CI: conformance release build is 30-35s with warm cache - investigate

[iainmcgin]

The conformance CI jobs spend 30-35s on cargo build --release --bin ...
even with Swatinem/rust-cache@v2 restoring in 3-5s and the post-step
skipping save (meaning: cache hit, nothing new to write back).

From the PR #6 merge run:

Job	Cache restore	Build	Cache save
Conformance (server)	3s	30s	0s
Conformance (client)	5s	35s	0s

This was ~50s before #5 switched from building the whole
connectrpc-conformance package to --bin ${{ matrix.bin }}.

Hypothesis

Swatinem/rust-cache caches target/*/deps/ but not the final crate
artifacts - so dependencies are warm but connectrpc-conformance
itself recompiles from scratch every time. The conformance crate is
large (includes all the checked-in generated protobuf code), so 30s
of release-mode compile for the leaf crate might just be the floor.

If that's the answer, there's no fix - this is working as designed.
Filing this to record that someone should confirm it rather than
assume.

Possible levers if it's not the floor

• Check if LTO is on for the workspace release profile (slow link)
• Check if the codegen units setting is working against parallelism
• sccache as a second layer (caches per-crate compile output, not
  just deps)

[paikend]

Investigation results

I profiled the conformance build locally (Apple Silicon) with dependencies already cached (simulating a warm rust-cache hit):

Binary	Rebuild time (local)	CPU utilization
conformance-server	12.7s	400%
conformance-client	19.9s	352%

CI runs on GitHub-hosted Ubuntu runners which are slower, so the 30-35s observed in CI is consistent with ~13-20s on local hardware — this confirms the hypothesis that the current time is effectively the floor.

Why it's the floor

• The conformance crate has ~18K LOC of generated protobuf code with complex trait implementations (Serde, Debug, Clone, Message, etc.)
• Release-mode compilation with opt-level=3 and codegen-units=16 (defaults) on that volume of code is inherently expensive
• rust-cache is already doing its job — dependencies are warm, only the leaf crate recompiles
• No LTO is enabled, so link time is not the bottleneck

Could sccache help?

sccache caches per-crate compiled artifacts, so it would skip recompilation when the conformance source hasn't changed. Since most PRs don't touch conformance code, this could reduce the 30s to near-zero on those runs. However, when conformance code does change, the full 30-35s is unavoidable.

Whether the added CI complexity of sccache is worth it for this one job is a judgment call — happy to submit a PR if desired.