Rust's "enum" mechanism is really an algebraic data type, and corresponds to Ada's enumeration types when applied as discriminants in variant record types. But Ada's enumeration types have a wider context of use, separate and independent from variant record types, including compile- and run-time features related directly to a) symbolic order and symbol names, b) binary representation mapped to these symbols, and c) subtyping/ranges.

I'm not aware of a good online presentation focused exclusively on Ada's enumeration types and their various uses. It's not even singled out in the Rationale documents for the design of the language and the (3?) design revisions since the 1980 launch; maybe the AARM (Annotated Ada Reference Manual) has more focused discussions? I'm not sure, I haven't looked at these since ~20y ago.