Bug 1527671 - Assign the correct start position to arrow functions. r=arai,jorendorff

This position ends up being used for source notes in some cases now, meaning that this can
cause breakpoints to be given the wrong position when assigned to a variable. This fixes
that by using the correct token for the position value.

This fix also ensures that `cur->pos.end` is updated when skipping functions because
expression statements set their end based on the end position of the expression, and
if the expression statements end token is the end of a skipped function, we would
otherwise read the wrong position triggering assertion failures.

Differential Revision: https://phabricator.services.mozilla.com/D19714

--HG--
extra : moz-landing-system : lando

js/src/frontend/Parser.cpp

@@ -8024,7 +8024,8 @@ typename ParseHandler::Node GeneralParser<ParseHandler, Unit>::assignExpr(
     if (!tokenStream.getToken(&next, TokenStream::Operand)) {
       return null();
     }
-    uint32_t toStringStart = pos().begin;
+    TokenPos startPos = pos();
+    uint32_t toStringStart = startPos.begin;
     anyChars.ungetToken();
 
     FunctionAsyncKind asyncKind = FunctionAsyncKind::SyncFunction;
@@ -8049,7 +8050,7 @@ typename ParseHandler::Node GeneralParser<ParseHandler, Unit>::assignExpr(
     }
 
     FunctionSyntaxKind syntaxKind = FunctionSyntaxKind::Arrow;
-    FunctionNodeType funNode = handler.newFunction(syntaxKind, pos());
+    FunctionNodeType funNode = handler.newFunction(syntaxKind, startPos);
     if (!funNode) {
       return null();
     }

js/src/frontend/TokenStream.cpp

@@ -1275,6 +1275,7 @@ bool TokenStreamSpecific<Unit, AnyCharsAccess>::advance(size_t position) {
   TokenStreamAnyChars& anyChars = anyCharsAccess();
   Token* cur = const_cast<Token*>(&anyChars.currentToken());
   cur->pos.begin = this->sourceUnits.offset();
+  cur->pos.end = cur->pos.begin;
   MOZ_MAKE_MEM_UNDEFINED(&cur->type, sizeof(cur->type));
   anyChars.lookahead = 0;
   return true;

js/src/jit-test/tests/debug/Script-getPossibleBreakpoints.js

@@ -259,7 +259,7 @@ assertBreakpoints(`
   }
 `);
 assertBreakpoints(`
-  var fn /*S*/= () => {
+  var fn = /*S*/() => {
     /*S*/console./*B*/log("fn");
     /*S*/return /*B*/new Proxy({ prop: 42 }, {
       deleteProperty() {
@@ -268,6 +268,21 @@ assertBreakpoints(`
     });
   /*B*/};
 `);
+assertBreakpoints(`
+  var fn = /*S*/async (arg) => {
+    /*S*/console./*B*/log("fn");
+  /*B*/};
+`);
+assertBreakpoints(`
+  var fn = /*S*/arg => {
+    /*S*/console./*B*/log("fn");
+  /*B*/};
+`);
+assertBreakpoints(`
+  var fn = /*S*/async arg => {
+    /*S*/console./*B*/log("fn");
+  /*B*/};
+`);
 assertBreakpoints(`
   if ((/*S*/delete /*B*/fn().prop) + /*B*/b()) {
     /*S*/console./*B*/log("foo");