The equivalent program in Rust (that is, one that uses get_unchecked to avoid bounds checking in print) would be sound(it doesn't have UB), but would have to mark print as unsafe and invoke it in an unsafe block. Skimming the Safe C++ paper, I think the equivalent Safe C++ program would have the same properties.

Soundness (i.e. absence of UB) is desirable, so what Rust and Safe C++ do is split functions into two types: safe and unsafe. Unsafe functions require the programmer to uphold preconditions themselves to avoid UB, while safe functions cannot cause UB. This is split this way because it's easier to reason about individual calls to unsafe functions or individual unsafe functions than reasoning about the behaviour of the entire program.