Macro __impl_altvec_flavor 

#[doc(hidden)]
macro_rules! __impl_altvec_flavor {
    ($ty:ty, dataptr($elem:ty)) => { ... };
    ($ty:ty, materializing($elem:ty)) => { ... };
    ($ty:ty, string_dataptr) => { ... };
    ($ty:ty, subset) => { ... };
    ({$($gen:tt)*} $ty:ty {$($whr:tt)*}, dataptr($elem:ty)) => { ... };
    ({$($gen:tt)*} $ty:ty {$($whr:tt)*}, materializing($elem:ty)) => { ... };
    ({$($gen:tt)*} $ty:ty {$($whr:tt)*}, string_dataptr) => { ... };
    ({$($gen:tt)*} $ty:ty {$($whr:tt)*}, subset) => { ... };
}

Internal macro: emit the impl AltVec for a given flavor.

Flavors:

• dataptr($elem) — typed direct pointer via AltrepDataptr<$elem>, falling back to data2 materialization.
• materializing($elem) — trivial AltrepDataptr<$elem> returning None plus the same __impl_altvec_dataptr! (pure data2 materialization).
• string_dataptr — whole-vector STRSXP materialization (string family).
• subset — Extract_subset support via AltrepExtractSubset.