Trailing comma after rest - The final fix

[nicolo-ribaudo]

Q	A
Fixed Issues?
Patch: Bug Fix?	Yes
Major: Breaking Change?
Minor: New Feature?
Tests Added + Pass?	Yes
Documentation PR Link
Any Dependency Changes?
License	MIT

Tracking a comma after a spread/rest element is ticky, because we don't know when we should reset the state. We can't do it in parseExpression because, for example, in [[...a,]] = [] parseExpression is finished after parsing the inner array.

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/11700/

[JLHwung]

By raising error at comma position we are not really consistent with where this error is throw in other situation, i.e. in toAssignableObjectExpressionProp.

babel/packages/babel-parser/src/parser/lval.js

Lines 132 to 134 in 11fa246

	} else if (prop.type === "SpreadElement" && !isLast) {
	this.raiseRestNotLast(prop.start);
	} else {

Hence we could throw at prop.start.

[JLHwung]

What if we instead record the traillingCommaAttachedNodeStart? So we can always throw at the node.

[nicolo-ribaudo]

I'd prefer to make trailingComma a boolean, and then throw at props[props.length - 1].start

[nicolo-ribaudo]

@JLHwung I ended up fixing the error location in a different way.
It now throws two different errors:

1. Rest element must be last element, pointing to the rest element
2. Unexpected trailing comma after rest element, pointing to the trailing comma

[nicolo-ribaudo]

#10371