Extensions

- Rust has impl blocks for as the sole mechanism for attaching members to nominal targets
- TS++ has its as an additional mechanism
- like impl in Rust but a little broader

- inherent, anonymous, named extensions
- E / T, T may be local or imported
- extension of T
- extension E of T
- export extension of T
- export extension E of T
- an inhernt extension beside its target is visible wherever the target is visible
- an anonymous extension on a foreign target is visible only in its declaring file
- a named foreign extension must be imported explicitly

- extension<T> of T: blanket extension
- structural types, unions, and intersections cannot receive extensions
- rustc coherence
- overlapping implementations of one interface for one type, including blanket overlap, are errors
- no orphan rule?
- extension members are lexical, but implements contributes a program-wide relation whenever its module is in the program
- global extensions considered for impls (not import order)
- member overloading only within a single declaration block (extension or itme declaration)
- also for @unsafe impls

newtype UserId = string;

extension of UserId {
    value(this): string {
        string(this)
    }
}
- extension members are lexical and import-scoped
- interface implementations participate in the whole program (even if not imported/exported)