Implement built-in await syntax

Adds support for .await under the existing async_await feature gate.
Moves macro-like await! syntax to the await_macro feature gate.
Removes support for `await` as a non-keyword under the `async_await`
feature.

Author: cramertj

src/librustc/error_codes.rs

@@ -2205,4 +2205,6 @@ register_diagnostics! {
     E0711, // a feature has been declared with conflicting stability attributes
 //  E0702, // replaced with a generic attribute input check
     E0726, // non-explicit (not `'_`) elided lifetime in unsupported position
+    E0727, // `async` generators are not yet supported
+    E0728, // `await` must be in an `async` function or block
 }

src/librustc/hir/lowering.rs

@@ -1606,6 +1606,8 @@ pub enum LocalSource {
     /// }
     /// ```
     AsyncFn,
+    /// A desugared `<expr>.await`.
+    AwaitDesugar,
 }
 
 /// Hints at the original code for a `match _ { .. }`.
@@ -1624,6 +1626,8 @@ pub enum MatchSource {
     ForLoopDesugar,
     /// A desugared `?` operator.
     TryDesugar,
+    /// A desugared `<expr>.await`.
+    AwaitDesugar,
 }
 
 /// The loop type that yielded an `ExprKind::Loop`.

src/librustc/hir/mod.rs

@@ -396,6 +396,7 @@ impl_stable_hash_for!(enum ::syntax_pos::hygiene::ExpnFormat {
 
 impl_stable_hash_for!(enum ::syntax_pos::hygiene::CompilerDesugaringKind {
     Async,
+    Await,
     QuestionMark,
     ExistentialReturnType,
     ForLoop,

src/librustc/ich/impls_syntax.rs

@@ -46,7 +46,6 @@ use syntax::symbol::{Symbol, keywords};
 use syntax::errors::{Applicability, DiagnosticBuilder};
 use syntax::print::pprust::expr_to_string;
 use syntax::visit::FnKind;
-use syntax::struct_span_err;
 
 use rustc::hir::{self, GenericParamKind, PatKind};
 
@@ -1438,15 +1437,10 @@ impl KeywordIdents {
                          UnderMacro(under_macro): UnderMacro,
                          ident: ast::Ident)
     {
-        let ident_str = &ident.as_str()[..];
-        let cur_edition = cx.sess.edition();
-        let is_raw_ident = |ident: ast::Ident| {
-            cx.sess.parse_sess.raw_identifier_spans.borrow().contains(&ident.span)
-        };
-        let next_edition = match cur_edition {
+        let next_edition = match cx.sess.edition() {
             Edition::Edition2015 => {
-                match ident_str {
-                    "async" | "try" => Edition::Edition2018,
+                match &ident.as_str()[..] {
+                    "async" | "await" | "try" => Edition::Edition2018,
 
                     // rust-lang/rust#56327: Conservatively do not
                     // attempt to report occurrences of `dyn` within
@@ -1462,43 +1456,16 @@ impl KeywordIdents {
                     // an identifier.
                     "dyn" if !under_macro => Edition::Edition2018,
 
-                    // Only issue warnings for `await` if the `async_await`
-                    // feature isn't being used. Otherwise, users need
-                    // to keep using `await` for the macro exposed by std.
-                    "await" if !cx.sess.features_untracked().async_await => Edition::Edition2018,
                     _ => return,
                 }
             }
 
             // There are no new keywords yet for the 2018 edition and beyond.
-            // However, `await` is a "false" keyword in the 2018 edition,
-            // and can only be used if the `async_await` feature is enabled.
-            // Otherwise, we emit an error.
-            _ => {
-                if "await" == ident_str
-                    && !cx.sess.features_untracked().async_await
-                    && !is_raw_ident(ident)
-                {
-                    let mut err = struct_span_err!(
-                        cx.sess,
-                        ident.span,
-                        E0721,
-                        "`await` is a keyword in the {} edition", cur_edition,
-                    );
-                    err.span_suggestion(
-                        ident.span,
-                        "you can use a raw identifier to stay compatible",
-                        "r#await".to_string(),
-                        Applicability::MachineApplicable,
-                    );
-                    err.emit();
-                }
-                return
-            },
+            _ => return,
         };
 
         // don't lint `r#foo`
-        if is_raw_ident(ident) {
+        if cx.sess.parse_sess.raw_identifier_spans.borrow().contains(&ident.span) {
             return;
         }
 

src/librustc_lint/builtin.rs

@@ -77,6 +77,7 @@ impl<'a, 'tcx> Visitor<'tcx> for MatchVisitor<'a, 'tcx> {
             hir::LocalSource::Normal => "local binding",
             hir::LocalSource::ForLoopDesugar => "`for` loop binding",
             hir::LocalSource::AsyncFn => "async fn binding",
+            hir::LocalSource::AwaitDesugar => "`await` future binding",
         });
 
