Use two bits of type meta byte for Comparable / IsBinary

[dgryski]

These two queries still require looping over all the fields in a struct, recursively. Caching them in the meta byte would make both these operations O(1).

[deadprogram]

This is the only other open issue for milestone 0.28 @dgryski 🐈

[deadprogram]

How about this one @dgryski ?

[dgryski]

So we only need these flags for structs -- everything else it's just an easy type switch. I suppose we could either choose to use these flags instead of the type switch or we could add a flag byte to the struct type that includes not only these two flags but also the alignment for the struct (so we don't need to recursively calculate that either).

[deadprogram]

Tagging @aykevl to answer this question, please.

[aykevl]

No strong preference from me.

So we only need these flags for structs -- everything else it's just an easy type switch.

True. Although, these bits are currently unused so we might as well generalize to all the types - not just structs (unless you know of a better use for these bits).

A third option would be to store the bits in the meta byte, but only for the struct type. I don't see a real advantage of that though - except perhaps saving a few bits for future use in non-struct types.

[dgryski]

I suppose since these are internal flags, we can always change it in the future if it turns out we have a better use for those bits.