wishcasting /ˈwɪʃˌkɑːstɪŋ/
n. — The act of interpreting information or a situation in a way that casts it as favorable or desired, although there is no evidence for such a conclusion; a wishful forecast.

pattern-wishcast

proc macro implementing some parts of pattern types, a proposed rust language feature which may be added soon™

here's to hoping that demonstrating the usefulness of predicate subtyping with this limited hack speeds the addition of real pattern types

what

compile-time subtyping relationships between enums with conditionally uninhabited variants. hopefully probably maybe safe transmute-based conversions. miri seems happy.

pattern_wishcast! {
    enum Value is <P: PatternFields> = StuckEvaluation | {
        Number { value: i32 },
        Boolean { value: bool },
        // ...
    };

    // Complete values: no stuck states
    type CompleteValue = Value is Number { .. } | Boolean { .. } | ...;

    // with real pattern types we wouldn't need explicit wildcards
    type PartialValue = Value is _;

    #[derive(SubtypingRelation(upcast=to_partial, downcast=try_to_complete))]
    impl CompleteValue : PartialValue;
}

generates transmute-based upcasts and runtime-checked downcasts. auto-generated safety tests.

limitations

• only patterns that make entire variants conditional work. can't restrict a field to a range like real rust patterns
• downcast gen has builtin support for only Vec<T>, Box<T>, Option<T> for generic containers containing Value types
  • requires #[unsafe_transmute_check(iter = ".values()")] for custom containers, don't mess up or you'll transmute never types into existence

examples

see examples/expression_evaluator.rs for stuck evaluation -> resolved evaluation demo using CompleteValue and PartialValue

status

works but hacky. would be much cleaner with native pattern types support in rustc.
pre-release for now, expect the API to change. might be small changes, might get reworked. have only been iterating on this for a few days.