Function generate_altrep_len 

fn generate_altrep_len(
    name: &Ident,
    generics: &Generics,
    len_field: &Ident,
) -> TokenStream

Generates an impl AltrepLen for T block that delegates to a named struct field.

The generated implementation returns self.{len_field} cast to usize as the ALTREP vector length.

Arguments

• name – The struct identifier.
• generics – Generic parameters for the struct.
• len_field – The identifier of the field that holds the length value.