new lint: using `let mut a = a` at start of function

[jyn514]

I inherited a codebase from someone new to Rust and there were lots of function definitions that looked like this:

fn f(a: Vec<u8>) {
    let mut a = a;
    // rest of code
}

It would be nice if there were a lint saying this would be better as fn f(mut a: Vec<u8>) instead.

[sonng]

Looks like a good first issue. Could this be assigned to me please?

Thanks

[sonng]

Hey guys,

I'm on the last leg of the work (cargo test) and making sure everything is fine. I noticed that while running that, our new lint gets triggered and causes an error, causing the whole thing to fail.

My question is, how do I turn this from an error to a warning?
or alternatively, I could update the problematic code and make them fn parameters mutable.

Cheers

[flip1995]

Can you open a WIP PR, so I can have a look at the code and the dogfood error?

[sonng]

I'm happy for any feedback on the overall change too.

[brightly-salty]

I'm willing to attempt to implement this and submit a PR if it would be accepted if @sonng isn't working on it. I expect to restart (to avoid merge conflicts) using their PR as a guide/inspiration?

[flip1995]

@brightly-salty Thanks! Yes you can use they're PR as inspiration. I think my comments on the PR should also help you with dos and don'ts.

[max-niederman]

@rustbot claim