async await desugaring and tests

38 files changed, 1282 insertions, 199 deletions

diff --git a/src/librustc/diagnostics.rs b/src/librustc/diagnostics.rs
index 1435957a5c1..6e6b15bdff7 100644
--- a/src/librustc/diagnostics.rs
+++ b/src/librustc/diagnostics.rs
@@ -2131,5 +2131,10 @@ register_diagnostics! {
     E0657, // `impl Trait` can only capture lifetimes bound at the fn level
     E0687, // in-band lifetimes cannot be used in `fn`/`Fn` syntax
     E0688, // in-band lifetimes cannot be mixed with explicit lifetime binders
-    E0697, // closures cannot be static
+
+    E0906, // closures cannot be static
+
+    E0703, // multiple different lifetimes used in arguments of `async fn`
+    E0704, // multiple elided lifetimes used in arguments of `async fn`
+    E0705, // `async` non-`move` closures with arguments are not currently supported
 }
diff --git a/src/librustc/hir/lowering.rs b/src/librustc/hir/lowering.rs
index 16405b602be..d0e6a5e2973 100644
--- a/src/librustc/hir/lowering.rs
+++ b/src/librustc/hir/lowering.rs
@@ -449,7 +449,7 @@ impl<'a> LoweringContext<'a> {
         }
     }
 
-    fn allocate_hir_id_counter<T: Debug>(&mut self, owner: NodeId, debug: &T) {
+    fn allocate_hir_id_counter<T: Debug>(&mut self, owner: NodeId, debug: &T) -> LoweredNodeId {
         if self.item_local_id_counters.insert(owner, 0).is_some() {
             bug!(
                 "Tried to allocate item_local_id_counter for {:?} twice",
@@ -457,7 +457,7 @@ impl<'a> LoweringContext<'a> {
             );
         }
         // Always allocate the first HirId for the owner itself
-        self.lower_node_id_with_owner(owner, owner);
+        self.lower_node_id_with_owner(owner, owner)
     }
 
     fn lower_node_id_generic<F>(&mut self, ast_node_id: NodeId, alloc_hir_id: F) -> LoweredNodeId
@@ -501,7 +501,7 @@ impl<'a> LoweringContext<'a> {
     {
         let counter = self.item_local_id_counters
             .insert(owner, HIR_ID_COUNTER_LOCKED)
-            .unwrap();
+            .unwrap_or_else(|| panic!("No item_local_id_counters entry for {:?}", owner));
         let def_index = self.resolver.definitions().opt_def_index(owner).unwrap();
         self.current_hir_id_owner.push((def_index, counter));
         let ret = f(self);
@@ -840,6 +840,46 @@ impl<'a> LoweringContext<'a> {
         result
     }
 
+    fn make_async_expr(
+        &mut self,
+        capture_clause: CaptureBy,
+        closure_node_id: NodeId,
+        ret_ty: Option<&Ty>,
+        body: impl FnOnce(&mut LoweringContext) -> hir::Expr,
+    ) -> hir::Expr_ {
+        let prev_is_generator = mem::replace(&mut self.is_generator, true);
+        let body_expr = body(self);
+        let span = body_expr.span;
+        let output = match ret_ty {
+            Some(ty) => FunctionRetTy::Ty(P(ty.clone())),
+            None => FunctionRetTy::Default(span),
+        };
+        let decl = FnDecl {
+            inputs: vec![],
+            output,
+            variadic: false
+        };
+        let body_id = self.record_body(body_expr, Some(&decl));
+        self.is_generator = prev_is_generator;
+
+        let capture_clause = self.lower_capture_clause(capture_clause);
+        let closure_hir_id = self.lower_node_id(closure_node_id).hir_id;
+        let decl = self.lower_fn_decl(&decl, None, /* impl trait allowed */ false, false);
+        let generator = hir::Expr {
+            id: closure_node_id,
+            hir_id: closure_hir_id,
+            node: hir::ExprClosure(capture_clause, decl, body_id, span,
+                Some(hir::GeneratorMovability::Static)),
+            span,
+            attrs: ThinVec::new(),
+        };
+
+        let unstable_span = self.allow_internal_unstable(CompilerDesugaringKind::Async, span);
+        let gen_future = self.expr_std_path(
+            unstable_span, &["raw", "future_from_generator"], ThinVec::new());
+        hir::ExprCall(P(gen_future), hir_vec![generator])
+    }
+
     fn lower_body<F>(&mut self, decl: Option<&FnDecl>, f: F) -> hir::BodyId
     where
         F: FnOnce(&mut LoweringContext) -> hir::Expr,
@@ -1067,7 +1107,7 @@ impl<'a> LoweringContext<'a> {
                                 ),
                                 unsafety: this.lower_unsafety(f.unsafety),
                                 abi: f.abi,
-                                decl: this.lower_fn_decl(&f.decl, None, false),
+                                decl: this.lower_fn_decl(&f.decl, None, false, false),
                                 arg_names: this.lower_fn_args_to_names(&f.decl),
                             }))
                         },
@@ -1132,92 +1172,10 @@ impl<'a> LoweringContext<'a> {
                 let span = t.span;
                 match itctx {
                     ImplTraitContext::Existential(fn_def_id) => {
-
-                        // We need to manually repeat the code of `next_id` because the lowering
-                        // needs to happen while the owner_id is pointing to the item itself,
-                        // because items are their own owners
-                        let exist_ty_node_id = self.sess.next_node_id();
-
-                        // Make sure we know that some funky desugaring has been going on here.
-                        // This is a first: there is code in other places like for loop
-                        // desugaring that explicitly states that we don't want to track that.
-                        // Not tracking it makes lints in rustc and clippy very fragile as
-                        // frequently opened issues show.
-                        let exist_ty_span = self.allow_internal_unstable(
-                            CompilerDesugaringKind::ExistentialReturnType,
-                            t.span,
-                        );
-
-                        // Pull a new definition from the ether
-                        let exist_ty_def_index = self
-                            .resolver
-                            .definitions()
-                            .create_def_with_parent(
-                            fn_def_id.index,
-                            exist_ty_node_id,
-                            DefPathData::ExistentialImplTrait,
-                            DefIndexAddressSpace::High,
-                            Mark::root(),
-                            exist_ty_span,
-                        );
-
-                        // the `t` is just for printing debug messages
-                        self.allocate_hir_id_counter(exist_ty_node_id, t);
-
-                        let hir_bounds = self.with_hir_id_owner(exist_ty_node_id, |lctx| {
-                            lctx.lower_param_bounds(bounds, itctx)
-                        });
-
-                        let (lifetimes, lifetime_defs) = self.lifetimes_from_impl_trait_bounds(
-                            exist_ty_node_id,
-                            exist_ty_def_index,
-                            &hir_bounds,
-                        );
-
-                        self.with_hir_id_owner(exist_ty_node_id, |lctx| {
-                            let exist_ty_item_kind = hir::ItemExistential(hir::ExistTy {
-                                generics: hir::Generics {
-                                    params: lifetime_defs,
-                                    where_clause: hir::WhereClause {
-                                        id: lctx.next_id().node_id,
-                                        predicates: Vec::new().into(),
-                                    },
-                                    span,
-                                },
-                                bounds: hir_bounds,
-                                impl_trait_fn: Some(fn_def_id),
-                            });
-                            let exist_ty_id = lctx.lower_node_id(exist_ty_node_id);
-                            // Generate an `existential type Foo: Trait;` declaration
-                            trace!("creating existential type with id {:#?}", exist_ty_id);
-                            // Set the name to `impl Bound1 + Bound2`
-                            let exist_ty_name = Symbol::intern(&pprust::ty_to_string(t));
-
-                            trace!("exist ty def index: {:#?}", exist_ty_def_index);
-                            let exist_ty_item = hir::Item {
-                                id: exist_ty_id.node_id,
-                                hir_id: exist_ty_id.hir_id,
-                                name: exist_ty_name,
-                                attrs: Default::default(),
-                                node: exist_ty_item_kind,
-                                vis: hir::Visibility::Inherited,
-                                span: exist_ty_span,
-                            };
-
-                            // Insert the item into the global list. This usually happens
-                            // automatically for all AST items. But this existential type item
-                            // does not actually exist in the AST.
-                            lctx.items.insert(exist_ty_id.node_id, exist_ty_item);
-
-                            // `impl Trait` now just becomes `Foo<'a, 'b, ..>`
-                            hir::TyImplTraitExistential(
-                                hir::ItemId {
-                                    id: exist_ty_id.node_id
-                                },
-                                DefId::local(exist_ty_def_index),
-                                lifetimes,
-                            )
-                        })
+                        // Set the name to `impl Bound1 + Bound2`
+                        let exist_ty_name = Symbol::intern(&pprust::ty_to_string(t));
+                        self.lower_existential_impl_trait(
+                            span, fn_def_id, exist_ty_name, |this| this.lower_bounds(bounds, itctx))
                     }
                     ImplTraitContext::Universal(def_id) => {
                         let def_node_id = self.next_id().node_id;
@@ -1281,6 +1239,95 @@ impl<'a> LoweringContext<'a> {
         })
     }
 
+    fn lower_existential_impl_trait(
+        &mut self,
+        span: Span,
+        fn_def_id: DefId,
+        exist_ty_name: Name,
+        lower_bounds: impl FnOnce(&mut LoweringContext) -> hir::TyParamBounds,
+    ) -> hir::Ty_ {
+        // We need to manually repeat the code of `next_id` because the lowering
+        // needs to happen while the owner_id is pointing to the item itself,
+        // because items are their own owners
+        let exist_ty_node_id = self.sess.next_node_id();
+
+        // Make sure we know that some funky desugaring has been going on here.
+        // This is a first: there is code in other places like for loop
+        // desugaring that explicitly states that we don't want to track that.
+        // Not tracking it makes lints in rustc and clippy very fragile as
+        // frequently opened issues show.
+        let exist_ty_span = self.allow_internal_unstable(
+            CompilerDesugaringKind::ExistentialReturnType,
+            span,
+        );
+
+        // Pull a new definition from the ether
+        let exist_ty_def_index = self
+            .resolver
+            .definitions()
+            .create_def_with_parent(
+            fn_def_id.index,
+            exist_ty_node_id,
+            DefPathData::ExistentialImplTrait,
+            DefIndexAddressSpace::High,
+            Mark::root(),
+            exist_ty_span,
+        );
+
+        self.allocate_hir_id_counter(exist_ty_node_id, &"existential impl trait");
+
+        let hir_bounds = self.with_hir_id_owner(exist_ty_node_id, lower_bounds);
+
+        let (lifetimes, lifetime_defs) = self.lifetimes_from_impl_trait_bounds(
+            exist_ty_node_id,
+            exist_ty_def_index,
+            &hir_bounds,
+        );
+
+        self.with_hir_id_owner(exist_ty_node_id, |lctx| {
+            let exist_ty_item_kind = hir::ItemExistential(hir::ExistTy {
+                generics: hir::Generics {
+                    params: lifetime_defs,
+                    where_clause: hir::WhereClause {
+                        id: lctx.next_id().node_id,
+                        predicates: Vec::new().into(),
+                    },
+                    span,
+                },
+                bounds: hir_bounds,
+                impl_trait_fn: Some(fn_def_id),
+            });
+            let exist_ty_id = lctx.lower_node_id(exist_ty_node_id);
+            // Generate an `existential type Foo: Trait;` declaration
+            trace!("creating existential type with id {:#?}", exist_ty_id);
+
+            trace!("exist ty def index: {:#?}", exist_ty_def_index);
+            let exist_ty_item = hir::Item {
+                id: exist_ty_id.node_id,
+                hir_id: exist_ty_id.hir_id,
+                name: exist_ty_name,
+                attrs: Default::default(),
+                node: exist_ty_item_kind,
+                vis: hir::Visibility::Inherited,
+                span: exist_ty_span,
+            };
+
+            // Insert the item into the global list. This usually happens
+            // automatically for all AST items. But this existential type item
+            // does not actually exist in the AST.
+            lctx.items.insert(exist_ty_id.node_id, exist_ty_item);
+
+            // `impl Trait` now just becomes `Foo<'a, 'b, ..>`
+            hir::TyImplTraitExistential(
+                hir::ItemId {
+                    id: exist_ty_id.node_id
+                },
+                DefId::local(exist_ty_def_index),
+                lifetimes,
+            )
+        })
+    }
+
     fn lifetimes_from_impl_trait_bounds(
         &mut self,
         exist_ty_id: NodeId,
@@ -1829,31 +1876,40 @@ impl<'a> LoweringContext<'a> {
             .collect()
     }
 
