From e4ac2743aaebcf96e20db1e75bf4f55f2422fc7b Mon Sep 17 00:00:00 2001
From: Paul Lancaster <paul@lancasterzone.com>
Date: Fri, 9 Oct 2020 22:50:02 +0100
Subject: [PATCH 1/7] Add async keyword

---
 boa/src/syntax/ast/keyword.rs | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/boa/src/syntax/ast/keyword.rs b/boa/src/syntax/ast/keyword.rs
index 566d25b3224..925b8c9c9c6 100644
--- a/boa/src/syntax/ast/keyword.rs
+++ b/boa/src/syntax/ast/keyword.rs
@@ -36,6 +36,16 @@ pub enum Keyword {
     /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/await
     Await,
 
+    /// The `async` keyword.
+    ///
+    /// More information:
+    ///  - [ECMAScript reference][spec]
+    ///  - [MDN documentation][mdn]
+    ///
+    /// [spec]: https://tc39.es/ecma262/#prod-AsyncMethod
+    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function
+    Async,
+
     /// The `break` keyword.
     ///
     /// More information:
@@ -454,6 +464,7 @@ impl Keyword {
     pub fn as_str(self) -> &'static str {
         match self {
             Self::Await => "await",
+            Self::Async => "async",
             Self::Break => "break",
             Self::Case => "case",
             Self::Catch => "catch",
@@ -526,6 +537,7 @@ impl FromStr for Keyword {
     fn from_str(s: &str) -> Result<Self, Self::Err> {
         match s {
             "await" => Ok(Self::Await),
+            "async" => Ok(Self::Async),
             "break" => Ok(Self::Break),
             "case" => Ok(Self::Case),
             "catch" => Ok(Self::Catch),

From f7ca45c3ff259e04a43d999c424df42e6561b577 Mon Sep 17 00:00:00 2001
From: Paul Lancaster <paul@lancasterzone.com>
Date: Fri, 9 Oct 2020 23:29:39 +0100
Subject: [PATCH 2/7] Created AsyncFunctionDecl/Expr nodes

---
 .../declaration/async_function_decl/mod.rs    | 92 ++++++++++++++++++
 .../declaration/async_function_expr/mod.rs    | 94 +++++++++++++++++++
 boa/src/syntax/ast/node/declaration/mod.rs    |  4 +
 boa/src/syntax/ast/node/mod.rs                | 16 +++-
 4 files changed, 203 insertions(+), 3 deletions(-)
 create mode 100644 boa/src/syntax/ast/node/declaration/async_function_decl/mod.rs
 create mode 100644 boa/src/syntax/ast/node/declaration/async_function_expr/mod.rs

