connectrpc-build: expose buffa message-type feature-gating options (gate_impls_on_crate_features, feature names)

[iainmcgin]

Summary

connectrpc-build's Config surfaces several buffa codegen options as first-class convenience setters (generate_json, strict_utf8_mapping, emit_register_fn, file_per_package), plus a wholesale buffa_config(CodeGenConfig) escape hatch. But it does not surface buffa's options for gating the generated message-type impls behind crate features:

• gate_impls_on_crate_features
• gate_reflect_on_crate_feature
• feature_gate_names (the FeatureGateNames struct, with the customizable json / reflect / etc. names)

So an integrator who wants their generated message types' serde (json) impls gated behind a custom-named feature has to call buffa_config(...) and build a whole CodeGenConfig by hand. Because buffa_config replaces the config wholesale, it does not compose with the other convenience setters — you lose generate_json, strict_utf8_mapping, etc. unless you re-set them on the CodeGenConfig yourself.

Proposal

Add convenience passthrough setters on connectrpc-build::Config that forward to the underlying buffa CodeGenConfig, e.g.:

• gate_impls_on_crate_features(bool)
• gate_reflect_on_crate_feature(bool)
• a setter (or builder) for the feature-gate names (json, reflect, ...)

so message-type feature gating can be configured alongside the existing convenience setters, without dropping to the wholesale buffa_config escape hatch.

Why

This is the integration point for buffa's customizable-feature-name support: the json gate that integrators may want to customize lives on the message types, which buffa generates and connectrpc-build drives. The capability already exists (via buffa_config); this issue is about first-class ergonomics and discoverability, and consistency with the other buffa options connectrpc-build already re-exports.

Notes / priority

Low priority and additive. A separate issue covers making connectrpc-codegen's own client feature gate name configurable; this one is specifically about the connectrpc-build → buffa passthrough for message-type gating.