rust - Rust宏: unit type?

Question

以下宏:

macro_rules! generate_parse_expression_ast_data {
    ($lit:literal) => ();
}

enum Ast {
    Foo (generate_parse_expression_ast_data!("bar")),
}

给出此错误:

error: macro expansion ends with an incomplete expression: expected type
 --> src/main.rs:6:10
  |
2 |     ($lit:literal) => ();
  |                       -- in this macro arm
...
6 |     Foo (generate_parse_expression_ast_data!("bar")),
  |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |          |
  |          expected type
  |          this macro call doesn't expand to a type

error: aborting due to previous error

error: could not compile `playground`.

Playground link
我特别想在某些枚举情况下使用unit type，根据定义，这应该是有效的类型。我将如何完成？

Answer 1

看起来像known issue。
我现在最好的建议是，除了发出零大小的类型(如单位)外，还应该重组宏，以便它完整地生成变体。例如:

macro_rules! generate_parse_expression_ast_data {(
    enum $name:ident {
        $( $variant:ident($lit:literal), )+
    }
) => (
    enum $name {
        $( $variant(), )+
    }
)}

generate_parse_expression_ast_data! {
    enum Ast {
        Foo("bar"),
    }
}