`let ()` has a poor error message

[steveklabnik]

fn main() {
    let () = 1i;
}

gives

<anon>:2:9: 2:11 error: mismatched types: expected `int`, found `()` (expected int, found ())
<anon>:2     let () = 1i;
                 ^~

Obviously, () isn't a valid name for a binding, but this message isn't very helpful.

[kmcallister]

It's a standard pattern type mismatch error, and seems pretty clear to me. What sort of message would you prefer? Is there a particular case of people writing let () that you have in mind?

[steveklabnik]

To me, the fact that () can't be a valid name for binding is more important than some kind of type mis-match. But this might not be worth special casing. I don't know, the weirdness of it threw me off, maybe it's not that bad.

[kmcallister]

I see. Conceptually it's not much different from

fn main() {
    let (a, b) = 1i;
}

which is more evidently a type mismatch. But I agree that it's not obvious () is a pattern in Rust. It was familiar to me because of my background in Haskell, where pattern-matching on () is fairly common (and has a run-time effect!)

[steveklabnik]

Right, I was seeing it as unit, not a pattern.

[kmcallister]

It's both. It's a pattern which matches the one and only value of the unit type.

[steveklabnik]

But this might not be worth special casing. I don't know, the weirdness of it threw me off, maybe it's not that bad.

Agree with my past self, I've never seen anyone else run into this since.