set the syntax edition in the driver's phase 1

src/librustc_driver/driver.rs

@@ -57,7 +57,7 @@ use syntax::ext::base::ExtCtxt;
 use syntax::fold::Folder;
 use syntax::parse::{self, PResult};
 use syntax::util::node_count::NodeCounter;
-use syntax_pos::FileName;
+use syntax_pos::{FileName, hygiene};
 use syntax_ext;
 
 use derive_registrar;
@@ -670,6 +670,7 @@ pub fn phase_1_parse_input<'a>(
 ) -> PResult<'a, ast::Crate> {
     sess.diagnostic()
         .set_continue_after_error(control.continue_parse_after_error);
+    hygiene::set_default_edition(sess.edition());
 
     if sess.profile_queries() {
         profile::begin(sess);

src/librustc_driver/lib.rs

@@ -109,7 +109,7 @@ use syntax::ast;
 use syntax::codemap::{CodeMap, FileLoader, RealFileLoader};
 use syntax::feature_gate::{GatedCfg, UnstableFeatures};
 use syntax::parse::{self, PResult};
-use syntax_pos::{hygiene, DUMMY_SP, MultiSpan, FileName};
+use syntax_pos::{DUMMY_SP, MultiSpan, FileName};
 
 #[cfg(test)]
 mod test;
@@ -477,7 +477,6 @@ pub fn run_compiler<'a>(args: &[String],
         };
 
         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)

src/librustdoc/test.rs

@@ -37,7 +37,7 @@ use syntax::codemap::CodeMap;
 use syntax::edition::Edition;
 use syntax::feature_gate::UnstableFeatures;
 use syntax::with_globals;
-use syntax_pos::{BytePos, DUMMY_SP, Pos, Span, FileName, hygiene};
+use syntax_pos::{BytePos, DUMMY_SP, Pos, Span, FileName};
 use errors;
 use errors::emitter::ColorConfig;
 
@@ -562,7 +562,6 @@ impl Collector {
                     rustc_driver::in_named_rustc_thread(name, move || with_globals(move || {
                         io::set_panic(panic);
                         io::set_print(print);
-                        hygiene::set_default_edition(edition);
                         run_test(&test,
                                  &cratename,
                                  &filename,

src/test/rustdoc/async-fn.rs

@@ -0,0 +1,24 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// edition:2018
+// compile-flags:-Z unstable-options
+
+// FIXME: once `--edition` is stable in rustdoc, remove that `compile-flags` directive
+
+#![feature(rust_2018_preview, async_await, futures_api)]
+
+// @has async_fn/struct.S.html
+// @has - '//code' 'pub async fn f()'
+pub struct S;
+
+impl S {
+    pub async fn f() {}
+}