diff --git a/boa/src/syntax/ast/node/declaration/async_function_decl/mod.rs b/boa/src/syntax/ast/node/declaration/async_function_decl/mod.rs
new file mode 100644
index 00000000000..44d47e36ba3
--- /dev/null
+++ b/boa/src/syntax/ast/node/declaration/async_function_decl/mod.rs
@@ -0,0 +1,92 @@
+use crate::{
+    exec::Executable,
+    syntax::ast::node::{join_nodes, FormalParameter, Node, StatementList},
+    BoaProfiler, Context, Result, Value,
+};
+use gc::{Finalize, Trace};
+use std::fmt;
+
+#[cfg(feature = "serde")]
+use serde::{Deserialize, Serialize};
+
+/// Async Function Declaration.
+///
+/// More information:
+///  - [ECMAScript reference][spec]
+///  - [MDN documentation][mdn]
+///
+/// [spec]: https://tc39.es/ecma262/#sec-async-function-prototype-properties
+/// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function
+#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
+#[derive(Clone, Debug, Trace, Finalize, PartialEq)]
+pub struct AsyncFunctionDecl {
+    name: Box<str>,
+    parameters: Box<[FormalParameter]>,
+    body: StatementList,
+}
+
+impl AsyncFunctionDecl {
+    /// Creates a new async function declaration.
+    pub(in crate::syntax) fn new<N, P, B>(name: N, parameters: P, body: B) -> Self
+    where
+        N: Into<Box<str>>,
+        P: Into<Box<[FormalParameter]>>,
+        B: Into<StatementList>,
+    {
+        Self {
+            name: name.into(),
+            parameters: parameters.into(),
+            body: body.into(),
+        }
+    }
+
+    /// Gets the name of the async function declaration.
+    pub fn name(&self) -> &str {
+        &self.name
+    }
+
+    /// Gets the list of parameters of the async function declaration.
+    pub fn parameters(&self) -> &[FormalParameter] {
+        &self.parameters
+    }
+
+    /// Gets the body of the async function declaration.
+    pub fn body(&self) -> &[Node] {
+        self.body.statements()
+    }
+
+    /// Implements the display formatting with indentation.
+    pub(in crate::syntax::ast::node) fn display(
+        &self,
+        f: &mut fmt::Formatter<'_>,
+        indentation: usize,
+    ) -> fmt::Result {
+        write!(f, "async function {}(", self.name)?;
+        join_nodes(f, &self.parameters)?;
+        f.write_str(") {{")?;
+
+        self.body.display(f, indentation + 1)?;
+
+        writeln!(f, "}}")
+    }
+}
+
+impl Executable for AsyncFunctionDecl {
+    fn run(&self, interpreter: &mut Context) -> Result<Value> {
+        let _timer = BoaProfiler::global().start_event("AsyncFunctionDecl", "exec");
+        unimplemented!("Execute AsyncFunctionDecl");
+        Ok(Value::undefined())
+    }
+}
+
+impl From<AsyncFunctionDecl> for Node {
+    fn from(decl: AsyncFunctionDecl) -> Self {
+        Self::AsyncFunctionDecl(decl)
+    }
+}
+
+impl fmt::Display for AsyncFunctionDecl {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.display(f, 0)
+    }
+}
diff --git a/boa/src/syntax/ast/node/declaration/async_function_expr/mod.rs b/boa/src/syntax/ast/node/declaration/async_function_expr/mod.rs
new file mode 100644
index 00000000000..2a1979d717e
--- /dev/null
+++ b/boa/src/syntax/ast/node/declaration/async_function_expr/mod.rs
@@ -0,0 +1,94 @@
+use crate::{
+    exec::Executable,
+    syntax::ast::node::{join_nodes, FormalParameter, Node, StatementList},
+    Context, Result, Value,
+};
+use gc::{Finalize, Trace};
+use std::fmt;
+
+#[cfg(feature = "serde")]
+use serde::{Deserialize, Serialize};
+
+/// AsyncFunctionExpr.
+///
+/// More information:
+///  - [ECMAScript reference][spec]
+///  - [MDN documentation][mdn]
+///
+/// [spec]: https://tc39.es/ecma262/#prod-AsyncFunctionExpression
+/// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/async_function
+#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
+#[derive(Clone, Debug, Trace, Finalize, PartialEq)]
+pub struct AsyncFunctionExpr {
+    name: Option<Box<str>>,
+    parameters: Box<[FormalParameter]>,
+    body: StatementList,
+}
+
+impl AsyncFunctionExpr {
+    /// Creates a new function expression
+    pub(in crate::syntax) fn new<N, P, B>(name: N, parameters: P, body: B) -> Self
+    where
+        N: Into<Option<Box<str>>>,
+        P: Into<Box<[FormalParameter]>>,
+        B: Into<StatementList>,
+    {
+        Self {
+            name: name.into(),
+            parameters: parameters.into(),
+            body: body.into(),
+        }
+    }
+
+    /// Gets the name of the function declaration.
+    pub fn name(&self) -> Option<&str> {
+        self.name.as_ref().map(Box::as_ref)
+    }
+
+    /// Gets the list of parameters of the function declaration.
+    pub fn parameters(&self) -> &[FormalParameter] {
+        &self.parameters
+    }
+
+    /// Gets the body of the function declaration.
+    pub fn body(&self) -> &[Node] {
+        self.body.statements()
+    }
+
+    /// Implements the display formatting with indentation.
+    pub(in crate::syntax::ast::node) fn display(
+        &self,
+        f: &mut fmt::Formatter<'_>,
+        indentation: usize,
+    ) -> fmt::Result {
+        f.write_str("function")?;
+        if let Some(ref name) = self.name {
+            write!(f, " {}", name)?;
+        }
+        f.write_str("(")?;
+        join_nodes(f, &self.parameters)?;
+        f.write_str(") {{")?;
+
+        self.body.display(f, indentation + 1)?;
+
+        writeln!(f, "}}")
+    }
+}
+
+impl Executable for AsyncFunctionExpr {
+    fn run(&self, interpreter: &mut Context) -> Result<Value> {
+        unimplemented!("Execute AsyncFunctionExpr");
+    }
+}
+
+impl fmt::Display for AsyncFunctionExpr {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.display(f, 0)
+    }
+}
+
+impl From<AsyncFunctionExpr> for Node {
+    fn from(expr: AsyncFunctionExpr) -> Self {
+        Self::AsyncFunctionExpr(expr)
+    }
+}
diff --git a/boa/src/syntax/ast/node/declaration/mod.rs b/boa/src/syntax/ast/node/declaration/mod.rs
index 9e5d181a81f..a256a75e87f 100644
--- a/boa/src/syntax/ast/node/declaration/mod.rs
+++ b/boa/src/syntax/ast/node/declaration/mod.rs
@@ -1,6 +1,8 @@
 //! Declaration nodes
 
