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

@nikomatsakis

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

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.

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.

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.

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.

When we turn on NLL migration in the 2018 edition, we need two-phase …

Auto merge of #52975 - pnkfelix:issue-52967-edition-2018-implies-2-ph…

Navigation Menu

Repository navigation

Erroneous borrow error from 2018 edition's NLL migration mode. #52967

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Erroneous borrow error from 2018 edition's NLL migration mode. #52967

Description

Activity

pnkfelix commented on Aug 2, 2018

pnkfelix commented on Aug 2, 2018

pnkfelix commented on Aug 2, 2018

pnkfelix commented on Aug 2, 2018

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Issue actions