Ensure that drop order of async fn matches fn.

This commit modifies the lowering of `async fn` arguments so that the
drop order matches the equivalent `fn`.

Previously, async function arguments were lowered as shown below:

    async fn foo(<pattern>: <ty>) {
      async move {
      }
    } // <-- dropped as you "exit" the fn

    // ...becomes...
    fn foo(__arg0: <ty>) {
      async move {
        let <pattern> = __arg0;
      } // <-- dropped as you "exit" the async block
    }

After this PR, async function arguments will be lowered as:

    async fn foo(<pattern>: <ty>, <pattern>: <ty>, <pattern>: <ty>) {
      async move {
      }
    } // <-- dropped as you "exit" the fn

    // ...becomes...
    fn foo(__arg0: <ty>, __arg1: <ty>, __arg2: <ty>) {
      async move {
        let __arg2 = __arg2;
        let <pattern> = __arg2;
        let __arg1 = __arg1;
        let <pattern> = __arg1;
        let __arg0 = __arg0;
        let <pattern> = __arg0;
      } // <-- dropped as you "exit" the async block
    }

If `<pattern>` is a simple ident, then it is lowered to a single
`let <pattern> = <pattern>;` statement as an optimization.

Author: davidtwco

src/librustc/hir/lowering.rs

@@ -2996,8 +2996,33 @@ impl<'a> LoweringContext<'a> {
             if let IsAsync::Async { closure_id, ref arguments, .. } = asyncness {
                 let mut body = body.clone();
 
+                // Async function arguments are lowered into the closure body so that they are
+                // captured and so that the drop order matches the equivalent non-async functions.
+                //
+                //     async fn foo(<pattern>: <ty>, <pattern>: <ty>, <pattern>: <ty>) {
+                //       async move {
+                //       }
+                //     }
+                //
+                //     // ...becomes...
+                //     fn foo(__arg0: <ty>, __arg1: <ty>, __arg2: <ty>) {
+                //       async move {
+                //         let __arg2 = __arg2;
+                //         let <pattern> = __arg2;
+                //         let __arg1 = __arg1;
+                //         let <pattern> = __arg1;
+                //         let __arg0 = __arg0;
+                //         let <pattern> = __arg0;
+                //       }
+                //     }
+                //
+                // If `<pattern>` is a simple ident, then it is lowered to a single
+                // `let <pattern> = <pattern>;` statement as an optimization.
                 for a in arguments.iter().rev() {
-                    body.stmts.insert(0, a.stmt.clone());
+                    if let Some(pat_stmt) = a.pat_stmt.clone() {
+                        body.stmts.insert(0, pat_stmt);
+                    }
+                    body.stmts.insert(0, a.move_stmt.clone());
                 }
 
                 let async_expr = this.make_async_expr(
@@ -3093,7 +3118,11 @@ impl<'a> LoweringContext<'a> {
                         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();
+                        for (i, a) in arguments.clone().drain(..).enumerate() {
+                            if let Some(arg) = a.arg {
+                                decl.inputs[i] = arg;
+                            }
+                        }
                         lower_fn(&decl)
                     } else {
                         lower_fn(decl)
@@ -3590,7 +3619,11 @@ impl<'a> LoweringContext<'a> {
                     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();
+                    for (i, a) in arguments.clone().drain(..).enumerate() {
+                        if let Some(arg) = a.arg {
+                            sig.decl.inputs[i] = arg;
+                        }
+                    }
                     lower_method(&sig)
                 } else {
                     lower_method(sig)

src/librustc/hir/map/def_collector.rs

@@ -94,7 +94,9 @@ impl<'a> DefCollector<'a> {
             // Walk the generated arguments for the `async fn`.
             for a in arguments {
                 use visit::Visitor;
-                this.visit_ty(&a.arg.ty);
+                if let Some(arg) = &a.arg {
+                    this.visit_ty(&arg.ty);
+                }
             }
 
