> “Future” values only become defined (“present” in your sense) at compile-time, but may occur before that at runtime. Note that this mostly presumes a language with separate compilation, [...]
Separate compilation is a technical implementation detail that shouldn't have an impact on semantics. Especially since LTO (link time optimisation) is becoming more and more common; 'thin' LTO is essentially free in Rust at least in terms of extra build time. LTO blurs the lines between separate compilation units.
On the flip side, Rust can use multiple codegen units even for the same crate, thus introducing separate compilation where a naive approach, like in classic C, would only use a single one.
Separate compilation is relevant, because it means the version of the interface you compile against may not be the same version you run against. This is fine if the newer version is compatible with the older version. And for the present discussion, we consider an added enum value to not constitute a compatibility break. Nevertheless, it means that the client code can now receive a value that it couldn’t receive before. And it’s useful to be able to define a case distinction for such unknown future values, while at the same time having the compiler check that all currently defined values have been duly considered.
In other words, you want to ensure that you have the most appropriate behavior for whatever values are currently known, and a fallback behavior for the future values that by definition you can’t possibly know at the present time. Of course, this is more or less only practical in languages where the interface version you compile against is only updated deliberately, while the implementation version at runtime can be any newer compatible version.
Separate compilation is a technical implementation detail that shouldn't have an impact on semantics. Especially since LTO (link time optimisation) is becoming more and more common; 'thin' LTO is essentially free in Rust at least in terms of extra build time. LTO blurs the lines between separate compilation units.
On the flip side, Rust can use multiple codegen units even for the same crate, thus introducing separate compilation where a naive approach, like in classic C, would only use a single one.