+    // Lowers a function declaration.
+    //
+    // decl: the unlowered (ast) function declaration.
+    // fn_def_id: if `Some`, impl Trait arguments are lowered into generic parameters on the
+    //      given DefId, otherwise impl Trait is disallowed. Must be `Some` if
+    //      make_ret_async is true.
+    // impl_trait_return_allow: determines whether impl Trait can be used in return position.
+    //      This guards against trait declarations and implementations where impl Trait is
+    //      disallowed.
+    // make_ret_async: if enabled, converts `-> T` into `-> impl Future<Output = T>` in the
+    //      return type. This is used for `async fn` declarations.
     fn lower_fn_decl(
         &mut self,
         decl: &FnDecl,
         fn_def_id: Option<DefId>,
         impl_trait_return_allow: bool,
+        make_ret_async: bool,
     ) -> P<hir::FnDecl> {
-        // NOTE: The two last parameters here have to do with impl Trait. If fn_def_id is Some,
-        //       then impl Trait arguments are lowered into generic parameters on the given
-        //       fn_def_id, otherwise impl Trait is disallowed. (for now)
-        //
-        //       Furthermore, if impl_trait_return_allow is true, then impl Trait may be used in
-        //       return positions as well. This guards against trait declarations and their impls
-        //       where impl Trait is disallowed. (again for now)
-        P(hir::FnDecl {
-            inputs: decl.inputs
-                .iter()
-                .map(|arg| {
-                    if let Some(def_id) = fn_def_id {
-                        self.lower_ty(&arg.ty, ImplTraitContext::Universal(def_id))
-                    } else {
-                        self.lower_ty(&arg.ty, ImplTraitContext::Disallowed)
-                    }
-                })
-                .collect(),
-            output: match decl.output {
+        let inputs = decl.inputs
+            .iter()
+            .map(|arg| {
+                if let Some(def_id) = fn_def_id {
+                    self.lower_ty(&arg.ty, ImplTraitContext::Universal(def_id))
+                } else {
+                    self.lower_ty(&arg.ty, ImplTraitContext::Disallowed)
+                }
+            })
+            .collect::<HirVec<_>>();
+
+        let output = if make_ret_async {
+            self.lower_async_fn_ret_ty(
+                &inputs, &decl.output, fn_def_id.expect("make_ret_async but no fn_def_id"))
+        } else {
+            match decl.output {
                 FunctionRetTy::Ty(ref ty) => match fn_def_id {
                     Some(def_id) if impl_trait_return_allow => {
                         hir::Return(self.lower_ty(ty, ImplTraitContext::Existential(def_id)))
@@ -1861,7 +1917,12 @@ impl<'a> LoweringContext<'a> {
                     _ => hir::Return(self.lower_ty(ty, ImplTraitContext::Disallowed)),
                 },
                 FunctionRetTy::Default(span) => hir::DefaultReturn(span),
-            },
+            }
+        };
+
+        P(hir::FnDecl {
+            inputs,
+            output,
             variadic: decl.variadic,
             has_implicit_self: decl.inputs.get(0).map_or(false, |arg| match arg.ty.node {
                 TyKind::ImplicitSelf => true,
@@ -1871,6 +1932,243 @@ impl<'a> LoweringContext<'a> {
         })
     }
 
+    // Transform `-> T` into `-> impl Future<Output = T>` for `async fn`
+    //
+    // fn_span: the span of the async function declaration. Used for error reporting.
+    // inputs: lowered types of arguments to the function. Used to collect lifetimes.
+    // output: unlowered output type (`T` in `-> T`)
+    // fn_def_id: DefId of the parent function. Used to create child impl trait definition.
+    fn lower_async_fn_ret_ty(
+        &mut self,
+        inputs: &[P<hir::Ty>],
+        output: &FunctionRetTy,
+        fn_def_id: DefId,
+    ) -> hir::FunctionRetTy {
+        // Get lifetimes used in the input arguments to the function. Our output type must also
+        // have the same lifetime. FIXME(cramertj) multiple different lifetimes are not allowed
+        // because `impl Trait + 'a + 'b` doesn't allow for capture `'a` and `'b` where neither
+        // is a subset of the other. We really want some new lifetime that is a subset of all input
+        // lifetimes, but that doesn't exist at the moment.
+
+        struct AsyncFnLifetimeCollector<'r, 'a: 'r> {
+            context: &'r mut LoweringContext<'a>,
+            // Lifetimes bound by HRTB
+            currently_bound_lifetimes: Vec<hir::LifetimeName>,
+            // Whether to count elided lifetimes.
+            // Disabled inside of `Fn` or `fn` syntax.
+            collect_elided_lifetimes: bool,
+            // The lifetime found.
+            // Multiple different or elided lifetimes cannot appear in async fn for now.
+            output_lifetime: Option<(hir::LifetimeName, Span)>,
+        }
+
+        impl<'r, 'a: 'r, 'v> hir::intravisit::Visitor<'v> for AsyncFnLifetimeCollector<'r, 'a> {
+            fn nested_visit_map<'this>(
+                &'this mut self,
+            ) -> hir::intravisit::NestedVisitorMap<'this, 'v> {
+                hir::intravisit::NestedVisitorMap::None
+            }
+
+            fn visit_path_parameters(&mut self, span: Span, parameters: &'v hir::PathParameters) {
+                // Don't collect elided lifetimes used inside of `Fn()` syntax.
+                if parameters.parenthesized {
+                    let old_collect_elided_lifetimes = self.collect_elided_lifetimes;
+                    self.collect_elided_lifetimes = false;
+                    hir::intravisit::walk_path_parameters(self, span, parameters);
+                    self.collect_elided_lifetimes = old_collect_elided_lifetimes;
+                } else {
+                    hir::intravisit::walk_path_parameters(self, span, parameters);
+                }
+            }
+
+            fn visit_ty(&mut self, t: &'v hir::Ty) {
+                // Don't collect elided lifetimes used inside of `fn()` syntax
+                if let &hir::Ty_::TyBareFn(_) = &t.node {
+                    let old_collect_elided_lifetimes = self.collect_elided_lifetimes;
+                    self.collect_elided_lifetimes = false;
+
+                    // Record the "stack height" of `for<'a>` lifetime bindings
+                    // to be able to later fully undo their introduction.
+                    let old_len = self.currently_bound_lifetimes.len();
+                    hir::intravisit::walk_ty(self, t);
+                    self.currently_bound_lifetimes.truncate(old_len);
+
+                    self.collect_elided_lifetimes = old_collect_elided_lifetimes;
+                } else {
+                    hir::intravisit::walk_ty(self, t);
+                }
+            }
+
+            fn visit_poly_trait_ref(
+                &mut self,
+                trait_ref: &'v hir::PolyTraitRef,
+                modifier: hir::TraitBoundModifier,
+            ) {
+                // Record the "stack height" of `for<'a>` lifetime bindings
+                // to be able to later fully undo their introduction.
+                let old_len = self.currently_bound_lifetimes.len();
+                hir::intravisit::walk_poly_trait_ref(self, trait_ref, modifier);
+                self.currently_bound_lifetimes.truncate(old_len);
+            }
+
+            fn visit_generic_param(&mut self, param: &'v hir::GenericParam) {
+                // Record the introduction of 'a in `for<'a> ...`
+                if let hir::GenericParam::Lifetime(ref lt_def) = *param {
+                    // Introduce lifetimes one at a time so that we can handle
+                    // cases like `fn foo<'d>() -> impl for<'a, 'b: 'a, 'c: 'b + 'd>`
+                    self.currently_bound_lifetimes.push(lt_def.lifetime.name);
+                }
+
+                hir::intravisit::walk_generic_param(self, param);
+            }
+
+            fn visit_lifetime(&mut self, lifetime: &'v hir::Lifetime) {
+                let name = match lifetime.name {
+                    hir::LifetimeName::Implicit | hir::LifetimeName::Underscore => {
+                        if self.collect_elided_lifetimes {
+                            // Use `'_` for both implicit and underscore lifetimes in
+                            // `abstract type Foo<'_>: SomeTrait<'_>;`
+                            hir::LifetimeName::Underscore
+                        } else {
+                            return;
+                        }
+                    }
+                    name @ hir::LifetimeName::Fresh(_) => name,
+                    name @ hir::LifetimeName::Name(_) => name,
+                    hir::LifetimeName::Static => return,
+                };
+
+                if !self.currently_bound_lifetimes.contains(&name) {
+                    if let Some((current_lt_name, current_lt_span)) = self.output_lifetime {
+                        // We don't currently have a reliable way to desugar `async fn` with
+                        // multiple potentially unrelated input lifetimes into
+                        // `-> impl Trait + 'lt`, so we report an error in this case.
+                        if current_lt_name != name {
+                            struct_span_err!(
+                                self.context.sess,
+                                current_lt_span.between(lifetime.span),
+                                E0703,
+                                "multiple different lifetimes used in arguments of `async fn`",
+                            )
+                                .span_label(current_lt_span, "first lifetime here")
+                                .span_label(lifetime.span, "different lifetime here")
+                                .help("`async fn` can only accept borrowed values \
+                                      identical lifetimes")
+                                .emit()
+                        } else if current_lt_name.is_elided() && name.is_elided() {
+                            struct_span_err!(
+                                self.context.sess,
+                                current_lt_span.between(lifetime.span),
+                                E0704,
+                                "multiple elided lifetimes used in arguments of `async fn`",
+                            )
+                                .span_label(current_lt_span, "first lifetime here")
+                                .span_label(lifetime.span, "different lifetime here")
+                                .help("consider giving these arguments named lifetimes")
+                                .emit()
+                        }
+                    } else {
+                        self.output_lifetime = Some((name, lifetime.span));
+                    }
+                }
+            }
+        }
+
+        let bound_lifetime = {
+            let mut lifetime_collector = AsyncFnLifetimeCollector {
+                context: self,
+                currently_bound_lifetimes: Vec::new(),
+                collect_elided_lifetimes: true,
+                output_lifetime: None,
+            };
+
+            for arg in inputs {
+                hir::intravisit::walk_ty(&mut lifetime_collector, arg);
+            }
+            lifetime_collector.output_lifetime
+        };
+
+        let output_ty_name_owned;
+        let (output_ty_name, span) = match output {
+            FunctionRetTy::Ty(ty) => {
+                output_ty_name_owned = pprust::ty_to_string(ty);
+                (&*output_ty_name_owned, ty.span)
+            },
+            FunctionRetTy::Default(span) => ("()", *span),
+        };
+
+        // FIXME(cramertj) add lifetimes (see FIXME below) to the name
+        let exist_ty_name = Symbol::intern(&format!("impl Future<Output = {}>", output_ty_name));
+        let impl_trait_ty = self.lower_existential_impl_trait(
+            span, fn_def_id, exist_ty_name, |this| {
+            let output_ty = match output {
+                FunctionRetTy::Ty(ty) =>
+                    this.lower_ty(ty, ImplTraitContext::Existential(fn_def_id)),
+                FunctionRetTy::Default(span) => {
+                    let LoweredNodeId { node_id, hir_id } = this.next_id();
+                    P(hir::Ty {
+                        id: node_id,
+                        hir_id: hir_id,
+                        node: hir::TyTup(hir_vec![]),
+                        span: *span,
+                    })
+                }
+            };
+
+            let hir::Path { def, segments, .. } = this.std_path(span, &["future", "Future"], false);
+            let future_path = hir::Path {
+                segments: segments.map_slice(|mut v| {
+                    v.last_mut().unwrap().parameters = Some(P(hir::PathParameters {
+                        lifetimes: hir_vec![],
+                        types: hir_vec![],
+                        bindings: hir_vec![hir::TypeBinding {
+                            name: Symbol::intern(FN_OUTPUT_NAME),
+                            ty: output_ty,
+                            id: this.next_id().node_id,
+                            span,
+                        }],
+                        parenthesized: false,
+                    }));
+                    v
+                }),
+                def, span
+            };
+
+            // FIXME(cramertj) collect input lifetimes to function and add them to
+            // the output `impl Trait` type here.
+            let mut bounds = vec![
+                hir::TyParamBound::TraitTyParamBound(
+                    hir::PolyTraitRef {
+                        trait_ref: hir::TraitRef {
+                            path: future_path,
+                            ref_id: this.next_id().node_id,
+                        },
+                        bound_generic_params: hir_vec![],
+                        span,
+                    },
+                    hir::TraitBoundModifier::None
+                ),
+            ];
+
+            if let Some((name, span)) = bound_lifetime {
+                bounds.push(hir::RegionTyParamBound(
+                    hir::Lifetime { id: this.next_id().node_id, name, span }));
+            }
+
+            hir::HirVec::from(bounds)
+        });
+
+        let LoweredNodeId { node_id, hir_id } = self.next_id();
+        let impl_trait_ty = P(hir::Ty {
+            id: node_id,
+            node: impl_trait_ty,
+            span,
+            hir_id,
+        });
+
+        hir::FunctionRetTy::Return(impl_trait_ty)
+    }
+
     fn lower_param_bound(
         &mut self,
         tpb: &GenericBound,
@@ -2286,16 +2584,32 @@ impl<'a> LoweringContext<'a> {
             }
             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| {
+                    // Note: we can use non-async decl here because lower_body
+                    // only cares about the input argument patterns,
+                    // not the return types.
                     let body_id = this.lower_body(Some(decl), |this| {
-                        let body = this.lower_block(body, false);
-                        this.expr_block(body, ThinVec::new())
+                        if let IsAsync::Async(async_node_id) = header.asyncness {
+                            let async_expr = this.make_async_expr(
+                                CaptureBy::Value, async_node_id, None,
+                                |this| {
+                                    let body = this.lower_block(body, false);
+                                    this.expr_block(body, ThinVec::new())
+                                });
+                            this.expr(body.span, async_expr, ThinVec::new())
+                        } else {
+                            let body = this.lower_block(body, false);
+                            this.expr_block(body, ThinVec::new())
+                        }
                     });
+
                     let (generics, fn_decl) = this.add_in_band_defs(
                         generics,
                         fn_def_id,
                         AnonymousLifetimeMode::PassThrough,
-                        |this| this.lower_fn_decl(decl, Some(fn_def_id), true),
+                        |this| this.lower_fn_decl(
+                            decl, Some(fn_def_id), true, header.asyncness.is_async())
                     );
 
