Issue #2559 - Implement hashbang grammar

This adds special-case #! comment handling at the top level, treating
any hashbang line as a single-line comment in the parser.
This only applies to Eval(), module script or global context.

js/src/frontend/Parser.cpp

@@ -4141,8 +4141,15 @@ Parser<ParseHandler>::statementList(YieldHandling yieldHandling)
         return null();
 
     bool canHaveDirectives = pc->atBodyLevel();
-    if (canHaveDirectives)
+    if (canHaveDirectives) {
         tokenStream.clearSawOctalEscape();
+    }
+
+    bool canHaveHashbangComment = pc->atTopLevel();
+    if (canHaveHashbangComment) {
+        tokenStream.consumeOptionalHashbangComment();
+    }
+
     bool afterReturn = false;
     bool warnedAboutStatementsAfterReturn = false;
     uint32_t statementBegin = 0;

js/src/frontend/Parser.h

@@ -470,6 +470,12 @@ class ParseContext : public Nestable<ParseContext>
         return atBodyLevel() && sc_->isModuleContext();
     }
 
+    // True if we are at the topmost level of an entire script or module.  For
+    // example, in the comment on |atBodyLevel()| above, we would encounter |f1|
+    // and the outermost |if (cond)| at top level, and everything else would not
+    // be at top level.
+    bool atTopLevel() { return atBodyLevel() && sc_->isTopLevelContext(); }
+
     void setIsStandaloneFunctionBody() {
         isStandaloneFunctionBody_ = true;
     }

js/src/frontend/SharedContext.h

@@ -289,6 +289,16 @@ class SharedContext
     bool isEvalContext() { return kind_ == Kind::Eval; }
     inline EvalSharedContext* asEvalContext();
 
+    bool isTopLevelContext() const {
+      switch (kind_) {
+        case Kind::Module:
+        case Kind::Global:
+        case Kind::Eval:
+          return true;
+      }
+      return false;
+    }
+
     ThisBinding thisBinding()          const { return thisBinding_; }
 
     bool hasModuleGoal()               const { return hasModuleGoal_; }

js/src/frontend/TokenStream.cpp

@@ -1300,6 +1300,21 @@ TokenStream::putIdentInTokenbuf(const char16_t* identStart)
     return true;
 }
 
+void
+TokenStream::consumeOptionalHashbangComment() {
+  int c = userbuf.getRawChar();
+  if (c == '#') {
+    if (matchChar('!')) {
+      // Hashbang; ignore rest of line as comment.
+      while ((c = getChar()) != EOF && c != '\n')
+          continue;
+    }
+  }
+  ungetChar(c);
+  cursor = (cursor - 1) & ntokensMask;
+}
+  
+
 enum FirstCharKind {
     // A char16_t has the 'OneChar' kind if it, by itself, constitutes a valid
     // token that cannot also be a prefix of a longer token.  E.g. ';' has the

js/src/frontend/TokenStream.h

@@ -428,6 +428,13 @@ class MOZ_STACK_CLASS TokenStream
     // asm.js reporter
     void reportAsmJSError(uint32_t offset, unsigned errorNumber, ...);
 
+    /**
+     * Consume any hashbang comment at the start of a Script or Module, if one is
+     * present.  Stops consuming just before any terminating LineTerminator or
+     * before an encoding error is encountered.
+     */
+    void consumeOptionalHashbangComment();
+
     JSAtom* getRawTemplateStringAtom() {
         MOZ_ASSERT(currentToken().type == TOK_TEMPLATE_HEAD ||
                    currentToken().type == TOK_NO_SUBS_TEMPLATE);