Rust: 🛠️ specialization permits empty impls when parent has no default items

[nikomatsakis]

It is potentially somewhat surprising that the following impls are accepted:

#![feature(specialization)]

trait Foo {
    fn bar();
}

impl<T> Foo for T {
    fn bar() { } // no default
}

impl Foo for i32 {
    // OK so long as we define no items:
    // fn bar() { }
}


fn main() {}

The original example was unearthed by @eddyb and @varkor as part of a PR.

To be clear, I don't see this as a soundness problem per se. Just a question of what we think it ought to do.

cc @aturon

[nikomatsakis]

Also, this affects type inference in creepy ways. For example, this program will not build:

#![feature(specialization)]

struct X;
struct Y;
impl From<X> for Y {
    fn from(_: X) -> Y { Y }
}

// impl Into<Y> for X {}

fn main() {
    let x = X.into();
}

but if you uncomment the impl, it will.

[nikomatsakis]

The main question is, is this even a bug?

[varkor]

In the case of specialisation affecting type inference, I think this is very counter-intuitive and makes the behaviour hard to fathom without actually compiling the program, which I think we'd rather avoid...

[nikomatsakis]

In the case of specialisation affecting type inference, I think this is very counter-intuitive and makes the behaviour hard to fathom without actually compiling the program, which I think we'd rather avoid...

That's true, though I think that's a sort of separate problem really. We should probably extract that into its own issue. I suspect it is a by-product of some of the details of how trait selection works -- details I hope to change. See e.g. https://github.com/rust-lang/rust/issues/41756. Also, I wonder if this aspect (the affects on type inference) is a duplicate of https://github.com/rust-lang/rust/issues/46363...

[Diggsey]

This doesn't seem like a bug per se - perhaps the best thing to do is to emit a warning when an impl is redundant in this way? Something like Warning: "impl" block is redundant because i32 already implements Foo