                     hir::ItemFn(
@@ -2863,7 +3177,7 @@ impl<'a> LoweringContext<'a> {
                         |this| {
                             (
                                 // Disallow impl Trait in foreign items
-                                this.lower_fn_decl(fdec, None, false),
+                                this.lower_fn_decl(fdec, None, false, false),
                                 this.lower_fn_args_to_names(fdec),
                             )
                         },
@@ -2890,7 +3204,7 @@ impl<'a> LoweringContext<'a> {
     ) -> hir::MethodSig {
         hir::MethodSig {
             header: self.lower_fn_header(sig.header),
-            decl: self.lower_fn_decl(&sig.decl, Some(fn_def_id), impl_trait_return_allow),
+            decl: self.lower_fn_decl(&sig.decl, Some(fn_def_id), impl_trait_return_allow, false),
         }
     }
 
@@ -2926,7 +3240,7 @@ impl<'a> LoweringContext<'a> {
 
     fn lower_asyncness(&mut self, a: IsAsync) -> hir::IsAsync {
         match a {
-            IsAsync::Async => hir::IsAsync::Async,
+            IsAsync::Async(_) => hir::IsAsync::Async,
             IsAsync::NotAsync => hir::IsAsync::NotAsync,
         }
     }
@@ -3218,46 +3532,101 @@ impl<'a> LoweringContext<'a> {
                 arms.iter().map(|x| self.lower_arm(x)).collect(),
                 hir::MatchSource::Normal,
             ),
-            ExprKind::Closure(capture_clause, movability, ref decl, ref body, fn_decl_span) => {
+            ExprKind::Async(capture_clause, closure_node_id, ref block) => {
+                self.make_async_expr(capture_clause, closure_node_id, None, |this| {
+                    this.with_new_scopes(|this| {
+                        let block = this.lower_block(block, false);
+                        this.expr_block(block, ThinVec::new())
+                    })
+                })
+            },
+            ExprKind::Closure(
+                capture_clause, asyncness, movability, ref decl, ref body, fn_decl_span) =>
+            {
                 self.with_new_scopes(|this| {
-                    let mut is_generator = false;
-                    let body_id = this.lower_body(Some(decl), |this| {
-                        let e = this.lower_expr(body);
-                        is_generator = this.is_generator;
-                        e
-                    });
-                    let generator_option = if is_generator {
-                        if !decl.inputs.is_empty() {
-                            span_err!(
+                    if let IsAsync::Async(async_closure_node_id) = asyncness {
+                        // FIXME(cramertj) allow `async` non-`move` closures with
+                        if capture_clause == CaptureBy::Ref &&
+                            !decl.inputs.is_empty()
+                        {
+                            struct_span_err!(
                                 this.sess,
                                 fn_decl_span,
-                                E0628,
-                                "generators cannot have explicit arguments"
-                            );
-                            this.sess.abort_if_errors();
+                                E0705,
+                                "`async` non-`move` closures with arguments \
+                                are not currently supported",
+                            )
+                                .help("consider using `let` statements to manually capture \
+                                        variables by reference before entering an \
+                                        `async move` closure")
+                                .emit();
                         }
-                        Some(match movability {
-                            Movability::Movable => hir::GeneratorMovability::Movable,
-                            Movability::Static => hir::GeneratorMovability::Static,
-                        })
+
+                        // Transform `async |x: u8| -> X { ... }` into
+                        // `|x: u8| future_from_generator(|| -> X { ... })`
+                        let outer_decl = FnDecl {
+                            inputs: decl.inputs.clone(),
+                            output: FunctionRetTy::Default(fn_decl_span),
+                            variadic: false,
+                        };
+                        let body_id = this.lower_body(Some(&outer_decl), |this| {
+                            let async_ret_ty = if let FunctionRetTy::Ty(ty) = &decl.output {
+                                Some(&**ty)
+                            } else { None };
+                            let async_body = this.make_async_expr(
+                                capture_clause, async_closure_node_id, async_ret_ty,
+                                |this| {
+                                    this.with_new_scopes(|this| this.lower_expr(body))
+                                });
+                            this.expr(fn_decl_span, async_body, ThinVec::new())
+                        });
+                        hir::ExprClosure(
+                            this.lower_capture_clause(capture_clause),
+                            this.lower_fn_decl(&outer_decl, None, false, false),
+                            body_id,
+                            fn_decl_span,
+                            None,
+                        )
                     } else {
-                        if movability == Movability::Static {
-                            span_err!(
-                                this.sess,
-                                fn_decl_span,
-                                E0697,
-                                "closures cannot be static"
-                            );
-                        }
-                        None
-                    };
-                    hir::ExprClosure(
-                        this.lower_capture_clause(capture_clause),
-                        this.lower_fn_decl(decl, None, false),
-                        body_id,
-                        fn_decl_span,
-                        generator_option,
-                    )
+                        let mut is_generator = false;
+                        let body_id = this.lower_body(Some(decl), |this| {
+                            let e = this.lower_expr(body);
+                            is_generator = this.is_generator;
+                            e
+                        });
+                        let generator_option = if is_generator {
+                            if !decl.inputs.is_empty() {
+                                span_err!(
+                                    this.sess,
+                                    fn_decl_span,
+                                    E0628,
+                                    "generators cannot have explicit arguments"
+                                );
+                                this.sess.abort_if_errors();
+                            }
+                            Some(match movability {
+                                Movability::Movable => hir::GeneratorMovability::Movable,
+                                Movability::Static => hir::GeneratorMovability::Static,
+                            })
+                        } else {
+                            if movability == Movability::Static {
+                                span_err!(
+                                    this.sess,
+                                    fn_decl_span,
+                                    E0906,
+                                    "closures cannot be static"
+                                );
+                            }
+                            None
+                        };
+                        hir::ExprClosure(
+                            this.lower_capture_clause(capture_clause),
+                            this.lower_fn_decl(decl, None, false, false),
+                            body_id,
+                            fn_decl_span,
+                            generator_option,
+                        )
+                    }
                 })
             }
             ExprKind::Block(ref blk, opt_label) => {
diff --git a/src/librustc/hir/map/def_collector.rs b/src/librustc/hir/map/def_collector.rs
index 8aa5dd4ad80..335e38bbe7e 100644
--- a/src/librustc/hir/map/def_collector.rs
+++ b/src/librustc/hir/map/def_collector.rs
@@ -99,6 +99,21 @@ impl<'a> visit::Visitor<'a> for DefCollector<'a> {
             ItemKind::Mod(..) if i.ident == keywords::Invalid.ident() => {
                 return visit::walk_item(self, i);
             }
+            ItemKind::Fn(_, FnHeader { asyncness: IsAsync::Async(async_node_id), .. }, ..) => {
+                // For async functions, we need to create their inner defs inside of a
+                // closure to match their desugared representation.
+                let fn_def_data = DefPathData::ValueNs(i.ident.name.as_interned_str());
+                let fn_def = self.create_def(i.id, fn_def_data, ITEM_LIKE_SPACE, i.span);
+                return self.with_parent(fn_def, |this| {
+                    let closure_def = this.create_def(async_node_id,
+                                          DefPathData::ClosureExpr,
+                                          REGULAR_SPACE,
+                                          i.span);
+                    this.with_parent(closure_def, |this| {
+                        visit::walk_item(this, i);
+                    })
+                });
+            }
             ItemKind::Mod(..) => DefPathData::Module(i.ident.name.as_interned_str()),
             ItemKind::Static(..) | ItemKind::Const(..) | ItemKind::Fn(..) =>
                 DefPathData::ValueNs(i.ident.name.as_interned_str()),
@@ -227,15 +242,32 @@ impl<'a> visit::Visitor<'a> for DefCollector<'a> {
 
