Constant TRAIT_FORMAL_UNSUPPORTED 

pub(crate) const TRAIT_FORMAL_UNSUPPORTED: &str = "`formal(...)` is not supported on trait \
     methods: a trait method's R formals are the trait declaration's parameter names, which \
     every implementing type shares. Declare the dotted name on a standalone function or an \
     inherent impl method instead";

The error of a formal(...) on a trait method (#1842), declared or implemented. A trait method’s R formals are its declaration’s parameter names, shared by every implementing type and the trait ABI’s metadata; declared formals cover standalone functions and inherent impl methods.