UB in Rust vs C

UB in Rust vs C

It's complicated, because Rust defines addition as "either wrap (in twos' complement if signed) or panic", and says that which of those behaviours you get is unspecified, just that it'll be one of those two. So, while it's a bug to depend on wrapping without using wrapping_add etc or Wrapping<T> to indicate that it's deliberate, it's not as bad as it is in C, where it's UB.

With that said, the kernel's current compiler flags for Rust include -C overflow-checks so that the chosen behaviour is locked at "panic", even in release mode, if the CONFIG_RUST_OVERFLOW_CHECKS option is set. Hopefully enough developers leave this at the current default that Rust for Linux code never actually has a dependency on wrapping behaviour that's not expressed in code.