Macro __impl_altvec_materializing_dataptr 

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

Internal macro: impl AltVec with a materializing dataptr for a given element type.

Thin wrapper, parameterised by element type: installs a trivial AltrepDataptr<$elem> (no direct pointer — dataptr returns None) and delegates to __impl_altvec_dataptr, which materializes into data2 via RNativeType::elt. The 5 per-family aliases below pin $elem so the derive and the public impl_alt*_from_data! macros can reference them by a stable per-family name.