Using `/WHOLEARCHIVE` on a COFF import library with an MSVC linker fails to link

[ChrisDenton]

Every COFF import library has a special object file containing the null import descriptor. This is exactly the same between import libraries, which causes a conflict if using /WHOLEARCHIVE.

The fact that it's exactly the same does however allow for a possible fix: add a COMDAT record to the special .idata$3 section so it behaves as if it's still in a library even if all objects are forced to be included. The downside would be that import libraries generated by this crate would no longer be byte-for-byte the same as LLVM import libraries.

Pseudo code:

// Placed before the `__NULL_IMPORT_DESCRIPTOR` in the COFF symbol table
ImageSymbol {
    name: *b".idata$3",
    section_number: 1,
    storage_class: IMAGE_SYM_CLASS_STATIC,
    number_of_aux_symbols: 1,
    ...
};
ImageAuxSymbolSection {
    length: mem::size_of::<ImageImportDescriptor>(),
    selection: IMAGE_COMDAT_SELECT_ANY,
    ...
};

cc @dpaoliello

[bjorn3]

Maybe we could have a flag fo enable/disable usage of COMDAT. That way all tests can keep asserting matches between LLVM and ar_archive_writer, while rustc can use COMDAT.