@jrose Rust does define the Rust ABI - that is, the extern "Rust" ABI, which is the default ABI for functions defined in Rust. However, Rust also uses ABIs like extern "C" to interoperate with other programs.
Ralf Jung found an example here of two 1-ZSTs, one of which is constructed as a repr(transparent) wrapper around the other, which don't have the same behavior in the extern "C" ABI.