Mitigate 128-bit TypeId hash

[direc85]

Obviously, this breaks Rust <1.72, but I couldn't find a neat way to conditionally include the changes using. It doesn't seem to be possible to have something like #[cfg(version >= "1.72")] or #[cfg(TYPE_ID_SIZE == 16)], or perhaps I just didn't find the correct keyword to use.

Not sure if either method is good enough for master, but at least there's something that works:

Approach 1: unsafe { transmute }

Approach 2: Custom hasher which utilizes TypeId.hash() which internally casts u128 → u64

Improvements and further ideas welcome 😅

Fixes #299

[rubdos]

Would it not be better to upsize the older Rust types, instead of chopping the new one?

[direc85]

As said in the issue, this is fixed already in master, I'm only affected because I'm running 0.2.5-ish due to Rust 1.52.1. Closing as irrelevant.

[ogoffart]

thanks for your patch anyway.