core: change `Collect::downcast_raw` to return `NonNull`

[hawkw]

Feature Request

Crates

• tracing-core

Motivation

Right now, Collect::downcast_raw returns an Option<*const ()>:

tracing/tracing-core/src/collect.rs

Line 448 in eadf2a2

unsafe fn downcast_raw(&self, id: TypeId) -> Option<*const ()> {

The void pointer is expected to never be null, but rustc doesn't know that. This means that we're returning an option discriminant as well as a pointer, so it's more than 1 word.

Proposal

If we changed this to Option<NonNull<()>> we could benefit from niche optimization, saving us the option discriminant and making this return a single word pointer. That may in turn let us benefit from additional optimizations. This could make downcasting slightly faster.

Also, callers wouldn't have to null-check the pointer manually.

This is technically a breaking change, even though this API isn't supposed to be used publicly. However, we can make this change as part of tracing-core 0.2.