Use PhantomData<fn() -> (T0, T1, ...)> everywhere

[addisoncrump]

This was a trick I saw here: astral-sh/ruff#10849 (comment)

This has a lot of nice qualities for derive that can help us with #2068:

• Eq
• Copy
• Debug
• Serialize/Deserialize (with #[serde(skip)])
• Send/Sync (in case we ever do any async)

If a lifetime is stored in the PhantomData, then use PhantomData<fn() -> &'a &'b ... (T0, T1, ...)>.

[domenukk]

Where exactly will this help us? Think I'm too stupid to understand

[addisoncrump]

No, this one's pretty arcane. I had no idea this trick existed until yesterday.

Basically, we currently require e.g. T: Debug in a lot of places because we hold PhantomData<T>, but this is actually an excessive restriction because T is never debug'd. Similarly with T: Copy, T: Eq, T: Serialize + Deserialize: we probably have lots of places where we can simplify the generic constraints because we never actually handle the type except in passing (e.g. in methods).

[fiveseven-lambda]

When deriving the Debug trait, the type parameter T is required to implement Debug as well. However, this requirement is imposed by the derive macro itself, not by the presence of PhantomData<T>. Therefore, changing PhantomData<T> to PhantomData<fn() -> T> will not alter this requirement. The derive macro will still enforce T: Debug regardless of which version of PhantomData is used.

[addisoncrump]

So it would seem. This is very annoying, I thought I checked that... 😕