write! should not need external `use`

[kmcallister]

I have to

use std::fmt::Writer;

outside the macro invocation.

[tomjakubowski]

Dupe of #21615 I think.

[alexcrichton]

Somewhat but not entirely I think. We could solve this by solving #21615, but I'm not sure we can solve that issue. This can, however, be independently solved by adding use $crate::fmt::Writer and use std::io::Writer imports to the relevant macro I believe, however.

[huonw]

This is still a reality, although the traits are now Write (no trailing r).

Adding the imports to write! has a few downsides/problems:

1. it requires being able to import duplicate names, where only one is used
2. it may trigger unused-import lints, and so would need to handle this properly (including in the presence of #[forbid(unused_imports)])
3. it would mean a type that implements both traits cannot be used with write! at all, whereas it is possible now (just have to ensure that only one of the traits is in scope)

Example (currently can only triggers 1, but if/when that is fixed 2 and 3 will be a problem):

mod foo {
    pub trait Write {
        fn write(&self) {}
    }

    impl Write for u8 {}
}
mod bar {
    pub trait Write {
        fn write(&self) {}
    }

    impl Write for u8 {}
}
macro_rules! write {
    ($x: expr) => { {
        use foo::Write;
        use bar::Write;

        $x.write()
    }}
}

fn main() {
    write!(1u8);
}

<anon>:18:13: 18:23 error: a trait named `Write` has already been imported in this module [E0252]
<anon>:18         use bar::Write;
                      ^~~~~~~~~~
<anon>:25:5: 25:17 note: in this expansion of write! (defined in <anon>)
<anon>:18:13: 18:23 help: see the detailed explanation for E0252
<anon>:17:9: 17:24 note: previous import of `Write` here
<anon>:17         use foo::Write;
                  ^~~~~~~~~~~~~~~
<anon>:25:5: 25:17 note: in this expansion of write! (defined in <anon>)

[steveklabnik]

Triage: no changes

[steveklabnik]

Triage: no changes. Given the above list of problems, and no comments since 2015, I'm going to give this issue a close.