Incorrect: this expression borrows a value the compiler would automatically borrow

[ritchie46]

Summary

Clippy claims that a compiler would automatically borrow specific user borrows. E.g. (&impl Struct). These explicit borrows are done to dispatch to a implementation on the reference. So that owned code can utilize the same implementation.

Lint Name

needless_borrow

Reproducer

I tried this code:

use std::ops::Add;

struct Bar {
    val: i32
}

impl Add for &Bar {
    type Output = Bar;

    fn add(self, rhs: Self) -> Self::Output {
        let new = self.val + rhs.val;
        Bar {
            val: new
        }
    }
}

impl Add for Bar {
    type Output = Bar;

    fn add(self, rhs: Self) -> Self::Output {
        // dispatch to ref impl
        (&self).add(&rhs)
    }
}

I saw this happen:

warning: this expression borrows a value the compiler would automatically borrow
  --> src/main.rs:58:9
   |
58 |         (&self).add(&rhs)
   |         ^^^^^^^ help: change this to: `self`
   |
   = note: `#[warn(clippy::needless_borrow)]` on by default
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrow

I expected to see this happen:

No warning.

Doing as the lint suggests, leads to recursive call site

warning: function cannot return without recursing
  --> src/main.rs:56:5
   |
56 |     fn add(self, rhs: Self) -> Self::Output {
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot return without recursing
57 |         // dispatch to ref impl
58 |         self.add(rhs)
   |         ------------- recursive call site
   |
   = note: `#[warn(unconditional_recursion)]` on by default
   = help: a `loop` may express intention better if this is on purpose

So that's a catch 22.

Version

rustc 1.60.0-nightly (0c292c966 2022-02-08)
binary: rustc
commit-hash: 0c292c9667f1b202a9150d58bdd2e89e3e803996
commit-date: 2022-02-08
host: x86_64-unknown-linux-gnu
release: 1.60.0-nightly
LLVM version: 13.0.0

Additional Labels

No response

[georgmu]

I have the same warning on a slightly different setup, which can be reduced to the following piece of code:

struct A (u32);
struct B (u32);

impl<'a> From<&'a A> for B {
    fn from(a: &A) -> Self {
        Self(a.0)
    }
}

fn main() {
    let a = A(1);
    let b: B = (&a).into(); // <- wrong warning here
    println!("b: {}", b.0);
}

No clippy warning with stable, but with nightly.