Confusing warn-unused-mut warnings since 1.27.0-nightly 2018-04-29

[Robbepop]

With the latest nightly (rustc 1.27.0-nightly (66363b288 2018-04-29)) I get confusing compile warnings about unused mut keywords where there are no mut keywords present.

The warnings do not happen on the previous nightly (rustc 1.27.0-nightly (66363b288 2018-04-28)) - so this might be a regression.

For further inspection the project where I receive those warnings can be found here.

Some examples

warning: variable does not need to be mutable
   --> src/ast/visitor.rs:158:56
    |
158 |     fn visit_cond(&mut self, _cond: &expr::IfThenElse, _: VisitEvent) {}
    |                                                        ^ help: remove this `mut`
    |
    = note: #[warn(unused_mut)] on by default

warning: variable does not need to be mutable
   --> src/simplifier/simplifications/normalizer.rs:100:17
    |
100 |           .all(|(lhs, rhs)| {
    |                 ------^^^^
    |                 |
    |                 help: remove this `mut`

[ExpHP]

Bizarre; this must be where all the missing "unused mut" warnings from #50321 are running off to!

[pietroalbini]

A quick bisect run points at #48605 as the cause of this. cc @KiChjang @nikomatsakis

[nikomatsakis]

Just to confirm: this is specific to crates using nll, right?

[realcr]

This problem breaks our build. Some examples:

error: variable does not need to be mutable
   --> src/utils/async_mutex.rs:154:35
    |
154 |                         .map_err(|_| AsyncMutexError::AwakenerCanceled)?
    |                                   ^ help: remove this `mut`

...

error: variable does not need to be mutable
   --> src/timer.rs:153:19
    |
153 |             .map(|_| tm.create_client())
    |                   ^ help: remove this `mut`
    |
note: lint level defined here
   --> src/timer.rs:40:9
    |
40  | #![deny(warnings)]
    |         ^^^^^^^^

[KiChjang]

@realcr Same question as Niko posted above -- does this only happen when you enable NLL?

[Robbepop]

In the case of my linked project nll is enabled but I cannot tell you if this bug is exclusive to nll enabled projects so I couldn't really answer niko's question.