connectrpc-build: add Config::emit_descriptor_set for gRPC server reflection

[christopherwxyz]

connectrpc-build: add Config::emit_descriptor_set for gRPC Server Reflection

Motivation

Config::compile() already parses the .proto inputs into a full FileDescriptorSet — run_protoc is invoked with --include_imports, run_buf with --as-file-descriptor-set, and the precompiled path is read as-is, so the descriptor_bytes it decodes for codegen always carry the complete transitive import closure.

Today that set can be read in (Config::descriptor_set(path)) but never written out. A build script that needs those bytes — to serve grpc.reflection.v1.ServerReflection to clients like grpcurl via include_bytes!(concat!(env!("OUT_DIR"), "…")) — has to invoke protoc a second time with --descriptor_set_out --include_imports, duplicating work connectrpc-build already did and re-requiring a protoc install even when generation used buf or a precompiled set.

This adds the symmetric Config::emit_descriptor_set(name): write the descriptor set connectrpc-build already computed to <out_dir>/<name>.

This is the connectrpc-build counterpart of the buffa#125 follow-up the maintainers invited — buffa-build went with the embedded buffa-reflect pool for in-process reflection, explicitly leaving open the "standalone .bin for tooling outside the Rust build" case. Emitting at the connectrpc-build layer is the natural home for that: the bytes are already in hand, with --include_imports guaranteed, for every descriptor source.

API

/// Also write the parsed `FileDescriptorSet` (the descriptors used for
/// codegen) to `<out_dir>/<name>` as wire-format bytes.
///
/// The set carries the full transitive import closure for every descriptor
/// source (`protoc --include_imports`, `buf --as-file-descriptor-set`, or a
/// precompiled set), so it is ready to back `grpc.reflection.v1.ServerReflection`.
/// The inverse of [`Config::descriptor_set`], which *reads* a precompiled set.
#[must_use]
pub fn emit_descriptor_set(mut self, name: impl Into<String>) -> Self

// build.rs
connectrpc_build::Config::new()
    .files(&["proto/svc.proto"])
    .includes(&["proto/"])
    .emit_descriptor_set("svc_descriptor.bin")
    .compile()?;

// src/lib.rs
pub const FILE_DESCRIPTOR_SET: &[u8] =
    include_bytes!(concat!(env!("OUT_DIR"), "/svc_descriptor.bin"));

Implementation (connectrpc-build/src/lib.rs)

Purely additive — no change to existing behavior; the field defaults to None.

1. Config struct — new field:

     include_file: Option<String>,
+    emit_descriptor_set: Option<String>,
     emit_rerun_directives: bool,

2. Config::new() — default:

             include_file: None,
+            emit_descriptor_set: None,
             emit_rerun_directives: true,

3. Builder method (next to descriptor_set):

+    #[must_use]
+    pub fn emit_descriptor_set(mut self, name: impl Into<String>) -> Self {
+        self.emit_descriptor_set = Some(name.into());
+        self
+    }

4. In compile(), right after create_dir_all(&out_dir) (with descriptor_bytes already in scope from the descriptor-source match):

+        // Emit the parsed descriptor set for reflection, if requested.
+        if let Some(name) = &self.emit_descriptor_set {
+            write_if_changed(&out_dir.join(name), &descriptor_bytes)?;
+        }

It reuses the existing write_if_changed (so it won't churn mtimes / trigger needless rebuilds), and works uniformly for the Protoc, Buf, and Precompiled sources because each already produces an import-complete descriptor_bytes.

Tests (included)

Two tests added to the existing mod tests, mirroring compile_precompiled_descriptor_set:

• config_emit_descriptor_set_toggle — builder-state check.
• emit_descriptor_set_writes_reflection_bin — end-to-end against the echo.fds.bin fixture (hermetic, Precompiled source, no protoc): asserts the file is written, FileDescriptorSet::decode_from_slice succeeds with a non-empty file list, and the bytes round-trip the source set exactly.

Verified locally: cargo build/clippy -p connectrpc-build clean; cargo test -p connectrpc-build → 20 passed, 0 failed.

Notes

• Additive and backward-compatible; no existing API changes.
• Complements buffa-reflect's in-process pool — this is the wire-bytes path for serving reflection v1 to external clients.

Follow-up to anthropics/buffa#125.

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[christopherwxyz]

I have read the CLA Document and I hereby sign the CLA

[christopherwxyz]

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