I suspect that it's more common to want to pass flags to `rustc` than to the generated binary. I'd rather type `:RustRun! -O` than `:RustRun -O --`. To that end, if I were to get rid of the bang, I'd have `:RustRun args` be equivalent to the current `:RustRun! args` and require `:RustRun -- args` to pass args to the binary. But I also thought that was a bit nonintuitive. For someone who knows the `:RustRun` command is there but hasn't read the docs in detail will likely assume that argument are passed to the binary.