multiboot2: properly type DST tags

[phip1611]

This PR introduces a proper DST typing of Multiboot2 tags that are DSTs. This includes all tags that have a dynamically size, usually used for a null-terminated string at the end of a tag.

[phip1611]

hey @nicholasbishop,
May I ask you for help, please? That's a tough one, I think. I want to have default implementations for both cases but the compiler tells me the following

However, I'd assume that both implementations are distinct, as Metadata has a different specialization..

I have to cover both cases, as at another place in the code, I do the following:

 unsafe fn from_base_tag<'a>(tag: &Tag) -> &'a Self {
        let ptr = tag as *const _ as *const ();
        // second parameter is either of type `()` or `usize`
        let ptr = ptr_meta::from_raw_parts(ptr, Self::dst_size(tag));
        &*ptr
    }

[nicholasbishop]

Possibly the advice in https://stackoverflow.com/questions/40392524 would help, although I had some difficulty following it.

I'm not quite clear on whether a blanket impl makes sense here anyway though. Do I have it right that all tag types must start with a 32-bit ID followed by a 32-bit size field? The blanket impl would mean that TagTrait is implemented for things like () and u32 which seems wrong.

[phip1611]

I just found out that there is an open issue for my use case: rust-lang/rust#20400

Do I have it right that all tag types must start with a 32-bit ID followed by a 32-bit size field

Yes

The blanket impl would mean that TagTrait is implemented for things like () and u32 which seems wrong.

I do want to have TagTrait implemented for all tags, sized and unsized ones. This is required as I need to create sized as well as unsized tag references from the same method:

// Self::dst_size() either returns `()` for sized types or a value of type `usize` for DSTs
let ptr = ptr_meta::from_raw_parts(ptr, Self::dst_size(tag));

[phip1611]

I had a chat with a colleque. After looking at rust-lang/rust#20400 and rust-lang/rfcs#1672 (comment), I came to the conclusion that the solution without 4a6964e is good enough.

Do you have maybe any other thoughts or another idea for a solution? The only problem with my solution without 4a6964e is that I have to repeat

impl crate::TagTrait for BootLoaderNameTag {
    fn dst_size(base_tag: &Tag) -> usize {
        // The size of the sized portion of the bootloader name tag.
        let tag_base_size = 8;
        assert!(base_tag.size >= 8);
        base_tag.size as usize - tag_base_size
    }
}

for every dynamically sized tag. But this only affects three tags. So I think, the solution is good enough..

[phip1611]

@YtvwlD any further thoughts on this?