There's no proposal to stabilize the Rust ABI. There are proposals to define a separate stable ABI, which would not be the default ABI. (Such a separate stable ABI would want to plan for long-term performance, but the default ABI could continue to improve.)
There is already a separate stable ABI, it's just the C ABI. There are also multiple crates that address the issue of stable ABIs for Rust code. It's not very clear why compiler involvement would be required for this.
Surely it would be nice to be able to specify using the repr mechanism that you explicitly want so-and-such ABI, in the same way that you can for the C ABI.
I haven't looked at the crates you're describing, but presumably they're providing a proc macro or something instead, which is not really the right layer to do this stuff.
