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.

Author: Without Boats

src/doc/book

@@ -41,7 +41,6 @@
 //! This order consistency is required in a few places in rustc, for
 //! example generator inference, and possibly also HIR borrowck.
 
-use rustc_target::spec::abi::Abi;
 use syntax::ast::{NodeId, CRATE_NODE_ID, Name, Attribute};
 use syntax_pos::Span;
 use hir::*;
@@ -55,7 +54,7 @@ use std::u32;
 #[derive(Copy, Clone, PartialEq, Eq)]
 pub enum FnKind<'a> {
     /// fn foo() or extern "Abi" fn foo()
-    ItemFn(Name, &'a Generics, Unsafety, Constness, Abi, &'a Visibility, &'a [Attribute]),
+    ItemFn(Name, &'a Generics, FnHeader, &'a Visibility, &'a [Attribute]),
 
     /// fn foo(&self)
     Method(Name, &'a MethodSig, Option<&'a Visibility>, &'a [Attribute]),
@@ -466,12 +465,10 @@ pub fn walk_item<'v, V: Visitor<'v>>(visitor: &mut V, item: &'v Item) {
             visitor.visit_ty(typ);
             visitor.visit_nested_body(body);
         }
-        ItemFn(ref declaration, unsafety, constness, abi, ref generics, body_id) => {
+        ItemFn(ref declaration, header, ref generics, body_id) => {
             visitor.visit_fn(FnKind::ItemFn(item.name,
                                             generics,
-                                            unsafety,
-                                            constness,
-                                            abi,
+                                            header,
                                             &item.vis,
                                             &item.attrs),
                              declaration,

src/librustc/hir/intravisit.rs

@@ -2194,7 +2194,7 @@ impl<'a> LoweringContext<'a> {
                 let value = self.lower_body(None, |this| this.lower_expr(e));
                 hir::ItemConst(self.lower_ty(t, ImplTraitContext::Disallowed), value)
             }
-            ItemKind::Fn(ref decl, unsafety, constness, abi, ref generics, ref body) => {
+            ItemKind::Fn(ref decl, header, ref generics, ref body) => {
                 let fn_def_id = self.resolver.definitions().local_def_id(id);
                 self.with_new_scopes(|this| {
                     let body_id = this.lower_body(Some(decl), |this| {
@@ -2210,9 +2210,7 @@ impl<'a> LoweringContext<'a> {
 
                     hir::ItemFn(
                         fn_decl,
-                        this.lower_unsafety(unsafety),
-                        this.lower_constness(constness),
-                        abi,
+                        this.lower_fn_header(header),
                         generics,
                         body_id,
                     )
@@ -2750,9 +2748,7 @@ impl<'a> LoweringContext<'a> {
         impl_trait_return_allow: bool,
     ) -> hir::MethodSig {
         hir::MethodSig {
-            abi: sig.abi,
-            unsafety: self.lower_unsafety(sig.unsafety),
-            constness: self.lower_constness(sig.constness),
+            header: self.lower_fn_header(sig.header),
             decl: self.lower_fn_decl(&sig.decl, Some(fn_def_id), impl_trait_return_allow),
         }
     }
@@ -2764,6 +2760,15 @@ impl<'a> LoweringContext<'a> {
         }
     }
 
+    fn lower_fn_header(&mut self, h: FnHeader) -> hir::FnHeader {
+        hir::FnHeader {
+            unsafety: self.lower_unsafety(h.unsafety),
+            asyncness: self.lower_asyncness(h.asyncness),
+            constness: self.lower_constness(h.constness),
+            abi: h.abi,
+        }
+    }
+
     fn lower_unsafety(&mut self, u: Unsafety) -> hir::Unsafety {
         match u {
             Unsafety::Unsafe => hir::Unsafety::Unsafe,
@@ -2778,6 +2783,13 @@ impl<'a> LoweringContext<'a> {
         }
     }
 
+    fn lower_asyncness(&mut self, a: IsAsync) -> hir::IsAsync {
+        match a {
+            IsAsync::Async => hir::IsAsync::Async,
+            IsAsync::NotAsync => hir::IsAsync::NotAsync,
+        }
+    }
+
     fn lower_unop(&mut self, u: UnOp) -> hir::UnOp {
         match u {
             UnOp::Deref => hir::UnDeref,

src/librustc/hir/lowering.rs

@@ -25,7 +25,6 @@ use hir as ast;
 use hir::map::{self, Node};
 use hir::{Expr, FnDecl};
 use hir::intravisit::FnKind;
-use rustc_target::spec::abi;
 use syntax::ast::{Attribute, Name, NodeId};
 use syntax_pos::Span;
 