 pub mod arrow_function_decl;
+pub mod async_function_decl;
+pub mod async_function_expr;
 pub mod const_decl_list;
 pub mod function_decl;
 pub mod function_expr;
@@ -9,6 +11,8 @@ pub mod var_decl_list;
 
 pub use self::{
     arrow_function_decl::ArrowFunctionDecl,
+    async_function_decl::AsyncFunctionDecl,
+    async_function_expr::AsyncFunctionExpr,
     const_decl_list::{ConstDecl, ConstDeclList},
     function_decl::FunctionDecl,
     function_expr::FunctionExpr,
diff --git a/boa/src/syntax/ast/node/mod.rs b/boa/src/syntax/ast/node/mod.rs
index 5180c1ad706..d65914c9e8f 100644
--- a/boa/src/syntax/ast/node/mod.rs
+++ b/boa/src/syntax/ast/node/mod.rs
@@ -26,8 +26,8 @@ pub use self::{
     call::Call,
     conditional::{ConditionalOp, If},
     declaration::{
-        ArrowFunctionDecl, ConstDecl, ConstDeclList, FunctionDecl, FunctionExpr, LetDecl,
-        LetDeclList, VarDecl, VarDeclList,
+        ArrowFunctionDecl, AsyncFunctionDecl, AsyncFunctionExpr, ConstDecl, ConstDeclList,
+        FunctionDecl, FunctionExpr, LetDecl, LetDeclList, VarDecl, VarDeclList,
     },
     field::{GetConstField, GetField},
     identifier::Identifier,
@@ -65,6 +65,12 @@ pub enum Node {
     /// An assignment operator node. [More information](./operator/struct.Assign.html).
     Assign(Assign),
 
+    /// An async function declaration node. [More information](./declaration/struct.AsyncFunctionDecl.html).
+    AsyncFunctionDecl(AsyncFunctionDecl),
+
+    /// An async function expression node. [More information](./declaration/struct.AsyncFunctionExpr.html).
+    AsyncFunctionExpr(AsyncFunctionExpr),
+
     /// A binary operator node. [More information](./operator/struct.BinOp.html).
     BinOp(BinOp),
 
@@ -104,7 +110,7 @@ pub enum Node {
     /// A function declaration node. [More information](./declaration/struct.FunctionDecl.html).
     FunctionDecl(FunctionDecl),
 
-    /// A function expressino node. [More information](./declaration/struct.FunctionExpr.html).
+    /// A function expression node. [More information](./declaration/struct.FunctionExpr.html).
     FunctionExpr(FunctionExpr),
 
     /// Provides access to an object types' constant properties. [More information](./declaration/struct.GetConstField.html).
@@ -243,6 +249,8 @@ impl Node {
             Self::Assign(ref op) => Display::fmt(op, f),
             Self::LetDeclList(ref decl) => Display::fmt(decl, f),
             Self::ConstDeclList(ref decl) => Display::fmt(decl, f),
+            Self::AsyncFunctionDecl(ref decl) => decl.display(f, indentation),
+            Self::AsyncFunctionExpr(ref expr) => expr.display(f, indentation),
         }
     }
 }
@@ -251,6 +259,8 @@ impl Executable for Node {
     fn run(&self, interpreter: &mut Context) -> Result<Value> {
         let _timer = BoaProfiler::global().start_event("Executable", "exec");
         match *self {
+            Node::AsyncFunctionDecl(ref decl) => decl.run(interpreter),
+            Node::AsyncFunctionExpr(ref function_expr) => function_expr.run(interpreter),
             Node::Call(ref call) => call.run(interpreter),
             Node::Const(Const::Null) => Ok(Value::null()),
             Node::Const(Const::Num(num)) => Ok(Value::rational(num)),

From c2bf1b3cfbcd157236ed06f381bd8e69078417ef Mon Sep 17 00:00:00 2001
From: Paul Lancaster <paul@lancasterzone.com>
Date: Fri, 9 Oct 2020 23:47:35 +0100
Subject: [PATCH 3/7] AsyncFunctionDecl/Expr parser created (not impl)

