rustdoc: issues with some async syntax

[ehuss]

Rustdoc seems to fail on async syntax in some situations.

With edition = "2018" in Cargo.toml, the following is a sample of some apparent issues.

#![feature(rust_2018_preview, async_await, futures_api)]
#![allow(unused)]

use std::future::Future;

struct S;

impl S {
    async fn f() {}
}

fn f() {
    async move { };
}

fn async_closure(x: u8) -> impl Future<Output = u8> {
    (async move |x: u8| -> u8 {
        x
    })(x)
}

This should build fine, but fails when running rustdoc:

cargo +376b60da8b65d7c4392f0a5e2b45c42e84bd01ce doc
 Documenting ed v0.1.0 (file:///Users/eric/Proj/rust/cargo/scratch/ed)
error: missing `fn`, `type`, or `const` for impl-item declaration
 --> src/lib.rs:8:9
  |
8 |   impl S {
  |  _________^
9 | |     async fn f() {}
  | |____^ missing `fn`, `type`, or `const`

error: expected one of `!`, `.`, `::`, `;`, `?`, `{`, `}`, or an operator, found `move`
  --> src/lib.rs:13:11
   |
13 |     async move { };
   |           ^^^^ expected one of 8 possible tokens here

error: expected one of `!`, `)`, `,`, `.`, `::`, `?`, `{`, or an operator, found `move`
  --> src/lib.rs:17:12
   |
17 |     (async move |x: u8| -> u8 {
   |            ^^^^ expected one of 8 possible tokens here

error[E0277]: the trait bound `(): std::future::Future` is not satisfied
  --> src/lib.rs:16:28
   |
16 | fn async_closure(x: u8) -> impl Future<Output = u8> {
   |                            ^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::future::Future` is not implemented for `()`
   |
   = note: the return type of a function must have a statically known size

error: Compilation failed, aborting rustdoc

error: Could not document `ed`.

Caused by:
  process didn't exit successfully: `rustdoc -Zunstable-options --edition=2018 --crate-name ed src/lib.rs -o /Users/eric/Proj/rust/cargo/scratch/ed/target/doc -L dependency=/Users/eric/Proj/rust/cargo/scratch/ed/target/debug/deps` (exit code: 1)

Tested with a few different nightlies (rustc 1.30.0-nightly (73c7873 2018-08-05)) and latest master (376b60d).

[QuietMisdreavus]

This is a bug between rustdoc and rustc_driver, where the only place that sessopts.edition is handed to libsyntax is this specific all-in-one entry point to the compiler:

rust/src/librustc_driver/lib.rs

Lines 467 to 486 in 80caa7f

	pub fn run_compiler<'a>(args: &[String],
	callbacks: Box<dyn CompilerCalls<'a> + sync::Send + 'a>,
	file_loader: Option<Box<dyn FileLoader + Send + Sync + 'static>>,
	emitter_dest: Option<Box<dyn Write + Send>>)
	-> (CompileResult, Option<Session>)
	{
	syntax::with_globals(|| {
	let matches = match handle_options(args) {
	Some(matches) => matches,
	None => return (Ok(()), None),
	};
	let (sopts, cfg) = config::build_session_options_and_crate_config(&matches);
	hygiene::set_default_edition(sopts.edition);
	driver::spawn_thread_pool(sopts, |sopts| {
	run_compiler_with_pool(matches, sopts, cfg, callbacks, file_loader, emitter_dest)
	})
	})
	}

However, since rustdoc manages the phases of compilation itself, this line is never hit, and thus the global state telling libsyntax what edition to use is not set. We use this call manually for building doctests (see #52385), but not anywhere else. I propose moving the hygiene::set_default_edition call from run_compiler to phase_1_parse_input, right before it sets up the parse session. That way, rustdoc can continue to set the edition in the session options, and other consumers of librustc_driver aren't caught by this gotcha. @rust-lang/compiler