#[flags]Expand description
Expands an enum into a compact bitflag type.
The #[flag] attributes on variants are helper markers consumed by this
attribute macro. They are intentionally not exported as standalone macros,
so using #[flag] without #[flags] fails at compile time.
#[flags] optionally accepts an explicit backing integer type, e.g.
#[flags(u32)]. Without an argument the smallest type that fits every
flag value is chosen automatically, matching prior behavior.