[SiVal]

It seems to me (FWIW as someone curious abt Rust but not seriously using it yet), that "as" should require a warning label. If "unsafe" means something else, then "unsound" could be used. If the types are always safe to convert, "as" is fine, but if the types involved allow for a lossy conversion in any case, the compiler would require you to write "unsound as". You are both acknowledging that you are aware of the issue and notifying future users of that code of a potential problem. Rust could go ahead with the saturating conversion to remove the UB AND require the "unsound" warning label.

If required to an an "unsound" label, that might be enough push for some developers to add their own explicit "if" guards or use another conversion method without having to deprecate "as".

[kibwen]

"unsound" has a particular definition that is separate from what people have issue with here; in Rust parlance, there's nothing unsound about the `as` operator choosing to saturate or overflow etc., because that has no ability to break the guarantees provided by the type system.

As for requiring an additional speedbump (like e.g. a "lossy" label) here to guard against misuse, I think this proposal is overlooking something: Rust can't just abruptly break all code that currently uses `as` in order to demand that something like `lossy as` be used instead. Any removal would have to first have a very long period where `lossy as` is syntactically valid and where the compiler instead warns for people using raw `as`. But if the compiler is already emitting a mere warning for `as` that suggests a better alternative, then it could just as easily suggest a method like `.try_into()`, which exists today. And once you're having the compiler warn about changing `as` into something else, that's already indistinguishable from deprecating `as` in those instances, so there's no point trying to avoid it.