---
 .../primary/async_function_expression.rs      | 42 +++++++++++++++++
 .../syntax/parser/expression/primary/mod.rs   |  8 +++-
 .../parser/statement/declaration/hoistable.rs | 47 ++++++++++++++++++-
 3 files changed, 94 insertions(+), 3 deletions(-)
 create mode 100644 boa/src/syntax/parser/expression/primary/async_function_expression.rs

diff --git a/boa/src/syntax/parser/expression/primary/async_function_expression.rs b/boa/src/syntax/parser/expression/primary/async_function_expression.rs
new file mode 100644
index 00000000000..61f24e84129
--- /dev/null
+++ b/boa/src/syntax/parser/expression/primary/async_function_expression.rs
@@ -0,0 +1,42 @@
+//! Async Function expression parsing.
+//!
+//! More information:
+//!  - [MDN documentation][mdn]
+//!  - [ECMAScript specification][spec]
+//!
+//! [mdn]:
+//! [spec]:
+
+use crate::{
+    syntax::{
+        ast::node::AsyncFunctionExpr,
+        parser::{Cursor, ParseError, TokenParser},
+    },
+    BoaProfiler,
+};
+
+use std::io::Read;
+
+/// Async Function expression parsing.
+///
+/// More information:
+///  - [MDN documentation][mdn]
+///  - [ECMAScript specification][spec]
+///
+/// [mdn]:
+/// [spec]:
+#[derive(Debug, Clone, Copy)]
+pub(super) struct AsyncFunctionExpression;
+
+impl<R> TokenParser<R> for AsyncFunctionExpression
+where
+    R: Read,
+{
+    type Output = AsyncFunctionExpr;
+
+    fn parse(self, cursor: &mut Cursor<R>) -> Result<Self::Output, ParseError> {
+        let _timer = BoaProfiler::global().start_event("AsyncFunctionExpression", "Parsing");
+
+        unimplemented!("Async function expression parse");
+    }
+}
diff --git a/boa/src/syntax/parser/expression/primary/mod.rs b/boa/src/syntax/parser/expression/primary/mod.rs
index 50cc95cc8af..9963df4b572 100644
--- a/boa/src/syntax/parser/expression/primary/mod.rs
+++ b/boa/src/syntax/parser/expression/primary/mod.rs
@@ -8,14 +8,15 @@
 //! [spec]: https://tc39.es/ecma262/#prod-PrimaryExpression
 
 mod array_initializer;
+mod async_function_expression;
 mod function_expression;
 mod object_initializer;
 #[cfg(test)]
 mod tests;
 
 use self::{
-    array_initializer::ArrayLiteral, function_expression::FunctionExpression,
-    object_initializer::ObjectLiteral,
+    array_initializer::ArrayLiteral, async_function_expression::AsyncFunctionExpression,
+    function_expression::FunctionExpression, object_initializer::ObjectLiteral,
 };
 use super::Expression;
 use crate::{
@@ -77,6 +78,9 @@ where
             TokenKind::Keyword(Keyword::Function) => {
                 FunctionExpression.parse(cursor).map(Node::from)
             }
+            TokenKind::Keyword(Keyword::Async) => {
+                AsyncFunctionExpression.parse(cursor).map(Node::from)
+            }
             TokenKind::Punctuator(Punctuator::OpenParen) => {
                 cursor.set_goal(InputElement::RegExp);
                 let expr =
diff --git a/boa/src/syntax/parser/statement/declaration/hoistable.rs b/boa/src/syntax/parser/statement/declaration/hoistable.rs
index 1f18c099762..a059bbe95db 100644
--- a/boa/src/syntax/parser/statement/declaration/hoistable.rs
+++ b/boa/src/syntax/parser/statement/declaration/hoistable.rs
@@ -7,7 +7,10 @@
 
