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> I'm not sure I understand this. Does it not produce a run time error? Why not? It’s not supposed to. Type casting with ‘as’ is supposed to be lightweight and

> I'm not sure I understand this. Does it not produce a run time error? Why not? It’s not supposed to. Type casting with ‘as’ is supposed to be lightweight and always succeed; there is no room in the type system to return an error. In case lossless casting is not possible, some value still has to be returned. Until now, this was outright UB — meaning the compiler is not even obligated to keep it consistent from one build to another. Saturating, while still not optimal, is at least deterministic. > This looks very dangerous, because it essentially does the "nearest to right" thing. That’s why the intention is to introduce more robust approximate conversion functions and eventually probably deprecate ‘as’ casts altogether. There has been a number of discussions about this; current disagreements seem to be about how to handle the various possible rounding modes.

chowells 6y ago
> meaning the compiler is not even obligated to keep it consistent from one build to another. Way worse than that. The compiler wasn't obligated to act like anything at all. It would be totally legal to compile it so that the first time the value was accessed you got 0, the next time you got 1 - within the same program execution, with no mutation of the value. That is the sort of thing that is observed behavior of UB in the worst cases, and why it's so terrible to just pretend that UB is innocuous.
- Sharlin 6y ago
  Way worse than that, even. UB poisons every state of the program that eventually results in UB. For example, the optimizer is well within its rights to remove as dead code any branch that, if taken, would provably lead to UB at some arbitrary future point of execution.
  - TomMarius 6y ago
    That could literally produce no output program?
    - rjeli 6y ago
      c.f. “nasal demons”
    - Spivak 6y ago
      Yep! Dumb example. main() x = get_from_some_external_data_source() if x: print("Hello World") trigger_ub() You might expect this code to always print if x is true but the optimizer can look at this and say "welp, if x is true then it would trigger ub, therefore it must be false, and since x must always be false we can just remove that entire branch."
      - kbenson 6y ago
        My favorite example along these lines (in C) is "Cap'n'Proto remote vuln: pointer overflow check optimized away by compiler"[1] which was covered here a few years back and shows all of these "theoretical" compiler behaviors coming to a head in a real bug which is thoroughly explained. 1: https://news.ycombinator.com/item?id=14163111 https://news.ycombinator.com/item?id=14163111
    - cesarb 6y ago
      > That could literally produce no output program? Way worse than even that (you might be noticing a theme here...). Once the optimizer has removed as dead code any branch that, if taken, would provably lead to UB at some arbitrary future point of execution, it can conclude that the other branch is now the only possible execution, and call it unconditionally, even if that leads to removing all your files (the classic example is https://kristerw.blogspot.com/2017/09/why-undefined-behavior-may-call-never.html https://kristerw.blogspot.com/2017/09/why-undefined-behavior...).
kd5bjo 6y ago
As it was UB anyway, wouldn’t the compiler also be within its rights to detect the problem and panic the thread?
- GolDDranks 6y ago
  Yes, it would. However, it rarely does because of how the incentives are: it's useful for optimization to pretend that UB never happens, and more optimization leads to faster binaries, which are what the compiler engineers often pursue.
  - kd5bjo 6y ago
    Right, but given that panicing was already allowable behavior, why was the new behavior chosen to be one likely to introduce subtle bugs? It seems much better to loudly proclaim the existence of an erroneous precondition, which is consistent with how things like array indexing behave. I guess I’ll have to go dig up the RFC discussion on this one; it should make for interesting reading.
    - GolDDranks 6y ago
      I thing you are right about that, I'd prefer panicking too. Also, reading the RFC will surely clear up the motivation. Without knowing this case, I'd wager a guess: it's about performance. Panicking introduces a branch and side-effects which, again, affects negstively optimization potential and performance. The saturating cast affects performance too, but less. If some old code has a lot of number crunching containing these operations, a big performance regression would be nasty.
      - kd5bjo 6y ago
        It looks like ‘as’ is generally defined as a truncating cast operator for other numeric types. Since it doesn’t panic for other overflow situations (like u64->u32), they chose consistency with those cases. https://github.com/rust-lang/rust/issues/10184#issuecomment-194885126 https://github.com/rust-lang/rust/issues/10184#issuecomment-...
jfkebwjsbx 6y ago
> there is no room in the type system to return an error There is: you panic, like in the array case. That would have been much robust (at the cost of performance).
- kibwen 6y ago
  For consistency you would also expect `257u32 as u8` to panic (which it doesn't, and never has); the `as` operator has always been about fast-and-lossy conversions, with the standard library ideally providing methods for more principled conversions.
quietbritishjim 6y ago
Seems a pity they didn't add a built-in at the same time that's a bit more nuanced. It could maybe return an enum with Success(value), Underflow, Overflow and NaN. That way it's up to the coder to decide whether they want a saturating cast or to check the result explicitly.
- estebank 6y ago
  There are conversations about doing exactly that with `.try_into()` doing exactly that.

