Feature request: Support functions where some recursive calls are non-tail recursive.

[aDifferentJT]

It's often nice to eliminate the tail call in a function that can also call itself in non-tail position, I don't believe that this is supported by this library, and I can see no fundamental reason why it couldn't be, though I confess my own ignorance of the subtleties of this problem.

[alecdotninja]

This is supported in v2.0.0.

The current API makes the distinction explicit:

• tailcall::call! { ... } means "this recursive call should go through the trampoline"
• a plain recursive call remains an ordinary Rust call

So a single #[tailcall] function can now mix both styles. For example:

use tailcall::tailcall;

#[tailcall]
fn mixed_recursion_sum(n: u64) -> u64 {
    match n {
        0 => 0,
        1 => tailcall::call! { mixed_recursion_sum(0) },
        _ if n % 2 == 0 => {
            let partial = mixed_recursion_sum(n - 1);
            n + partial
        }
        _ => tailcall::call! { mixed_recursion_sum(n - 1) },
    }
}

In that example, the odd-numbered path is trampoline-backed, while the even-numbered path is just a normal recursive call.

This behavior is now covered by tests and documented in the crate docs/README as the supported way to handle mixed recursion.