Cannot do single subparse()

[mkeeter]

This is a simplified test case, showing that we can't do a single-element subparse()

#[derive(Ingot)]
pub struct Packet {
    pub bla: u8,

    #[ingot(subparse())]
    pub next_packet: InnerPacket, // works with `Repeated<InnerPacket>`
}

#[derive(Clone, Ingot)]
pub struct InnerPacket {
    pub baaaa: u8,
}

It fails with a cryptic message:

error[E0282]: type annotations needed
  --> src/lib.rs:76:10
   |
76 | #[derive(Ingot)]
   |          ^^^^^ cannot infer type
   |
   = note: this error originates in the derive macro `Ingot` (in Nightly builds, run with -Z macro-backtrace for more info)