             // We do not invoke `walk_fn_decl` as this will walk the arguments that are being
@@ -105,10 +107,13 @@ impl<'a> DefCollector<'a> {
                 *closure_id, DefPathData::ClosureExpr, REGULAR_SPACE, span,
             );
             this.with_parent(closure_def, |this| {
+                use visit::Visitor;
+                // Walk each of the generated statements before the regular block body.
                 for a in arguments {
-                    use visit::Visitor;
-                    // Walk each of the generated statements before the regular block body.
-                    this.visit_stmt(&a.stmt);
+                    this.visit_stmt(&a.move_stmt);
+                    if let Some(pat_stmt) = &a.pat_stmt {
+                        this.visit_stmt(&pat_stmt);
+                    }
                 }
 
                 visit::walk_block(this, &body);

src/librustc/lint/context.rs

@@ -1334,14 +1334,19 @@ impl<'a, T: EarlyLintPass> ast_visit::Visitor<'a> for EarlyContextAndPass<'a, T>
         if let ast::IsAsync::Async { ref arguments, .. } = header.asyncness.node {
             for a in arguments {
                 // Visit the argument..
-                self.visit_pat(&a.arg.pat);
-                if let ast::ArgSource::AsyncFn(pat) = &a.arg.source {
-                    self.visit_pat(pat);
+                if let Some(arg) = &a.arg {
+                    self.visit_pat(&arg.pat);
+                    if let ast::ArgSource::AsyncFn(pat) = &arg.source {
+                        self.visit_pat(pat);
+                    }
+                    self.visit_ty(&arg.ty);
                 }
-                self.visit_ty(&a.arg.ty);
 
                 // ..and the statement.
-                self.visit_stmt(&a.stmt);
+                self.visit_stmt(&a.move_stmt);
+                if let Some(pat_stmt) = &a.pat_stmt {
+                    self.visit_stmt(&pat_stmt);
+                }
             }
         }
     }

src/librustc_resolve/lib.rs

@@ -862,7 +862,13 @@ impl<'a, 'tcx> Visitor<'tcx> for Resolver<'a> {
         // Walk the generated async arguments if this is an `async fn`, otherwise walk the
         // normal arguments.
         if let IsAsync::Async { ref arguments, .. } = asyncness {
-            for a in arguments { add_argument(&a.arg); }
+            for (i, a) in arguments.iter().enumerate() {
+                if let Some(arg) = &a.arg {
+                    add_argument(&arg);
+                } else {
+                    add_argument(&declaration.inputs[i]);
+                }
+            }
         } else {
             for a in &declaration.inputs { add_argument(a); }
         }
@@ -882,8 +888,11 @@ impl<'a, 'tcx> Visitor<'tcx> for Resolver<'a> {
                     let mut body = body.clone();
                     // Insert the generated statements into the body before attempting to
                     // resolve names.
-                    for a in arguments {
-                        body.stmts.insert(0, a.stmt.clone());
+                    for a in arguments.iter().rev() {
+                        if let Some(pat_stmt) = a.pat_stmt.clone() {
+                            body.stmts.insert(0, pat_stmt);
+                        }
+                        body.stmts.insert(0, a.move_stmt.clone());
                     }
                     self.visit_block(&body);
                 } else {

src/libsyntax/ast.rs

@@ -1865,10 +1865,14 @@ pub enum Unsafety {
 pub struct AsyncArgument {
     /// `__arg0`
     pub ident: Ident,
-    /// `__arg0: <ty>` argument to replace existing function argument `<pat>: <ty>`.
-    pub arg: Arg,
-    /// `let <pat>: <ty> = __arg0;` statement to be inserted at the start of the block.
-    pub stmt: Stmt,
+    /// `__arg0: <ty>` argument to replace existing function argument `<pat>: <ty>`. Only if
+    /// argument is not a simple binding.
+    pub arg: Option<Arg>,
+    /// `let __arg0 = __arg0;` statement to be inserted at the start of the block.
+    pub move_stmt: Stmt,
+    /// `let <pat> = __arg0;` statement to be inserted at the start of the block, after matching
+    /// move statement. Only if argument is not a simple binding.
+    pub pat_stmt: Option<Stmt>,
 }
 
