fix: babel-traverse constant flag in bindings

[rufman]

flag bindings in loops as not constant.

Fixes: #13760

Q	A
Fixed Issues?	Fixes #1, Fixes #2
Patch: Bug Fix?
Major: Breaking Change?
Minor: New Feature?
Tests Added + Pass?	Yes
Documentation PR Link
Any Dependency Changes?
License	MIT

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/48833/

[codesandbox-ci]

This pull request is automatically built and testable in CodeSandbox.

To see build info of the built libraries, click here or the icon next to each commit SHA.

Latest deployment of this branch, based on commit 82b2d96:

Sandbox	Source
babel-repl-custom-plugin	Configuration
babel-plugin-multi-config	Configuration

[JLHwung]

I don't think we should fix the issue on @babel/traverse, see #13760 (comment).

[nicolo-ribaudo]

1. When node.isFunction() we can break the loop, since variables don't cross function boundaries:

   for (;;) () => { var x };

   is constant.
2. We only need to do this if we have a var, not for const or let
3. We don't need to collect the whole ancestry, since we'll only use part of it (up to a function). We can do something like

   let { parentPath } = path;
   while (parentPath && !parentPath.isFunction()) {
     // ...
     ({ parentPath } = parentPath);
   }

[JLHwung]

I think it's a bit dangerous to mark all bindings within for loop as constant violations. The const within loop is constant anyway.

We can handle it in

babel/packages/babel-traverse/src/scope/index.ts

Lines 772 to 780 in 3d1481f

	if (local) {
	this.registerConstantViolation(bindingPath);
	} else {
	this.bindings[name] = new Binding({
	identifier: id,
	scope: this,
	path: bindingPath,
	kind: kind,
	});

After we initialize the binding for var n within the for loop, we should also register the constant violation if we see a Loop before a Function when we walk up the ancestry.

In this way the constantViolations are also correct: The binding declaration itself violates the constant because the declaration is effectively hoisted to a function parent and the initializer assignment are executed for more than one time.