MovingPtr<'_,B: Bundle> impls Bundle

[janis-bhm]

(I messed up by renaming the branch and I couldn't figure out how to change the branch and reopen #20984)
relevant issue: #20976

Solution

In order to spawn bundles inside MovingPtrs alongside other bundles, we need to be able to name MovingPtr as a bundle, which the new associated type Name on Bundle allows. This type is basically a canonicalized bundle name-type that allows types like SpawnRelatedBundle to appear to the Bundles as if they were the bundle they are wrapping: R::RelationshipTarget. (I need to write tests for this).

Other parts of the engine, like observers, rely on Bundle being 'static in order to pass around the type without being constrained by lifetimes, which is why I've renamed Bundle to BundleImpl and made trait Bundle: BundleImpl + 'static.
Not sure how much I like this personally.
Observers only use the associated trait functions of Bundle which shouldn't be affected by the lifetime of the type implementing Bundle (I think).
An alternative would be to enforce Bundle::Name to impl Bundle itself (which it does, since it's just a tuple of bundles) and have the same components/component order as Self so that Bundles methods are available 'static (though using DynamicBundle this way would certainly be unsafe). That could allow types carrying Bundles in phantom datas to use the associated functions for component order and ids.

Alternative Solutions

@james7132 pointed out the typeid crate, which is already in the dependency graph, and which allows getting TypeIds from non-static types, notably at the cost of dynamic dispatch. This could allow for a simple trait method that returns the type id rather than using the associated type Name.
Note:
As far as I can tell, rustc can inline the dynamic dispatch here into a static call to core::any::TypeId::of, meaning there is potentially no added cost to using typeid.

Click to expand

fn main() {
    struct X {
        a: u32,
        b: u64,
    }
    let x = X { a: 1, b: 2 };
    move_as_ptr!(x);
    let _x = black_box(get_typeid_of(x));
    assert_eq!(TypeId::of::<X>(), _x);
}

fn get_typeid_of<T: Debug>(_: T) -> TypeId {
    typeid::of::<T>()
}

disassembly:

playground::main:
	.cfi_startproc
	sub rsp, 88
	.cfi_def_cfa_offset 96
	movups xmm0, xmmword ptr [rip + .Lanon.e23240f1cfa1af806bfe11aec7d94188.5]
	movaps xmmword ptr [rsp], xmm0
	mov rax, rsp
	#APP
	#NO_APP
	movups xmm0, xmmword ptr [rip + .Lanon.e23240f1cfa1af806bfe11aec7d94188.0]
	movaps xmmword ptr [rsp + 16], xmm0
	movdqa xmm0, xmmword ptr [rsp]
	pcmpeqb xmm0, xmmword ptr [rip + .LCPI0_0]
	pmovmskb eax, xmm0
	cmp eax, 65535
	jne .LBB0_2
	add rsp, 88
	.cfi_def_cfa_offset 8
	ret

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