Rust macro_rules嵌套重复

Question

在macro_rules中！转录，嵌套重复处理不正确。效果很好：

macro_rules! demo_macro {
    ($op:tt; $($arg:tt),*) {
        ($($op * $arg),*)
    }
}

fn main() {
    println!("{?:}", demo_macro!(2; 1,2,3));
}

并输出(2, 4, 6)，但这不能编译：

macro_rules! demo_macro {
    ([$($op:tt)*] $($arg:tt),*) {
        ($($($op)* $arg),*)
    }
}

fn main() {
    println!("{?:}", demo_macro!([2*] 1,2,3));
}

并导致此错误消息：

error: meta-variable `op` repeats 2 times, but `arg` repeats 3 times
 --> src/main.rs:3:11
  |
3 |         ($($($op)* $arg),*)
  |           ^^^^^^^^^^^^^^

如果我将1,2,3更改为1,2，因此重复计数匹配，则会收到此错误消息：

error: attempted to repeat an expression containing no syntax variables matched as repeating at this depth
 --> src/main.rs:3:13
  |
3 |         ($($($op)* $arg),*)
  |             ^^^^^

知道如何将demo_macro!([2*] 1,2,3)转换为(2, 4, 6)吗？

Answer 1

是，这可行！

macro_rules! unbracket {
    (() [$($tt1:tt)*]) => { ($($tt1)*) };
    ($tt0:tt [$($tt1:tt)*] @unbracket [$($tt2:tt)*] $($tt3:tt)*) => { unbracket!($tt0 [$($tt1)* $($tt2)*] $($tt3)*) };
    ($tt0:tt [$($tt1:tt)*] $tt2:tt $($tt3:tt)*) => { unbracket!($tt0 [$($tt1)* $tt2] $($tt3)*) };
}

macro_rules! demo_macro {
    ($op:tt $($arg:tt),*) => {
        unbracket!(() [] $(@unbracket $op $arg),*)
    }
}

fn main() {
    println!("{:?}", demo_macro!([2*] 1,2,3));
}