$( <$ty>::BITS )+*

示例

循环展开

使用 循环展开 计算数组元素的总和

let array = b"oh my, I am getting summed!";
let mut acc = 0u32;
let mut i = 0;
while i <= array.len()-4 {
    syntactic_for!{ offset in [ 0, 1, 2, 3 ] {$(
        acc += array[i + $offset] as u32;
    )*}}
    i += 4;
}
for j in i..array.len() {
    acc += array[j] as u32;
}
assert_eq!(acc, 2366);

匹配

找到给定位大小的整型类型中的最大值

let max_size = syntactic_for!{ ty in [ u8, u16, u32, u64, u128 ] {
    match bit_size {
        $(<$ty>::BITS => <$ty>::MAX as u128,)*
        other => panic!("No integer of size {other}"),
    }
}};

impl 块

为一系列类型实现一个特质

syntactic_for!{ ty in [ u8, u16, u32, u64, u128 ] {$(
    impl MyTrait for $ty {
        // snip.
    }
)*}}

自定义语法循环

一个有用的设计模式是定义一个自定义宏，它展开为对给定表达式集的语法循环

#[doc(hidden)]
pub extern crate syntactic_for;

#[macro_export]
macro_rules! for_each_custom_type {
    ($ident:ident { $($tt:tt)* }) => {
        $crate::syntactic_for::syntactic_for! { $ident in [
            $crate::CustomType1,
            $crate::CustomType2,
            // etc.
        ] { $($tt)* } }
    }
}

例如，一个库可以通过将 for_each_custom_type 暴露出来，让用户能够编写在库中定义的类型集上的语法循环。这样，就可以在库内部添加类型到循环中，而不需要用户端进行任何更改

// Try and parse each library type in succession, stopping at the first
// success:
fn can_parse(input: &str) -> bool {
    my_library::for_each_custom_type! { ty {
        $(if let Ok(parsed) = <$ty>::parse(input) {
            return true;
        })*
    }}
    return false;
}