[wg-async-await] Drop async fn arguments in async block

src/librustc/hir/intravisit.rs

@@ -254,6 +254,9 @@ pub trait Visitor<'v> : Sized {
     fn visit_pat(&mut self, p: &'v Pat) {
         walk_pat(self, p)
     }
+    fn visit_argument_source(&mut self, s: &'v ArgSource) {
+        walk_argument_source(self, s)
+    }
     fn visit_anon_const(&mut self, c: &'v AnonConst) {
         walk_anon_const(self, c)
     }
@@ -391,10 +394,17 @@ pub fn walk_body<'v, V: Visitor<'v>>(visitor: &mut V, body: &'v Body) {
     for argument in &body.arguments {
         visitor.visit_id(argument.hir_id);
         visitor.visit_pat(&argument.pat);
+        visitor.visit_argument_source(&argument.source);
     }
     visitor.visit_expr(&body.value);
 }
 
+pub fn walk_argument_source<'v, V: Visitor<'v>>(visitor: &mut V, source: &'v ArgSource) {
+    if let ArgSource::AsyncFn(pat) = source {
+        visitor.visit_pat(pat);
+    }
+}
+
 pub fn walk_local<'v, V: Visitor<'v>>(visitor: &mut V, local: &'v Local) {
     // Intentionally visiting the expr first - the initialization expr
     // dominates the local's definition.

src/librustc/hir/lowering.rs

@@ -2025,10 +2025,17 @@ impl<'a> LoweringContext<'a> {
             init: l.init.as_ref().map(|e| P(self.lower_expr(e))),
             span: l.span,
             attrs: l.attrs.clone(),
-            source: hir::LocalSource::Normal,
+            source: self.lower_local_source(l.source),
         }, ids)
     }
 
+    fn lower_local_source(&mut self, ls: LocalSource) -> hir::LocalSource {
+        match ls {
+            LocalSource::Normal => hir::LocalSource::Normal,
+            LocalSource::AsyncFn => hir::LocalSource::AsyncFn,
+        }
+    }
+
     fn lower_mutability(&mut self, m: Mutability) -> hir::Mutability {
         match m {
             Mutability::Mutable => hir::MutMutable,
@@ -2041,6 +2048,14 @@ impl<'a> LoweringContext<'a> {
         hir::Arg {
             hir_id,
             pat: self.lower_pat(&arg.pat),
+            source: self.lower_arg_source(&arg.source),
+        }
+    }
+
+    fn lower_arg_source(&mut self, source: &ArgSource) -> hir::ArgSource {
+        match source {
+            ArgSource::Normal => hir::ArgSource::Normal,
+            ArgSource::AsyncFn(pat) => hir::ArgSource::AsyncFn(self.lower_pat(pat)),
         }
     }
 
@@ -2809,15 +2824,21 @@ impl<'a> LoweringContext<'a> {
     fn lower_async_body(
         &mut self,
         decl: &FnDecl,
-        asyncness: IsAsync,
+        asyncness: &IsAsync,
         body: &Block,
     ) -> hir::BodyId {
-        self.lower_body(Some(decl), |this| {
-            if let IsAsync::Async { closure_id, .. } = asyncness {
+        self.lower_body(Some(&decl), |this| {
+            if let IsAsync::Async { closure_id, ref arguments, .. } = asyncness {
+                let mut body = body.clone();
+
+                for a in arguments.iter().rev() {
+                    body.stmts.insert(0, a.stmt.clone());
+                }
+
                 let async_expr = this.make_async_expr(
-                    CaptureBy::Value, closure_id, None,
+                    CaptureBy::Value, *closure_id, None,
                     |this| {
-                        let body = this.lower_block(body, false);
+                        let body = this.lower_block(&body, false);
                         this.expr_block(body, ThinVec::new())
                     });
                 this.expr(body.span, async_expr, ThinVec::new())
@@ -2876,26 +2897,42 @@ impl<'a> LoweringContext<'a> {
                     value
                 )
             }
-            ItemKind::Fn(ref decl, header, ref generics, ref body) => {
+            ItemKind::Fn(ref decl, ref header, ref generics, ref body) => {
                 let fn_def_id = self.resolver.definitions().local_def_id(id);
                 self.with_new_scopes(|this| {
-                    // Note: we don't need to change the return type from `T` to
-                    // `impl Future<Output = T>` here because lower_body
-                    // only cares about the input argument patterns in the function
-                    // declaration (decl), not the return types.
-                    let body_id = this.lower_async_body(decl, header.asyncness.node, body);
+                    let mut lower_fn = |decl: &FnDecl| {
+                        // Note: we don't need to change the return type from `T` to
+                        // `impl Future<Output = T>` here because lower_body
+                        // only cares about the input argument patterns in the function
+                        // declaration (decl), not the return types.
+                        let body_id = this.lower_async_body(&decl, &header.asyncness.node, body);
+
+                        let (generics, fn_decl) = this.add_in_band_defs(
+                            generics,
+                            fn_def_id,
+                            AnonymousLifetimeMode::PassThrough,
+                            |this, idty| this.lower_fn_decl(
+                                &decl,
+                                Some((fn_def_id, idty)),
+                                true,
+                                header.asyncness.node.opt_return_id()
+                            ),
+                        );
 
