Exhaustiveness

[nrc]

• Previous discussion, HackMD
• Previous discussion, Zulip

Exhaustiveness checking could be incorrect in the presence of an incorrectly implemented deref function. This could lead to UB. Some possible solutions:

• The compiler adds an implicit _ => unreachable!(), match arm for all sub-patterns which use deref. The match arm would not be generated if the user supplied one. The actual pattern would have _ as a sub-pattern, as required. E.g., when matching a scrutinee with type Option<Rc<bool>>, the generated arm would be equivalent to Some(_) => unreachable!(). Obviously the generated arm should not trigger any unreachable code lints. This is my personally preferred option.
• We require the user to have an explicit _ match arm. This is boilerplatey.
• Only allow deref in patterns for trusted types, e.g., a subset of std types.
• Add a DerefPure trait or similar attribute to indicate to the compiler that a smart pointer is safe to be used with deref patterns. This option would require a lot of work to design, implement, and stabilise such a trait.