dev: replace unmaintained derivative crate with manual impls

[sshaplygin]

Problem

derivative is flagged unmaintained by RUSTSEC-2024-0388 (upstream is no longer maintained; the advisory suggests derive_more, derive-where or educe). Every cargo audit run against a project depending on ydb reports it.

Change

It was used in exactly three places, all for the same thing: derive Debug while skipping fields that are not Debug themselves. Each becomes a hand-written impl listing the remaining fields, which produces identical output, and the dependency is dropped.

Type	Skipped fields
TableSession	on_drop_callbacks, channel_pool
RacyRoundRobinState	both ConnectionTask queues
GrpcConnectionManagerGeneric	interceptor

GrpcConnectionManagerGeneric also used derivative(Clone(bound = "BalancerT: Clone")), so its Clone becomes a manual impl too, preserving the balancer-only bound — ConnectionT sits behind an Arc and does not need Clone. Its Debug keeps the ConnectionT: Debug bound that derivative generated implicitly.

No behaviour change: derivative's Debug = "ignore" omits the field entirely, which is what debug_struct(..).field(..).finish() emits.

Verification

cargo tree -p ydb -e normal -i derivative   # no longer in the tree
cargo fmt --check && cargo clippy --workspace --all-targets --no-deps --exclude=ydb-grpc -- -D warnings
cargo test --workspace     # 214 passed, 88 ignored

The Cargo.lock diff is exactly the removal of derivative 2.2.0 — no incidental version bumps.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 89.68254% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.89%. Comparing base (a6d7911) to head (4d48673).

Files with missing lines	Patch %	Lines
ydb/src/grpc_connection_manager.rs	92.06%	5 Missing ⚠️
ydb/src/session.rs	84.37%	5 Missing ⚠️
ydb/src/connection_pool.rs	90.32%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #587      +/-   ##
==========================================
- Coverage   86.91%   86.89%   -0.02%     
==========================================
  Files         198      198              
  Lines       19492    19615     +123     
==========================================
+ Hits        16941    17044     +103     
- Misses       2551     2571      +20     

Flag	Coverage Δ
rust-1.88.0	86.89% <89.68%> (-0.02%)	⬇️
rust-1.96.1	87.23% <100.00%> (+0.08%)	⬆️
tests	86.89% <89.68%> (-0.02%)	⬇️
ubuntu	86.89% <89.68%> (-0.02%)	⬇️

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[sshaplygin]

Pushed 4d48673 to raise patch coverage: 25% → 100%.

The diff started at 25% because nothing in the suite ever formatted the three types whose derives this PR replaces. That is worth closing for its own sake, not just for the number — derivative(Debug = "ignore") enforced the skipped fields structurally, while a hand-written debug_struct chain can silently start or stop reporting a field. Each test pins both halves of the contract.

Type	Must report	Must never report
TableSession	id, can_pooled, timeouts	channel_pool, on_drop_callbacks
RacyRoundRobinState	addrs, first_connection	the two connection queues
GrpcConnectionManagerGeneric	balancer, connections_pool, database	interceptor

Two extra tests pin the Clone side: the BalancerT: Clone-only bound that derivative(Clone(bound = ...)) gave us, and that clones keep sharing one Arc pool — checked for both NoBalancer and the SharedLoadBalancer used in production.

Nothing contacts a server: RacyRoundRobinState uses a lazily connected channel (async only because connect_lazy needs a reactor), and the session reuses the construction discard_from_pool_clears_can_pooled already relied on — now extracted into a shared test_session helper.

I checked the tests are not vacuous: dropping .field("timeouts", ..) from the TableSession impl fails the test with timeouts missing: TableSession { id: "session-42", can_pooled: true }.

Coverage

Patch coverage 100% (28/28 changed lines) under cargo llvm-cov --lib. Module coverage: grpc_connection_manager.rs 39.3% → 71.6% lines, session.rs 67.1% → 74.0%, connection_pool.rs 29.9% → 39.8%.

cargo fmt --check, clippy -D warnings and cargo test --workspace all pass (219 passed, 88 ignored).

🤖 Generated with Claude Code