         // Check legality of move bindings and `@` patterns.
@@ -412,8 +413,9 @@ fn check_arms<'a, 'tcx>(
                             err.emit();
                         }
 
-                        // Unreachable patterns in try expressions occur when one of the arms
-                        // are an uninhabited type. Which is OK.
+                        // Unreachable patterns in try and await expressions occur when one of
+                        // the arms are an uninhabited type. Which is OK.
+                        hir::MatchSource::AwaitDesugar |
                         hir::MatchSource::TryDesugar => {}
                     }
                 }

src/librustc_mir/hair/pattern/check_match.rs

@@ -1065,6 +1065,7 @@ impl Expr {
             ExprKind::Block(..) => ExprPrecedence::Block,
             ExprKind::TryBlock(..) => ExprPrecedence::TryBlock,
             ExprKind::Async(..) => ExprPrecedence::Async,
+            ExprKind::Await(..) => ExprPrecedence::Await,
             ExprKind::Assign(..) => ExprPrecedence::Assign,
             ExprKind::AssignOp(..) => ExprPrecedence::AssignOp,
             ExprKind::Field(..) => ExprPrecedence::Field,
@@ -1186,6 +1187,9 @@ pub enum ExprKind {
     /// created during lowering cannot be made the parent of any other
     /// preexisting defs.
     Async(CaptureBy, NodeId, P<Block>),
+    /// An await expression (`my_future.await`).
+    Await(AwaitOrigin, P<Expr>),
+
     /// A try block (`try { ... }`).
     TryBlock(P<Block>),
 
@@ -1287,6 +1291,15 @@ pub enum Movability {
     Movable,
 }
 
+/// Whether an `await` comes from `await!` or `.await` syntax.
+/// FIXME: this should be removed when support for legacy `await!` is removed.
+/// https://github.com/rust-lang/rust/issues/60610
+#[derive(Clone, PartialEq, RustcEncodable, RustcDecodable, Debug, Copy)]
+pub enum AwaitOrigin {
+    FieldLike,
+    MacroLike,
+}
+
 pub type Mac = Spanned<Mac_>;
 
 /// Represents a macro invocation. The `Path` indicates which macro

src/libsyntax/ast.rs

@@ -485,6 +485,10 @@ declare_features! (
     // Allows async and await syntax.
     (active, async_await, "1.28.0", Some(50547), None),
 
+    // Allows await! macro-like syntax.
+    // This will likely be removed prior to stabilization of async/await.
+    (active, await_macro, "1.28.0", Some(50547), None),
+
     // Allows reinterpretation of the bits of a value of one type as another type during const eval.
     (active, const_transmute, "1.29.0", Some(53605), None),
 
@@ -2104,6 +2108,20 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
             ast::ExprKind::Async(..) => {
                 gate_feature_post!(&self, async_await, e.span, "async blocks are unstable");
             }
+            ast::ExprKind::Await(origin, _) => {
+                match origin {
+                    ast::AwaitOrigin::FieldLike =>
+                        gate_feature_post!(&self, async_await, e.span, "async/await is unstable"),
+                    ast::AwaitOrigin::MacroLike =>
+                        gate_feature_post!(
+                            &self,
+                            await_macro,
+                            e.span,
+                            "`await!(<expr>)` macro syntax is unstable, and will soon be removed \
+                            in favor of `<expr>.await` syntax."
+                        ),
+                }
+            }
             _ => {}
         }
         visit::walk_expr(self, e);

src/libsyntax/feature_gate.rs

@@ -1185,6 +1185,7 @@ pub fn noop_visit_expr<T: MutVisitor>(Expr { node, id, span, attrs }: &mut Expr,
             vis.visit_id(node_id);
             vis.visit_block(body);
         }
+        ExprKind::Await(_origin, expr) => vis.visit_expr(expr),
         ExprKind::Assign(el, er) => {
             vis.visit_expr(el);
             vis.visit_expr(er);

src/libsyntax/mut_visit.rs

@@ -2751,6 +2751,14 @@ impl<'a> Parser<'a> {
                     db.span_label(self.span, "expected expression");
                     db.note("variable declaration using `let` is a statement");
                     return Err(db);
+                } else if self.span.rust_2018() && self.eat_keyword(keywords::Await) {
+                    // FIXME: remove this branch when `await!` is no longer supported
+                    // https://github.com/rust-lang/rust/issues/60610
+                    self.expect(&token::Not)?;
+                    self.expect(&token::OpenDelim(token::Paren))?;
+                    let expr = self.parse_expr()?;
+                    self.expect(&token::CloseDelim(token::Paren))?;
+                    ex = ExprKind::Await(ast::AwaitOrigin::MacroLike, expr);
                 } else if self.token.is_path_start() {
                     let path = self.parse_path(PathStyle::Expr)?;
 
