needless_borrow should trigger when taking a reference to Option<&_>

[jplatte]

Lint Name

needless_borrow

Reproducer

I tried this code:

struct Object;

impl Object {
    fn str_getter(&self) -> Option<&str> {
        None
    }
}

fn main() {
    let o = Object;
    if let Some(s) = &o.str_getter() {
        println!("{s}");
    }
}

I expected to see this happen: needless_borrow triggering for &o.str_getter()

Instead, this happened: nothing

Version

Tested on Rust playground, hence less info. Let me know if you need me to reproduce locally.

Nightly version: 1.73.0-nightly
(2023-08-10 439d066bcf9496b1b8c8)

[samueltardieu]

Just to make sure I understand the issue at hand, on which part of code would the needless_borrow lint apply?

If this is about the & in &o.str_getter(), removing it would change the type of s from &&str to &str due to match ergonomics. Even if it would work here as Display is implemented the same way on &&str and &str, it is not equivalent if s is later used in a larger expression.

Of course, in (&o).str_getter(), the lint would trigger, as this borrow would indeed be needless.

[jplatte]

I didn't think of that. I guess involving extra analysis on whether the bound variable from the if let needs to be a double ref would be become pretty complex pretty fast. Feel free to close this.