This file displays the "mutable variables cannot be implicitly captured" error

captured-mutable.rs

pub struct Bogo {
    x: fn@() -> uint
}

pub fn bugfun2(_exts: @mut uint)
    -> uint { 3 }

pub fn bugfun() {
    let mut exts = 146;
    let _f_pre = @Bogo {
        x: || bugfun2(@mut exts),
    };
}