And this is why this argument persists. This is, to people outside the community, a semantic evasion:

1. It relies on a Rust-internal definition for "actually valid" (conforming to Rust's specific set of provability requirements) that doesn't correspond to what the rest of the world views as "correct" (not behaving incorrectly). Think about stuff like allocate-through-a-session-and-free-in-a-block paradigms (Apache was famous for this), or run-once-and-exit, or garbage collection, etc... Those things aren't "invalid" in any reasonable sense, they're just not what Rust programs do.

2. The definition for "valid" is (and this is my point above) entirely Rust-internal and ad hoc. It's not that we refuse to conform to your rules, really, it's that we don't know what they are!

    let (left, right) = (&mut foo[..split], &mut foo[split..]);

I see these are limitations but not show-stoppers in any way.