Function decoder 

pub fn decoder<T: CodecTarget>(
    ast: &Ast,
    key: LayoutKey,
    options: CodecOptions,
) -> Result<Vec<T::Decl>, EncodeError>

Build the decoder for every type in the model, under one plan.

Existing data entry points keep their published names and initialization.

Errors

EncodeError when the layout cannot be planned or a construct has no C reader — never a body that leaves a member out.