@@ -105,9 +104,7 @@ impl<'a> Code<'a> {
 struct ItemFnParts<'a> {
     name:     Name,
     decl:     &'a ast::FnDecl,
-    unsafety: ast::Unsafety,
-    constness: ast::Constness,
-    abi:      abi::Abi,
+    header:   ast::FnHeader,
     vis:      &'a ast::Visibility,
     generics: &'a ast::Generics,
     body:     ast::BodyId,
@@ -183,31 +180,31 @@ impl<'a> FnLikeNode<'a> {
 
     pub fn constness(self) -> ast::Constness {
         match self.kind() {
-            FnKind::ItemFn(_, _, _, constness, ..) => {
-                constness
-            }
-            FnKind::Method(_, m, ..) => {
-                m.constness
-            }
+            FnKind::ItemFn(_, _, header, ..) => header.constness,
+            FnKind::Method(_, m, ..) => m.header.constness,
             _ => ast::Constness::NotConst
         }
     }
 
+    pub fn asyncness(self) -> ast::IsAsync {
+        match self.kind() {
+            FnKind::ItemFn(_, _, header, ..) => header.asyncness,
+            FnKind::Method(_, m, ..) => m.header.asyncness,
+            _ => ast::IsAsync::NotAsync
+        }
+    }
+
     pub fn unsafety(self) -> ast::Unsafety {
         match self.kind() {
-            FnKind::ItemFn(_, _, unsafety, ..) => {
-                unsafety
-            }
-            FnKind::Method(_, m, ..) => {
-                m.unsafety
-            }
+            FnKind::ItemFn(_, _, header, ..) => header.unsafety,
+            FnKind::Method(_, m, ..) => m.header.unsafety,
             _ => ast::Unsafety::Normal
         }
     }
 
     pub fn kind(self) -> FnKind<'a> {
         let item = |p: ItemFnParts<'a>| -> FnKind<'a> {
-            FnKind::ItemFn(p.name, p.generics, p.unsafety, p.constness, p.abi, p.vis, p.attrs)
+            FnKind::ItemFn(p.name, p.generics, p.header, p.vis, p.attrs)
         };
         let closure = |c: ClosureParts<'a>| {
             FnKind::Closure(c.attrs)
@@ -232,19 +229,17 @@ impl<'a> FnLikeNode<'a> {
     {
         match self.node {
             map::NodeItem(i) => match i.node {
-                ast::ItemFn(ref decl, unsafety, constness, abi, ref generics, block) =>
+                ast::ItemFn(ref decl, header, ref generics, block) =>
                     item_fn(ItemFnParts {
                         id: i.id,
                         name: i.name,
                         decl: &decl,
-                        unsafety,
                         body: block,
-                        generics,
-                        abi,
                         vis: &i.vis,
-                        constness,
                         span: i.span,
                         attrs: &i.attrs,
+                        header,
+                        generics,
                     }),
                 _ => bug!("item FnLikeNode that is not fn-like"),
             },

src/librustc/hir/map/blocks.rs

@@ -176,7 +176,7 @@ impl<'hir> MapEntry<'hir> {
                 match item.node {
                     ItemConst(_, body) |
                     ItemStatic(.., body) |
-                    ItemFn(_, _, _, _, _, body) => Some(body),
+                    ItemFn(_, _, _, body) => Some(body),
                     _ => None,
                 }
             }

src/librustc/hir/map/mod.rs

@@ -1558,9 +1558,7 @@ pub struct MutTy {
 /// Represents a method's signature in a trait declaration or implementation.
 #[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug)]
 pub struct MethodSig {
-    pub unsafety: Unsafety,
-    pub constness: Constness,
-    pub abi: Abi,
+    pub header: FnHeader,
     pub decl: P<FnDecl>,
 }
 
@@ -1787,7 +1785,13 @@ pub enum IsAuto {
     No
 }
 
-#[derive(Copy, Clone, PartialEq, Eq,PartialOrd, Ord, RustcEncodable, RustcDecodable, Hash, Debug)]
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, RustcEncodable, RustcDecodable, Hash, Debug)]
+pub enum IsAsync {
+    Async,
+    NotAsync,
+}
+
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, RustcEncodable, RustcDecodable, Hash, Debug)]
 pub enum Unsafety {
     Unsafe,
     Normal,
@@ -2063,6 +2067,25 @@ pub struct Item {
     pub span: Span,
 }
 
