Panics
Explicit panics — the macros that abort the current task by design rather than through an unexpected fault. They share a mechanism (unwinding or aborting the thread) but differ in intent: panic! signals an unrecoverable condition the author chose not to return as an error, while todo!, unimplemented!, and unreachable! are placeholders and assertions that a path is unfinished or should never run.
Each is legitimate in the right place, yet each becomes a defect when it reaches production on a path that real input can drive. A todo! left in a shipped function, or an unreachable! that turns out to be reachable, is a latent crash that the type system will not catch for you.