Make ComponentDescriptor const

[alice-i-cecile]

What problem does this solve or what need does it fill?

The ComponentDescriptor is always the same for each component type.

What solution would you like?

Create an associated constant on the Component and Resource trait, which stores the existing information.

Constify all of the methods on ComponentDescriptor, then use those to initialize this information at compile-time.

We can also do this for NonSend resources by creating a simple trait for them.

What alternative(s) have you considered?

Enable the feature flags and do it now!

Additional context

This is blocked by several (!) unstable rustc features.

1. trait bounds other than Sized on const fn parameters are unstable
2. std::any::type_name is not yet stable as a const fn
3. TypeId::of is not yet stable as a const fn
4. function pointer casts are not allowed in constant functions

rust-lang/rust#57563 is the relevant tracking issue

[ItsDoot]

Additional context

This is blocked by several (!) unstable rustc features.

1. trait bounds other than Sized on const fn parameters are unstable

2. std::any::type_name is not yet stable as a const fn

3. TypeId::of is not yet stable as a const fn

4. function pointer casts are not allowed in constant functions

As of writing, 2. and 3. have been const-ified. 1. is allowed for declaration, but not for usage in the function (until const traits). Unsure of 4..

[james7132]

const std::any::type_name is still unstable and so is const TypeId::of. Marking this as blocked again.

[BenjaminBrienen]

Still blocked on rust-lang/rust#77125