Disallow 'for (using of of ...)'

[rbuckton]

This was first noticed in the Babel PR for using: babel/babel#14968 (review). Rather than introduce additional complexity with a cover grammar, this instead prevents of from being considered as a BindingIdentifier in a for (using ...).

The intent is that the source:

for (using of of [x]) 

will be parsed as:

for ( using of of [x] )
      ^^^^^ ^^ ^^ ^^^
      |     |  |
      |     |  AssignmentExpression->MemberExpression (`of[x]`)
      |     |
      |     `of` keyword
      |
      LeftHandSideExpression->Identifier `using`

This is to preserve backwards compatibility with existing JS.

Fixes #107

[github-actions]

A preview of this PR can be found at https://tc39.es/proposal-explicit-resource-management/pr/108.

[bakkot]

As currently written I believe this also makes for (using of foo) illegal (because using of is the lookahead after the (), so that production can't be used.

I think you can instead put lookahead ≠ of inside of the new RHS of ForDeclaration, as in ForDeclaration : using [no LineTerminator here] [lookahead != of] ForBinding. That should just rule out the using of of case.

[rbuckton]

As currently written I believe this also makes for (using of foo) illegal (because using of is the lookahead after the (), so that production can't be used.

I think you can instead put lookahead ≠ of inside of the new RHS of ForDeclaration, as in ForDeclaration : using [no LineTerminator here] [lookahead != of] ForBinding. That should just rule out the using of of case.

It shouldn't, since for (using of foo) would be parsed under this RHS:

for ( [lookahead ∉ { let, async of }] LeftHandSideExpression[?Yield, ?Await] of AssignmentExpression[+In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return]

[bakkot]

Oh, quite right, I was looking at the wrong production.

[rbuckton]

@syg, @waldemarhorwat: Since this is a fairly small change, I'm planning to merge this later today. If you have any feedback after that, please feel free to file it here and I can put up a separate PR.