Macro expansion recurses endlessly?

The following macro + invocation(taken from https://github.com/rust-lang/rust/issues/88577#issuecomment-914665202) seems to recurse endlessly(or just takes a very long time?) even emitting a warning in name res, [WARN hir_def::nameres::collector] macro expansion is too deep

macro_rules! many_args {
    ([$($t:tt)*]#$($h:tt)*) => {
        many_args!{[$($t)*$($t)*]$($h)*}
    };
    ([$($t:tt)*]) => {
        fn _f($($t: ()),*) {}
    }
}

many_args!{[_]########## ######}

Start by reproducing the macro_rules! definition and invocation shown in the issue, then observe the [WARN hir_def::nameres::collector] macro expansion is too deep warning. Trace the expansion until the excessive recursion or long runtime is understood; done means this input no longer recurses endlessly or takes an impractical amount of time.