Rust's enum lacks of these features:

Ad-hoc enums. To refer an ad-hoc enum via enumerating all its variants. As an analogy, Rust's tuples are ad-hoc struct types.
Implement traits for enums via delegating to variants.
Allow functions to return different types which implement a common trait.
Enum exchange. Conversion between enums which share a common set of variant types.
Checked exceptions. Enumerating all possible error types in function signature, or hiding them via Result<_, impl Trait>.

Four crates categorized into the fowllowing sub projects:

EnumX, the enum extension library.

Type definitions in enumx crate and proc-macro derives in enumx_derive crate.

See enumx/README.md for more.

CeX, for Checked EXception.

Type definitions in cex crate and proc-macro derives in cex_derive crate.

See cex/README.md for more.

Licensed under MIT.