Function decoder_from 

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

The decoder for the types one root reaches, and no others.

What a probe’s translation unit wants: a decoder for a type the root cannot reach is a static function nothing calls, and a generated file that warns is one whose warnings stop being read.

Errors

As decoder.