Implement PartialOrd and Ord for callsite::Identifier

[Robo210]

Feature Request

Crates

tracing-core

Motivation

The tracing-etw crate needs to associate extra metadata with each callsite. Currently this can be done with a hashmap, as the callsite identity implements Hash, but a hash map is not ideal due to the unnecessary overhead (the list of callsites never changes once populated, so we don't really need mutability and all the performance costs associated with it, etc.). A simple fixed-size, sorted array of callsite identities and the extra metadata will be leaner, if only there was a way to sort and compare the Identifiers without hashing them.

Proposal

Implement PartialOrd and Ord (and Eq) on callsite::Identifier. Since an Identifier is currently an opaque wrapper around a pointer, these can be implemented via core::ptr::[partial_]cmp, just like how PartialEq is currently implemented.

I can send a PR to do this if there are no objections to this idea.

Alternatives

Currently tracing-etw is using the hashed value of the callsite Identifier to compare and sort them. This means it has to have a global hasher and spend CPU cycles every time something is logged to compute a hash of what is really just a single pointer value, just so it can perform a lookup in its sorted array. This is an improvement over prior iterations, but still not ideal.

Prior iterations used DashMap<callsite::Identifier, metadata>, but this required the map to acquire a reader-lock every lookup and uses multiple heap allocations, neither of which are truly necessary in this scenario since the map is effectively completely immutable.