box syntax does not allow an expression?

[FranklinChen]

    let foo = box (1i+2);

<anon>:3:25: 3:26 error: unexpected token: `;`
<anon>:3     let foo = box (1i+2);

[tomjakubowski]

I expect it's trying to parse (1i+2) as an identifier since the box operator can take an optional "argument" in parentheses defaulting to std::owned::HEAP. This should compile:

fn main() {
    use std::owned::HEAP;
    let foo = box(HEAP) (1i + 2);
}

[FranklinChen]

Ah, yes, that makes sense now. But I suggest something be done about the syntax because people will get quite confused when just using the default.

[huonw]

identifier

Actually an arbitrary 'allocator' expression. Currently we only support two builtin ones, but we will likely have some form of generalisation in the future. In any case, you can choose the default via box() (...) too (i.e. omitting the expression is the same as the explicit HEAP).

I do think we can improve the error messages here.

[ftxqxd]

Is this a dupe of #15386?

I do agree that the syntax is awful and probably should be changed, but that’s probably for another issue.

[alexcrichton]

Yes, I believe this is a dupe of #15386, which also has to do with diagnostics. Thanks @P1start!