Always transform for-await in async functions[rebase of #6953].

[Gvozd]

Q	A
Fixed Issues?	#4969
Patch: Bug Fix?	yes
Major: Breaking Change?	yes?
Minor: New Feature?
Tests Added + Pass?	Yes
Documentation PR
Any Dependency Changes?
License	MIT

for-await was transformed in @babel/helper-remap-async-to-generator, which was called
by @babel/plugin-transform-async-to-generator and
@babel/plugin-proposal-async-generator-functions. This prevented for-await statements
in async functions to be transpiled if the transform-async-to-generator plugin was't
enabled.

This might be considered a breaking change, in case someone used for-await in async functions using just the async functions transform.

This PR is rebase(and fix merge conflicts) of #6953 on actual master

[Gvozd]

@nicolo-ribaudo I've rebase your fix.
And I really need this fix)

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/6979/

[nicolo-ribaudo]

Thank you! 🙂

[danez]

Do I understand this correctly: To get the same result as before you need to have proposal-async-generator-functions (which name does not seem to make sense after this change?) and transform-async-to-generator active?

[nicolo-ribaudo]

Before:

• async function* () {} was parsed and transformed by proposal-async-generators
• async function () {} was transformed by transform-async-to-generator
• for await inside async function* () {} was parsed and transformed by proposal-async-generator
• for await inside async function () {} was parsed by proposal-async-generators and transformed by transform-async-to-generator

After:

• async function* () {} is parsed and transformed by proposal-async-generators
• async function () {} is transformed by transform-async-to-generator
• for await is parsed and transformed by proposal-async-generator

So to fully transpile this code yes, you need both the plugins:

async function fn() {
  for await (const x of gen()) {}
}

But it makes sense, because for await and asunc function* fn() {} are part of the same proposal and for await will almost always be used to iterate over async function* fn() {}

On the other hand, without this PR it is impossible to only transpile the for await statement in that code.

[danez]

Thanks for the clarification.

[nicolo-ribaudo]

I don't know if I should label this PR as a bug fix or as a breaking change 🤔

[loilo]

Is there any hint when this will go into @babel/preset-stage-3?