changed marker struct to const generic parameter

[mysteriouslyseeing]

Objective

Fixes #18085.

Solution

Implementation as described in issue:

• Replaced Aligned and Unaligned structs with ALIGNED and UNALIGNED constants
• Changed A: IsAligned = Aligned to const IS_ALIGNED: bool = ALIGNED in Ptr, PtrMut and OwningPtr

Testing

• cargo check --workspace --all-targets passes
• cargo test --workspace --all-targets passes

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Migration Guide

• Replace all uses of bevy_ptr::Aligned and bevy_ptr::Unaligned in the context of Ptr, PtrMut and OwningPtr with ALIGNED and UNALIGNED, respectively.
• Replace generic type parameters using the now removed IsAligned trait with a const generic parameter of type bool.

[alice-i-cecile]

I like that the two options are exhaustively enumerated, but I don't like the const ALIGNED = true stuff. Pointer<true> is particularly egregious, and that's the obvious way to use these types. I don't think this is a net improvement after seeing the ultimate effect, but thanks for poking at this.

Maybe with const generic enums and Pointer<Aligned::Yes> or something this would be better, but even that reads pretty poorly.

[mysteriouslyseeing]

Maybe with const generic enums and Pointer<Aligned::Yes> or something this would be better, but even that reads pretty poorly.

I think you would be able to do pub use Aligned::Yes; like how the standard library reexports Some and None. Then it would be Pointer<Yes> - so if you name things carefully (something like IsAligned::Aligned) you could keep the usage almost exactly the same as it is now.

[mysteriouslyseeing]

I don't think this is a net improvement after seeing the ultimate effect, but thanks for poking at this.

Makes sense! Good to close it then?