Function codec 

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

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

Under per_sample the named non-@nested aggregates additionally get a public entry that bakes their own sample decision in; every other type is reached through the body of the type that declares it.

Errors

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