I am currently trying to build
https://github.com/firecracker-microvm/firecracker.git (but other rust project
show the same issue) on tumbleweed and always get this:

----------------------
~/firecracker> cargo build 
    Updating crates.io index
error: failed to fetch `https://github.com/rust-lang/crates.io-index`

Caused by:
  invalid version 3 on git_proxy_options; class=Invalid (3)
----------------------

Digging through the upstream bugs report I came accross:
https://github.com/rust-lang/rust/issues/63476

Which points at an issue with the libgit2 integration. Which itself leads to
https://github.com/rust-lang/git2-rs/issues/458. I am by no means a rust expert
but I if I understand correctly the recommendation is to build rust with the
bundled libgit2 instead of using the system provide one. Which means we'd need
to remove the LIBGIT2_SYS_USE_PKG_CONFIG=1 from our spec files (AFAIU this is
also how the package is build on Fedora)

Another possible fix is outlined here (and parently in use on FreeBSD):
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=239785
using this patch:
https://svnweb.freebsd.org/ports/head/lang/rust-nightly/files/patch-vendor_libgit2-sys_lib.rs?view=log&pathrev=509173