I'm building a high performance merkle tree for a research paper. I'm using the arena approach, i.e.: I allocate a very big array and then each node use a type ArenaID = (u8, u16) as pointer. To simplify I have 3 main types of nodes:

• Real node
• Sentinel node: contains metadata for up to 16 sibling nodes (hash, guardian offset, ...)
• Guardian node: contains metadata for up to 16 sibling sentinel nodes (parent, height, sentinels offsets, ... to treat it as a skip list)

Now all 3 type of nodes are represented as type NodeUnion = [u8, 16] (more or less), with the actual type encoded in the first u8.

I would like now to write a generic function which takes a generic NodeUnion and changes its behaviour according to the actual node type.

My questions are:

• Could I use an enum but with a custom discriminant, to guarantee packed representation (no more than 128 bits) while reusing Rust idioms?
• Should I use the union type instead maybe? Never used those, didn't know Rust had them, I always used enums before
• Should I do a manual casts with as or into? But then I would not use the Rust compiler guarantees in terms of exhaustive matching
• Is there another smarter/more idiomatic approach?

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