remove empty field list in unions

[nefelitav]

Rust unions cannot have zero fields, in contrast to enums and structs. (https://github.com/ferrocene/ferrocene/blob/main/tests/ui/union/union-empty.rs)

[Veykril]

Good catch! Though this is a semantic restriction, not a syntactic one (that is macros can observe the syntax still), as can be seen by the following compiling https://play.rust-lang.org/?version=stable&mode=debug&edition=2021

Could you phrase this as a legality rule? (like is done for unsafe tokens for modules in https://spec.ferrocene.dev/program-structure-and-compilation.html#fls_whgv72emrm47)

[nefelitav]

Good catch! Though this is a semantic restriction, not a syntactic one (that is macros can observe the syntax still), as can be seen by the following compiling https://play.rust-lang.org/?version=stable&mode=debug&edition=2021

Could you phrase this as a legality rule? (like is done for unsafe tokens for modules in https://spec.ferrocene.dev/program-structure-and-compilation.html#fls_whgv72emrm47)

Thank you for the feedback and the clarification! It seems there might have been an issue with the link you provided on the playground. Could you please resend it?

Also, based on my understanding of the Rust Reference, it seems like the Union Syntax requires at least one field, in contrast to Struct and Enum. Could you please confirm if this is correct?

[Veykril]

Whoops, here is the link https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=a3c7c2e0cafb924ead10ded4dd93a4b1

[Veykril]

Reading through the reference, it seems to actually not describe this restriction at all. The grammar lists StructFields like we do, but they are also lacking a phrasing that requires at least on field in the declaration.

[nefelitav]

Reading through the reference, it seems to actually not describe this restriction at all. The grammar lists StructFields like we do, but they are also lacking a phrasing that requires at least on field in the declaration.

Thank you for the link! I apologize in advance if I'm wrong, but I still think the Reference syntax is different from the Ferrocene one. In the reference, for unions, there is no "?" after the StructFields, while for structs and enums there is a "?", showing that fields are optional. And StructFields are defined like this:

StructFields :
StructField (, StructField)* ,?

So, I think that according to the Reference syntax, there should be at least one StructField in unions .

[Veykril]

Ah I see, I didn't look at the definition of StructFields properly, in that case the reference is wrong given you can have zero fields in the grammar as shown by my snippet. It is a semantic rule, not syntactic, so both documents are missing the semantic rule disallowing this state post macro expansion.

[nefelitav]

Ah I see, I didn't look at the definition of StructFields properly, in that case the reference is wrong given you can have zero fields in the grammar as shown by my snippet. It is a semantic rule, not syntactic, so both documents are missing the semantic rule disallowing this state post macro expansion.

Thank you for the explanation! I hope it is okay now.

[Veykril]

Sounds good, thanks!
bors merge

[bors-ferrocene]

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