@@ -3014,6 +3022,15 @@ impl<'a> Parser<'a> {
 
     // Assuming we have just parsed `.`, continue parsing into an expression.
     fn parse_dot_suffix(&mut self, self_arg: P<Expr>, lo: Span) -> PResult<'a, P<Expr>> {
+        if self.span.rust_2018() && self.eat_keyword(keywords::Await) {
+            let span = lo.to(self.prev_span);
+            let await_expr = self.mk_expr(
+                span,
+                ExprKind::Await(ast::AwaitOrigin::FieldLike, self_arg),
+                ThinVec::new(),
+            );
+            return Ok(await_expr);
+        }
         let segment = self.parse_path_segment(PathStyle::Expr)?;
         self.check_trailing_angle_brackets(&segment, token::OpenDelim(token::Paren));
 

src/libsyntax/parse/parser.rs

@@ -99,6 +99,11 @@ pub(crate) fn ident_can_begin_expr(ident: ast::Ident, is_raw: bool) -> bool {
     ident_token.is_path_segment_keyword() ||
     [
         keywords::Async.name(),
+
+        // FIXME: remove when `await!(..)` syntax is removed
+        // https://github.com/rust-lang/rust/issues/60610
+        keywords::Await.name(),
+
         keywords::Do.name(),
         keywords::Box.name(),
         keywords::Break.name(),

src/libsyntax/parse/token.rs

@@ -2250,6 +2250,18 @@ impl<'a> State<'a> {
                 self.ibox(0)?;
                 self.print_block_with_attrs(blk, attrs)?;
             }
+            ast::ExprKind::Await(origin, ref expr) => {
+                match origin {
+                    ast::AwaitOrigin::MacroLike => {
+                        self.s.word("await!")?;
+                        self.print_expr_maybe_paren(expr, parser::PREC_FORCE_PAREN)?;
+                    }
+                    ast::AwaitOrigin::FieldLike => {
+                        self.print_expr_maybe_paren(expr, parser::PREC_POSTFIX)?;
+                        self.s.word(".await")?;
+                    }
+                }
+            }
             ast::ExprKind::Assign(ref lhs, ref rhs) => {
                 let prec = AssocOp::Assign.precedence() as i8;
                 self.print_expr_maybe_paren(lhs, prec + 1)?;

src/libsyntax/print/pprust.rs

@@ -267,6 +267,7 @@ pub enum ExprPrecedence {
     TryBlock,
     Struct,
     Async,
+    Await,
     Err,
 }
 
@@ -301,6 +302,7 @@ impl ExprPrecedence {
             ExprPrecedence::Unary => PREC_PREFIX,
 
             // Unary, postfix
+            ExprPrecedence::Await |
             ExprPrecedence::Call |
             ExprPrecedence::MethodCall |
             ExprPrecedence::Field |
@@ -346,6 +348,7 @@ pub fn contains_exterior_struct_lit(value: &ast::Expr) -> bool {
             // X { y: 1 } + X { y: 2 }
             contains_exterior_struct_lit(&lhs) || contains_exterior_struct_lit(&rhs)
         }
+        ast::ExprKind::Await(_, ref x) |
         ast::ExprKind::Unary(_, ref x) |
         ast::ExprKind::Cast(ref x, _) |
         ast::ExprKind::Type(ref x, _) |

src/libsyntax/util/parser.rs

@@ -768,6 +768,7 @@ pub fn walk_expr<'a, V: Visitor<'a>>(visitor: &mut V, expression: &'a Expr) {
         ExprKind::Async(_, _, ref body) => {
             visitor.visit_block(body);
         }
+        ExprKind::Await(_, ref expr) => visitor.visit_expr(expr),
         ExprKind::Assign(ref left_hand_expression, ref right_hand_expression) => {
             visitor.visit_expr(left_hand_expression);
             visitor.visit_expr(right_hand_expression);

src/libsyntax/visit.rs

@@ -598,13 +598,15 @@ pub enum CompilerDesugaringKind {
     /// `impl Trait` with `Foo`.
     ExistentialReturnType,
     Async,
+    Await,
     ForLoop,
 }
 
 impl CompilerDesugaringKind {
     pub fn name(self) -> Symbol {
         Symbol::intern(match self {
             CompilerDesugaringKind::Async => "async",
+            CompilerDesugaringKind::Await => "await",
             CompilerDesugaringKind::QuestionMark => "?",
             CompilerDesugaringKind::TryBlock => "try block",
             CompilerDesugaringKind::ExistentialReturnType => "existential type",

src/libsyntax_pos/hygiene.rs

@@ -84,6 +84,7 @@ symbols! {
 
         // Edition-specific keywords that are used in unstable Rust or reserved for future use.
         Async:              "async", // >= 2018 Edition only
+        Await:              "await", // >= 2018 Edition only
         Try:                "try", // >= 2018 Edition only
 
         // Special lifetime names