zerocopy 'static lifetime issue for mapping raw data to rust structures

[valoh]

Hi,

I have a problem using zerocopy for mapping raw data to rust structures:

#[repr(C)]
#[derive(Debug, KnownLayout, Immutable, FromBytes, IntoBytes)]
struct FileVersion {
    major: u8,
    minor: u8,
    patch: u8,
    pad0: u8,
}

#[derive(Debug, KnownLayout, Immutable, FromBytes, IntoBytes)]
#[repr(C)]
struct Header {
    magic: u32,
    version: FileVersion,
    total_bytes: u32,
    num_sections: u32,
}

#[repr(C)]
#[derive(Debug, KnownLayout, Immutable, FromBytes, AsBytes)]
struct SectionInfo {
    type_flag: u32,
    pad0: u32,
    offset: u64,
    length: u64,
}

#[repr(C)]
#[derive(Debug, KnownLayout, Immutable, FromBytes, AsBytes)]
struct Sections {
    section: [SectionInfo],
}


struct LoadedFile<'a> {
    data: &'a [u8],
    header: &'a Header,
    sections_info: &'a Sections,
    // a lot of more mapped data from data
}

impl<'a> LoadedFile<'a> {

    pub fn from(data: &'a [u8]) -> Result<LoadedFile<'a>> {

        let mut offset = 0;
        let bytes = size_of::<Header>();

        let header = Header::ref_from_bytes(&data[offset..offset + bytes])?;
        offset += bytes;

        let bytes = size_of::<SectionInfo>() * header.num_sections as usize;
        let sections = Sections::ref_from_bytes(&data[offset..offset + bytes])?;
        offset += bytes;

        Ok(LoadedFile {
            data,
            header,
            sections_info: sections,
        })
    }
}

I get following lifetime error:

error[E0521]: borrowed data escapes outside of associated function
--> src/optimized_chunk.rs:565:22
|
558 | impl<'a> LoadedFile<'a> {
| -- lifetime 'a defined here
559 |
560 | pub fn from(data: &'a [u8]) -> Result<LoadedFile<'a>> {
| ---- data is a reference that is only valid in the associated function body
...
565 | let header = Header::ref_from_bytes(&data[offset..offset + bytes])?;
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
| |
| data escapes the associated function body here
| argument requires that 'a must outlive 'static

I'm not sure where this 'static lifetime requirements come from. I guess from zerocopy but haven't found the source. Also I wonder how to implement this pattern to in the context of loading a complete file into a large Vec and put a mapped LoadedFile next to it for access the complete file data just via mapping. To my understanding the purpose of zerocopy is exactly to enable something like this but I haven't found a none-trivial example in docs or somewhere else showing how to do this.

Best regards,

Vasco

[valoh]

ok, I think I somewhat understand the problem now, the self-referential struct seem to be a problem which can be solved via self_cell. at least this compiles:

#[derive(Debug)]
struct LoadedFileView<'a> {
    pub header: &'a Header,
    pub sections_info: &'a Sections,
}

self_cell!(
    struct LoadedFile {
        owner: Vec<u8>,

        #[covariant]
        dependent: LoadedFileView,
    }

    impl {Debug}
);


impl LoadedFile {

    fn from(data: Vec<u8>) -> Result<LoadedFile> {
        LoadedFile::try_new(
            data,
            |data| {
                let mut offset = 0;
                let bytes = size_of::<Header>();

                let header = Header::ref_from_bytes(&data[offset..offset + bytes]).unwrap();
                offset += bytes;

                let bytes = size_of::<SectionInfo>() * header.num_sections as usize;
                let sections = Sections::ref_from_bytes(&data[offset..offset + bytes]).unwrap();
                offset += bytes;

                Ok(LoadedFileView {
                    header: header,
                    sections_info: sections,
                })
            })
    }
}

haven't done much testing with it yet though.

[joshlf]

Seems like this is resolved, so I'm closing the discussion, but feel free to reopen!