         match expr.node {
             ExprKind::Mac(..) => return self.visit_macro_invoc(expr.id),
-            ExprKind::Closure(..) => {
-                let def = self.create_def(expr.id,
+            ExprKind::Closure(_, asyncness, ..) => {
+                let closure_def = self.create_def(expr.id,
                                           DefPathData::ClosureExpr,
                                           REGULAR_SPACE,
                                           expr.span);
-                self.parent_def = Some(def);
+                self.parent_def = Some(closure_def);
+
+                // Async closures desugar to closures inside of closures, so
+                // we must create two defs.
+                if let IsAsync::Async(async_id) = asyncness {
+                    let async_def = self.create_def(async_id,
+                                                    DefPathData::ClosureExpr,
+                                                    REGULAR_SPACE,
+                                                    expr.span);
+                    self.parent_def = Some(async_def);
+                }
+            }
+            ExprKind::Async(_, async_id, _) => {
+                let async_def = self.create_def(async_id,
+                                                DefPathData::ClosureExpr,
+                                                REGULAR_SPACE,
+                                                expr.span);
+                self.parent_def = Some(async_def);
             }
             _ => {}
-        }
+        };
 
         visit::walk_expr(self, expr);
         self.parent_def = parent_def;
diff --git a/src/librustc/hir/mod.rs b/src/librustc/hir/mod.rs
index 9479234cf11..bd6bef29c29 100644
--- a/src/librustc/hir/mod.rs
+++ b/src/librustc/hir/mod.rs
@@ -245,6 +245,16 @@ pub enum LifetimeName {
 }
 
 impl LifetimeName {
+    pub fn is_elided(self) -> bool {
+        match self {
+            LifetimeName::Implicit
+            | LifetimeName::Underscore => true,
+            LifetimeName::Fresh(_)
+            | LifetimeName::Static
+            | LifetimeName::Name(_) => false,
+        }
+    }
+
     pub fn name(&self) -> Name {
         use self::LifetimeName::*;
         match *self {
diff --git a/src/librustc/ich/impls_syntax.rs b/src/librustc/ich/impls_syntax.rs
index 7b14831cf95..0f4603be39d 100644
--- a/src/librustc/ich/impls_syntax.rs
+++ b/src/librustc/ich/impls_syntax.rs
@@ -409,6 +409,7 @@ impl_stable_hash_for!(enum ::syntax_pos::hygiene::ExpnFormat {
 });
 
 impl_stable_hash_for!(enum ::syntax_pos::hygiene::CompilerDesugaringKind {
+    Async,
     DotFill,
     QuestionMark,
     ExistentialReturnType,
diff --git a/src/librustc_metadata/encoder.rs b/src/librustc_metadata/encoder.rs
index 7ed3abe66c2..ce270006a9d 100644
--- a/src/librustc_metadata/encoder.rs
+++ b/src/librustc_metadata/encoder.rs
@@ -1245,7 +1245,10 @@ impl<'a, 'b: 'a, 'tcx: 'b> IsolatedEncoder<'a, 'b, 'tcx> {
                         (has_types || tcx.codegen_fn_attrs(def_id).requests_inline()) &&
                             !self.metadata_output_only();
                     let always_encode_mir = self.tcx.sess.opts.debugging_opts.always_encode_mir;
-                    if needs_inline || header.constness == hir::Constness::Const || always_encode_mir {
+                    if needs_inline
+                        || header.constness == hir::Constness::Const
+                        || always_encode_mir
+                    {
                         self.encode_optimized_mir(def_id)
                     } else {
                         None
diff --git a/src/librustc_resolve/lib.rs b/src/librustc_resolve/lib.rs
index e311701ac05..d3c19291594 100644
--- a/src/librustc_resolve/lib.rs
+++ b/src/librustc_resolve/lib.rs
@@ -55,7 +55,7 @@ use syntax::util::lev_distance::find_best_match_for_name;
 
 use syntax::visit::{self, FnKind, Visitor};
 use syntax::attr;
-use syntax::ast::{Arm, BindingMode, Block, Crate, Expr, ExprKind};
+use syntax::ast::{Arm, IsAsync, BindingMode, Block, Crate, Expr, ExprKind, FnHeader};
 use syntax::ast::{FnDecl, ForeignItem, ForeignItemKind, GenericParamKind, Generics};
 use syntax::ast::{Item, ItemKind, ImplItem, ImplItemKind};
 use syntax::ast::{Label, Local, Mutability, Pat, PatKind, Path};
@@ -2054,13 +2054,54 @@ impl<'a> Resolver<'a> {
         self.check_proc_macro_attrs(&item.attrs);
 
         match item.node {
+            ItemKind::Fn(ref declaration,
+                         FnHeader { asyncness: IsAsync::Async(async_closure_id), .. },
+                         ref generics,
+                         ref body) => {
+                // Async functions are desugared from `async fn foo() { .. }`
+                // to `fn foo() { future_from_generator(move || ... ) }`,
+                // so we have to visit the body inside the closure scope
+                self.with_type_parameter_rib(HasTypeParameters(generics, ItemRibKind), |this| {
+                    this.visit_vis(&item.vis);
+                    this.visit_ident(item.ident);
+                    this.visit_generics(generics);
+                    let rib_kind = ItemRibKind;
+                    this.ribs[ValueNS].push(Rib::new(rib_kind));
+                    this.label_ribs.push(Rib::new(rib_kind));
+                    let mut bindings_list = FxHashMap();
+                    for argument in &declaration.inputs {
+                        this.resolve_pattern(
+                            &argument.pat, PatternSource::FnParam, &mut bindings_list);
+                        this.visit_ty(&*argument.ty);
+                    }
+                    visit::walk_fn_ret_ty(this, &declaration.output);
+
+                    // Now resolve the inner closure
+                    {
+                        let rib_kind = ClosureRibKind(async_closure_id);
+                        this.ribs[ValueNS].push(Rib::new(rib_kind));
+                        this.label_ribs.push(Rib::new(rib_kind));
+                        // No need to resolve either arguments nor return type,
+                        // as this closure has neither
+
+                        // Resolve the body
+                        this.visit_block(body);
+                        this.label_ribs.pop();
+                        this.ribs[ValueNS].pop();
+                    }
+                    this.label_ribs.pop();
+                    this.ribs[ValueNS].pop();
+
+                    walk_list!(this, visit_attribute, &item.attrs);
+                })
+            }
             ItemKind::Enum(_, ref generics) |
             ItemKind::Ty(_, ref generics) |
             ItemKind::Struct(_, ref generics) |
             ItemKind::Union(_, ref generics) |
-            ItemKind::Fn(.., ref generics, _) => {
+            ItemKind::Fn(_, _, ref generics, _) => {
                 self.with_type_parameter_rib(HasTypeParameters(generics, ItemRibKind),
-                                             |this| visit::walk_item(this, item));
+                                         |this| visit::walk_item(this, item));
             }
 
             ItemKind::Impl(.., ref generics, ref opt_trait_ref, ref self_type, ref impl_items) =>
@@ -3888,6 +3929,49 @@ impl<'a> Resolver<'a> {
                 visit::walk_expr(self, expr);
                 self.current_type_ascription.pop();
             }
+            // Resolve the body of async exprs inside the async closure to which they desugar
+            ExprKind::Async(_, async_closure_id, ref block) => {
+                let rib_kind = ClosureRibKind(async_closure_id);
+                self.ribs[ValueNS].push(Rib::new(rib_kind));
+                self.label_ribs.push(Rib::new(rib_kind));
+                self.visit_block(&block);
+                self.label_ribs.pop();
+                self.ribs[ValueNS].pop();
+            }
+            // `async |x| ...` gets desugared to `|x| future_from_generator(|| ...)`, so we need to
+            // resolve the arguments within the proper scopes so that usages of them inside the
+            // closure are detected as upvars rather than normal closure arg usages.
+            ExprKind::Closure(
+                _, IsAsync::Async(inner_closure_id), _, ref fn_decl, ref body, _span) =>
+            {
+                let rib_kind = ClosureRibKind(expr.id);
+                self.ribs[ValueNS].push(Rib::new(rib_kind));
+                self.label_ribs.push(Rib::new(rib_kind));
+                // Resolve arguments:
+                let mut bindings_list = FxHashMap();
+                for argument in &fn_decl.inputs {
+                    self.resolve_pattern(&argument.pat, PatternSource::FnParam, &mut bindings_list);
+                    self.visit_ty(&argument.ty);
+                }
+                // No need to resolve return type-- the outer closure return type is
+                // FunctionRetTy::Default
+
+                // Now resolve the inner closure
+                {
+                    let rib_kind = ClosureRibKind(inner_closure_id);
+                    self.ribs[ValueNS].push(Rib::new(rib_kind));
+                    self.label_ribs.push(Rib::new(rib_kind));
+                    // No need to resolve arguments: the inner closure has none.
+                    // Resolve the return type:
+                    visit::walk_fn_ret_ty(self, &fn_decl.output);
+                    // Resolve the body
+                    self.visit_expr(body);
+                    self.label_ribs.pop();
+                    self.ribs[ValueNS].pop();
+                }
+                self.label_ribs.pop();
+                self.ribs[ValueNS].pop();
+            }
             _ => {
                 visit::walk_expr(self, expr);
             }
diff --git a/src/librustc_save_analysis/dump_visitor.rs b/src/librustc_save_analysis/dump_visitor.rs
index 7da5b1668b3..262c0e40abc 100644
--- a/src/librustc_save_analysis/dump_visitor.rs
+++ b/src/librustc_save_analysis/dump_visitor.rs
@@ -1555,7 +1555,7 @@ impl<'l, 'tcx: 'l, 'll, O: DumpOutput + 'll> Visitor<'l> for DumpVisitor<'l, 'tc
                     }
                 }
             }
-            ast::ExprKind::Closure(_, _, ref decl, ref body, _fn_decl_span) => {
+            ast::ExprKind::Closure(_, _, _, ref decl, ref body, _fn_decl_span) => {
                 let mut id = String::from("$");
                 id.push_str(&ex.id.to_string());
 
diff --git a/src/librustc_save_analysis/sig.rs b/src/librustc_save_analysis/sig.rs
index b532aaf90df..9f2ca20276c 100644
--- a/src/librustc_save_analysis/sig.rs
+++ b/src/librustc_save_analysis/sig.rs
@@ -385,7 +385,7 @@ impl Sig for ast::Item {
                 if header.constness.node == ast::Constness::Const {
                     text.push_str("const ");
                 }
-                if header.asyncness == ast::IsAsync::Async {
+                if header.asyncness.is_async() {
                     text.push_str("async ");
                 }
                 if header.unsafety == ast::Unsafety::Unsafe {
@@ -920,7 +920,7 @@ fn make_method_signature(
     if m.header.constness.node == ast::Constness::Const {
         text.push_str("const ");
     }
-    if m.header.asyncness == ast::IsAsync::Async {
+    if m.header.asyncness.is_async() {
         text.push_str("async ");
     }
     if m.header.unsafety == ast::Unsafety::Unsafe {
diff --git a/src/librustc_typeck/check/mod.rs b/src/librustc_typeck/check/mod.rs
index 366420cfcab..f149a9fe571 100644
--- a/src/librustc_typeck/check/mod.rs
+++ b/src/librustc_typeck/check/mod.rs
@@ -1164,7 +1164,7 @@ fn check_fn<'a, 'gcx, 'tcx>(inherited: &'a Inherited<'a, 'gcx, 'tcx>,
                     }
 
