Fix eaten async for function with labelled args

[mununki]

This PR fixes #674

[mununki]

I don't know the history of why letting only the pexp_fun with nolabel arg keeps the attributes.

syntax/src/res_parsetree_viewer.ml

Line 176 in a3c27ef

pexp_desc = Pexp_fun (Nolabel, _defaultExpr, _pattern, _returnExpr);

[mununki]

This PR allows the Pexp_fun with labeled and optional to keep the res.async attribute only.

[cknitt]

Looks good to me, thanks a lot for fixing this! 🎉

(I don't know why Pexp_fun with Nolabel keeps all attributes either though.)

[cristianoc]

Oops, I had another fix done on the plane:
#677

[cristianoc]

Basically, for (@foo ~a) => ... when one wants to put an attribute on the label, there's no way to represent this directly in the AST, as labelled args don't have attributes.
So the convention is to take the attributes on the function, and consider them applied to the label.

This does not fit well with the fact that async is put on the function, and some adjustment is needed.

[cristianoc]

This throws away all the other attributes, if there are some.

[mununki]

You are correct.

[mununki]

Basically, for (@foo ~a) => ... when one wants to put an attribute on the label, there's no way to represent this directly in the AST, as labelled args don't have attributes.
So the convention is to take the attributes on the function, and consider them applied to the label.

This does not fit well with the fact that async is put on the function, and some adjustment is needed.

There was a reason! Thank you for the explanation. I couldn't guess it. 👍

[mununki]

I'm going to close this PR, #677 will fix the issue.