Parse using declaration (explicit resource management)

[JLHwung]

Q	A
Fixed Issues?	Implement parsing/printing support of the stage-2 Explicit Resource Management proposal
Patch: Bug Fix?
Major: Breaking Change?
Minor: New Feature?	Yes
Tests Added + Pass?	Yes
Documentation PR Link	babel/website#2684
Any Dependency Changes?
License	MIT

This PR parses / generates the using declaration

{
  using handler = getHandler()
}

introduced by the Explicit Resource Management proposal. We also create the new syntax plugin and add it to the babel-standalone.

I suggest reviewing this PR by commits.

/cc @rbuckton

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/53239/

[JLHwung]

In the worst scenario, we have to do triple lookahead to differentiate between using of of of and using of of.

@rbuckton Maybe we can forbid of as a using binding in for-of? So for (using of must be a for-LHS and we don't have to peek more.

[nicolo-ribaudo]

Isn't it the same as let of of of? Or is of disallowed in that case?

[JLHwung]

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

for-of has disabled let and async as LHS so for(let of of must start a for-of with a let-binding of. But using comes after for-of so we have to recognize for(using of of) as for-of with using as LHS.

[JLHwung]

Per tc39/proposal-explicit-resource-management#107, I will disallow of as a for-using-of binding name, hopefully we will get rid of extra lookahead, too.

[rbuckton]

There is now a PR that addresses this in the proposal repository.

[nicolo-ribaudo]

Shouldn't we also unset _noLineTerminator after printing?

[JLHwung]

Do you mean unset _noLineTerminator after printing the using keyword? The variable _noLineTerminator here stores the value of this.noLineTerminator before we print using, and then we restore this._noLineTerminator after the first declarator is printed.

[liuxingbaoyu]

The comments and indentation look fine.😃

[liuxingbaoyu]

Just a small question: I noticed that the newly added containsEsc property is only doing reads and no writes, is this expected?

[JLHwung]

Just a small question: I noticed that the newly added containsEsc property is only doing reads and no writes, is this expected?

The containsEsc will be set by the tokenizer by readWord

babel/packages/babel-parser/src/tokenizer/index.ts

Line 1380 in 0b0f083

this.state.containsEsc = false;

So if we see an identifier, lookahead.containsEsc will be a boolean, otherwise lookahead will not have a containsEsc property.