for(const x in y)

[classilla]

This is needed for full functionality in Github's current release, and probably fixes other sites. Unfortunately it also seems to have been fixed by the frontend binding rewrite that is the crux of #533.

It may be possible to hack in something that allows const as a synonym for var as a temporary fix. The underlying issues, which were never fully repaired, are https://bugzilla.mozilla.org/show_bug.cgi?id=449811 and https://bugzilla.mozilla.org/show_bug.cgi?id=1101653 .

[classilla]

In Parser.cpp, look at lines 4585 and 4791. Then, in BytecodeEmitter.cpp, fix the assertions in lines 5499 and remove the assertion at 5521. This should be enough to get const treated as let which should have similar enough semantics for a first pass. However, this is too risky to put in FPR12.

[roytam1]

so these changes "hackfixes" for(const x in y) problems:

diff --git a/js/src/frontend/BytecodeEmitter.cpp b/js/src/frontend/BytecodeEmitter.cpp
index df0d8a7b0..dafb61f05 100644
--- a/js/src/frontend/BytecodeEmitter.cpp
+++ b/js/src/frontend/BytecodeEmitter.cpp
@@ -5496,7 +5496,7 @@ BytecodeEmitter::emitIterator()
 bool
 BytecodeEmitter::emitForInOrOfVariables(ParseNode* pn)
 {
-    MOZ_ASSERT(pn->isKind(PNK_VAR) || pn->isKind(PNK_LET));
+    MOZ_ASSERT(pn->isKind(PNK_VAR) || pn->isKind(PNK_LET) || pn->isKind(PNK_CONST));
 
     // ES6 specifies that loop variables get a fresh binding in each iteration.
     // This is currently implemented for C-style for(;;) loops, but not
@@ -5518,7 +5518,7 @@ BytecodeEmitter::emitForInOrOfVariables(ParseNode* pn)
         if (!emitVariables(pn, DefineVars))
             return false;
     } else {
-        MOZ_ASSERT(pn->isKind(PNK_LET));
+        MOZ_ASSERT(pn->isKind(PNK_LET) || pn->isKind(PNK_CONST));
         if (!emitVariables(pn, InitializeVars))
             return false;
     }
diff --git a/js/src/frontend/Parser.cpp b/js/src/frontend/Parser.cpp
index 5830e7d90..3d6757fd9 100644
--- a/js/src/frontend/Parser.cpp
+++ b/js/src/frontend/Parser.cpp
@@ -4583,10 +4583,10 @@ Parser<ParseHandler>::declarationPattern(Node decl, TokenKind tt, BindData<Parse
         if (*forHeadKind != PNK_FORHEAD) {
             // |for (const ... in ...);| and |for (const ... of ...);| are
             // syntax errors for now.  We'll fix this in bug 449811.
-            if (handler.declarationIsConst(decl)) {
+            /*if (handler.declarationIsConst(decl)) {
                 report(ParseError, false, pattern, JSMSG_BAD_CONST_DECL);
                 return null();
-            }
+            }*/
 
             if (!checkDestructuringPattern(data, pattern))
                 return null();
@@ -4788,7 +4788,7 @@ Parser<ParseHandler>::declarationName(Node decl, TokenKind tt, BindData<ParseHan
             if (isForIn || isForOf) {
                 // XXX Uncomment this when fixing bug 449811.  Until then,
                 //     |for (const ... in/of ...)| remains an error.
-                //constRequiringInitializer = false;
+                constRequiringInitializer = false;
 
                 *forHeadKind = isForIn ? PNK_FORIN : PNK_FOROF;
             } else {

and now complaining async/await keyword.
xref: #521

[classilla]

This isn't worth it, might regress other code, and doesn't fix enough to get Github working properly. Wontfixing for TenFourFox.

[classilla]

If we can get async functions working, we might reexamine this hack.

[classilla]

This may also fix Discord, see https://tenfourfox.tenderapp.com/discussions/problems/8804-discord-problem

[classilla]

This enables login to Discord, but now it just "crashes" (the page complains Discord has crashed). Changing the user agent makes no difference.

