Basic arithmetic without overflow-checks undocumented

[orlp]

It is documented what the semantics are of basic arithmetic when overflow-checks is turned on: https://doc.rust-lang.org/reference/expressions/operator-expr.html#overflow.

However as far as I can see it is nowhere actually specified what arithmetic is supposed to do when overflow-checks is turned off, or whether this is subject to change, hardware specifics, etc.

[mattheww]

It's mentioned on the "Behavior not considered unsafe" page:

In the case of implicitly-wrapped overflow, implementations must provide well-defined (even if still considered erroneous) results by using two’s complement overflow conventions.

But I think it would be better if the "Operator expressions" page said this.

[orlp]

@mattheww That does not cover the shift operations.

[mattheww]

I suspect that the sentence "Remember that signed integers are always represented using two's complement." was intended to convey this information (partly because I can't see what else it's doing there).

But I don't think that's a good way of saying it. It would be better to be more direct.

[mattheww]

#949 is supposed to track unfinished business of this sort.