                     if let Node::NodeItem(item) = fcx.tcx.hir.get(fn_id) {
-                        if let Item_::ItemFn(_, _, _, _, ref generics, _) = item.node {
+                        if let Item_::ItemFn(_, _, ref generics, _) = item.node {
                             if !generics.params.is_empty() {
                                 fcx.tcx.sess.span_err(
                                     span,
diff --git a/src/librustdoc/html/render.rs b/src/librustdoc/html/render.rs
index 5989694116c..a0f29f5918e 100644
--- a/src/librustdoc/html/render.rs
+++ b/src/librustdoc/html/render.rs
@@ -2585,7 +2585,8 @@ fn item_function(w: &mut fmt::Formatter, cx: &Context, it: &clean::Item,
     write!(w, "{}<pre class='rust fn'>", render_spotlight_traits(it)?)?;
     render_attributes(w, it)?;
     write!(w,
-           "{vis}{constness}{asyncness}{unsafety}{abi}fn {name}{generics}{decl}{where_clause}</pre>",
+           "{vis}{constness}{asyncness}{unsafety}{abi}fn \
+           {name}{generics}{decl}{where_clause}</pre>",
            vis = VisSpace(&it.visibility),
            constness = ConstnessSpace(f.header.constness),
            asyncness = AsyncSpace(f.header.asyncness),
diff --git a/src/libstd/lib.rs b/src/libstd/lib.rs
index a6061e96ae5..c74cd3feca3 100644
--- a/src/libstd/lib.rs
+++ b/src/libstd/lib.rs
@@ -263,6 +263,7 @@
 #![feature(fn_traits)]
 #![feature(fnbox)]
 #![feature(futures_api)]
+#![feature(generator_trait)]
 #![feature(hashmap_internals)]
 #![feature(int_error_internals)]
 #![feature(integer_atomics)]
@@ -410,8 +411,6 @@ pub use core::ops;
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use core::ptr;
 #[stable(feature = "rust1", since = "1.0.0")]
-pub use core::raw;
-#[stable(feature = "rust1", since = "1.0.0")]
 pub use core::result;
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use core::option;
@@ -496,6 +495,7 @@ pub mod os;
 pub mod panic;
 pub mod path;
 pub mod process;
+pub mod raw;
 pub mod sync;
 pub mod time;
 
diff --git a/src/libstd/macros.rs b/src/libstd/macros.rs
index 8da70f5717e..812b0b9fdda 100644
--- a/src/libstd/macros.rs
+++ b/src/libstd/macros.rs
@@ -213,6 +213,26 @@ macro_rules! eprintln {
     ($fmt:expr, $($arg:tt)*) => (eprint!(concat!($fmt, "\n"), $($arg)*));
 }
 
+#[macro_export]
+#[unstable(feature = "await_macro", issue = "50547")]
+#[allow_internal_unstable]
+macro_rules! await {
+    ($e:expr) => { {
+        let mut pinned = $e;
+        let mut pinned = unsafe { ::core::mem::PinMut::new_unchecked(&mut pinned) };
+        loop {
+            match ::std::raw::with_get_cx(|cx|
+                      ::core::future::Future::poll(pinned.reborrow(), cx))
+            {
+                // FIXME(cramertj) prior to stabilizing await, we have to ensure that this
+                // can't be used to create a generator on stable via `|| await!()`.
+                ::core::task::Poll::Pending => yield,
+                ::core::task::Poll::Ready(x) => break x,
+            }
+        }
+    } }
+}
+
 /// A macro to select an event from a number of receivers.
 ///
 /// This macro is used to wait for the first event to occur on a number of
diff --git a/src/libstd/raw.rs b/src/libstd/raw.rs
new file mode 100644
index 00000000000..62fd42c4de7
--- /dev/null
+++ b/src/libstd/raw.rs
@@ -0,0 +1,110 @@
+// Copyright 2013 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.
+
+#![allow(missing_docs)]
+#![unstable(feature = "raw", issue = "27751")]
+
+//! Contains struct definitions for the layout of compiler built-in types.
+//!
+//! They can be used as targets of transmutes in unsafe code for manipulating
+//! the raw representations directly.
+//!
+//! Their definition should always match the ABI defined in `rustc::back::abi`.
+
+use core::cell::Cell;
+use core::future::Future;
+use core::marker::Unpin;
+use core::mem::PinMut;
+use core::option::Option;
+use core::ptr::NonNull;
+use core::task::{self, Poll};
+use core::ops::{Drop, Generator, GeneratorState};
+
+#[stable(feature = "rust1", since = "1.0.0")]
+pub use core::raw::*;
+
+/// Wrap a future in a generator.
+///
+/// This function returns a `GenFuture` underneath, but hides it in `impl Trait` to give
+/// better error messages (`impl Future` rather than `GenFuture<[closure.....]>`).
+#[unstable(feature = "gen_future", issue = "50547")]
+pub fn future_from_generator<T: Generator<Yield = ()>>(x: T) -> impl Future<Output = T::Return> {
+    GenFuture(x)
+}
+
+/// A wrapper around generators used to implement `Future` for `async`/`await` code.
+#[unstable(feature = "gen_future", issue = "50547")]
+#[derive(Copy, Clone, Debug, Eq, PartialEq, Ord, PartialOrd, Hash)]
+struct GenFuture<T: Generator<Yield = ()>>(T);
+
+// We rely on the fact that async/await futures are immovable in order to create
+// self-referential borrows in the underlying generator.
+impl<T: Generator<Yield = ()>> !Unpin for GenFuture<T> {}
+
+#[unstable(feature = "gen_future", issue = "50547")]
+impl<T: Generator<Yield = ()>> Future for GenFuture<T> {
+    type Output = T::Return;
+    fn poll(self: PinMut<Self>, cx: &mut task::Context) -> Poll<Self::Output> {
+        with_set_cx(cx, || match unsafe { PinMut::get_mut(self).0.resume() } {
+            GeneratorState::Yielded(()) => Poll::Pending,
+            GeneratorState::Complete(x) => Poll::Ready(x),
+        })
+    }
+}
+
+thread_local! {
+    static TLS_CX: Cell<Option<NonNull<task::Context<'static>>>> = Cell::new(None);
+}
+
+struct SetOnDrop(Option<NonNull<task::Context<'static>>>);
+
+impl Drop for SetOnDrop {
+    fn drop(&mut self) {
+        TLS_CX.with(|tls_cx| {
+            tls_cx.set(self.0.take());
+        });
+    }
+}
+
+#[unstable(feature = "gen_future", issue = "50547")]
+pub fn with_set_cx<F, R>(cx: &mut task::Context, f: F) -> R
+where
+    F: FnOnce() -> R
+{
+    let old_cx = TLS_CX.with(|tls_cx| {
+        let old_cx = tls_cx.get();
+        tls_cx.set(NonNull::new(
+                cx as *mut task::Context as *mut () as *mut task::Context<'static>));
+        old_cx
+    });
+    let _reset_cx = SetOnDrop(old_cx);
+    let res = f();
+    res
+}
+
+#[unstable(feature = "gen_future", issue = "50547")]
+pub fn with_get_cx<F, R>(f: F) -> R
+where
+    F: FnOnce(&mut task::Context) -> R
+{
+    let cx_ptr = TLS_CX.with(|tls_cx| {
+        let cx_ptr = tls_cx.get();
+        // Clear the entry so that nested `with_get_cx` calls
+        // will fail or set their own value.
+        tls_cx.set(None);
+        cx_ptr
+    });
+    let _reset_cx = SetOnDrop(cx_ptr);
+
+    let mut cx_ptr = cx_ptr.expect(
+        "TLS task::Context not set. This is a rustc bug. \
+        Please file an issue on https://github.com/rust-lang/rust.");
+    unsafe { f(cx_ptr.as_mut()) }
+}
diff --git a/src/libsyntax/ast.rs b/src/libsyntax/ast.rs
index 55c21c64c83..a57a9b95e5f 100644
--- a/src/libsyntax/ast.rs
+++ b/src/libsyntax/ast.rs
@@ -987,6 +987,7 @@ impl Expr {
             ExprKind::Closure(..) => ExprPrecedence::Closure,
             ExprKind::Block(..) => ExprPrecedence::Block,
             ExprKind::Catch(..) => ExprPrecedence::Catch,
+            ExprKind::Async(..) => ExprPrecedence::Async,
             ExprKind::Assign(..) => ExprPrecedence::Assign,
             ExprKind::AssignOp(..) => ExprPrecedence::AssignOp,
             ExprKind::Field(..) => ExprPrecedence::Field,
@@ -1094,9 +1095,18 @@ pub enum ExprKind {
     /// A closure (for example, `move |a, b, c| a + b + c`)
     ///
     /// The final span is the span of the argument block `|...|`
-    Closure(CaptureBy, Movability, P<FnDecl>, P<Expr>, Span),
+    Closure(CaptureBy, IsAsync, Movability, P<FnDecl>, P<Expr>, Span),
     /// A block (`'label: { ... }`)
     Block(P<Block>, Option<Label>),
+    /// An async block (`async move { ... }`)
+    ///
+    /// The `NodeId` is the `NodeId` for the closure that results from
+    /// desugaring an async block, just like the NodeId field in the
+    /// `IsAsync` enum. This is necessary in order to create a def for the
+    /// closure which can be used as a parent of any child defs. Defs
+    /// created during lowering cannot be made the parent of any other
+    /// preexisting defs.
+    Async(CaptureBy, NodeId, P<Block>),
     /// A catch block (`catch { ... }`)
     Catch(P<Block>),
 
@@ -1708,10 +1718,20 @@ pub enum Unsafety {
 
 #[derive(Copy, Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug)]
 pub enum IsAsync {
-    Async,
+    Async(NodeId),
     NotAsync,
 }
 
+impl IsAsync {
+    pub fn is_async(self) -> bool {
+        if let IsAsync::Async(_) = self {
+            true
+        } else {
+            false
+        }
+    }
+}
+
 #[derive(Copy, Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug)]
 pub enum Constness {
     Const,
diff --git a/src/libsyntax/ext/build.rs b/src/libsyntax/ext/build.rs
index 6bb7535544b..61356507665 100644
--- a/src/libsyntax/ext/build.rs
+++ b/src/libsyntax/ext/build.rs
@@ -915,6 +915,7 @@ impl<'a> AstBuilder for ExtCtxt<'a> {
                       fn_decl_span: Span) // span of the `|...|` part
                       -> P<ast::Expr> {
         self.expr(span, ast::ExprKind::Closure(ast::CaptureBy::Ref,
+                                               ast::IsAsync::NotAsync,
                                                ast::Movability::Movable,
                                                fn_decl,
                                                body,
@@ -935,6 +936,7 @@ impl<'a> AstBuilder for ExtCtxt<'a> {
         // the entire lambda body. Probably we should extend the API
         // here, but that's not entirely clear.
         self.expr(span, ast::ExprKind::Closure(ast::CaptureBy::Ref,
+                                               ast::IsAsync::NotAsync,
                                                ast::Movability::Movable,
                                                fn_decl,
                                                body,
diff --git a/src/libsyntax/feature_gate.rs b/src/libsyntax/feature_gate.rs
index a9679f86ddb..ccb2f51f964 100644
--- a/src/libsyntax/feature_gate.rs
+++ b/src/libsyntax/feature_gate.rs
@@ -308,7 +308,7 @@ declare_features! (
     // Declarative macros 2.0 (`macro`).
     (active, decl_macro, "1.17.0", Some(39412), None),
 
