Function rustc_codegen_ssa::back::metadata::create_wrapper_file

pub fn create_wrapper_file(
    sess: &Session,
    section_name: Vec<u8>,
    data: &[u8]
) -> (Vec<u8>, MetadataPosition)

For rlibs we “pack” rustc metadata into a dummy object file.

Historically it was needed because rustc linked rlibs as whole-archive in some cases. In that case linkers try to include all files located in an archive, so if metadata is stored in an archive then it needs to be of a form that the linker is able to process. Now it’s not clear whether metadata still needs to be wrapped into an object file or not.

Note, though, that we don’t actually want this metadata to show up in any final output of the compiler. Instead this is purely for rustc’s own metadata tracking purposes.

With the above in mind, each “flavor” of object format gets special handling here depending on the target:

• MachO - macos-like targets will insert the metadata into a section that is sort of fake dwarf debug info. Inspecting the source of the macos linker this causes these sections to be skipped automatically because it’s not in an allowlist of otherwise well known dwarf section names to go into the final artifact.

• WebAssembly - we actually don’t have any container format for this target. WebAssembly doesn’t support the dylib crate type anyway so there’s no need for us to support this at this time. Consequently the metadata bytes are simply stored as-is into an rlib.

• COFF - Windows-like targets create an object with a section that has the IMAGE_SCN_LNK_REMOVE flag set which ensures that if the linker ever sees the section it doesn’t process it and it’s removed.

• ELF - All other targets are similar to Windows in that there’s a SHF_EXCLUDE flag we can set on sections in an object file to get automatically removed from the final output.