 use crate::{
     syntax::{
-        ast::{node::FunctionDecl, Keyword, Node, Punctuator},
+        ast::{
+            node::{AsyncFunctionDecl, FunctionDecl},
+            Keyword, Node, Punctuator,
+        },
         parser::{
             function::FormalParameters, function::FunctionBody, statement::BindingIdentifier,
             AllowAwait, AllowDefault, AllowYield, Cursor, ParseError, ParseResult, TokenParser,
@@ -119,3 +122,45 @@ where
         Ok(FunctionDecl::new(name, params, body))
     }
 }
+
+/// Async Function declaration parsing.
+///
+/// More information:
+///  - [MDN documentation][mdn]
+///  - [ECMAScript specification][spec]
+///
+/// [mdn]:
+/// [spec]:
+#[derive(Debug, Clone, Copy)]
+struct AsyncFunctionDeclaration {
+    allow_yield: AllowYield,
+    allow_await: AllowAwait,
+    is_default: AllowDefault,
+}
+
+impl AsyncFunctionDeclaration {
+    /// Creates a new `FunctionDeclaration` parser.
+    fn new<Y, A, D>(allow_yield: Y, allow_await: A, is_default: D) -> Self
+    where
+        Y: Into<AllowYield>,
+        A: Into<AllowAwait>,
+        D: Into<AllowDefault>,
+    {
+        Self {
+            allow_yield: allow_yield.into(),
+            allow_await: allow_await.into(),
+            is_default: is_default.into(),
+        }
+    }
+}
+
+impl<R> TokenParser<R> for AsyncFunctionDeclaration
+where
+    R: Read,
+{
+    type Output = AsyncFunctionDecl;
+
+    fn parse(self, cursor: &mut Cursor<R>) -> Result<Self::Output, ParseError> {
+        unimplemented!("AsyncFunctionDecl parse");
+    }
+}

From 5bc89413debbbb0211c4a99e55e6cc009767250b Mon Sep 17 00:00:00 2001
From: Paul Lancaster <paul@lancasterzone.com>
Date: Sat, 10 Oct 2020 00:00:55 +0100
Subject: [PATCH 4/7] Add async to HoistableDeclaration

---
 .../parser/statement/declaration/hoistable.rs | 22 ++++++++++++++-----
 .../parser/statement/declaration/mod.rs       |  2 +-
 2 files changed, 18 insertions(+), 6 deletions(-)

diff --git a/boa/src/syntax/parser/statement/declaration/hoistable.rs b/boa/src/syntax/parser/statement/declaration/hoistable.rs
index a059bbe95db..1fb5fe1fc56 100644
--- a/boa/src/syntax/parser/statement/declaration/hoistable.rs
+++ b/boa/src/syntax/parser/statement/declaration/hoistable.rs
@@ -11,6 +11,7 @@ use crate::{
             node::{AsyncFunctionDecl, FunctionDecl},
             Keyword, Node, Punctuator,
         },
+        lexer::TokenKind,
         parser::{
             function::FormalParameters, function::FunctionBody, statement::BindingIdentifier,
             AllowAwait, AllowDefault, AllowYield, Cursor, ParseError, ParseResult, TokenParser,
@@ -18,7 +19,6 @@ use crate::{
     },
     BoaProfiler,
 };
-
 use std::io::Read;
 
 /// Hoistable declaration parsing.
@@ -58,10 +58,21 @@ where
 
     fn parse(self, cursor: &mut Cursor<R>) -> ParseResult {
         let _timer = BoaProfiler::global().start_event("HoistableDeclaration", "Parsing");
-        // TODO: check for generators and async functions + generators
-        FunctionDeclaration::new(self.allow_yield, self.allow_await, self.is_default)
-            .parse(cursor)
-            .map(Node::from)
+        let tok = cursor.peek(0)?.ok_or(ParseError::AbruptEnd)?;
+
+        match tok.kind() {
+            TokenKind::Keyword(Keyword::Function) => {
+                FunctionDeclaration::new(self.allow_yield, self.allow_await, self.is_default)
+                    .parse(cursor)
+                    .map(Node::from)
+            }
+            TokenKind::Keyword(Keyword::Async) => {
+                AsyncFunctionDeclaration::new(self.allow_yield, self.allow_await, false)
+                    .parse(cursor)
+                    .map(Node::from)
+            }
+            _ => unreachable!("unknown token found: {:?}", tok),
+        }
     }
 }
 
@@ -161,6 +172,7 @@ where
     type Output = AsyncFunctionDecl;
 
     fn parse(self, cursor: &mut Cursor<R>) -> Result<Self::Output, ParseError> {
+        cursor.expect(Keyword::Async, "async function declaration")?;
         unimplemented!("AsyncFunctionDecl parse");
     }
 }
diff --git a/boa/src/syntax/parser/statement/declaration/mod.rs b/boa/src/syntax/parser/statement/declaration/mod.rs
index d1c1c9dd985..518748d2e7a 100644
--- a/boa/src/syntax/parser/statement/declaration/mod.rs
+++ b/boa/src/syntax/parser/statement/declaration/mod.rs
@@ -63,7 +63,7 @@ where
         let tok = cursor.peek(0)?.ok_or(ParseError::AbruptEnd)?;
 
         match tok.kind() {
-            TokenKind::Keyword(Keyword::Function) => {
+            TokenKind::Keyword(Keyword::Function) | TokenKind::Keyword(Keyword::Async) => {
                 HoistableDeclaration::new(self.allow_yield, self.allow_await, false).parse(cursor)
             }
             TokenKind::Keyword(Keyword::Const) | TokenKind::Keyword(Keyword::Let) => {