-                    let (generics, fn_decl) = this.add_in_band_defs(
-                        generics,
-                        fn_def_id,
-                        AnonymousLifetimeMode::PassThrough,
-                        |this, idty| this.lower_fn_decl(
-                            decl,
-                            Some((fn_def_id, idty)),
-                            true,
-                            header.asyncness.node.opt_return_id()
-                        ),
-                    );
+                        (body_id, generics, fn_decl)
+                    };
+
+                    let (body_id, generics, fn_decl) = if let IsAsync::Async {
+                        arguments, ..
+                    } = &header.asyncness.node {
+                        let mut decl = decl.clone();
+                        // Replace the arguments of this async function with the generated
+                        // arguments that will be moved into the closure.
+                        decl.inputs = arguments.clone().drain(..).map(|a| a.arg).collect();
+                        lower_fn(&decl)
+                    } else {
+                        lower_fn(decl)
+                    };
 
                     hir::ItemKind::Fn(
                         fn_decl,
@@ -3384,15 +3421,33 @@ impl<'a> LoweringContext<'a> {
                 )
             }
             ImplItemKind::Method(ref sig, ref body) => {
-                let body_id = self.lower_async_body(&sig.decl, sig.header.asyncness.node, body);
-                let impl_trait_return_allow = !self.is_in_trait_impl;
-                let (generics, sig) = self.lower_method_sig(
-                    &i.generics,
-                    sig,
-                    impl_item_def_id,
-                    impl_trait_return_allow,
-                    sig.header.asyncness.node.opt_return_id(),
-                );
+                let mut lower_method = |sig: &MethodSig| {
+                    let body_id = self.lower_async_body(
+                        &sig.decl, &sig.header.asyncness.node, body
+                    );
+                    let impl_trait_return_allow = !self.is_in_trait_impl;
+                    let (generics, sig) = self.lower_method_sig(
+                        &i.generics,
+                        sig,
+                        impl_item_def_id,
+                        impl_trait_return_allow,
+                        sig.header.asyncness.node.opt_return_id(),
+                    );
+                    (body_id, generics, sig)
+                };
+
+                let (body_id, generics, sig) = if let IsAsync::Async {
+                    ref arguments, ..
+                } = sig.header.asyncness.node {
+                    let mut sig = sig.clone();
+                    // Replace the arguments of this async function with the generated
+                    // arguments that will be moved into the closure.
+                    sig.decl.inputs = arguments.clone().drain(..).map(|a| a.arg).collect();
+                    lower_method(&sig)
+                } else {
+                    lower_method(sig)
+                };
+
                 (generics, hir::ImplItemKind::Method(sig, body_id))
             }
             ImplItemKind::Type(ref ty) => (
@@ -3582,7 +3637,7 @@ impl<'a> LoweringContext<'a> {
         impl_trait_return_allow: bool,
         is_async: Option<NodeId>,
     ) -> (hir::Generics, hir::MethodSig) {
-        let header = self.lower_fn_header(sig.header);
+        let header = self.lower_fn_header(&sig.header);
         let (generics, decl) = self.add_in_band_defs(
             generics,
             fn_def_id,
@@ -3604,10 +3659,10 @@ impl<'a> LoweringContext<'a> {
         }
     }
 
