Bug 1685482 - Part 2: Disallow identifiers named "async" in for-of lo…

…ops. r=yulia

`for-of` loops mustn't start with the token sequence `async of`, because that
leads to a shift-reduce conflict when parsing `for (async of => {};;)` or
`for (async of [])`. This restriction doesn't apply to `for-await-of` loops,
because `async` in `for await (async of ...)` is always parsed as an identifier.

Parsing `for (async of ...)` already results in a SyntaxError, but that happens
because `assignExpr()` always tries to parse the sequence `async [no LineTerminator] of`
as the start of an async arrow function. That means `forHeadStart()` still needs
to handle the case when `async` and `of` are separated by a line terminator.

Part 3 will update the parser to allow `for await (async of ...)`.

Spec change: tc39/ecma262#2256

Depends on D100994

Differential Revision: https://phabricator.services.mozilla.com/D100995

UltraBlame original commit: c260009506931dfe9e8c75906cd200e3ff687b79

js/public/friend/ErrorNumbers.msg

@@ -3089,8 +3089,8 @@ binding
 )
 MSG_DEF
 (
-JSMSG_LET_STARTING_FOROF_LHS
-0
+JSMSG_BAD_STARTING_FOROF_LHS
+1
 JSEXN_SYNTAXERR
 "
 an
@@ -3110,7 +3110,9 @@ not
 start
 with
 '
-let
+{
+0
+}
 '
 "
 )

js/src/frontend/Parser.cpp

@@ -36058,6 +36058,8 @@ forHeadStart
 (
 YieldHandling
 yieldHandling
+IteratorKind
+iterKind
 ParseNodeKind
 *
 forHeadKind
@@ -36352,6 +36354,65 @@ a
 declaration
 .
 )
+/
+/
+/
+/
+For
+-
+of
+loops
+can
+'
+t
+start
+with
+the
+token
+sequence
+"
+async
+of
+"
+because
+that
+/
+/
+leads
+to
+a
+shift
+-
+reduce
+conflict
+when
+parsing
+|
+for
+(
+async
+of
+=
+>
+{
+}
+;
+;
+)
+|
+or
+/
+/
+|
+for
+(
+async
+of
+[
+]
+)
+|
+.
 bool
 parsingLexicalDeclaration
 =
@@ -36362,6 +36423,11 @@ letIsIdentifier
 =
 false
 ;
+bool
+startsWithForOf
+=
+false
+;
 if
 (
 tt
@@ -36508,6 +36574,83 @@ true
 ;
 }
 }
+else
+if
+(
+tt
+=
+=
+TokenKind
+:
+:
+Async
+&
+&
+iterKind
+=
+=
+IteratorKind
+:
+:
+Sync
+)
+{
+tokenStream
+.
+consumeKnownToken
+(
+TokenKind
+:
+:
+Async
+TokenStream
+:
+:
+SlashIsRegExp
+)
+;
+TokenKind
+next
+;
+if
+(
+!
+tokenStream
+.
+peekToken
+(
+&
+next
+)
+)
+{
+return
+false
+;
+}
+if
+(
+next
+=
+=
+TokenKind
+:
+:
+Of
+)
+{
+startsWithForOf
+=
+true
+;
+}
+anyChars
+.
+ungetToken
+(
+)
+;
+}
 if
 (
 parsingLexicalDeclaration
@@ -36933,7 +37076,65 @@ letIsIdentifier
 errorAt
 (
 exprOffset
-JSMSG_LET_STARTING_FOROF_LHS
+JSMSG_BAD_STARTING_FOROF_LHS
+"
+let
+"
+)
+;
+return
+false
+;
+}
+/
+/
+In
+a
+for
+-
+of
+loop
+the
+LeftHandSideExpression
+isn
+'
+t
+allowed
+to
+be
+an
+/
+/
+identifier
+named
+"
+async
+"
+per
+the
+[
+lookahead
+async
+of
+]
+restriction
+.
+if
+(
+isForOf
+&
+&
+startsWithForOf
+)
+{
+errorAt
+(
+exprOffset
+JSMSG_BAD_STARTING_FOROF_LHS
+"
+async
+of
+"
 )
 ;
 return
@@ -37804,6 +38005,7 @@ if
 forHeadStart
 (
 yieldHandling
+iterKind
 &
 headKind
 &

js/src/frontend/Parser.h

@@ -1599,6 +1599,15 @@ include
 "
 frontend
 /
+IteratorKind
+.
+h
+"
+#
+include
+"
+frontend
+/
 NameAnalysisTypes
 .
 h
@@ -6608,6 +6617,8 @@ forHeadStart
 (
 YieldHandling
 yieldHandling
+IteratorKind
+iterKind
 ParseNodeKind
 *
 forHeadKind