枚举解析

  1. macro_rules! parse_unitary_variants {
  2.     (@as_expr $e:expr) => {$e};
  3.     (@as_item $($i:item)+) => {$($i)+};
  5.     // Exit rules.
  6.     (
  7.         @collect_unitary_variants ($callback:ident ( $($args:tt)* )),
  8.         ($(,)*) -> ($($var_names:ident,)*)
  9.     ) => {
  10.         parse_unitary_variants! {
  11.             @as_expr
  12.             $callback!{ $($args)* ($($var_names),*) }
  13.         }
  14.     };
  16.     (
  17.         @collect_unitary_variants ($callback:ident { $($args:tt)* }),
  18.         ($(,)*) -> ($($var_names:ident,)*)
  19.     ) => {
  20.         parse_unitary_variants! {
  21.             @as_item
  22.             $callback!{ $($args)* ($($var_names),*) }
  23.         }
  24.     };
  26.     // Consume an attribute.
  27.     (
  28.         @collect_unitary_variants $fixed:tt,
  29.         (#[$_attr:meta] $($tail:tt)*) -> ($($var_names:tt)*)
  30.     ) => {
  31.         parse_unitary_variants! {
  32.             @collect_unitary_variants $fixed,
  33.             ($($tail)*) -> ($($var_names)*)
  34.         }
  35.     };
  37.     // Handle a variant, optionally with an with initialiser.
  38.     (
  39.         @collect_unitary_variants $fixed:tt,
  40.         ($var:ident $(= $_val:expr)*, $($tail:tt)*) -> ($($var_names:tt)*)
  41.     ) => {
  42.         parse_unitary_variants! {
  43.             @collect_unitary_variants $fixed,
  44.             ($($tail)*) -> ($($var_names)* $var,)
  45.         }
  46.     };
  48.     // Abort on variant with a payload.
  49.     (
  50.         @collect_unitary_variants $fixed:tt,
  51.         ($var:ident $_struct:tt, $($tail:tt)*) -> ($($var_names:tt)*)
  52.     ) => {
  53.         const _error: () = "cannot parse unitary variants from enum with non-unitary variants";
  54.     };
  56.     // Entry rule.
  57.     (enum $name:ident {$($body:tt)*} => $callback:ident $arg:tt) => {
  58.         parse_unitary_variants! {
  59.             @collect_unitary_variants
  60.             ($callback $arg), ($($body)*,) -> ()
  61.         }
  62.     };
  63. }
  64. # 
  65. # fn main() {
  66. #     assert_eq!(
  67. #         parse_unitary_variants!(
  68. #             enum Dummy { A, B, C }
  69. #             => stringify(variants:)
  70. #         ),
  71. #         "variants : ( A , B , C )"
  72. #     );
  73. # }

此宏展示了如何使用标记树撕咬机与下推累积来解析类C枚举的变量。完成后的parse_unitary_variants!宏将调用一个回调宏，为后者提供枚举中的所有选择(以及任何附加参数)。

经过改造后，此宏将也能用于解析struct的域，或为枚举变量计算标签值，甚至是将任意一个枚举中的所有变量名称都提取出来。