-    fn lower_fn_header(&mut self, h: FnHeader) -> hir::FnHeader {
+    fn lower_fn_header(&mut self, h: &FnHeader) -> hir::FnHeader {
         hir::FnHeader {
             unsafety: self.lower_unsafety(h.unsafety),
-            asyncness: self.lower_asyncness(h.asyncness.node),
+            asyncness: self.lower_asyncness(&h.asyncness.node),
             constness: self.lower_constness(h.constness),
             abi: h.abi,
         }
@@ -3627,7 +3682,7 @@ impl<'a> LoweringContext<'a> {
         }
     }
 
-    fn lower_asyncness(&mut self, a: IsAsync) -> hir::IsAsync {
+    fn lower_asyncness(&mut self, a: &IsAsync) -> hir::IsAsync {
         match a {
             IsAsync::Async { .. } => hir::IsAsync::Async,
             IsAsync::NotAsync => hir::IsAsync::NotAsync,
@@ -3940,7 +3995,7 @@ impl<'a> LoweringContext<'a> {
                 })
             }
             ExprKind::Closure(
-                capture_clause, asyncness, movability, ref decl, ref body, fn_decl_span
+                capture_clause, ref asyncness, movability, ref decl, ref body, fn_decl_span
             ) => {
                 if let IsAsync::Async { closure_id, .. } = asyncness {
                     let outer_decl = FnDecl {
@@ -3978,7 +4033,7 @@ impl<'a> LoweringContext<'a> {
                                 Some(&**ty)
                             } else { None };
                             let async_body = this.make_async_expr(
-                                capture_clause, closure_id, async_ret_ty,
+                                capture_clause, *closure_id, async_ret_ty,
                                 |this| {
                                     this.with_new_scopes(|this| this.lower_expr(body))
                                 });

src/librustc/hir/map/def_collector.rs

@@ -68,16 +68,17 @@ impl<'a> DefCollector<'a> {
         id: NodeId,
         name: Name,
         span: Span,
-        header: &FnHeader,
+        header: &'a FnHeader,
         generics: &'a Generics,
         decl: &'a FnDecl,
         body: &'a Block,
     ) {
-        let (closure_id, return_impl_trait_id) = match header.asyncness.node {
+        let (closure_id, return_impl_trait_id, arguments) = match &header.asyncness.node {
             IsAsync::Async {
                 closure_id,
                 return_impl_trait_id,
-            } => (closure_id, return_impl_trait_id),
+                arguments,
+            } => (closure_id, return_impl_trait_id, arguments),
             _ => unreachable!(),
         };
 
@@ -86,17 +87,31 @@ impl<'a> DefCollector<'a> {
         let fn_def_data = DefPathData::ValueNs(name.as_interned_str());
         let fn_def = self.create_def(id, fn_def_data, ITEM_LIKE_SPACE, span);
         return self.with_parent(fn_def, |this| {
-            this.create_def(return_impl_trait_id, DefPathData::ImplTrait, REGULAR_SPACE, span);
+            this.create_def(*return_impl_trait_id, DefPathData::ImplTrait, REGULAR_SPACE, span);
 
             visit::walk_generics(this, generics);
-            visit::walk_fn_decl(this, decl);
 
-            let closure_def = this.create_def(closure_id,
-                                              DefPathData::ClosureExpr,
-                                              REGULAR_SPACE,
-                                              span);
+            // Walk the generated arguments for the `async fn`.
+            for a in arguments {
+                use visit::Visitor;
+                this.visit_ty(&a.arg.ty);
+            }
+
+            // We do not invoke `walk_fn_decl` as this will walk the arguments that are being
+            // replaced.
+            visit::walk_fn_ret_ty(this, &decl.output);
+
+            let closure_def = this.create_def(
+                *closure_id, DefPathData::ClosureExpr, REGULAR_SPACE, span,
+            );
             this.with_parent(closure_def, |this| {
-                visit::walk_block(this, body);
+                for a in arguments {
+                    use visit::Visitor;
+                    // Walk each of the generated statements before the regular block body.
+                    this.visit_stmt(&a.stmt);
+                }
+
+                visit::walk_block(this, &body);
             })
         })
     }
@@ -288,7 +303,7 @@ impl<'a> visit::Visitor<'a> for DefCollector<'a> {
 