 #[derive(Clone, RustcEncodable, RustcDecodable, Debug)]

src/libsyntax/ext/placeholders.rs

@@ -199,7 +199,10 @@ impl<'a, 'b> MutVisitor for PlaceholderExpander<'a, 'b> {
 
         if let ast::IsAsync::Async { ref mut arguments, .. } = a {
             for argument in arguments.iter_mut() {
-                self.next_id(&mut argument.stmt.id);
+                self.next_id(&mut argument.move_stmt.id);
+                if let Some(ref mut pat_stmt) = &mut argument.pat_stmt {
+                    self.next_id(&mut pat_stmt.id);
+                }
             }
         }
     }

src/libsyntax/mut_visit.rs

@@ -694,13 +694,21 @@ pub fn noop_visit_asyncness<T: MutVisitor>(asyncness: &mut IsAsync, vis: &mut T)
         IsAsync::Async { closure_id, return_impl_trait_id, ref mut arguments } => {
             vis.visit_id(closure_id);
             vis.visit_id(return_impl_trait_id);
-            for AsyncArgument { ident, arg, stmt } in arguments.iter_mut() {
+            for AsyncArgument { ident, arg, pat_stmt, move_stmt } in arguments.iter_mut() {
                 vis.visit_ident(ident);
-                vis.visit_arg(arg);
-                visit_clobber(stmt, |stmt| {
+                if let Some(arg) = arg {
+                    vis.visit_arg(arg);
+                }
+                visit_clobber(move_stmt, |stmt| {
                     vis.flat_map_stmt(stmt)
                         .expect_one("expected visitor to produce exactly one item")
                 });
+                visit_opt(pat_stmt, |stmt| {
+                    visit_clobber(stmt, |stmt| {
+                        vis.flat_map_stmt(stmt)
+                            .expect_one("expected visitor to produce exactly one item")
+                    })
+                });
             }
         }
         IsAsync::NotAsync => {}

src/libsyntax/parse/parser.rs

@@ -8880,25 +8880,44 @@ impl<'a> Parser<'a> {
                 let name = format!("__arg{}", index);
                 let ident = Ident::from_str(&name);
 
+                // Check if this is a ident pattern, if so, we can optimize and avoid adding a
+                // `let <pat> = __argN;` statement, instead just adding a `let <pat> = <pat>;`
+                // statement.
+                let (ident, is_simple_pattern) = match input.pat.node {
+                    PatKind::Ident(_, ident, _) => (ident, true),
+                    _ => (ident, false),
+                };
+
                 // Construct an argument representing `__argN: <ty>` to replace the argument of the
-                // async function.
-                let arg = Arg {
-                    ty: input.ty.clone(),
-                    id,
+                // async function if it isn't a simple pattern.
+                let arg = if is_simple_pattern {
+                    None
+                } else {
+                    Some(Arg {
+                        ty: input.ty.clone(),
+                        id,
+                        pat: P(Pat {
+                            id,
+                            node: PatKind::Ident(
+                                BindingMode::ByValue(Mutability::Immutable), ident, None,
+                            ),
+                            span,
+                        }),
+                        source: ArgSource::AsyncFn(input.pat.clone()),
+                    })
+                };
+
+                // Construct a `let __argN = __argN;` statement to insert at the top of the
+                // async closure. This makes sure that the argument is captured by the closure and
+                // that the drop order is correct.
+                let move_local = Local {
                     pat: P(Pat {
                         id,
                         node: PatKind::Ident(
                             BindingMode::ByValue(Mutability::Immutable), ident, None,
                         ),
                         span,
                     }),
-                    source: ArgSource::AsyncFn(input.pat.clone()),
-                };
-
-                // Construct a `let <pat> = __argN;` statement to insert at the top of the
-                // async closure.
-                let local = P(Local {
-                    pat: input.pat.clone(),
                     // We explicitly do not specify the type for this statement. When the user's
                     // argument type is `impl Trait` then this would require the
                     // `impl_trait_in_bindings` feature to also be present for that same type to
@@ -8918,10 +8937,25 @@ impl<'a> Parser<'a> {
                     span,
                     attrs: ThinVec::new(),
                     source: LocalSource::AsyncFn,
-                });
-                let stmt = Stmt { id, node: StmtKind::Local(local), span, };
+                };
+
+                // Construct a `let <pat> = __argN;` statement to insert at the top of the
+                // async closure if this isn't a simple pattern.
+                let pat_stmt = if is_simple_pattern {
+                    None
+                } else {
+                    Some(Stmt {
+                        id,
+                        node: StmtKind::Local(P(Local {
+                            pat: input.pat.clone(),
+                            ..move_local.clone()
+                        })),
+                        span,
+                    })
+                };
 
-                arguments.push(AsyncArgument { ident, arg, stmt });
+                let move_stmt = Stmt { id, node: StmtKind::Local(P(move_local)), span };
+                arguments.push(AsyncArgument { ident, arg, pat_stmt, move_stmt });
             }
         }
     }