+#[derive(Clone, Copy, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug)]
+pub struct FnHeader {
+    pub unsafety: Unsafety,
+    pub constness: Constness,
+    pub asyncness: IsAsync,
+    pub abi: Abi,
+}
+
+impl Default for FnHeader {
+    fn default() -> FnHeader {
+        FnHeader {
+            unsafety: Unsafety::Normal,
+            constness: Constness::NotConst,
+            asyncness: IsAsync::NotAsync,
+            abi: Abi::Rust,
+        }
+    }
+}
+
 #[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug)]
 pub enum Item_ {
     /// An `extern crate` item, with optional *original* crate name if the crate was renamed.
@@ -2082,7 +2105,7 @@ pub enum Item_ {
     /// A `const` item
     ItemConst(P<Ty>, BodyId),
     /// A function declaration
-    ItemFn(P<FnDecl>, Unsafety, Constness, Abi, Generics, BodyId),
+    ItemFn(P<FnDecl>, FnHeader, Generics, BodyId),
     /// A module
     ItemMod(Mod),
     /// An external module
@@ -2144,7 +2167,7 @@ impl Item_ {
 
     pub fn generics(&self) -> Option<&Generics> {
         Some(match *self {
-            ItemFn(_, _, _, _, ref generics, _) |
+            ItemFn(_, _, ref generics, _) |
             ItemTy(_, ref generics) |
             ItemEnum(_, ref generics) |
             ItemStruct(_, ref generics) |

src/librustc/hir/mod.rs

@@ -446,9 +446,7 @@ impl<'a> State<'a> {
             hir::ForeignItemFn(ref decl, ref arg_names, ref generics) => {
                 self.head("")?;
                 self.print_fn(decl,
-                              hir::Unsafety::Normal,
-                              hir::Constness::NotConst,
-                              Abi::Rust,
+                              hir::FnHeader::default(),
                               Some(item.name),
                               generics,
                               &item.vis,
@@ -585,12 +583,10 @@ impl<'a> State<'a> {
                 self.s.word(";")?;
                 self.end()?; // end the outer cbox
             }
-            hir::ItemFn(ref decl, unsafety, constness, abi, ref typarams, body) => {
+            hir::ItemFn(ref decl, header, ref typarams, body) => {
                 self.head("")?;
                 self.print_fn(decl,
-                              unsafety,
-                              constness,
-                              abi,
+                              header,
                               Some(item.name),
                               typarams,
                               &item.vis,
@@ -887,9 +883,7 @@ impl<'a> State<'a> {
                             body_id: Option<hir::BodyId>)
                             -> io::Result<()> {
         self.print_fn(&m.decl,
-                      m.unsafety,
-                      m.constness,
-                      m.abi,
+                      m.header,
                       Some(name),
                       generics,
                       vis,
@@ -1924,16 +1918,14 @@ impl<'a> State<'a> {
 
     pub fn print_fn(&mut self,
                     decl: &hir::FnDecl,
-                    unsafety: hir::Unsafety,
-                    constness: hir::Constness,
-                    abi: Abi,
+                    header: hir::FnHeader,
                     name: Option<ast::Name>,
                     generics: &hir::Generics,
                     vis: &hir::Visibility,
                     arg_names: &[Spanned<ast::Name>],
                     body_id: Option<hir::BodyId>)
                     -> io::Result<()> {
-        self.print_fn_header_info(unsafety, constness, abi, vis)?;
+        self.print_fn_header_info(header, vis)?;
 
         if let Some(name) = name {
             self.nbsp()?;
@@ -2189,9 +2181,10 @@ impl<'a> State<'a> {
             span: syntax_pos::DUMMY_SP,
         };
         self.print_fn(decl,
-                      unsafety,
-                      hir::Constness::NotConst,
-                      abi,
+                      hir::FnHeader {
+                          unsafety, abi,
+                          ..hir::FnHeader::default()
+                      },
                       name,
                       &generics,
                       &hir::Inherited,
@@ -2262,22 +2255,26 @@ impl<'a> State<'a> {
     }
 
     pub fn print_fn_header_info(&mut self,
-                                unsafety: hir::Unsafety,
-                                constness: hir::Constness,
-                                abi: Abi,
+                                header: hir::FnHeader,
                                 vis: &hir::Visibility)
                                 -> io::Result<()> {
         self.s.word(&visibility_qualified(vis, ""))?;
-        self.print_unsafety(unsafety)?;
 
-        match constness {
+        match header.constness {
             hir::Constness::NotConst => {}
             hir::Constness::Const => self.word_nbsp("const")?,
         }
 
-        if abi != Abi::Rust {
+        match header.asyncness {
+            hir::IsAsync::NotAsync => {}
+            hir::IsAsync::Async => self.word_nbsp("async")?,
+        }
+
+        self.print_unsafety(header.unsafety)?;
+
+        if header.abi != Abi::Rust {
             self.word_nbsp("extern")?;
-            self.word_nbsp(&abi.to_string())?;
+            self.word_nbsp(&header.abi.to_string())?;
         }
 
         self.s.word("fn")