From 8849bacbd55b52139fea8c8b1ef0f0592d30688a Mon Sep 17 00:00:00 2001
From: Paul Lancaster <paul@lancasterzone.com>
Date: Sat, 10 Oct 2020 10:21:08 +0100
Subject: [PATCH 5/7] Added expect_no_lineterminator, updated some cursor docs

---
 boa/src/syntax/parser/cursor/mod.rs           | 31 ++++++++++++++-----
 .../parser/statement/declaration/hoistable.rs |  7 +++++
 2 files changed, 31 insertions(+), 7 deletions(-)

diff --git a/boa/src/syntax/parser/cursor/mod.rs b/boa/src/syntax/parser/cursor/mod.rs
index b33851f7fd9..0d938741f7a 100644
--- a/boa/src/syntax/parser/cursor/mod.rs
+++ b/boa/src/syntax/parser/cursor/mod.rs
@@ -67,10 +67,6 @@ where
     }
 
     /// Returns an error if the next token is not of kind `kind`.
-    ///
-    /// Note: it will consume the next token only if the next token is the expected type.
-    ///
-    /// If skip_line_terminators is true then line terminators will be discarded.
     #[inline]
     pub(super) fn expect<K>(&mut self, kind: K, context: &'static str) -> Result<Token, ParseError>
     where
@@ -134,7 +130,7 @@ where
     ///
     /// It expects that the token stream does not end here.
     ///
-    /// This is just syntatic sugar for a .peek(skip_n, false) call followed by a check that the result is not a line terminator or None.
+    /// This is just syntatic sugar for a .peek(skip_n) call followed by a check that the result is not a line terminator or None.
     #[inline]
     pub(super) fn peek_expect_no_lineterminator(
         &mut self,
@@ -151,13 +147,34 @@ where
         }
     }
 
+    /// Returns an error if the next token is not of kind `kind`.
+    #[inline]
+    pub(super) fn expect_no_skip_lineterminator<K>(
+        &mut self,
+        kind: K,
+        context: &'static str,
+    ) -> Result<Token, ParseError>
+    where
+        K: Into<TokenKind>,
+    {
+        let next_token = self
+            .buffered_lexer
+            .next(false)?
+            .ok_or(ParseError::AbruptEnd)?;
+        let kind = kind.into();
+
+        if next_token.kind() == &kind {
+            Ok(next_token)
+        } else {
+            Err(ParseError::expected(vec![kind], next_token, context))
+        }
+    }
+
     /// Advance the cursor to the next token and retrieve it, only if it's of `kind` type.
     ///
     /// When the next token is a `kind` token, get the token, otherwise return `None`.
     ///
     /// No next token also returns None.
-    ///
-    /// If skip_line_terminators is true then line terminators will be discarded.
     #[inline]
     pub(super) fn next_if<K>(&mut self, kind: K) -> Result<Option<Token>, ParseError>
     where
diff --git a/boa/src/syntax/parser/statement/declaration/hoistable.rs b/boa/src/syntax/parser/statement/declaration/hoistable.rs
index 1fb5fe1fc56..66fa732b1b2 100644
--- a/boa/src/syntax/parser/statement/declaration/hoistable.rs
+++ b/boa/src/syntax/parser/statement/declaration/hoistable.rs
@@ -173,6 +173,13 @@ where
 
     fn parse(self, cursor: &mut Cursor<R>) -> Result<Self::Output, ParseError> {
         cursor.expect(Keyword::Async, "async function declaration")?;
+        let tok = cursor.peek_expect_no_lineterminator(0)?;
+
+        match tok.kind() {
+            TokenKind::Keyword(Keyword::Function) => {}
+            _ => {}
+        }
+
         unimplemented!("AsyncFunctionDecl parse");
     }
 }

From 0bfe141b6ae88fb1f034a4f1cf8c8f18cc4ea80e Mon Sep 17 00:00:00 2001
From: Paul Lancaster <paul@lancasterzone.com>
Date: Sat, 10 Oct 2020 11:07:36 +0100
Subject: [PATCH 6/7] AsyncFunctionDecl parsing

