fix(build): Correctly convert Empty to () (#734)

tests/wellknown-compiled/Cargo.toml

@@ -16,4 +16,5 @@ tonic = { path = "../../tonic" }
 prost = "0.9"
 
 [build-dependencies]
+prost-build = "0.9"
 tonic-build = { path = "../../tonic-build" }

tests/wellknown-compiled/build.rs

@@ -1,6 +1,13 @@
 fn main() {
+    let mut config = prost_build::Config::new();
+    config.extern_path(".google.protobuf.Empty", "()");
+
     tonic_build::configure()
         .compile_well_known_types(true)
-        .compile(&["proto/google.proto"], &["proto"])
+        .compile_with_config(
+            config,
+            &["proto/google.proto", "proto/test.proto"],
+            &["proto"],
+        )
         .unwrap();
 }

tests/wellknown-compiled/proto/test.proto

@@ -0,0 +1,11 @@
+syntax = "proto3";
+package test;
+
+import "google/protobuf/empty.proto";
+
+message Input {
+}
+
+service Service {
+  rpc Call(Input) returns (google.protobuf.Empty);
+}

tests/wellknown-compiled/src/lib.rs

@@ -1,9 +1,18 @@
-pub mod google {
-    pub mod protobuf {
-        tonic::include_proto!("google.protobuf");
+pub mod gen {
+    pub mod google {
+        pub mod protobuf {
+            tonic::include_proto!("google.protobuf");
+        }
+    }
+
+    pub mod test {
+        tonic::include_proto!("test");
     }
 }
 
 pub fn grok() {
-    let _empty = crate::google::protobuf::Empty {};
+    let _any = crate::gen::google::protobuf::Any {
+        type_url: "foo".to_owned(),
+        value: Vec::new(),
+    };
 }