[OneWingedShark]

>>say arrays that "know their own length"

>

> There's a fair amount of C code that does just that and does it for a long time.

No, there isn't.

There can't be because of how arrays in C degenerate into pointers/addresses; see Walter Bright's "C's Biggest Mistake" -- here: https://www.digitalmars.com/articles/C-biggest-mistake.html

>> or numeric-types that are range-constrained.

> Those don't need astral and can be passed through C ABI just fine.

No, they can't.

If you're passing a "Positive" through C's ABI you lose the fact that the value can be neither negative, nor zero. (Unless you mean "passed through" as in, "not mangled", but this is setting the bar so low as to be laughable.)