Enum Discriminant Access

To read the discriminant of an enum value today, one must use core::mem::discriminant(&place). This unfortunately requires a &-borrow, which may introduce UB in unsafe contexts. Moreover there's no stable way to get a discriminant value without having an enum value with that discriminant around.

To solve this, I propose to combine ideas from two existing RFCs:

• from RFC 3607 we add a new place expression $place.enum#discriminant evaluates to the discriminant value at that place;
• from RFC 3727, we add discriminant_of!($enum_type, $variant_name) that returns the discriminant value for that variant.

Using both, we have that matches!($place, Some(_)) is equivalent to $place.enum#discriminant == discriminant_of!(Option<_>, Some).

Those two will also come useful for Phased Initialization.

Backlinks

• Pattern Unnesting
• The Final Language
• Phased Initialization