Macro match_each_varbin_builder 

macro_rules! match_each_varbin_builder {
    ($builder:expr, | $typed:ident | $body:expr) => { ... };
}

Recovers a concrete VarBinBuilder from a &mut dyn ArrayBuilder.

Evaluates to Some($body) with $typed bound to the &mut VarBinBuilder<O> when the builder is one, and None otherwise, so an encoding can fall through to its other output paths:

if let Some(result) = match_each_varbin_builder!(builder, |builder| {
    append_my_encoding(array, builder, ctx)
}) {
    return result;
}