Ran into a weird Protobuf issue when compiling the Rust part of the Pants codebase - curious if anyone else has run into this:

--- stderr
  Error: Custom { kind: Other, error: "protoc failed: google/bytestream/bytestream.proto: warning: Import google/protobuf/wrappers.proto but not used.\ncache.proto:39:12: Explicit 'optional' labels are disallowed in the Proto3 syntax. To define 'optional' fields in Proto3, simply remove the 'optional' label, as fields are 'optional' by default.\n" }

I had the same problem! and bisected to https://github.com/pantsbuild/pants/commit/1ec5fea40bc51ee6195fad0fc9cb185aea0cc335 but my protoc was apparently some random(!?) binary in

~/bin

, and it worked after I got rid of that. (For reasons relating to my not knowing rust at all, just deleting

optional

was not enough.)

proto3 originally got rid of the optional keyword, but they added it back in version 3.15 https://github.com/protocolbuffers/protobuf/releases/tag/v3.15.0

Huh okay, thanks! It doesn’t look like I have that protoc problem:

which protoc
/usr/bin/protoc

... or figure out how to add that flag to the protoc invocation...

--experimental_allow_proto3_optional

Managed to get around this by installing protoc directly from GitHub releases: https://github.com/protocolbuffers/protobuf/releases Thanks everyone for the help!