multiboot2: properly type DST tags #134
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multiboot2/src/lib.rs
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| // All sized tags automatically have a Pointee implementation where | ||
| // Pointee::Metadata is (). Hence, the TagTrait is implemented automatically for | ||
| // all tags that are sized. | ||
| impl<T: Pointee<Metadata = usize>> TagTrait for T { |
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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
}
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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.
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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));There was a problem hiding this comment.
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..
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@YtvwlD any further thoughts on this? |
This commit transforms a few tags where applicable to DSTs. This better fits into the Rust type system and makes a few things easier, such as parsing the cmdline string from the command line tag. Depending on how users used the tags, this change is not even breaking. Additionally, there is now a public trait which allows custom tag users to also benefit from DSTs.
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I had enough time to think about this, talk with people about it, and I think it is the best way we have at the moment. I'll merge it so the dependent MRs can make progress. |
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Sorry for responding that late, but yes, this looks fine to me. :) |
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.