition opt-in, rather than turning on NLL via debug switches.

[pnkfelix]

In particular, you see the error with -Z borrowck=mir, but not with -Z borrowck=mir -Z two-phase-borrows. That's a pretty strong hint that I forgot to have edition=2008 imply two-phase-borrows.