Box::new requires `use std::boxed::Box` despite prelude injection

[pnkfelix]

In my branch that has #20723 landed, this oddity comes up:

#[cfg(workaround)]
use std::boxed::Box;

fn main() {
    let x: Box<char>;
    x = Box::new('c');
    println!(" x: {}", x);
}

Ready? Watch this:

% ./x86_64-apple-darwin/stage2/bin/rustc /tmp/boxnew2.rs
/tmp/boxnew2.rs:6:9: 6:17 error: unresolved name `Box::new`
/tmp/boxnew2.rs:6     x = Box::new('c');
                          ^~~~~~~~
error: aborting due to previous error
% ./x86_64-apple-darwin/stage2/bin/rustc /tmp/boxnew2.rs --cfg workaround
% 

That is, the Box::new call fails, unless one includes a use std::boxed::Box in one's code. Truly weird.

I spent much time today trying to dig into this (when I should have been dealing with the other fallout from feature gating box), but in the end I decided that this is "just a bug" and its an easy enough work around for the purposes of the alpha.

[alexcrichton]

I ended up fixing this in the rollup, so the bug never actually hit master!

[alexcrichton]

fix

[pnkfelix]

i wish i understood the bug well enough to see how that change to coherence is the true fix, but I will trust you.