Invalid warning unused_mut

[Boscop]

I just updated my nightly and now I'm getting this warning:

But it's invalid, the binding has to be mutable, otherwise:

[pietroalbini]

Thanks for reporting this @Boscop! Unfortunately, I can't reproduce this locally, can you give us a reduced snippet that causes the regression?

[Boscop]

Hm, this snippet causes the same warning on my machine (nightly 05-04) but not in the playground (nightly 05-07):
https://play.rust-lang.org/?gist=424a607550c14b3cb27ce4994bf9fd6c&version=nightly&mode=debug

I'm updating my nightly now to check with the most recent one..

[Boscop]

Hm, even with the most recent nightly 05-07 I get that warning on my machine!

[pietroalbini]

That's strange, the snippet you provided also compiled fine on my local machine. Can you get the output of rustc +nightly -Vv?

[Boscop]

Hm, it's only when it's part of the larger project (where the warning first appeared).
When I extract it into a new project (same code, as in the playground) it doesn't give that warning!
Unfortunately, I don't know how I can go from here, the large project is 20k lines, but this code is literally the same as in the playground example! Just pasted into the project (without the main function).

[pietroalbini]

Is the project available somewhere so we can see which PR caused the regression?

[Boscop]

No, it's closed source (for my job) :/

[pietroalbini]

Sure. If you hop into #rust-release on irc.mozilla.org and ping me I can guide you through bisecting the project yourself.

[nikomatsakis]

@Boscop -- is this project by any chance using #![feature(nll)]?

[nikomatsakis]

Were you ever able to bisect? The https://github.com/rust-lang-nursery/cargo-bisect-rustc tool might be useful to you. =)

[oli-obk]

I can confirm that #![feature(nll)] produces said bug https://play.rust-lang.org/?gist=183af0c42d164e5434852c9e6ad81317&version=nightly&mode=debug

[nikomatsakis]

cc @KiChjang -- quite likely another regression to track down

[Boscop]

Ah thanks! Yes, in the big project where it originally occurred I'm using #![feature(nll)].

[alexcrichton]

I think I've encountered this as well at https://play.rust-lang.org/?gist=72cb9080cedd5cced73b23e8454344bd&version=nightly&mode=debug

[pietroalbini]

OK, this is not a regression then.

[alexcrichton]

I've bisected my comment above to #48605 (cc @KiChjang)

[KiChjang]

I believe it's the same underlying cause as #50461.