[kllrnohj]

No, that's exactly what I'm aware of, and is exactly the wrong behavior I'm talking about. "Sometimes crashes, sometimes two's compliment" are extremely different behaviors, and not meaningfully different from just saying it's UB. It should always panic, with no way to disable it. The wrap around in release mode is simply bad behavior. It can't be relied upon (because it panics in debug), and it's not useful behavior for nearly anyone's logic (wrap around almost never is logically correct behavior)

It lets Rust claim to be UB free without delivering the actual value of being UB free. You still can't rely on a given behavior because it doesn't have one behavior, it has two, and the two behaviors are wildly incompatible with each other.

[Groxx]

It's surprising and potentially buggy behavior, but that's very different from undefined behavior. To such a degree that I think you honestly might not understand what it means, and what the risks are around undefined behavior, especially in the presence of an optimizing compiler.

As a starter / refresher perhaps, both of these are perfectly permissible and happen in practice with UB, but never with "wrap or panic" / "implementation defined" behavior: https://mohitmv.github.io/blog/Shocking-Undefined-Behaviour-... This kind of thing is an example of the "time travel" stevekablanik is referring to, stuff that is literally impossible as written, that absolutely no human would consider to be a reasonable execution of the code, but occurs regularly with UB.

[kllrnohj]

-fsantize=integer and whambo bambo C/C++'s "time traveling UB" is now more consistent, better defined, and safer than Rust's release build behavior.

I don't know if Rust will ever attempt to fix this mistake, but I seriously hope they do.

[afdbcreid]

In C/C++ the default behavior, always, is very dangerous UB. Some compilers offer an option for guaranteed wrap-around, and some compilers offer an option for controlled crash. None of them is standardized or the default.

In Rust the standard mandates a guaranteed wrap-around or a controlled crash. The default behavior is crashing in debug mode and wrapping-around in release mode, but you can control that with a compiler flag.

No matter how you look at it, the Rust behavior is safer. The default behavior? Way better in Rust than in C++. Want a guaranteed wrap-around? In Rust, check. In C++, depending on the compiler. Want a guaranteed crash? In Rust, check. In C++, depending on the compiler.

Also worth noting that overflowing has much more severe consequences in C++ than in Rust, due to bounds checks.

> I don't know if Rust will ever attempt to fix this mistake

It was discussed many times, and the conclusion is: it is too expensive, and the default will only change if that will change (due to better optimizations and/or better hardware). It is not a mistake, but a conscious decision.

[Groxx]

> > I don't know if Rust will ever attempt to fix this mistake

> It was discussed many times, and the conclusion is: it is too expensive, and the default will only change if that will change (due to better optimizations and/or better hardware). It is not a mistake, but a conscious decision.

There's also wrapping_add / saturating_add for anyone concerned about this. Ya want a specific behavior? Just tell the compiler! You can also use the type wrappers so it happens implicitly! https://doc.rust-lang.org/std/num/struct.Saturating.html (presumably this is optimizable on hardware that has operators for this, but idk if that happens yet)

[steveklabnik]

If you're talking about compiler flags,

  rustc -C overflow-checks=yes

or in your project config,

  [profile.release]
  overflow-checks = true

gives you consistent panics on overflow.

[steveklabnik]

> and not meaningfully different from just saying it's UB.

It is extremely meaningfully different, because the range of options of what can happen is bounded in one case (either two's compliment wrapping, or panic) and unbounded in the other case (literally anything is allowed to happen, including time travel).

This is "implementation defined behavior" in C and C++'s terms, not "undefined behavior."

[Dylan16807]

If you want guaranteed wrapping or panic you can choose one and then rely on it just fine.

The default behavior helps you avoid wrapping without permanently bogging down your performance. It makes sense as an option.