Rust suggests me to make a mutable variable mutable?

[jD91mZM2]

error[E0594]: cannot assign to captured outer variable in an `Fn` closure
   --> main.rs:232:9
    |
225 |     let mut typing_last = Instant::now();
    |         --------------- help: consider making `mut typing_last` mutable: `mut mut typing_last`
...
232 |         typing_last = Instant::now();
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
help: consider changing this closure to take self by mutable reference
   --> main.rs:228:40
    |
228 |       input.connect_property_text_notify(move |input| {
    |  ________________________________________^
229 | |         if typing_last.elapsed() < typing_duration {
230 | |             return;
231 | |         }
...   |
234 | |         println!("{:?}", text);
235 | |     });
    | |_____^

help: consider making mut typing_last mutable: mut mut typing_last

Sadly I can't seems to find a minimal reproduction example where this appears, but I commited all my code to a separate branch so you can look at the code (or try to compile it yourself): rust-weird-error.

I'm sorry, but that's all I have on this error.

[matthewjasper]

Minified:

pub fn bar<F: Fn()>(_f: F) {}

pub fn foo() {
    let mut x = 0;
    bar(move || x = 1); // Removing `move` produces a different, unhelpful error message
}

If the help message showed anything if should point to the Fn() bound.

cc #46020

[jD91mZM2]

@matthewjasper That doesn't seem to give the hint in the playground. Ooooh, maybe the reason I couldn't make a minimal example is because the playground has a different rust version?

[matthewjasper]

Looks like this is only an issue on beta/nightly.

[jD91mZM2]

When I encoutered this I wasn't using nightly.
Ugh never mind, when trying to install redox my toolchain was switched to nightly by default.

[estebank]

Current output:

error[E0594]: cannot assign to captured outer variable in an `Fn` closure
 --> src/main.rs:5:17
  |
5 |     bar(move || x = 1);
  |                 ^^^^^
  |
  = note: `Fn` closures cannot capture their enclosing environment for modifications
help: consider changing this closure to take self by mutable reference
 --> src/main.rs:5:9
  |
5 |     bar(move || x = 1);
  |         ^^^^^^^^^^^^^

warning: variable does not need to be mutable
 --> src/main.rs:4:9
  |
4 |     let mut x = 0;
  |         ----^
  |         |
  |         help: remove this `mut`
  |
  = note: #[warn(unused_mut)] on by default

error: aborting due to previous error