Can't create box of parenthesised expression (documentation, error reporting)

[ghost]

Contents of test.rs:

fn main() {
    let x: Box<int> = box (2 + 2);
}

in 0.11.0, this yields:

test.rs:2:31: 2:32 error: unexpected token: `;`
test.rs:2   let x: Box<int> = box (2 + 2);
                                         ^

However, the following code compiles:

fn main() {
    let temp = 2 + 2;
    let x: Box<int> = box temp;
}

Even doing box (2) is considered invalid syntax.

EDIT: Apparently, box()(2) is valid syntax, but this isn't written down anywhere.

[steveklabnik]

Yeah, if you add the parens, box currently expects that you're putting in a different allocator or whatever.

[o11c]

box () (2 + 2)

[ghost]

I'm going to mark this as missing documentation and poor error reporting, then. The box()(...) syntax is really confusing, although I don't think that I've been following the project enough to put up an RFC for it. It seems to me that it should be reasonable to infer whether something is an allocator or an expression.

[liigo]

box ... in HEAP ?

[ftxqxd]

Is it possible that this ambiguity could be resolved, at least in the majority of cases? The parser could check after parsing box(EXPR) that there is an expression following it, and if there isn’t, parse it as box() (EXPR). The only ambiguity I can think of is with boxed blocks in ifs, whiles, and fors, which could just be parsed greedily:

if box(foo) {
    ...
}
{
    ...
}

would be parsed as

if (box(foo) { ... }) {
    ...
}

as it is today.

I made a small patch to syntax that does this, and it works fine. I’ll open up a pull request if this behaviour is desirable.