Make std available to proc macro root in phase 1

[dtolnay]

Following up on #38356 (comment).

Right now, the prelude is in scope at the definition site (unless the proc-macro crate is #![no_implicit_prelude]) and this is an accident (in some sense) due to the phase 0 vs phase 1 distinction as you point out.

However, I think that phase 1 should implicitly contain std at the proc-macro root (so that you can always quote!(use std::...);) and the prelude for convenience/ergonomics and since these are already implicit in phase 0. There will be a PR to add std at the root in phase 1 soon.

The following procedural macro invocation should compile and print None.

#![feature(proc_macro)]

extern crate proc_macro;
use proc_macro::{quote, TokenStream};

#[proc_macro]
pub fn none(input: TokenStream) -> TokenStream {
    quote! {
        // Works currently, because `None` is in the prelude.
        //None::<$input>

        // Does not work, but should.
        std::option::Option::None::<$input>
    }
}

#![feature(proc_macro)]

extern crate mac;

fn main() {
    println!("{:?}", mac::none!(u8));
}

@jseyfried

[petrochenkov]

Another alternative (was decided somewhere in rust-lang/rfcs#2126, but never put into the RFC text) is to add the name std into the libstd prelude.

This can be done either through old $crate (pub use $crate as std;), or new crate (pub use crate as std;), but not right now because there are some bugs.

[petrochenkov]

This is

• fixed on 2018 edition, ::std::foo will certainly resolve to an extern crate named std
• mostly fixed on 2015 edition, std::foo will resolve to an extern crate named std unless it's shadowed by something also named std.