chowells 6y ago

> meaning the compiler is not even obligated to keep it consistent from one build to another. Way worse than that. The compiler wasn't obligated to act like anything at all. It would be totally legal to compile it so that the first time the value was accessed you got 0, the next time you got 1 - within the same program execution, with no mutation of the value. That is the sort of thing that is observed behavior of UB in the worst cases, and why it's so terrible to just pretend that UB is innocuous.

Sharlin 6y ago
Way worse than that, even. UB poisons every state of the program that eventually results in UB. For example, the optimizer is well within its rights to remove as dead code any branch that, if taken, would provably lead to UB at some arbitrary future point of execution.
- TomMarius 6y ago
  That could literally produce no output program?
  - rjeli 6y ago
    c.f. “nasal demons”
  - Spivak 6y ago
    Yep! Dumb example. main() x = get_from_some_external_data_source() if x: print("Hello World") trigger_ub() You might expect this code to always print if x is true but the optimizer can look at this and say "welp, if x is true then it would trigger ub, therefore it must be false, and since x must always be false we can just remove that entire branch."
    - kbenson 6y ago
      My favorite example along these lines (in C) is "Cap'n'Proto remote vuln: pointer overflow check optimized away by compiler"[1] which was covered here a few years back and shows all of these "theoretical" compiler behaviors coming to a head in a real bug which is thoroughly explained. 1: https://news.ycombinator.com/item?id=14163111 https://news.ycombinator.com/item?id=14163111
  - cesarb 6y ago
    > That could literally produce no output program? Way worse than even that (you might be noticing a theme here...). Once the optimizer has removed as dead code any branch that, if taken, would provably lead to UB at some arbitrary future point of execution, it can conclude that the other branch is now the only possible execution, and call it unconditionally, even if that leads to removing all your files (the classic example is https://kristerw.blogspot.com/2017/09/why-undefined-behavior-may-call-never.html https://kristerw.blogspot.com/2017/09/why-undefined-behavior...).

kd5bjo 6y ago

As it was UB anyway, wouldn’t the compiler also be within its rights to detect the problem and panic the thread?

GolDDranks 6y ago
Yes, it would. However, it rarely does because of how the incentives are: it's useful for optimization to pretend that UB never happens, and more optimization leads to faster binaries, which are what the compiler engineers often pursue.
- kd5bjo 6y ago
  Right, but given that panicing was already allowable behavior, why was the new behavior chosen to be one likely to introduce subtle bugs? It seems much better to loudly proclaim the existence of an erroneous precondition, which is consistent with how things like array indexing behave. I guess I’ll have to go dig up the RFC discussion on this one; it should make for interesting reading.
  - GolDDranks 6y ago
    I thing you are right about that, I'd prefer panicking too. Also, reading the RFC will surely clear up the motivation. Without knowing this case, I'd wager a guess: it's about performance. Panicking introduces a branch and side-effects which, again, affects negstively optimization potential and performance. The saturating cast affects performance too, but less. If some old code has a lot of number crunching containing these operations, a big performance regression would be nasty.
    - kd5bjo 6y ago
      It looks like ‘as’ is generally defined as a truncating cast operator for other numeric types. Since it doesn’t panic for other overflow situations (like u64->u32), they chose consistency with those cases. https://github.com/rust-lang/rust/issues/10184#issuecomment-194885126 https://github.com/rust-lang/rust/issues/10184#issuecomment-...

jfkebwjsbx 6y ago

> there is no room in the type system to return an error There is: you panic, like in the array case. That would have been much robust (at the cost of performance).

kibwen 6y ago
For consistency you would also expect `257u32 as u8` to panic (which it doesn't, and never has); the `as` operator has always been about fast-and-lossy conversions, with the standard library ideally providing methods for more principled conversions.

quietbritishjim 6y ago

Seems a pity they didn't add a built-in at the same time that's a bit more nuanced. It could maybe return an enum with Success(value), Underflow, Overflow and NaN. That way it's up to the coder to decide whether they want a saturating cast or to check the result explicitly.

estebank 6y ago
There are conversations about doing exactly that with `.try_into()` doing exactly that.