self doesn't need to be mutable lint

[Detector-I]

What it does

Having a function with &mut self as an argument, and actually never use self as mutable inside it wont give us any compiler or clippy warning...
here is an example

fn method(&mut self) -> String {
    self.a_str.to_string()
}

I think in this case clippy should say self doesn't need to be mutable.

Lint Name

unused_mutable_self

Category

correctness

Advantage

• self wont be borrowed anymore in this function when its dont needed

Drawbacks

No response

Example

fn method(&mut self) -> String {
    self.a_str.to_string()
}

Could be written as:

fn method(&self) -> String {
    self.a_str.to_string()
}

[kraktus]

Same as #5546 ?