Allow macros to determine the length of multiplicative arguments

This would allow http://static.rust-lang.org/doc/master/std/macros/macro.vec.html to use Vec::with_capacity to avoid unnecessary reallocations of the vector.

I'll be happy to take this on but I'm not entirely sure what a good syntax for this would be. I was thinking of:

macro_rules! vec(
    ($($e:expr),*) => ({
        // leading _ to allow empty construction without a warning.
        let mut _temp = ::std::vec_ng::Vec::with_capacity(#($e));
        $(_temp.push($e);)*
        _temp
    })
)

Since the individual elements in a multiplicative pattern can contain an arbitrary tree of expressions, # would recursively count all the elements that would expand to a specific $identifier.

[huonw]

I like this idea.

Note a version of this is currently possible with a recursive macro:

macro_rules! count_expr {
    () => { 0 };
    ($_e: expr $(, $rest: expr)*) => { 1 + count_expr!($($rest),*) }
}

Of course, this is slow (possibly even O(n^2), due to some implementation details/bugs), and a separate version of this is needed for every possible non-terminal, e.g. the expr one can't (easily) be reused to count items.

That said, I have wanted a macro/thing like this rather than the count_expr hack, but I think there are some subtleties to consider, e.g.

macro_rules! foo {
    ( $( $( $e: expr ),* );* ) => {
        (#( $($e)* ), 
         #($e))
    }
}

foo!(1, 2, 3; 4, 5, 6, 7) // == (2, 7) ?

• is #($e) at the top level valid (i.e. when $e has two levels of repeats)? if so, what does it count? (the total number of $e's across all copies of the outer repeat? )
• is #($($e)) valid as a way to count the number of exterior repeats?

Also, I don't quite understand

Since the individual elements in a multiplicative pattern can contain an arbitrary tree of expressions, # would recursively count all the elements that would expand to a specific $identifier.

could you explain it with an example?

@huonw What I meant is exactly your first bullet point. That, in your example, it should count "the total number of $e's across all copies of the outer repeat".

is #($($e)) valid as a way to count the number of exterior repeats?

I think so. An alternative would be to allow you to bind a whole sequence of items to an identifier:

macro_rules! foo {
    ( $( $outer( $e: expr ),* );* ) => {
        (#($outer), 
         #($e))
    }
}

Not sure if that would create any ambiguity in the syntax.

[alexcrichton]

My relevant comments are here: #12916 (comment), we've decided that this should go through the official RFC process now that we have one in place.