-    // Allows #[link(kind="static-nobundle"...]
+    // Allows #[link(kind="static-nobundle"...)]
     (active, static_nobundle, "1.16.0", Some(37403), None),
 
     // `extern "msp430-interrupt" fn()`
@@ -474,7 +474,7 @@ declare_features! (
     (active, doc_keyword, "1.28.0", Some(51315), None),
 
     // Allows async and await syntax
-    (active, async_await, "1.28.0", Some(0), Some(Edition::Edition2018)),
+    (active, async_await, "1.28.0", Some(50547), None),
 );
 
 declare_features! (
@@ -1722,6 +1722,12 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                                     "labels on blocks are unstable");
                 }
             }
+            ast::ExprKind::Closure(_, ast::IsAsync::Async(_), ..) => {
+                gate_feature_post!(&self, async_await, e.span, "async closures are unstable");
+            }
+            ast::ExprKind::Async(..) => {
+                gate_feature_post!(&self, async_await, e.span, "async blocks are unstable");
+            }
             _ => {}
         }
         visit::walk_expr(self, e);
@@ -1764,7 +1770,7 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
         match fn_kind {
             FnKind::ItemFn(_, header, _, _) => {
                 // check for const fn and async fn declarations
-                if header.asyncness == ast::IsAsync::Async {
+                if header.asyncness.is_async() {
                     gate_feature_post!(&self, async_await, span, "async fn is unstable");
                 }
                 if header.constness.node == ast::Constness::Const {
diff --git a/src/libsyntax/fold.rs b/src/libsyntax/fold.rs
index 941d2180fd1..ffea713f4ec 100644
--- a/src/libsyntax/fold.rs
+++ b/src/libsyntax/fold.rs
@@ -76,6 +76,10 @@ pub trait Folder : Sized {
         noop_fold_item_simple(i, self)
     }
 
+    fn fold_fn_header(&mut self, header: FnHeader) -> FnHeader {
+        noop_fold_fn_header(header, self)
+    }
+
     fn fold_struct_field(&mut self, sf: StructField) -> StructField {
         noop_fold_struct_field(sf, self)
     }
@@ -883,6 +887,7 @@ pub fn noop_fold_item_kind<T: Folder>(i: ItemKind, folder: &mut T) -> ItemKind {
         }
         ItemKind::Fn(decl, header, generics, body) => {
             let generics = folder.fold_generics(generics);
+            let header = folder.fold_fn_header(header);
             let decl = folder.fold_fn_decl(decl);
             let body = folder.fold_block(body);
             ItemKind::Fn(decl, header, generics, body)
@@ -990,6 +995,14 @@ pub fn noop_fold_impl_item<T: Folder>(i: ImplItem, folder: &mut T)
     })
 }
 
+pub fn noop_fold_fn_header<T: Folder>(mut header: FnHeader, folder: &mut T) -> FnHeader {
+    header.asyncness = match header.asyncness {
+        IsAsync::Async(node_id) => IsAsync::Async(folder.new_id(node_id)),
+        IsAsync::NotAsync => IsAsync::NotAsync,
+    };
+    header
+}
+
 pub fn noop_fold_mod<T: Folder>(Mod {inner, items}: Mod, folder: &mut T) -> Mod {
     Mod {
         inner: folder.new_span(inner),
@@ -1082,7 +1095,7 @@ pub fn noop_fold_foreign_item_simple<T: Folder>(ni: ForeignItem, folder: &mut T)
 
 pub fn noop_fold_method_sig<T: Folder>(sig: MethodSig, folder: &mut T) -> MethodSig {
     MethodSig {
-        header: sig.header,
+        header: folder.fold_fn_header(sig.header),
         decl: folder.fold_fn_decl(sig.decl)
     }
 }
@@ -1235,8 +1248,13 @@ pub fn noop_fold_expr<T: Folder>(Expr {id, node, span, attrs}: Expr, folder: &mu
                 ExprKind::Match(folder.fold_expr(expr),
                           arms.move_map(|x| folder.fold_arm(x)))
             }
-            ExprKind::Closure(capture_clause, movability, decl, body, span) => {
+            ExprKind::Closure(capture_clause, asyncness, movability, decl, body, span) => {
+                let asyncness = match asyncness {
+                    IsAsync::Async(node_id) => IsAsync::Async(folder.new_id(node_id)),
+                    IsAsync::NotAsync => IsAsync::NotAsync,
+                };
                 ExprKind::Closure(capture_clause,
+                                  asyncness,
                                   movability,
                                   folder.fold_fn_decl(decl),
                                   folder.fold_expr(body),
@@ -1246,6 +1264,9 @@ pub fn noop_fold_expr<T: Folder>(Expr {id, node, span, attrs}: Expr, folder: &mu
                 ExprKind::Block(folder.fold_block(blk),
                                 opt_label.map(|label| folder.fold_label(label)))
             }
+            ExprKind::Async(capture_clause, node_id, body) => {
+                ExprKind::Async(capture_clause, folder.new_id(node_id), folder.fold_block(body))
+            }
             ExprKind::Assign(el, er) => {
                 ExprKind::Assign(folder.fold_expr(el), folder.fold_expr(er))
             }
diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index d897aaadf43..e5ac0b8ed7c 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -43,7 +43,7 @@ use ast::{BinOpKind, UnOp};
 use ast::{RangeEnd, RangeSyntax};
 use {ast, attr};
 use codemap::{self, CodeMap, Spanned, respan};
-use syntax_pos::{self, Span, MultiSpan, BytePos, FileName, DUMMY_SP};
+use syntax_pos::{self, Span, MultiSpan, BytePos, FileName, DUMMY_SP, edition::Edition};
 use errors::{self, Applicability, DiagnosticBuilder};
 use parse::{self, SeqSep, classify, token};
 use parse::lexer::TokenAndSpan;
@@ -2258,6 +2258,15 @@ impl<'a> Parser<'a> {
                     hi = path.span;
                     return Ok(self.mk_expr(lo.to(hi), ExprKind::Path(Some(qself), path), attrs));
                 }
+                if syntax_pos::hygiene::default_edition() >= Edition::Edition2018 &&
+                    self.check_keyword(keywords::Async)
+                {
+                    if self.is_async_block() { // check for `async {` and `async move {`
+                        return self.parse_async_block(attrs);
+                    } else {
+                        return self.parse_lambda_expr(attrs);
+                    }
+                }
                 if self.check_keyword(keywords::Move) || self.check_keyword(keywords::Static) {
                     return self.parse_lambda_expr(attrs);
                 }
@@ -3252,6 +3261,13 @@ impl<'a> Parser<'a> {
         } else {
             Movability::Movable
         };
+        let asyncness = if syntax_pos::hygiene::default_edition() >= Edition::Edition2018
+            && self.eat_keyword(keywords::Async)
+        {
+            IsAsync::Async(ast::DUMMY_NODE_ID)
+        } else {
+            IsAsync::NotAsync
+        };
         let capture_clause = if self.eat_keyword(keywords::Move) {
             CaptureBy::Value
         } else {
@@ -3274,7 +3290,7 @@ impl<'a> Parser<'a> {
 
         Ok(self.mk_expr(
             lo.to(body.span),
-            ExprKind::Closure(capture_clause, movability, decl, body, lo.to(decl_hi)),
+            ExprKind::Closure(capture_clause, asyncness, movability, decl, body, lo.to(decl_hi)),
             attrs))
     }
 
@@ -3352,6 +3368,24 @@ impl<'a> Parser<'a> {
         Ok(self.mk_expr(span, ExprKind::Loop(body, opt_label), attrs))
     }
 
+    /// Parse an `async move {...}` expression
+    pub fn parse_async_block(&mut self, mut attrs: ThinVec<Attribute>)
+        -> PResult<'a, P<Expr>>
+    {
+        let span_lo = self.span;
+        self.expect_keyword(keywords::Async)?;
+        let capture_clause = if self.eat_keyword(keywords::Move) {
+            CaptureBy::Value
+        } else {
+            CaptureBy::Ref
+        };
+        let (iattrs, body) = self.parse_inner_attrs_and_block()?;
+        attrs.extend(iattrs);
+        Ok(self.mk_expr(
+            span_lo.to(body.span),
+            ExprKind::Async(capture_clause, ast::DUMMY_NODE_ID, body), attrs))
+    }
+
     /// Parse a `do catch {...}` expression (`do catch` token already eaten)
     fn parse_catch_expr(&mut self, span_lo: Span, mut attrs: ThinVec<Attribute>)
         -> PResult<'a, P<Expr>>
@@ -4286,6 +4320,18 @@ impl<'a> Parser<'a> {
         })
     }
 
+    fn is_async_block(&mut self) -> bool {
+        self.token.is_keyword(keywords::Async) &&
+        (
+            ( // `async move {`
+                self.look_ahead(1, |t| t.is_keyword(keywords::Move)) &&
+                self.look_ahead(2, |t| *t == token::OpenDelim(token::Brace))
+            ) || ( // `async {`
+                self.look_ahead(1, |t| *t == token::OpenDelim(token::Brace))
+            )
+        )
+    }
+
     fn is_catch_expr(&mut self) -> bool {
         self.token.is_keyword(keywords::Do) &&
         self.look_ahead(1, |t| t.is_keyword(keywords::Catch)) &&
@@ -6698,14 +6744,18 @@ impl<'a> Parser<'a> {
                                     maybe_append(attrs, extra_attrs));
             return Ok(Some(item));
         }
-        if self.eat_keyword(keywords::Async) {
+        if self.check_keyword(keywords::Async) &&
+            (self.look_ahead(1, |t| t.is_keyword(keywords::Fn)) ||
+             self.look_ahead(1, |t| t.is_keyword(keywords::Unsafe)))
+        {
             // ASYNC FUNCTION ITEM
+            self.expect_keyword(keywords::Async)?;
             let unsafety = self.parse_unsafety();
             self.expect_keyword(keywords::Fn)?;
             let fn_span = self.prev_span;
             let (ident, item_, extra_attrs) =
                 self.parse_item_fn(unsafety,
-                                   IsAsync::Async,
+                                   IsAsync::Async(ast::DUMMY_NODE_ID),
                                    respan(fn_span, Constness::NotConst),
                                    Abi::Rust)?;
             let prev_span = self.prev_span;
diff --git a/src/libsyntax/print/pprust.rs b/src/libsyntax/print/pprust.rs
index 210742c55a5..743d36d8db5 100644
--- a/src/libsyntax/print/pprust.rs
+++ b/src/libsyntax/print/pprust.rs
@@ -2171,8 +2171,10 @@ impl<'a> State<'a> {
                 }
                 self.bclose_(expr.span, INDENT_UNIT)?;
             }
-            ast::ExprKind::Closure(capture_clause, movability, ref decl, ref body, _) => {
+            ast::ExprKind::Closure(
+                capture_clause, asyncness, movability, ref decl, ref body, _) => {
                 self.print_movability(movability)?;
+                self.print_asyncness(asyncness)?;
                 self.print_capture_clause(capture_clause)?;
 
                 self.print_fn_block_args(decl)?;
@@ -2196,6 +2198,12 @@ impl<'a> State<'a> {
                 self.ibox(0)?;
                 self.print_block_with_attrs(blk, attrs)?;
             }
+            ast::ExprKind::Async(capture_clause, _, ref blk) => {
+                self.word_nbsp("async")?;
+                self.print_capture_clause(capture_clause)?;
+                self.s.space()?;
+                self.print_block_with_attrs(blk, attrs)?;
+            }
             ast::ExprKind::Assign(ref lhs, ref rhs) => {
                 let prec = AssocOp::Assign.precedence() as i8;
                 self.print_expr_maybe_paren(lhs, prec + 1)?;
@@ -2792,6 +2800,14 @@ impl<'a> State<'a> {
         }
     }
 
