ash builder pattern is error prone

[yshui]

Many of the builder methods take references with lifetime, such as DeviceCreateInfoBuilder::queue_create_infos, but have a final .build() which returns something without a lifetime. This leads me to believe falsely the returned struct holds a copy of the passed arguments internally, thus not limited by the source lifetime. But this is not the case, ash converts the passed references to pointers, which means the passed arguments must live longer than the returned value.

No where in documentation was this mentioned. And the API is designed in such a way that makes leveraging the borrow checker impossible. I suggest the return from builders should retain the lifetime of the builder.

[yshui]

Real example of misuse: vulkano-rs/vulkano#2465

[Friz64]

This has been fixed in #602, which will be published on crates.io in a future release.

No where in documentation was this mentioned.

It was previously explained in the README, but that was removed in #743 as that information is no longer applicable on the master branch.

[Ralith]

It is also warned against in the API docs for every single build method.

[yshui]

darn I was a bit late then 😅 feel free to close it now or when the change is released.

[tomaka]

#602 doesn't really seem to prevent these mistakes, unless I misunderstand something. As an example, the code below compiles just fine (using the latest git commit 92084df):

// Compiles fine but really shouldn't.
fn test() -> ash::vk::SubpassDescription<'static> {
    ash::vk::SubpassDescription::default().color_attachments(&[
        ash::vk::AttachmentReference {
            attachment: 0,
            layout: ash::vk::ImageLayout::UNDEFINED,
        },
    ])
}

[MarijnS95]

@tomaka what is wrong about that example? The slice has a 'static lifetime, so it is sound to return a ash::vk::SubpassDescription with a pointer to that ever-lasting slice.

[Ralith]

In your example, the array literal is getting promoted to having a static lifetime. Try using a dynamic value in it to prevent this, and observe that it will no longer compile.