Implement `for await...of` loops (#2286)

This Pull Request changes the following:

- Implement `for await...of` loop parsing
- Implement `for await...of` execution

boa_engine/src/builtins/iterable/mod.rs

@@ -233,6 +233,11 @@ pub struct IteratorResult {
 }
 
 impl IteratorResult {
+    /// Create a new `IteratorResult`.
+    pub(crate) fn new(object: JsObject) -> Self {
+        Self { object }
+    }
+
     /// `IteratorComplete ( iterResult )`
     ///
     /// The abstract operation `IteratorComplete` takes argument `iterResult` (an `Object`) and

boa_engine/src/bytecompiler/mod.rs

@@ -1578,7 +1578,11 @@ impl<'b> ByteCompiler<'b> {
                     self.emit_opcode(Opcode::PopEnvironment);
                 }
 
+                if for_of_loop.r#await() {
+                    self.emit_opcode(Opcode::InitIteratorAsync);
+                } else {
                     self.emit_opcode(Opcode::InitIterator);
+                }
 
                 self.emit_opcode(Opcode::LoopStart);
                 let start_address = self.next_opcode_location();
@@ -1588,7 +1592,15 @@ impl<'b> ByteCompiler<'b> {
                 self.context.push_compile_time_environment(false);
                 let push_env =
                     self.emit_opcode_with_two_operands(Opcode::PushDeclarativeEnvironment);
-                let exit = self.emit_opcode_with_operand(Opcode::ForInLoopNext);
+
+                let exit = if for_of_loop.r#await() {
+                    self.emit_opcode(Opcode::ForAwaitOfLoopIterate);
+                    self.emit_opcode(Opcode::Await);
+                    self.emit_opcode(Opcode::GeneratorNext);
+                    self.emit_opcode_with_operand(Opcode::ForAwaitOfLoopNext)
+                } else {
+                    self.emit_opcode_with_operand(Opcode::ForInLoopNext)
+                };
 
                 match for_of_loop.init() {
                     IterableLoopInitializer::Identifier(ref ident) => {

boa_engine/src/syntax/ast/node/iteration/for_of_loop/mod.rs

@@ -11,11 +11,12 @@ pub struct ForOfLoop {
     iterable: Box<Node>,
     body: Box<Node>,
     label: Option<Sym>,
+    r#await: bool,
 }
 
 impl ForOfLoop {
     /// Creates a new "for of" loop AST node.
-    pub fn new<I, B>(init: IterableLoopInitializer, iterable: I, body: B) -> Self
+    pub fn new<I, B>(init: IterableLoopInitializer, iterable: I, body: B, r#await: bool) -> Self
     where
         I: Into<Node>,
         B: Into<Node>,
@@ -25,6 +26,7 @@ impl ForOfLoop {
             iterable: Box::new(iterable.into()),
             body: Box::new(body.into()),
             label: None,
+            r#await,
         }
     }
 
@@ -48,6 +50,11 @@ impl ForOfLoop {
         self.label = Some(label);
     }
 
+    /// Returns true if this "for...of" loop is an "for await...of" loop.
+    pub(crate) fn r#await(&self) -> bool {
+        self.r#await
+    }
+
     /// Converts the "for of" loop to a string with the given indentation.
     pub(in crate::syntax::ast::node) fn to_indented_string(
         &self,

boa_engine/src/syntax/parser/statement/iteration/for_statement.rs

@@ -79,10 +79,34 @@ where
     ) -> Result<Self::Output, ParseError> {
         let _timer = Profiler::global().start_event("ForStatement", "Parsing");
         cursor.expect((Keyword::For, false), "for statement", interner)?;
-        let init_position = cursor
-            .expect(Punctuator::OpenParen, "for statement", interner)?
+
+        let mut r#await = false;
+
+        let next = cursor.next(interner)?.ok_or(ParseError::AbruptEnd)?;
+        let init_position = match next.kind() {
+            TokenKind::Punctuator(Punctuator::OpenParen) => next.span().end(),
+            TokenKind::Keyword((Keyword::Await, _)) if !self.allow_await.0 => {
+                return Err(ParseError::unexpected(
+                    next.to_string(interner),
+                    next.span(),
+                    "for await...of is only valid in async functions or async generators",
+                ));
+            }
+            TokenKind::Keyword((Keyword::Await, _)) => {
+                r#await = true;
+                cursor
+                    .expect(Punctuator::OpenParen, "for await...of", interner)?
                     .span()
-            .end();
+                    .end()
+            }
+            _ => {
+                return Err(ParseError::unexpected(
+                    next.to_string(interner),
+                    next.span(),
+                    "for statement",
+                ));
+            }
+        };
 
         let init = match cursor
             .peek(0, interner)?
@@ -233,7 +257,7 @@ where
                     }
                 }
 