+    pub fn print_asyncness(&mut self, asyncness: ast::IsAsync)
+                                -> io::Result<()> {
+        if asyncness.is_async() {
+            self.word_nbsp("async")?;
+        }
+        Ok(())
+    }
+
     pub fn print_capture_clause(&mut self, capture_clause: ast::CaptureBy)
                                 -> io::Result<()> {
         match capture_clause {
@@ -3122,11 +3138,7 @@ impl<'a> State<'a> {
             ast::Constness::Const => self.word_nbsp("const")?
         }
 
-        match header.asyncness {
-            ast::IsAsync::NotAsync => {}
-            ast::IsAsync::Async => self.word_nbsp("async")?
-        }
-
+        self.print_asyncness(header.asyncness)?;
         self.print_unsafety(header.unsafety)?;
 
         if header.abi != Abi::Rust {
diff --git a/src/libsyntax/ptr.rs b/src/libsyntax/ptr.rs
index 25d916af77d..ed10857e4c5 100644
--- a/src/libsyntax/ptr.rs
+++ b/src/libsyntax/ptr.rs
@@ -95,6 +95,16 @@ impl<T: 'static> P<T> {
     }
 }
 
+impl<T: 'static> P<[T]> {
+    pub fn map_slice<F>(self, f: F) -> P<[T]> where
+        F: FnOnce(Vec<T>) -> Vec<T>
+    {
+        P {
+            ptr: f(self.ptr.into()).into(),
+        }
+    }
+}
+
 impl<T: ?Sized> Deref for P<T> {
     type Target = T;
 
diff --git a/src/libsyntax/test.rs b/src/libsyntax/test.rs
index cfb29f562d7..77225585141 100644
--- a/src/libsyntax/test.rs
+++ b/src/libsyntax/test.rs
@@ -127,11 +127,17 @@ impl<'a> fold::Folder for TestHarnessGenerator<'a> {
                 ast::ItemKind::Fn(_, header, _, _) => {
                     if header.unsafety == ast::Unsafety::Unsafe {
                         let diag = self.cx.span_diagnostic;
-                        diag.span_fatal(i.span, "unsafe functions cannot be used for tests").raise();
+                        diag.span_fatal(
+                            i.span,
+                            "unsafe functions cannot be used for tests"
+                        ).raise();
                     }
-                    if header.asyncness == ast::IsAsync::Async {
+                    if header.asyncness.is_async() {
                         let diag = self.cx.span_diagnostic;
-                        diag.span_fatal(i.span, "async functions cannot be used for tests").raise();
+                        diag.span_fatal(
+                            i.span,
+                            "async functions cannot be used for tests"
+                        ).raise();
                     }
                 }
                 _ => {},
diff --git a/src/libsyntax/util/parser.rs b/src/libsyntax/util/parser.rs
index 51b535275d6..15d910b33b0 100644
--- a/src/libsyntax/util/parser.rs
+++ b/src/libsyntax/util/parser.rs
@@ -277,6 +277,7 @@ pub enum ExprPrecedence {
     Block,
     Catch,
     Struct,
+    Async,
 }
 
 impl PartialOrd for ExprPrecedence {
@@ -346,6 +347,7 @@ impl ExprPrecedence {
             ExprPrecedence::Match |
             ExprPrecedence::Block |
             ExprPrecedence::Catch |
+            ExprPrecedence::Async |
             ExprPrecedence::Struct => PREC_PAREN,
         }
     }
diff --git a/src/libsyntax/visit.rs b/src/libsyntax/visit.rs
index 3df96ff07c5..41e3ad9d4f4 100644
--- a/src/libsyntax/visit.rs
+++ b/src/libsyntax/visit.rs
@@ -235,7 +235,7 @@ pub fn walk_item<'a, V: Visitor<'a>>(visitor: &mut V, item: &'a Item) {
         }
         ItemKind::Fn(ref declaration, header, ref generics, ref body) => {
             visitor.visit_generics(generics);
-            visitor.visit_fn(FnKind::ItemFn(item.ident, header, 
+            visitor.visit_fn(FnKind::ItemFn(item.ident, header,
                                             &item.vis, body),
                              declaration,
                              item.span,
@@ -735,7 +735,7 @@ pub fn walk_expr<'a, V: Visitor<'a>>(visitor: &mut V, expression: &'a Expr) {
             visitor.visit_expr(subexpression);
             walk_list!(visitor, visit_arm, arms);
         }
-        ExprKind::Closure(_, _, ref function_declaration, ref body, _decl_span) => {
+        ExprKind::Closure(_, _, _, ref function_declaration, ref body, _decl_span) => {
             visitor.visit_fn(FnKind::Closure(body),
                              function_declaration,
                              expression.span,
@@ -745,6 +745,9 @@ pub fn walk_expr<'a, V: Visitor<'a>>(visitor: &mut V, expression: &'a Expr) {
             walk_list!(visitor, visit_label, opt_label);
             visitor.visit_block(block);
         }
+        ExprKind::Async(_, _, ref body) => {
+            visitor.visit_block(body);
+        }
         ExprKind::Assign(ref left_hand_expression, ref right_hand_expression) => {
             visitor.visit_expr(left_hand_expression);
             visitor.visit_expr(right_hand_expression);
diff --git a/src/libsyntax_pos/hygiene.rs b/src/libsyntax_pos/hygiene.rs
index 6dd910fba82..0ca42169b8e 100644
--- a/src/libsyntax_pos/hygiene.rs
+++ b/src/libsyntax_pos/hygiene.rs
@@ -493,12 +493,14 @@ pub enum CompilerDesugaringKind {
     /// to an `existential type Foo: Trait;` + replacing the
     /// `impl Trait` with `Foo`.
     ExistentialReturnType,
+    Async,
 }
 
 impl CompilerDesugaringKind {
     pub fn as_symbol(&self) -> Symbol {
         use CompilerDesugaringKind::*;
         let s = match *self {
+            Async => "async",
             DotFill => "...",
             QuestionMark => "?",
             Catch => "do catch",
diff --git a/src/test/run-pass/async-await.rs b/src/test/run-pass/async-await.rs
new file mode 100644
index 00000000000..6b2c01f5769
--- /dev/null
+++ b/src/test/run-pass/async-await.rs
@@ -0,0 +1,133 @@
+// 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.
+
+// compile-flags: --edition=2018
+
+#![feature(arbitrary_self_types, async_await, await_macro, futures_api, pin)]
+
+use std::boxed::PinBox;
+use std::mem::PinMut;
+use std::future::Future;
+use std::sync::{
+    Arc,
+    atomic::{self, AtomicUsize},
+};
+use std::task::{
+    Context, Poll, Wake,
+    Executor, TaskObj, SpawnObjError,
+    local_waker_from_nonlocal,
+};
+
+struct Counter {
+    wakes: AtomicUsize,
+}
+
+impl Wake for Counter {
+    fn wake(this: &Arc<Self>) {
+        this.wakes.fetch_add(1, atomic::Ordering::SeqCst);
+    }
+}
+
+struct NoopExecutor;
+impl Executor for NoopExecutor {
+    fn spawn_obj(&mut self, _: TaskObj) -> Result<(), SpawnObjError> {
+        Ok(())
+    }
+}
+
+struct WakeOnceThenComplete(bool);
+
+fn wake_and_yield_once() -> WakeOnceThenComplete { WakeOnceThenComplete(false) }
+
+impl Future for WakeOnceThenComplete {
+    type Output = ();
+    fn poll(mut self: PinMut<Self>, cx: &mut Context) -> Poll<()> {
+        if self.0 {
+            Poll::Ready(())
+        } else {
+            cx.waker().wake();
+            self.0 = true;
+            Poll::Pending
+        }
+    }
+}
+
+fn async_block(x: u8) -> impl Future<Output = u8> {
+    async move {
+        await!(wake_and_yield_once());
+        x
+    }
+}
+
+fn async_nonmove_block(x: u8) -> impl Future<Output = u8> {
+    async move {
+        let future = async {
+            await!(wake_and_yield_once());
+            x
+        };
+        await!(future)
+    }
+}
+
+fn async_closure(x: u8) -> impl Future<Output = u8> {
+    (async move |x: u8| -> u8 {
+        await!(wake_and_yield_once());
+        x
+    })(x)
+}
+
+async fn async_fn(x: u8) -> u8 {
+    await!(wake_and_yield_once());
+    x
+}
+
+async fn async_fn_with_borrow(x: &u8) -> u8 {
+    await!(wake_and_yield_once());
+    *x
+}
+
+fn async_fn_with_internal_borrow(y: u8) -> impl Future<Output = u8> {
+    async move {
+        await!(async_fn_with_borrow(&y))
+    }
+}
+
+fn test_future_yields_once_then_returns<F, Fut>(f: F)
+where
+    F: FnOnce(u8) -> Fut,
+    Fut: Future<Output = u8>,
+{
+    let mut fut = PinBox::new(f(9));
+    let counter = Arc::new(Counter { wakes: AtomicUsize::new(0) });
+    let waker = local_waker_from_nonlocal(counter.clone());
+    let executor = &mut NoopExecutor;
+    let cx = &mut Context::new(&waker, executor);
+
+    assert_eq!(0, counter.wakes.load(atomic::Ordering::SeqCst));
+    assert_eq!(Poll::Pending, fut.as_pin_mut().poll(cx));
+    assert_eq!(1, counter.wakes.load(atomic::Ordering::SeqCst));
+    assert_eq!(Poll::Ready(9), fut.as_pin_mut().poll(cx));
+}
+
+fn main() {
+    macro_rules! test {
+        ($($fn_name:ident,)*) => { $(
+            test_future_yields_once_then_returns($fn_name);
+        )* }
+    }
+
+    test! {
+        async_block,
+        async_nonmove_block,
+        async_closure,
+        async_fn,
+        async_fn_with_internal_borrow,
+    }
+}
diff --git a/src/test/ui/async-fn-multiple-lifetimes.rs b/src/test/ui/async-fn-multiple-lifetimes.rs
new file mode 100644
index 00000000000..ce062ded7f7
--- /dev/null
+++ b/src/test/ui/async-fn-multiple-lifetimes.rs
@@ -0,0 +1,30 @@
+// 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.
+
+// compile-flags: --edition=2018
+
+#![feature(arbitrary_self_types, async_await, await_macro, futures_api, pin)]
+
+use std::ops::Add;
+
+async fn multiple_named_lifetimes<'a, 'b>(_: &'a u8, _: &'b u8) {}
+//~^ ERROR multiple different lifetimes used in arguments of `async fn`
+
+async fn multiple_hrtb_and_single_named_lifetime_ok<'c>(
+    _: impl for<'a> Add<&'a u8>,
+    _: impl for<'b> Add<&'b u8>,
+    _: &'c u8,
+) {}
+
+async fn multiple_elided_lifetimes(_: &u8, _: &u8) {}
+//~^ ERROR multiple elided lifetimes used
+//~^^ ERROR missing lifetime specifier
+
+fn main() {}
diff --git a/src/test/ui/async-fn-multiple-lifetimes.stderr b/src/test/ui/async-fn-multiple-lifetimes.stderr
new file mode 100644
index 00000000000..227dd4483f2
--- /dev/null
+++ b/src/test/ui/async-fn-multiple-lifetimes.stderr
@@ -0,0 +1,32 @@
+error[E0703]: multiple different lifetimes used in arguments of `async fn`
+  --> $DIR/async-fn-multiple-lifetimes.rs:17:49
+   |
+LL | async fn multiple_named_lifetimes<'a, 'b>(_: &'a u8, _: &'b u8) {}
+   |                                               --^^^^^^^^^-- different lifetime here
+   |                                               |
+   |                                               first lifetime here
+   |
+   = help: `async fn` can only accept borrowed values identical lifetimes
+
+error[E0704]: multiple elided lifetimes used in arguments of `async fn`
+  --> $DIR/async-fn-multiple-lifetimes.rs:26:39
+   |
+LL | async fn multiple_elided_lifetimes(_: &u8, _: &u8) {}
+   |                                       -^^^^^^^- different lifetime here
+   |                                       |
+   |                                       first lifetime here
+   |
+   = help: consider giving these arguments named lifetimes
+
+error[E0106]: missing lifetime specifier
+  --> $DIR/async-fn-multiple-lifetimes.rs:26:39
+   |
+LL | async fn multiple_elided_lifetimes(_: &u8, _: &u8) {}
+   |                                       ^ expected lifetime parameter
+   |
+   = help: this function's return type contains a borrowed value, but the signature does not say whether it is borrowed from `_` or `_`
+
+error: aborting due to 3 previous errors
+
+Some errors occurred: E0106, E0703, E0704.
+For more information about an error, try `rustc --explain E0106`.
diff --git a/src/test/ui/edition-keywords-2018-2015-parsing.rs b/src/test/ui/edition-keywords-2018-2015-parsing.rs
index 29c5ea41f1f..02dc8c87956 100644
--- a/src/test/ui/edition-keywords-2018-2015-parsing.rs
+++ b/src/test/ui/edition-keywords-2018-2015-parsing.rs
@@ -25,7 +25,7 @@ pub fn check_async() {
     r#async = consumes_async_raw!(async); //~ ERROR no rules expected the token `async`
     r#async = consumes_async_raw!(r#async); // OK
 
