However, a vector's length is inherently a runtime concept, and is a user-defined library type, so the compiler does not attempt to directly reason about this at compile time. However, for this example, post-optimizations, it doesn't do a runtime check at all: it calls the panic directly, because the optimizer does in fact reason that this always panics and removes the dynamic check.

It is true that Rust inserts dynamic checks for things that it can't prove statically, but so do dependently typed languages. "Please read an integer from stdin and then load that element of an array" is not possible to statically check, it must rely on runtime checks, definitionally.