Heads-up: unsupported_calling_conventions will become a hard error soon

[RalfJung]

This crate showed up in the crater run for rust-lang/rust#129935 because it uses a calling convention that does not exist on x86-64-Linux. See the error log here:

https://crater-reports.s3.amazonaws.com/pr-129935/try%23498fce24f39c1ecbb0abe08824e72162da853341/reg/vtable_gen-1.1.0/log.txt

A warning has been shown for this issue for many years now, and this will be turned into a hard error fairly soon. Please update your crate to avoid it failing to build with a future version of Rust. :)