-    if passes_ident!(async) == 1 {} //~ ERROR expected expression, found reserved keyword `async`
+    if passes_ident!(async) == 1 {}
     if passes_ident!(r#async) == 1 {} // OK
     module::async(); //~ ERROR expected identifier, found reserved keyword `async`
     module::r#async(); // OK
diff --git a/src/test/ui/edition-keywords-2018-2015-parsing.stderr b/src/test/ui/edition-keywords-2018-2015-parsing.stderr
index 0b3ca57bfab..ceab5ea730e 100644
--- a/src/test/ui/edition-keywords-2018-2015-parsing.stderr
+++ b/src/test/ui/edition-keywords-2018-2015-parsing.stderr
@@ -22,11 +22,11 @@ error: no rules expected the token `async`
 LL |     r#async = consumes_async_raw!(async); //~ ERROR no rules expected the token `async`
    |                                   ^^^^^
 
-error: expected expression, found reserved keyword `async`
-  --> $DIR/edition-keywords-2018-2015-parsing.rs:28:22
+error: expected one of `move`, `|`, or `||`, found `<eof>`
+  --> <passes_ident macros>:1:22
    |
-LL |     if passes_ident!(async) == 1 {} //~ ERROR expected expression, found reserved keyword `async`
-   |                      ^^^^^ expected expression
+LL | ( $ i : ident ) => ( $ i )
+   |                      ^^^ expected one of `move`, `|`, or `||` here
 
 error: aborting due to 5 previous errors
 
diff --git a/src/test/ui/edition-keywords-2018-2018-parsing.rs b/src/test/ui/edition-keywords-2018-2018-parsing.rs
index a94808eb224..f9b4d0e18c1 100644
--- a/src/test/ui/edition-keywords-2018-2018-parsing.rs
+++ b/src/test/ui/edition-keywords-2018-2018-parsing.rs
@@ -25,7 +25,7 @@ pub fn check_async() {
     r#async = consumes_async_raw!(async); //~ ERROR no rules expected the token `async`
     r#async = consumes_async_raw!(r#async); // OK
 
-    if passes_ident!(async) == 1 {} //~ ERROR expected expression, found reserved keyword `async`
+    if passes_ident!(async) == 1 {}
     if passes_ident!(r#async) == 1 {} // OK
     module::async(); //~ ERROR expected identifier, found reserved keyword `async`
     module::r#async(); // OK
diff --git a/src/test/ui/edition-keywords-2018-2018-parsing.stderr b/src/test/ui/edition-keywords-2018-2018-parsing.stderr
index 1b18d8a39be..e48aac0a2bb 100644
--- a/src/test/ui/edition-keywords-2018-2018-parsing.stderr
+++ b/src/test/ui/edition-keywords-2018-2018-parsing.stderr
@@ -22,11 +22,11 @@ error: no rules expected the token `async`
 LL |     r#async = consumes_async_raw!(async); //~ ERROR no rules expected the token `async`
    |                                   ^^^^^
 
-error: expected expression, found reserved keyword `async`
-  --> $DIR/edition-keywords-2018-2018-parsing.rs:28:22
+error: expected one of `move`, `|`, or `||`, found `<eof>`
+  --> <passes_ident macros>:1:22
    |
-LL |     if passes_ident!(async) == 1 {} //~ ERROR expected expression, found reserved keyword `async`
-   |                      ^^^^^ expected expression
+LL | ( $ i : ident ) => ( $ i )
+   |                      ^^^ expected one of `move`, `|`, or `||` here
 
 error: aborting due to 5 previous errors
 
diff --git a/src/test/ui/feature-gate-async-await-2015-edition.rs b/src/test/ui/feature-gate-async-await-2015-edition.rs
new file mode 100644
index 00000000000..b1dd6a77e97
--- /dev/null
+++ b/src/test/ui/feature-gate-async-await-2015-edition.rs
@@ -0,0 +1,20 @@
+// 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.
+
+// compile-flags: --edition=2015
+
+#![feature(futures_api)]
+
+async fn foo() {} //~ ERROR async fn is unstable
+
+fn main() {
+    let _ = async {}; //~ ERROR cannot find struct, variant or union type `async`
+    let _ = async || {}; //~ ERROR cannot find value `async` in this scope
+}
diff --git a/src/test/ui/feature-gate-async-await-2015-edition.stderr b/src/test/ui/feature-gate-async-await-2015-edition.stderr
new file mode 100644
index 00000000000..3db90a48e90
--- /dev/null
+++ b/src/test/ui/feature-gate-async-await-2015-edition.stderr
@@ -0,0 +1,24 @@
+error[E0422]: cannot find struct, variant or union type `async` in this scope
+  --> $DIR/feature-gate-async-await-2015-edition.rs:18:13
+   |
+LL |     let _ = async {}; //~ ERROR cannot find struct, variant or union type `async`
+   |             ^^^^^ not found in this scope
+
+error[E0425]: cannot find value `async` in this scope
+  --> $DIR/feature-gate-async-await-2015-edition.rs:19:13
+   |
+LL |     let _ = async || {}; //~ ERROR cannot find value `async` in this scope
+   |             ^^^^^ not found in this scope
+
+error[E0658]: async fn is unstable (see issue #50547)
+  --> $DIR/feature-gate-async-await-2015-edition.rs:15:1
+   |
+LL | async fn foo() {} //~ ERROR async fn is unstable
+   | ^^^^^^^^^^^^^^^^^
+   |
+   = help: add #![feature(async_await)] to the crate attributes to enable
+
+error: aborting due to 3 previous errors
+
+Some errors occurred: E0422, E0425, E0658.
+For more information about an error, try `rustc --explain E0422`.
diff --git a/src/test/ui/feature-gate-async-await.rs b/src/test/ui/feature-gate-async-await.rs
new file mode 100644
index 00000000000..971b75c6dd0
--- /dev/null
+++ b/src/test/ui/feature-gate-async-await.rs
@@ -0,0 +1,19 @@
+// 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.
+
+// compile-flags: --edition=2018
+#![feature(futures_api)]
+
+async fn foo() {} //~ ERROR async fn is unstable
+
+fn main() {
+    let _ = async {}; //~ ERROR async blocks are unstable
+    let _ = async || {}; //~ ERROR async closures are unstable
+}
diff --git a/src/test/ui/feature-gate-async-await.stderr b/src/test/ui/feature-gate-async-await.stderr
new file mode 100644
index 00000000000..fdee6e20575
--- /dev/null
+++ b/src/test/ui/feature-gate-async-await.stderr
@@ -0,0 +1,27 @@
+error[E0658]: async fn is unstable (see issue #50547)
+  --> $DIR/feature-gate-async-await.rs:14:1
+   |
+LL | async fn foo() {} //~ ERROR async fn is unstable
+   | ^^^^^^^^^^^^^^^^^
+   |
+   = help: add #![feature(async_await)] to the crate attributes to enable
+
+error[E0658]: async blocks are unstable (see issue #50547)
+  --> $DIR/feature-gate-async-await.rs:17:13
+   |
+LL |     let _ = async {}; //~ ERROR async blocks are unstable
+   |             ^^^^^^^^
+   |
+   = help: add #![feature(async_await)] to the crate attributes to enable
+
+error[E0658]: async closures are unstable (see issue #50547)
+  --> $DIR/feature-gate-async-await.rs:18:13
+   |
+LL |     let _ = async || {}; //~ ERROR async closures are unstable
+   |             ^^^^^^^^^^^
+   |
+   = help: add #![feature(async_await)] to the crate attributes to enable
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0658`.
diff --git a/src/test/ui/no-args-non-move-async-closure.rs b/src/test/ui/no-args-non-move-async-closure.rs
new file mode 100644
index 00000000000..f2ecc447718
--- /dev/null
+++ b/src/test/ui/no-args-non-move-async-closure.rs
@@ -0,0 +1,18 @@
+// 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.
+
+// compile-flags: --edition=2018
+
+#![feature(arbitrary_self_types, async_await, await_macro, futures_api, pin)]
+
+fn main() {
+    let _ = async |x: u8| {};
+    //~^ ERROR `async` non-`move` closures with arguments are not currently supported
+}
diff --git a/src/test/ui/no-args-non-move-async-closure.stderr b/src/test/ui/no-args-non-move-async-closure.stderr
new file mode 100644
index 00000000000..1e49cb2e93c
--- /dev/null
+++ b/src/test/ui/no-args-non-move-async-closure.stderr
@@ -0,0 +1,11 @@
+error[E0705]: `async` non-`move` closures with arguments are not currently supported
+  --> $DIR/no-args-non-move-async-closure.rs:16:13
+   |
+LL |     let _ = async |x: u8| {};
+   |             ^^^^^^^^^^^^^
+   |
+   = help: consider using `let` statements to manually capture variables by reference before entering an `async move` closure
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0705`.