feat(es/transforms/compat): Add loose mode to parameters

[Austaras]

closes #2825, closes #2800

[Austaras]

I would need some help @kdy1
The test case of issue_2811 is failing because it inject _this to wrong scope, so the result will be something like

var f = function () { ... } 
if (true) {
} else {
   var _this = this
}

Why is this happening? I guess it's because of Parallel, how does that work and how should I fix it?

[kdy1]

#[parallel] only works if the number of statements (at the same level) is more than 64, so I don't think it's the case.

I think visit_mut_stmts should store some 'shared' variables like self.vars to somewhere, because otherwise variables from different statements will be injected by visit_mut_stmts.

(That's why I mentioned creating a new visitor can be better than storing variables)

[kdy1]

var f = function () { ... } 
if (true) {
} else {
   var _this = this
}

for this case, visit_mut_stmts for the top level generates variable, and visit_mut_stmts for

{
   var _this = this
}

takes the variables, just as those variables are generated by the nested block.

[Austaras]

But parallel doesn't allow a custom visit_mut_stmts, what should I do?

[kdy1]

You can remove #[parallel] for now. I'll modify #[parallel] in future.

[kdy1]

Thanks! Almost LGTM.

[kdy1]

Thank you!
I'm bit worried because circular dependency (in dev-dependencies) may block publising, but let's try.