From rust-lang/rfcs#1861 (comment)

@briansmith

Maybe we're talking past each other, but I would expect an extern type T to be guaranteed to not be a function pointer, just like struct T; is guaranteed to not point to a function in C.

Ah, I was not aware of that! (But also slight terminology tweak: It'd be *const T, not T, that would be a function pointer.) For anyone else who may have been in the same position as I was, the C99 standard says:

6.3.2.3:8 A pointer to a function of one type may be converted to a pointer to a function of another type and back again; the result shall compare equal to the original pointer. If a converted pointer is used to call a function whose type is not compatible with the pointed-to type, the behavior is undefined.

(As this gives no conversions between data pointer types and function pointer types, such conversions are undefined, and thus a pointer to one can't possibly be valid as a pointer to the other.)

The rationale I saw given when I looked was that data and function pointers may not be the same size.

However, Rust is not C, and this should probably fall under the aegis of the Rust unsafe semantics - permitting it may be of value. I've filed an issue raising that question.

Note that this requires having function pointer types, which we currently do not , and was something I thought RFC 1861 might make possible - the closest is a kind of "function reference of static lifetime" that is syntactically not a reference at all :(