`using` and `await using` parse issue

[fisker]

6 valid cases not correctly parsed.

async function f() {
  await using in foo;
}

async function f() {
  await using instanceof foo;
}

async function f() {
  await using[x];
  await using.x + await using(x) ? await using?.x : await using`x`;
}

for (using in []);
for (using.foo in []);
for (using().foo in []);
for (using``.foo in []);

using in using instanceof using;

{ using 𠮷 = foo(); }

1 invalid cases not rejected.

using x = bar();

Found during updating acorn in Prettier prettier/prettier#17580

[marijnh]

Attached patch should help with the identifier lookahead checks.

But why is this invalid?

using x = bar();

[fisker]

But why is this invalid?

Not in a block.

[ota-meshi]

As far as I know, it's valid without a block if it's a module.
Where did the spec say that "using" has to be within a block?

[fisker]

Actually, I'm not sure, the proposal seems mentioned allowed in Module, but Prettier support this for a long time(2022), all examples was wrapped in {}, so I assumed it's required.

Anyway, https://v8.dev/features/explicit-resource-management#using-and-await-using-declarations mentioned

The using and await using keywords can be used inside braces {} (such as blocks, for loops and function bodies), and cannot be used in top-levels.

IDK, and Prettier don't really care much 😄

[RReverser]

The proposal does say this at least for await using:

An await using declaration can appear in the following contexts:

The top level of a Module anywhere VariableStatement is allowed, as long as it is not immediately nested inside of a CaseClause or DefaultClause.

https://github.com/tc39/proposal-explicit-resource-management?tab=readme-ov-file#await-using-declarations

For regular using it only says it's not allowed at top level of Script, but I'd assume same rule as for await using applies.

[baseballyama]

So, according to the current proposal, it means that at the top level of a module, both using and await using can be used. Is it correct to understand that the current V8 implementation is incorrect in this regard?

[ota-meshi]

Maybe the blog is wrong, and I think it works as specified in Chrome (V8).

[fisker]

It's explicitly pointed out v8/v8.dev#806 (comment), @syg is it a mistake?

[RReverser]

I think the blog post is incorrect one. The V8 implementation does allow using at the top level of a module, matching the proposal:

[fisker]

If I understand correctly, it should not allowed in script top-level.

But in CommonJS module, the code is wrapped in a function to execute, maybe we need a new option like allowReturnOutsideFunction to allow?

[ota-meshi]

I tried a top-level using in the .cjs file in Node.js v24.2.0 and it seems to work.

I think it might be better to request acorn to add the option.