rust - Rust宏: capture exactly matching tokens

Question

我的目标是编写一个宏expand!，使得:

struct A;
struct B;
struct Mut<T>;

expand!()         => ()
expand!(A)        => (A,)
expand!(mut A)    => (Mut<A>,)
expand!(A, mut B) => (A, Mut<B>,) 
// etc

[编辑]添加了结尾逗号，以实现一致的元组语法。
到目前为止，我已经写了这个宏:

macro_rules! to_type {
    ( $ty:ty ) => { $ty };
    ( mut $ty:ty ) => { Mut<$ty> };
}

macro_rules! expand {
    ( $( $(mut)? $ty:ty ),* ) => {
        (
        $( to_type!($ty) ),*
        ,)
    };
}

我正在努力的是捕获 mut token 。如何将其分配给变量并在宏主体中重用？一次可以处理多个 token 吗？

Answer 1

像this一样？

macro_rules! expand {
    (@phase2($($ty_final:ty),*),) => {
        ($($ty_final,)*)
    };
    (@phase2($($ty_final:ty),*), mut $ty:ty, $($rest:tt)*) => {
        expand!(@phase2($($ty_final,)* Mut::<$ty>), $($rest)*)
    };
    (@phase2($($ty_final:ty),*), $ty:ty, $($rest:tt)*) => {
        expand!(@phase2($($ty_final,)* $ty), $($rest)*)
    };
    ($($t:tt)*) => {
        expand!(@phase2(), $($t)*)
    };
}

struct A;
struct B;
struct Mut<T>(std::marker::PhantomData<T>);

fn main() {
    #[allow(unused_parens)]
    let _: expand!() = ();
    #[allow(unused_parens)]
    let _: expand!(A,) = (A,);
    #[allow(unused_parens)]
    let _: expand!(mut B,)    = (Mut::<B>(Default::default()),);
    #[allow(unused_parens)]
    let _: expand!(A, mut B,) = (A, Mut::<B>(Default::default()));
}