Invalid generated code when using a nested message with a map field

[FoxLisk]

Using a protobuf file with these contents:

syntax = "proto3";
message M {
  message SubMessage {
    map<string, string> item = 1;
  }
  SubMessage reply = 2;
}

I'm getting invalid code:

#[allow(clippy::derive_partial_eq_without_eq)]
#[derive(Clone, PartialEq, ::prost::Message)]
pub struct M {
    #[prost(message, optional, tag = "2")]
    pub reply: ::core::option::Option<m::SubMessage>,
}
/// Nested message and enum types in `M`.
pub mod m {
    #[allow(clippy::derive_partial_eq_without_eq)]
    #[derive(Clone, PartialEq, ::prost::Message)]
    pub struct SubMessage {
        #[prost(message, repeated, tag = "1")]
        pub item: ::prost::alloc::vec::Vec<sub_message::ItemEntry>,
    }
}

the error:

error[E0433]: failed to resolve: use of undeclared crate or module `sub_message`
  --> D:\code\sni_things\target\debug\build\sni_things-e5c2e47c00096f71\out/_.rs:13:44
   |
13 |         pub item: ::prost::alloc::vec::Vec<sub_message::ItemEntry>,
   |                                            ^^^^^^^^^^^ use of undeclared crate or module `sub_message`

This works if I change the map in the proto definition into a scalar, or if I move SubMessage out of M (so it's no longer nested). (of course in real life I don't control the protobuf file, so this solution doesn't work very well).

[hsyan2008]

I have the same problem