vulkano/TROUBLES.md

# Troubles encountered with Rust during the making of this library

- No way to store an object and its borrowing in the same struct. This is the reason why the whole API uses `Arc`s.

- No pointer-to-struct-member. When you request the GPU to perform an operation on some data, you have to give a pointer to the data that
  must be manipulated. However regular Rust pointers can't be used, as the buffers are not necessarily in the memory accessible from your
  program. Instead vulkano provides its own pointer-like struct named `BufferSlice`. The problem is that there is no way to safely convert
  a `BufferSlice<Foo>` (equivalent of a virtual `&Foo`), where `struct Foo { a: i32 }`, into a `BufferSlice<i32>` (equivalent of a virtual
  `&i32`) pointing to the `a` member.

- Lack of plugins means that you have to use a build script to compile your shaders instead of inlining them directly where they are used.
  In addition to this, lots of existing macros would be much simpler to implement with plugins. The code generated by macros is currently
  inefficient because using the efficient way would make the code very difficult to read.

- Having a trait that defines which types can be put inside buffers is very annoying, as users have to implement it for every single struct
  they create. The best solution would be to use `impl BufferContent for ... {}`, but this syntax is still unstable. Vulkano currently doesn't
  enforce any trait for buffers contents, but that's unsafe.

- When `T` is unsized, there's no way to create a `*mut T` pointer from a `*mut c_void` and a size. This had to be implemented in a hacky
  (and very dangerous) way with a custom trait.

- https://github.com/rust-lang/rust/issues/29701 The fact that is legal to implement `Deref` and make `deref()` return a different object every
  time means that it is dangerous to interface with a `Foo` through a `P where P: Deref<Foo>`. That `P` could return several different
  `Foo`s every time you deref it, and the implementation can't rely on the fact that the object will be the same every time.

- This library was designed with specialization in mind. There are several `is_compatible` trait methods that perform deep comparisons between
  layouts. With specialization available, these methods could be specialized as `true` for layouts that are known to always be compatible.

- https://github.com/rust-lang/rust/issues/29328

- "Source trait is private" errors when traits are declared in private modules but reexported publicly from a parent module. Not sure if that's
  a bug or a feature from the privacy system, but in both cases it's annoying.

- Some trait implementations have an associated type that looks like `type T = (Arc<Foo>, Arc<Bar>);`. HKTs would allow this parameter to take
  references to the Arcs instead, and avoid having to clone them. This problem could by bypassed by making the code more ugly, but it's not worth
  it just to avoid cloning some Arcs.

- Visibility rules mean that you can't write `struct Error; pub mod foo { pub struct Foo; impl From<Error> for Foo { ... } }`. Rustc complains
  that `Error` is private and exported in `Foo`'s signature, even though that's in the author's opinion a totally legitimate usage.

- This repository contains the `vulkano-shaders` library, which generates Rust code that uses the `vulkano` library. If the API of `vulkano` gets
  a breaking change, there is no way to enforce or to check the fact that the user uses a correct combination of versions for `vulkano-shaders`
  and `vulkano`. Procedural macros declared in `vulkano` itself would solve this (https://github.com/rust-lang/rfcs/pull/1566).

- No way to set the alignment of a struct member, or to force the size of a struct to be a multiple of a certain size. Some parts of the Vulkan
  API enforce some limitations regarding the alignment, and either the user or the `vulkano-shaders` code generator currently has to manually
  add dummy padding fields.

- [No way to create dynamic-sized arrays on the stack](https://github.com/rust-lang/rfcs/issues/618). A lot of Vulkan functions require
  passing an array of small elements (small structs or integers). Building such an array with a `Vec` can be expensive, especially
  when most of the time the array only contains a single element. This is currently solved with the `SmallVec` type, but doing so wastes
  memory.