LazyMap::new as const fn

[robertvazan]

Please consider making LazyMap::new a const fn. It saves extra Lazy wrapper when declaring a static variable.

[a1phyr]

Currently this is not possible because it uses OnceMap under the hood, which uses sharding to reduce lock contention. Several hashmaps are allocated on the heap, which is not allowed at compile time. Also, it would be nice to add runtime parallelism availability detection in the future.

However, I could add a type with a constant amount of shards that could be built at compile time.

[robertvazan]

Couldn't you just wrap it in Lazy internally given that this is one of the main applications? Or incorporate Lazy-like initialization logic? Or is that too much of an overhead? I guess you must have internal pointer or reference somewhere (to support variable number of shards), which you could access via volatile or atomic and check for non-null value. That should be cheap enough.

[a1phyr]

I'm interested by this but I don't want to introduce a new dependency (eg once_cell) or break MSRV so I think that I will introduce that in 0.5 and use std's OnceLock.