[classilla]

There are various failures, but they aren't unexpected:

/Volumes/BruceDeuce/src/tenfourfox/js/src/jit-test/tests/parser/letContextualKeyword.js:7:5 Error: Assertion failed: got false, expected true
Stack:
expectError@/Volumes/BruceDeuce/src/tenfourfox/js/src/jit-test/tests/parser/letContextualKeyword.js:7:5
@/Volumes/BruceDeuce/src/tenfourfox/js/src/jit-test/tests/parser/letContextualKeyword.js:21:1

ecma_6/LexicalEnvironment/const-declaration-in-for-loop.js: rc = 3, run time = 0.723338

1146644: Don't crash compiling a non-body-level for-loop whose loop declaration is a const
ecma_6/LexicalEnvironment/const-declaration-in-for-loop.js:95:3 Error: Assertion failed: got false, expected true: unexpected error: expected SyntaxError, got Error: Congratulations on making for (const & in/of &) work! Please remove the try/catch and this throw.
Stack:
@ecma_6/LexicalEnvironment/const-declaration-in-for-loop.js:95:3

ecma_6/LexicalEnvironment/const-declaration-in-for-loop.js: rc = 3, run time = 1.333542

1146644: Don't crash compiling a non-body-level for-loop whose loop declaration is a const
ecma_6/LexicalEnvironment/const-declaration-in-for-loop.js:95:3 Error: Assertion failed: got false, expected true: unexpected error: expected SyntaxError, got Error: Congratulations on making for (const & in/of &) work! Please remove the try/catch and this throw.
Stack:
@ecma_6/LexicalEnvironment/const-declaration-in-for-loop.js:95:3

js1_8_5/reflect-parse/for-loop-destructuring.js: rc = 3, run time = 1.013104

assertError@js1_8_5/reflect-parse/PatternAsserts.js:92:11
test@js1_8_5/reflect-parse/for-loop-destructuring.js:29:1
runtest@js1_8_5/reflect-parse/shell.js:59:9
@js1_8_5/reflect-parse/for-loop-destructuring.js:51:1

js1_8_5/reflect-parse/PatternAsserts.js:92:11 Error: expected SyntaxError for "for (const x in foo);"
Stack:
assertError@js1_8_5/reflect-parse/PatternAsserts.js:92:11
test@js1_8_5/reflect-parse/for-loop-destructuring.js:29:1
runtest@js1_8_5/reflect-parse/shell.js:59:9
@js1_8_5/reflect-parse/for-loop-destructuring.js:51:1

js1_8_5/reflect-parse/for-loop-destructuring.js: rc = 3, run time = 5.893012

assertError@js1_8_5/reflect-parse/PatternAsserts.js:92:11
test@js1_8_5/reflect-parse/for-loop-destructuring.js:29:1
runtest@js1_8_5/reflect-parse/shell.js:59:9
@js1_8_5/reflect-parse/for-loop-destructuring.js:51:1

js1_8_5/reflect-parse/PatternAsserts.js:92:11 Error: expected SyntaxError for "for (const x in foo);"
Stack:
assertError@js1_8_5/reflect-parse/PatternAsserts.js:92:11
test@js1_8_5/reflect-parse/for-loop-destructuring.js:29:1
runtest@js1_8_5/reflect-parse/shell.js:59:9
@js1_8_5/reflect-parse/for-loop-destructuring.js:51:1

[classilla]

No offense, but I don't make one-offs since we don't have continuous integration and builds are lengthy (and they're too large to attach here anyway). The patch above is essentially what's on tree, so you could do it off the FPR14 tag with that added.

[classilla]

Since the latest attempt at #521 ended in failure again, and this might add some partial functionality on some sites, we'll try it (with test fixes) for FPR15.

[classilla]

Leaving open while I ponder if more work is needed.

[classilla]

This is now causing github to go into infinite errors, essentially seizing the browser (primarily on issues) due to an uncaught check op. I removed this op as a temporary measure but this makes the hack even more disgusting since const will be valid to assign to pretty much anywhere. It will also break a lot of tests. I haven't thought of a better way around it.

[roytam1]

maybe create a pref to change behavior in runtime?

[classilla]

Maybe, except the check op is generated in the frontend bytecode emitter, so a previously compiled script wouldn't change its behaviour. (Conversely, it would be highly impractical (and slow) to actually make the check during execution depend on a runtime setting, so this would have to live in the frontend if I did it at all.)