Extension structs have lifetimes even when they don't need one

[Rua]

Extension structs do contain pointers, but their lifetime cannot ever be used if the only pointer they contain is p_next. Structs like this don't actually borrow anything, all structures in a p_next chain are borrowed by the parent. They have no push_next method, and there are no other methods that return Self with a different lifetime than the input struct. Technically you can set the lifetime using the PhantomData member, but that seems a bit silly and pointless.

Take ExternalMemoryImageCreateInfo for example. It has no methods that borrow data.

[MarijnS95]

Cross-posting from #969 (comment): I don't think this issue is actionable if we don't want a breaking release any time such a struct starts being referenced in a new structextends. This is much less likely for ExternalMemoryImageCreateInfo given that it is already part of a chain defined by a parent struct, but very possible for "new" parent structs that are defined with pNext up-front with no extensions available right now, but possibility for extensions in future Vulkan releases.

[Ralith]

Could we classify structs into "always root" or "always extension" and omit lifetimes from the latter? Not sure Khronos would respect that...

[MarijnS95]

I doubt they would want to commit to that, but we could try.

I have no reason to pursue this (kept it out of the 0.39 milestone) as it's very easy to represent with 'static lifetime in user code.