[protoc-gen-tonic] Support tonic-build generate_default_stubs option

[fivetran-auslacroix]

Summary

Feature request.

tonic-build 0.14 exposes CodeGenBuilder::generate_default_stubs, but protoc-gen-tonic does not accept or forward the equivalent plugin option.

This is useful when a service implementation should be able to override only selected RPCs while the generated trait supplies UNIMPLEMENTED defaults for the rest.

Tested with protoc-gen-tonic 0.5.0 built from current main at 3e44d8a, using protoc 36.0.

Reproduction

service.proto:

syntax = "proto3";
package example;

message PingRequest {}
message PingResponse {}

service PingService {
  rpc Ping(PingRequest) returns (PingResponse);
}

With protoc-gen-tonic on PATH:

mkdir out
protoc -I . service.proto \
  --tonic_out=out \
  --tonic_opt=no_include,generate_default_stubs=true

no_include keeps this standalone Tonic invocation from requiring a Prost insertion target.

Actual behavior

Generation fails with exit code 1:

--tonic_out: invalid parameter: generate_default_stubs=true

Expected behavior

generate_default_stubs (bare or =true) should be accepted and forwarded to tonic_build::CodeGenBuilder::generate_default_stubs. Omitting it, or setting it to false, should preserve the current required-method behavior.

Proposed patch

I compiled this patch and reran the reproduction. Generation succeeded, and the server trait's ping method had a default body returning tonic::Status::unimplemented("Not yet implemented").

Patch against current main

diff --git a/protoc-gen-tonic/README.md b/protoc-gen-tonic/README.md
index 5dd1b56..8416ec8 100644
--- a/protoc-gen-tonic/README.md
+++ b/protoc-gen-tonic/README.md
@@ -44,6 +44,8 @@ see the related documentation from `tonic-build`:
 
 In addition, the following options can also be specified:
 
+* `generate_default_stubs(=<boolean>)`: Generates default server method stubs
+  that return an unimplemented gRPC status
 * `no_server(=<boolean>)`: Disables generation of the server modules
 * `no_client(=<boolean>)`: Disables generation of the client modules
 * `no_transport(=<boolean>)`: Disables generation of connect method using `tonic::transport::Channel`
diff --git a/protoc-gen-tonic/src/generator.rs b/protoc-gen-tonic/src/generator.rs
index 6eefc8b..7938171 100644
--- a/protoc-gen-tonic/src/generator.rs
+++ b/protoc-gen-tonic/src/generator.rs
@@ -13,6 +13,7 @@ use crate::{resolver::Resolver, util};
 pub(crate) struct TonicGenerator {
     pub(crate) resolver: Resolver,
     pub(crate) generate_server: bool,
+    pub(crate) generate_default_stubs: bool,
     pub(crate) generate_client: bool,
     pub(crate) generate_transport: bool,
     pub(crate) server_attributes: Attributes,
@@ -161,6 +162,7 @@ impl TonicGenerator {
                                 .build_transport(self.generate_transport)
                                 .compile_well_known_types(self.resolver.compile_well_known_types())
                                 .attributes(self.server_attributes.clone())
+                                .generate_default_stubs(self.generate_default_stubs)
                                 .generate_server(&service, PROTO_PATH)
                         });
 
diff --git a/protoc-gen-tonic/src/lib.rs b/protoc-gen-tonic/src/lib.rs
index 9b3574c..a754890 100644
--- a/protoc-gen-tonic/src/lib.rs
+++ b/protoc-gen-tonic/src/lib.rs
@@ -30,6 +30,7 @@ pub fn execute(raw_request: &[u8]) -> protoc_gen_prost::Result {
     let mut generator = TonicGenerator {
         resolver,
         generate_server: !params.no_server,
+        generate_default_stubs: params.generate_default_stubs,
         generate_client: !params.no_client,
         generate_transport: !params.no_transport,
         server_attributes: params.server_attributes,
@@ -54,6 +55,7 @@ struct Parameters {
     client_attributes: Attributes,
     compile_well_known_types: bool,
     disable_package_emission: bool,
+    generate_default_stubs: bool,
     no_server: bool,
     no_client: bool,
     no_transport: bool,
@@ -101,6 +103,17 @@ impl str::FromStr for Parameters {
                     param: "disable_package_emission",
                     value: "false",
                 } => (),
+                Param::Parameter {
+                    param: "generate_default_stubs",
+                }
+                | Param::Value {
+                    param: "generate_default_stubs",
+                    value: "true",
+                } => ret_val.generate_default_stubs = true,
+                Param::Value {
+                    param: "generate_default_stubs",
+                    value: "false",
+                } => (),
                 Param::Parameter { param: "no_server" }
                 | Param::Value {
                     param: "no_server",