Macro set_span 

set_span!() { /* proc-macro */ }

Set the span of certain tokens in the code block to the span of the input token

• grammar := span , { code* }
• span := #mixed / any-token index*
• index := [ num-literal ]
• code := #set_span { … }
  / #set_index_span { index* { … } }
  / any-token

{, } is general bracket, contain () [] {}

• set_span!((a, b, (c))[0], {...}) set a span
• set_span!((a, b, (c))[1], {...}) set b span
• set_span!((a, b, (c))[2], {...}) set (c) span
• set_span!((a, b, (c))[2][0], {...}) set c span

Similarly, there is also set_span!((a), {#set_index_span([0]{...})}) set a span

Example

macro_rules! foo {
    ($t:tt) => {
        foo! { ($t) ($t) }
    };
    ($t:tt (0)) => {
        set_span::set_span! {$t[0], {
            #set_span {
                compile_error! {"input by zero"}
            }
        }}
    };
    ($_:tt ($lit:literal)) => { /*...*/ };
}
// foo!(0); // test error msg