Presence of Add impl for unrelated type prevents compilation

[djc]

Simplified version of bodil/smartstring#7.

I tried this code:

fn main() {
    println!("{}", "Hello".to_string() + &(" World".to_string()));
}

struct FakeString {}

impl std::ops::Add<FakeString> for String {
    type Output = Self;
    fn add(mut self, rhs: FakeString) -> Self::Output {
        unreachable!()
    }
}

I expected to see this happen: this should compile correctly.

Instead, it fails like this:

error[E0277]: cannot add `&std::string::String` to `std::string::String`
 --> src/main.rs:2:40
  |
2 |     println!("{}", "Hello".to_string() + &(" World".to_string()));
  |                                        ^ no implementation for `std::string::String + &std::string::String`
  |
  = help: the trait `std::ops::Add<&std::string::String>` is not implemented for `std::string::String`

Removing the Add impl makes it work.

Meta

This is on current stable, 1.46.0.

[jonas-schievink]

This seems like expected behavior

[djc]

I'm confused -- why would String + &String start failing in the presence of an impl for String + FakeString?

[jonas-schievink]

Because the trait system then has two impls available to select from, impl Add<&str> for String and impl Add<FakeString> for String, which means that it doesn't know to apply autoderef to the &String argument to get the &str.

[djc]

Hmm, okay. So I guess this is a duplicate for #44619.

[andy128k]

@jonas-schievink It doesn't compile even if a FakeString and an Add implementation are in a separated private module.

mod x {
    struct FakeString {}

    impl std::ops::Add<FakeString> for String {
        type Output = Self;
        fn add(mut self, rhs: FakeString) -> Self::Output {
            unreachable!()
        }
    }
}

mod y {
    pub fn hw() {
        println!("{}", "Hello".to_string() + &(" World".to_string()));
    }
}

[jonas-schievink]

Yeah, that doesn't affect trait selection to my knowledge

[erenon]

Is there a workaround for this issue, short of removing the offending Add implementations? (as they can be in dependencies. e.g: I'm unable to add prometheus_client, as it breaks other code in the project)

[mattfbacon]

Deref manually. https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=7c82ca60a0948542d77fde6431e40643