Wrong syntax suggested by "consider changing this to be a mutable reference" with mut parameter in closure when iterating a vector

[chylex]

Code

fn main() {}

pub trait Layer {
	fn process(&mut self) -> u32;
}

pub struct State {
	layers: Vec<Box<dyn Layer>>,
}

impl State {
	pub fn process(&mut self) -> u32 {
		self.layers.iter().fold(0, |result, mut layer| result + layer.process())
	}
}

Current output

error[E0596]: cannot borrow `**layer` as mutable, as it is behind a `&` reference
  --> src/main.rs:13:59
   |
13 |         self.layers.iter().fold(0, |result, mut layer| result + layer.process())
   |                                                                 ^^^^^^^^^^^^^^^ `layer` is a `&` reference, so the data it refers to cannot be borrowed as mutable
   |
help: consider changing this to be a mutable reference
   |
13 |         self.layers.iter().fold(0, |result, mut lmut ayer| result + layer.process())
   |                                                  +++

Desired output

No response

Rationale and extra context

Initially, my State.process function looked like this:

self.layers.iter().fold(0, |result, layer| result + layer.process())

This led to a diagnostic that already has an open issue #62387 (it should suggest iter -> iter_mut).

I didn't notice I was not using iter_mut and tried changing layer -> mut layer, which led to the compiler somehow adding a second mut in the middle of the closure parameter.

I don't know what the desired output should be in this case; I don't think mut parameter is correct here either way, but it might be worth investigating why the compiler is making a syntactically wrong suggestion.

Other cases

No response

Anything else?

Tested on 1.71.0 and 1.72.0 (stable-x86_64-unknown-linux-gnu).

[asquared31415]

Minimized, it does not need the trait or dyn, just needs to try to use a & arg as a &mut and have the arg be mut binding:

fn take_f(_: impl FnMut(&String)) {}

fn main() {
    #[allow(unused_mut)] // `mut layer` needed to reproduce the diagnostic
    take_f(|mut layer| {
        layer.push('\n');
    });
}

@rustbot label -needs-triage +A-closures +A-suggestion-diagnostics +D-invalid-suggestion

[chenyukang]

The syntactically wrong suggestion is removed in nightly(in both test cases).

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=8e4ce576c014f50452c9234bbd01acca

but the current suggestion also seems not so good,
it's best if we could suggest iter_mut from the beginning.

[chylex]

Suggesting iter_mut is already covered by #62387, so perhaps this should be closed unless there's a possible improvement to the compile error in asquared31415's minimized case that doesn't use vectors? I don't know enough about Rust to understand what the compiler wants, and there is no suggestion: