Can't use `vec!()` without std

[kmcallister]

#![feature(phase)]
#![no_std]

extern crate native;

#[phase(plugin, link)]
extern crate collections;

use collections::vec::Vec;

fn main() {
    let x: Vec<uint> = vec!();
}

fails with

foo.rs:12:24: 12:27 error: macro undefined: 'vec!'

which requires this workaround similar to #16803:

#[phase(plugin)]
extern crate std;

mod std {
    pub use collections::vec;
}

And that has various other negative consequences.

I see that libcollections/macros.rs implements vec!() but it's not exported from the crate, probably because there's no clean way to re-export the macro in libstd.