---
 .../declaration/async_function_decl/mod.rs    | 17 ++++++---
 .../parser/statement/declaration/hoistable.rs | 38 ++++++++++++++++---
 2 files changed, 44 insertions(+), 11 deletions(-)

diff --git a/boa/src/syntax/ast/node/declaration/async_function_decl/mod.rs b/boa/src/syntax/ast/node/declaration/async_function_decl/mod.rs
index 44d47e36ba3..f38a6ef0ff4 100644
--- a/boa/src/syntax/ast/node/declaration/async_function_decl/mod.rs
+++ b/boa/src/syntax/ast/node/declaration/async_function_decl/mod.rs
@@ -20,7 +20,7 @@ use serde::{Deserialize, Serialize};
 #[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
 #[derive(Clone, Debug, Trace, Finalize, PartialEq)]
 pub struct AsyncFunctionDecl {
-    name: Box<str>,
+    name: Option<Box<str>>,
     parameters: Box<[FormalParameter]>,
     body: StatementList,
 }
@@ -29,7 +29,7 @@ impl AsyncFunctionDecl {
     /// Creates a new async function declaration.
     pub(in crate::syntax) fn new<N, P, B>(name: N, parameters: P, body: B) -> Self
     where
-        N: Into<Box<str>>,
+        N: Into<Option<Box<str>>>,
         P: Into<Box<[FormalParameter]>>,
         B: Into<StatementList>,
     {
@@ -41,8 +41,8 @@ impl AsyncFunctionDecl {
     }
 
     /// Gets the name of the async function declaration.
-    pub fn name(&self) -> &str {
-        &self.name
+    pub fn name(&self) -> Option<&str> {
+        self.name.as_deref()
     }
 
     /// Gets the list of parameters of the async function declaration.
@@ -61,7 +61,14 @@ impl AsyncFunctionDecl {
         f: &mut fmt::Formatter<'_>,
         indentation: usize,
     ) -> fmt::Result {
-        write!(f, "async function {}(", self.name)?;
+        match &self.name {
+            Some(name) => {
+                write!(f, "async function {}(", name)?;
+            }
+            None => {
+                write!(f, "async function (")?;
+            }
+        }
         join_nodes(f, &self.parameters)?;
         f.write_str(") {{")?;
 
diff --git a/boa/src/syntax/parser/statement/declaration/hoistable.rs b/boa/src/syntax/parser/statement/declaration/hoistable.rs
index 66fa732b1b2..04e5704f9f1 100644
--- a/boa/src/syntax/parser/statement/declaration/hoistable.rs
+++ b/boa/src/syntax/parser/statement/declaration/hoistable.rs
@@ -173,13 +173,39 @@ where
 
     fn parse(self, cursor: &mut Cursor<R>) -> Result<Self::Output, ParseError> {
         cursor.expect(Keyword::Async, "async function declaration")?;
-        let tok = cursor.peek_expect_no_lineterminator(0)?;
+        cursor.expect_no_skip_lineterminator(Keyword::Function, "async function declaration")?;
+        let tok = cursor.peek(0)?;
+
+        let name = if let Some(token) = tok {
+            match token.kind() {
+                TokenKind::Punctuator(Punctuator::OpenParen) => {
+                    if !self.is_default.0 {
+                        return Err(ParseError::unexpected(
+                            token.clone(),
+                            "Unexpected missing identifier for async function decl",
+                        ));
+                    }
+                    None
+                }
+                _ => {
+                    Some(BindingIdentifier::new(self.allow_yield, self.allow_await).parse(cursor)?)
+                }
+            }
+        } else {
+            return Err(ParseError::AbruptEnd);
+        };
 
-        match tok.kind() {
-            TokenKind::Keyword(Keyword::Function) => {}
-            _ => {}
-        }
+        cursor.expect(Punctuator::OpenParen, "async function declaration")?;
+
+        let params = FormalParameters::new(!self.allow_yield.0, true).parse(cursor)?;
+
+        cursor.expect(Punctuator::CloseParen, "async function declaration")?;
+        cursor.expect(Punctuator::OpenBlock, "async function declaration")?;
+
+        let body = FunctionBody::new(!self.allow_yield.0, true).parse(cursor)?;
+
+        cursor.expect(Punctuator::CloseBlock, "async function declaration")?;
 
-        unimplemented!("AsyncFunctionDecl parse");
+        Ok(AsyncFunctionDecl::new(name, params, body))
     }
 }

