Parse async fn header.

This is gated on edition 2018 & the `async_await` feature gate.

The parser will accept `async fn` and `async unsafe fn` as fn
items. Along the same lines as `const fn`, only `async unsafe fn`
is permitted, not `unsafe async fn`.The parser will not accept
`async` functions as trait methods.

To do a little code clean up, four fields of the function type
struct have been merged into the new `FnHeader` struct: constness,
asyncness, unsafety, and ABI.

Also, a small bug in HIR printing is fixed: it previously printed
`const unsafe fn` as `unsafe const fn`, which is grammatically
incorrect.

src/libsyntax/ast.rs

@@ -17,7 +17,7 @@ pub use util::ThinVec;
 pub use util::parser::ExprPrecedence;
 
 use syntax_pos::{Span, DUMMY_SP};
-use codemap::{respan, Spanned};
+use codemap::{dummy_spanned, respan, Spanned};
 use rustc_target::spec::abi::Abi;
 use ext::hygiene::{Mark, SyntaxContext};
 use print::pprust;
@@ -1325,9 +1325,7 @@ pub struct MutTy {
 /// or in an implementation.
 #[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug)]
 pub struct MethodSig {
-    pub unsafety: Unsafety,
-    pub constness: Spanned<Constness>,
-    pub abi: Abi,
+    pub header: FnHeader,
     pub decl: P<FnDecl>,
 }
 
@@ -1708,6 +1706,12 @@ pub enum Unsafety {
     Normal,
 }
 
+#[derive(Copy, Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug)]
+pub enum IsAsync {
+    Async,
+    NotAsync,
+}
+
 #[derive(Copy, Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug)]
 pub enum Constness {
     Const,
@@ -2009,6 +2013,29 @@ pub struct Item {
     pub tokens: Option<TokenStream>,
 }
 
+/// A function header
+///
+/// All the information between the visibility & the name of the function is
+/// included in this struct (e.g. `async unsafe fn` or `const extern "C" fn`)
+#[derive(Clone, Copy, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug)]
+pub struct FnHeader {
+    pub unsafety: Unsafety,
+    pub asyncness: IsAsync,
+    pub constness: Spanned<Constness>,
+    pub abi: Abi,
+}
+
+impl Default for FnHeader {
+    fn default() -> FnHeader {
+        FnHeader {
+            unsafety: Unsafety::Normal,
+            asyncness: IsAsync::NotAsync,
+            constness: dummy_spanned(Constness::NotConst),
+            abi: Abi::Rust,
+        }
+    }
+}
+
 #[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug)]
 pub enum ItemKind {
     /// An `extern crate` item, with optional *original* crate name if the crate was renamed.
@@ -2030,7 +2057,7 @@ pub enum ItemKind {
     /// A function declaration (`fn` or `pub fn`).
     ///
     /// E.g. `fn foo(bar: usize) -> usize { .. }`
-    Fn(P<FnDecl>, Unsafety, Spanned<Constness>, Abi, Generics, P<Block>),
+    Fn(P<FnDecl>, FnHeader, Generics, P<Block>),
     /// A module declaration (`mod` or `pub mod`).
     ///
     /// E.g. `mod foo;` or `mod foo { .. }`