Allow derived structs in packets in new rust generator

The legacy rust generator did not support derived structs because of
lack of common interface for parsing packets.
All packets under the new generator implement Packet::decode which
make it trivial to support.

pdl-compiler/src/backends/rust/parser.rs

@@ -524,10 +524,6 @@ impl<'a> FieldParser<'a> {
         assert_eq!(self.shift, 0, "Typedef field does not start on an octet boundary");
 
         let decl = self.scope.typedef[type_id];
-        if let ast::DeclDesc::Struct { parent_id: Some(_), .. } = &decl.desc {
-            panic!("Derived struct used in typedef field");
-        }
-
         let span = self.span;
         let id = id.to_ident();
         let type_id = type_id.to_ident();
@@ -555,11 +551,8 @@ impl<'a> FieldParser<'a> {
                         }
                     }
                     ast::DeclDesc::Struct { .. } => {
-                        let width = syn::Index::from(width / 8);
                         quote! {
-                            let (head, tail) = #span.split_at(#width);
-                            #span = tail;
-                            let #id = #type_id::decode_full(head)?;
+                            let (#id, mut #span) = #type_id::decode(#span)?;
                         }
                     }
                     _ => unreachable!(),

pdl-compiler/src/backends/rust/serializer.rs

@@ -115,10 +115,6 @@ impl Encoder {
         assert_eq!(self.bit_shift, 0, "Typedef field does not start on an octet boundary");
 
         let decl = scope.typedef[type_id];
-        if let ast::DeclDesc::Struct { parent_id: Some(_), .. } = &decl.desc {
-            panic!("Derived struct used in typedef field");
-        }
-
         let id = id.to_ident();
         let buf = &self.buf;