From 35fb7455cac402452b5d4d1d4bb93cef50e51d94 Mon Sep 17 00:00:00 2001
From: Paul Lancaster <paul@lancasterzone.com>
Date: Sat, 10 Oct 2020 11:22:27 +0100
Subject: [PATCH 7/7] AsyncFunctionExpr parsing

---
 .../primary/async_function_expression.rs      | 51 +++++++++++++++++--
 .../syntax/parser/expression/primary/mod.rs   |  6 +--
 2 files changed, 49 insertions(+), 8 deletions(-)

diff --git a/boa/src/syntax/parser/expression/primary/async_function_expression.rs b/boa/src/syntax/parser/expression/primary/async_function_expression.rs
index 61f24e84129..d030f848d62 100644
--- a/boa/src/syntax/parser/expression/primary/async_function_expression.rs
+++ b/boa/src/syntax/parser/expression/primary/async_function_expression.rs
@@ -6,11 +6,15 @@
 //!
 //! [mdn]:
 //! [spec]:
-
 use crate::{
     syntax::{
-        ast::node::AsyncFunctionExpr,
-        parser::{Cursor, ParseError, TokenParser},
+        ast::{node::AsyncFunctionExpr, Keyword, Punctuator},
+        lexer::TokenKind,
+        parser::{
+            function::{FormalParameters, FunctionBody},
+            statement::BindingIdentifier,
+            AllowYield, Cursor, ParseError, TokenParser,
+        },
     },
     BoaProfiler,
 };
@@ -26,7 +30,21 @@ use std::io::Read;
 /// [mdn]:
 /// [spec]:
 #[derive(Debug, Clone, Copy)]
-pub(super) struct AsyncFunctionExpression;
+pub(super) struct AsyncFunctionExpression {
+    allow_yield: AllowYield,
+}
+
+impl AsyncFunctionExpression {
+    /// Creates a new `AsyncFunctionExpression` parser.
+    pub(super) fn new<Y>(allow_yield: Y) -> Self
+    where
+        Y: Into<AllowYield>,
+    {
+        Self {
+            allow_yield: allow_yield.into(),
+        }
+    }
+}
 
 impl<R> TokenParser<R> for AsyncFunctionExpression
 where
@@ -36,7 +54,30 @@ where
 
     fn parse(self, cursor: &mut Cursor<R>) -> Result<Self::Output, ParseError> {
         let _timer = BoaProfiler::global().start_event("AsyncFunctionExpression", "Parsing");
+        cursor.expect_no_skip_lineterminator(Keyword::Function, "async function expression")?;
+
+        let tok = cursor.peek(0)?;
+
+        let name = if let Some(token) = tok {
+            match token.kind() {
+                TokenKind::Punctuator(Punctuator::OpenParen) => None,
+                _ => Some(BindingIdentifier::new(self.allow_yield, true).parse(cursor)?),
+            }
+        } else {
+            return Err(ParseError::AbruptEnd);
+        };
+
+        cursor.expect(Punctuator::OpenParen, "function expression")?;
+
+        let params = FormalParameters::new(!self.allow_yield.0, true).parse(cursor)?;
+
+        cursor.expect(Punctuator::CloseParen, "function expression")?;
+        cursor.expect(Punctuator::OpenBlock, "function expression")?;
+
+        let body = FunctionBody::new(!self.allow_yield.0, true).parse(cursor)?;
+
+        cursor.expect(Punctuator::CloseBlock, "function expression")?;
 
-        unimplemented!("Async function expression parse");
+        Ok(AsyncFunctionExpr::new(name, params, body))
     }
 }
diff --git a/boa/src/syntax/parser/expression/primary/mod.rs b/boa/src/syntax/parser/expression/primary/mod.rs
index 9963df4b572..7f55346663e 100644
--- a/boa/src/syntax/parser/expression/primary/mod.rs
+++ b/boa/src/syntax/parser/expression/primary/mod.rs
@@ -78,9 +78,9 @@ where
             TokenKind::Keyword(Keyword::Function) => {
                 FunctionExpression.parse(cursor).map(Node::from)
             }
-            TokenKind::Keyword(Keyword::Async) => {
-                AsyncFunctionExpression.parse(cursor).map(Node::from)
-            }
+            TokenKind::Keyword(Keyword::Async) => AsyncFunctionExpression::new(self.allow_yield)
+                .parse(cursor)
+                .map(Node::from),
             TokenKind::Punctuator(Punctuator::OpenParen) => {
                 cursor.set_goal(InputElement::RegExp);
                 let expr =