Fix export bindings not updated by 'for ... in' and 'for ... of'

[vedantroy]

Q	A
Fixed Issues?	Fixes #10941
Patch: Bug Fix?	Yes
Major: Breaking Change?	No?
Minor: New Feature?	No
Tests Added + Pass?	Yes
Documentation PR Link
Any Dependency Changes?	No
License	MIT

This PR makes Babel correctly transform exported variables in for of and for in loops.

[nicolo-ribaudo]

Maybe it would be easier if we transform

for (PATTERN in {}) {}

to

for (let _foo in {}) {
  PATTERN = _foo
}

Then all the existing logic would work for this case, without needing to distinguish the case when PATTERN is a simple identifier or when it's using destructuring.

[nicolo-ribaudo]

Btw, you can use t.getOuterBindingIdentifiers(left) to get all the bindings updated by the destructuring pattern, and then rename the bindings declared in the nested scope. By doing so, you don't need the custom transformArrayPattern (which currently would also be needed for object patterns).

[vedantroy]

@nicolo-ribaudo I think moving PATTERN into the loop body would break the following case:

for ({foo} of [{foo: 1}]) {
  let foo;
}

because this would be transformed into

for (let _foo of [{foo: 1}]) {
  ({foo} = _foo)
  let foo;
}

and Babel would transform that into

for (let _foo of [{foo: 1}]) {
  ({ foo } = _foo);
  exports.foo = foo;
  let foo;
}

which results in an error, since you are assigning to foo before it has been declared.

[nicolo-ribaudo]

I was proposing to do something like this:

1. Let bindings be t.getOuterBindingIdentifiers(left)
2. For each binding in bindings,
   a. If the inner scope declares binding, call rename(binding) on the inner scope
3. Move PATTERN to PATTERN = _foo inside the loop body.

[vedantroy]

@nicolo-ribaudo

I changed the code to use your method, it's a lot better now!

[nicolo-ribaudo]

Thanks!