Macro core_extensions::count_tts

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

Available on crate feature macro_utils only.

Counts the amount of token trees passed to this macro, passing the amount to an (optional) callback macro.

Note that macro parameters (eg: $foo) are one token tree, and matched pairs of []/()/{} count as one token tree regardless of the tokens inside.

Callback

You need to pass a callback macro whenever the macro expects a literal.

If you only need the count for an expression(ie: the length of an array), then no callback macro is necessary.

Version compatibility

This macro requires Rust 1.45.0 to be invoked with a callback parameter, inside an expression.

Example

use core_extensions::count_tts;
 
fn main() {
    // The counted tokens must be wrapped in parentheses,
    // otherwise passing a callback macro would be syntactically ambiguous.
    assert_eq!(count_tts!(()), 0);
    assert_eq!(count_tts!((zero)), 1);
    assert_eq!(count_tts!((zero one)), 2);
    assert_eq!(count_tts!((zero (one two three) four)), 3);
     
    assert_eq!(hello(), "hello");
}

macro_rules! expects_5{
    (
        foo $ident:ident baz
        5
    ) => {
        fn $ident() -> &'static str {
            stringify!($ident) 
        }
    }
}
 
// Calls the `expects_5` macro.
count_tts!{
    // The invoked macro, and the first arguments passed to it
    expects_5!{foo hello baz}

    // The token trees to count
    (a [b c d] (e f) {g h i} 10 )
}