Unecessary `mut` reference in parameter

[emilk]

What it does

It warns against using &mut in an parameter where & would suffice.

Lint Name

unnecessary-mut-ref

Category

suspicious

Advantage

Less mut means less fights with the borrow checker. It can also lead to more opportunities for parallelization.

This lint could also catch bugs where the user intended to mutate an argument but forgot too.

Drawbacks

A user may want to create an API where they reserve the right to mutate an argument, but does not do so yet.

We should not warn when implementing a trait.

Example

fn foo(&mut self, y: &mut i32) -> i32 {
    self.x + *y
}

Could be written as:

fn foo(&self, y: &i32) -> i32 {
    self.x + *y
}

[junderw]

I wonder if this could be also made into a separate lint where ownership is passed when a &mut or & would suffice.

Basically, warning when your fn is taking more than it needs.

I like it!

[Centri3]

I wonder if this could be also made into a separate lint where ownership is passed when a &mut or & would suffice.

Basically, warning when your fn is taking more than it needs.

I like it!

Is this already covered by needless_pass_by_value ?

[junderw]

Good point.

Maybe instead of unnecessary-mut-ref it should be proposed that this issue's lint is needless_pass_by_mut_ref

[junderw]

Referencing the pass_by_value PR in case someone wants to look at it for inspiration: #1556

[GuillaumeGomez]

I'll take a look.

[GuillaumeGomez]

I'm done with the implementation. I'll open a draft PR after the cleanup but I'll need people to test it so I can add more cases into the UI test.

[GuillaumeGomez]

I opened the draft here. Please test it so we can uncover cases I didn't think about. Thanks in advance! :)