> [...] but I would guess that those cases are less common than than the cases of "seemingly valid" Rust code that is in reality problematic if it were accepted.
I would guess that most people starting to try out Rust are hitting the latter more often than the former. "Valid" Rust code that `rustc` rejects is uncommon (and for the most part, a bug), but wanted to acknowledge it happens. A "common" case of it would self-referencing structs, but that is considered a bug to be solved. I've only personally experienced problems with valid code being rejected when expressing complex trees involving associated types with their own independent lifetimes[1][2][3].
> There is a subset of "valid" Rust code that `rustc` rejects because it isn't smart enough to understand it would be safe
Way to tell him he's wrong only to then proceed to directly contradict yourself...