Protoc failed: --experimental_allow_proto3_optional was not set

[dtolnay]

On Ubuntu 22.04.1 LTS with protobuf-compiler installed via apt, protoc --version is libprotoc 3.12.4 and I get the following errors from cargo check:

error: failed to run custom build command for `buck2_test_proto v0.1.0 (buck2/buck2_test_proto)`
Caused by:
  process didn't exit successfully: `buck2/target/debug/build/buck2_test_proto-d224bbb82df65000/build-script-build` (exit status: 1)
  --- stdout
  cargo:rerun-if-env-changed=PROTOC
  --- stderr
  Error: Custom { kind: Other, error: "protoc failed: test.proto: This file contains proto3 optional fields, but --experimental_allow_proto3_optional was not set.\n" }

error: failed to run custom build command for `buck2_data v0.1.0 (buck2/buck2_data)`
Caused by:
  process didn't exit successfully: `buck2/target/debug/build/buck2_data-db49426b304377a7/build-script-build` (exit status: 101)
  --- stdout
  cargo:rerun-if-env-changed=PROTOC
  --- stderr
  thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Custom { kind: Other, error: "protoc failed: data.proto: This file contains proto3 optional fields, but --experimental_allow_proto3_optional was not set.\n" }', buck2_data/build.rs:136:10
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

I found that this patch makes the build succeed:

diff --git a/buck2_data/build.rs b/buck2_data/build.rs
index 08790b9a..acbc7c4e 100644
--- a/buck2_data/build.rs
+++ b/buck2_data/build.rs
@@ -16,6 +16,7 @@ fn main() -> io::Result<()> {
     println!("cargo:rerun-if-env-changed=PROTOC");
     buck2_protoc_dev::maybe_setup_protoc("../../..");
     tonic_build::configure()
+        .protoc_arg("--experimental_allow_proto3_optional")
         .type_attribute(
             "buck.data.BuckEvent.data",
             "#[allow(clippy::large_enum_variant)]",
diff --git a/buck2_test_proto/build.rs b/buck2_test_proto/build.rs
index 69f5b6fc..5401c498 100644
--- a/buck2_test_proto/build.rs
+++ b/buck2_test_proto/build.rs
@@ -6,7 +6,9 @@ fn main() -> io::Result<()> {
     // Tonic build uses PROTOC to determine the protoc path.
     println!("cargo:rerun-if-env-changed=PROTOC");
     buck2_protoc_dev::maybe_setup_protoc("../../..");
-    tonic_build::configure().compile(proto_files, &["."])?;
+    tonic_build::configure()
+        .protoc_arg("--experimental_allow_proto3_optional")
+        .compile(proto_files, &["."])?;
 
     // Tell Cargo that if the given file changes, to rerun this build script.
     for proto_file in proto_files {

[ndmitchell]

Thanks - I've raised D39723187 to apply your patch.

[ndmitchell]

Landed, so closing this issue (also following up with a diff to apply this to our other proto generations, since I'm sure we will use optional and not realise it before long).