         match expr.node {
             ExprKind::Mac(..) => return self.visit_macro_invoc(expr.id),
-            ExprKind::Closure(_, asyncness, ..) => {
+            ExprKind::Closure(_, ref asyncness, ..) => {
                 let closure_def = self.create_def(expr.id,
                                           DefPathData::ClosureExpr,
                                           REGULAR_SPACE,
@@ -298,7 +313,7 @@ impl<'a> visit::Visitor<'a> for DefCollector<'a> {
                 // Async closures desugar to closures inside of closures, so
                 // we must create two defs.
                 if let IsAsync::Async { closure_id, .. } = asyncness {
-                    let async_def = self.create_def(closure_id,
+                    let async_def = self.create_def(*closure_id,
                                                     DefPathData::ClosureExpr,
                                                     REGULAR_SPACE,
                                                     expr.span);

src/librustc/hir/mod.rs

@@ -1583,6 +1583,17 @@ pub enum LocalSource {
     Normal,
     /// A desugared `for _ in _ { .. }` loop.
     ForLoopDesugar,
+    /// When lowering async functions, we create locals within the `async move` so that
+    /// all arguments are dropped after the future is polled.
+    ///
+    /// ```ignore (pseudo-Rust)
+    /// async fn foo(<pattern> @ x: Type) {
+    ///     async move {
+    ///         let <pattern> = x;
+    ///     }
+    /// }
+    /// ```
+    AsyncFn,
 }
 
 /// Hints at the original code for a `match _ { .. }`.
@@ -1871,6 +1882,26 @@ pub struct InlineAsm {
 pub struct Arg {
     pub pat: P<Pat>,
     pub hir_id: HirId,
+    pub source: ArgSource,
+}
+
+impl Arg {
+    /// Returns the pattern representing the original binding for this argument.
+    pub fn original_pat(&self) -> &P<Pat> {
+        match &self.source {
+            ArgSource::Normal => &self.pat,
+            ArgSource::AsyncFn(pat) => &pat,
+        }
+    }
+}
+
+/// Represents the source of an argument in a function header.
+#[derive(Clone, RustcEncodable, RustcDecodable, Debug, HashStable)]
+pub enum ArgSource {
+    /// Argument as specified by the user.
+    Normal,
+    /// Generated argument from `async fn` lowering, contains the original binding pattern.
+    AsyncFn(P<Pat>),
 }
 
 /// Represents the header (not the body) of a function declaration.

src/librustc/ich/impls_hir.rs

@@ -435,4 +435,3 @@ impl<'hir> HashStable<StableHashingContext<'hir>> for attr::OptimizeAttr {
         mem::discriminant(self).hash_stable(hcx, hasher);
     }
 }
-

src/librustc/infer/error_reporting/nice_region_error/different_lifetimes.rs

@@ -86,19 +86,16 @@ impl<'a, 'gcx, 'tcx> NiceRegionError<'a, 'gcx, 'tcx> {
         let sub_is_ret_type =
             self.is_return_type_anon(scope_def_id_sub, bregion_sub, ty_fndecl_sub);
 
-        let span_label_var1 = if let Some(simple_ident) = anon_arg_sup.pat.simple_ident() {
-            format!(" from `{}`", simple_ident)
-        } else {
-            String::new()
+        let span_label_var1 = match anon_arg_sup.original_pat().simple_ident() {
+            Some(simple_ident) => format!(" from `{}`", simple_ident),
+            None => String::new(),
         };
 
-        let span_label_var2 = if let Some(simple_ident) = anon_arg_sub.pat.simple_ident() {
-            format!(" into `{}`", simple_ident)
-        } else {
-            String::new()
+        let span_label_var2 = match anon_arg_sub.original_pat().simple_ident() {
+            Some(simple_ident) => format!(" into `{}`", simple_ident),
+            None => String::new(),
         };
 
-
         let (span_1, span_2, main_label, span_label) = match (sup_is_ret_type, sub_is_ret_type) {
             (None, None) => {
                 let (main_label_1, span_label_1) = if ty_sup.hir_id == ty_sub.hir_id {