-                return Ok(ForOfLoop::new(init, iterable, body).into());
+                return Ok(ForOfLoop::new(init, iterable, body, r#await).into());
             }
             (Some(Node::ConstDeclList(list)), _) => {
                 // Reject const declarations without initializers inside for loops

boa_engine/src/vm/code_block.rs

@@ -204,6 +204,7 @@ impl CodeBlock {
             | Opcode::SuperCall
             | Opcode::ForInLoopInitIterator
             | Opcode::ForInLoopNext
+            | Opcode::ForAwaitOfLoopNext
             | Opcode::ConcatToString
             | Opcode::GeneratorNextDelegate => {
                 let result = self.read::<u32>(*pc).to_string();
@@ -374,6 +375,7 @@ impl CodeBlock {
             | Opcode::CallSpread
             | Opcode::NewSpread
             | Opcode::SuperCallSpread
+            | Opcode::ForAwaitOfLoopIterate
             | Opcode::Nop => String::new(),
         }
     }

boa_engine/src/vm/mod.rs

@@ -6,7 +6,7 @@ use crate::{
     builtins::{
         async_generator::{AsyncGenerator, AsyncGeneratorState},
         function::{ConstructorKind, Function},
-        iterable::{IteratorHint, IteratorRecord},
+        iterable::{IteratorHint, IteratorRecord, IteratorResult},
         Array, ForInIterator, JsArgs, Number, Promise,
     },
     environments::EnvironmentSlots,
@@ -2129,6 +2129,46 @@ impl Context {
                     self.vm.push(done);
                 }
             }
+            Opcode::ForAwaitOfLoopIterate => {
+                let _done = self
+                    .vm
+                    .pop()
+                    .as_boolean()
+                    .expect("iterator [[Done]] was not a boolean");
+                let next_method = self.vm.pop();
+                let next_method_object = if let Some(object) = next_method.as_callable() {
+                    object
+                } else {
+                    return self.throw_type_error("iterable next method not a function");
+                };
+                let iterator = self.vm.pop();
+                let next_result = next_method_object.call(&iterator, &[], self)?;
+                self.vm.push(iterator);
+                self.vm.push(next_method);
+                self.vm.push(next_result);
+            }
+            Opcode::ForAwaitOfLoopNext => {
+                let address = self.vm.read::<u32>();
+
+                let next_result = self.vm.pop();
+                let next_result = if let Some(next_result) = next_result.as_object() {
+                    IteratorResult::new(next_result.clone())
+                } else {
+                    return self.throw_type_error("next value should be an object");
+                };
+
+                if next_result.complete(self)? {
+                    self.vm.frame_mut().pc = address as usize;
+                    self.vm.frame_mut().loop_env_stack_dec();
+                    self.vm.frame_mut().try_env_stack_dec();
+                    self.realm.environments.pop();
+                    self.vm.push(true);
+                } else {
+                    self.vm.push(false);
+                    let value = next_result.value(self)?;
+                    self.vm.push(value);
+                }
+            }
             Opcode::ConcatToString => {
                 let value_count = self.vm.read::<u32>();
                 let mut strings = Vec::with_capacity(value_count as usize);

boa_engine/src/vm/opcode.rs

@@ -1085,6 +1085,20 @@ pub enum Opcode {
     /// Stack: iterator, next_method, done **=>** iterator, next_method, done, next_result
     ForInLoopNext,
 
+    /// Move to the next value in a for await..of loop.
+    ///
+    /// Operands:
+    ///
+    /// Stack: iterator, next_method, done **=>** iterator, next_method, next_result
+    ForAwaitOfLoopIterate,
+
+    /// Get the value from a for await..of loop next result.
+    ///
+    /// Operands: address: `u32`
+    ///
+    /// Stack: next_result **=>** done, value
+    ForAwaitOfLoopNext,
+
     /// Concat multiple stack objects into a string.
     ///
     /// Operands: value_count: `u32`
@@ -1337,6 +1351,8 @@ impl Opcode {
             Self::IteratorClose => "IteratorClose",
             Self::IteratorToArray => "IteratorToArray",
             Self::ForInLoopNext => "ForInLoopNext",
+            Self::ForAwaitOfLoopNext => "ForAwaitOfLoopNext",
+            Self::ForAwaitOfLoopIterate => "ForAwaitOfLoopIterate",
             Self::ConcatToString => "ConcatToString",
             Self::RequireObjectCoercible => "RequireObjectCoercible",
             Self::ValueNotNullOrUndefined => "ValueNotNullOrUndefined",
@@ -1481,6 +1497,8 @@ impl Opcode {
             Self::IteratorClose => "INST - IteratorClose",
             Self::IteratorToArray => "INST - IteratorToArray",
             Self::ForInLoopNext => "INST - ForInLoopNext",
+            Self::ForAwaitOfLoopIterate => "INST - ForAwaitOfLoopIterate",
+            Self::ForAwaitOfLoopNext => "INST - ForAwaitOfLoopNext",
             Self::ConcatToString => "INST - ConcatToString",
             Self::RequireObjectCoercible => "INST - RequireObjectCoercible",
             Self::ValueNotNullOrUndefined => "INST - ValueNotNullOrUndefined",

boa_engine/src/vm/tests.rs

@@ -154,7 +154,7 @@ fn run_super_method_in_object() {
     assert_eq!(
         Context::default().eval(source.as_bytes()),
         Ok(JsValue::from("super"))
-    )
+    );
 }
 
 #[test]
@@ -180,5 +180,5 @@ fn get_reference_by_super() {
     assert_eq!(
         Context::default().eval(source.as_bytes()),
         Ok(JsValue::from("ab"))
-    )
+    );
 }