llvm-objcopy does not work on Rust .rlib archive files

[jeffy1009]

Rust .rlib archive files contains object files and .rmeta files. Tools for working with .rlib file has been requested for several years but there's still no work on it yet. (rust-lang/rust#25820)

Most of the commands like nm, objdump, etc. work on .rlib file, although there is an error message about unrecognized format of .rmeta files. objcopy is one of them, and commands like objcopy -W foo libxxx.rlib still work, even though it complains about unrecognized .rmeta file.

But it seems that llvm-objcopy does not commit the changes to .rlib files if an error occurs.

I don't know if the policy is to mimic GNU tools' behavior, but it would be a lot more convenient if llvm-objcopy would behave similarly to objcopy in cases like this, provided that the side-effects caused by this change is expected to be small.
Otherwise, I have to extract the archive, apply the change to individual object files, and recreate the archive.

[llvmbot]

@llvm/issue-subscribers-tools-llvm-objcopy-strip

[jh7370]

I've not given this much thought, but we could probably demote this error to a warning, in the case of archives. I'm not sure we want to remove the error entirely, as that could hide genuine issues people run into with their build-system (e.g. they try to use llvm-objcopy on an archive consisting entirely of unsupported outputs).

An alternative option would be to add specific "support" for .rmeta files, whereby llvm-objcopy ignores them specifically. I'm unfamiliar with Rust though, so I don't know if this would be safe to do. Someone with more Rust familiarity would have to sort that out.

[bjorn3]

Since rust-lang/rust#91604 the crate metadata is wrapped in a regular object file on most platforms. Wasm being the notable exception.