prisma-risk / tsoracle / 26299252624

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feat: add opt-in bt cargo feature for error backtraces (#120)

* feat: add opt-in bt cargo feature for error backtraces

Add a workspace-wide `bt` Cargo feature that wraps
`std::backtrace::Backtrace` in a tiny `Bt` newtype embedded inline in
selected error variants. Without the feature, `Bt` is a ZST and
`Bt::capture()` is a no-op — embedding it in an enum variant costs
zero bytes. With the feature on, capture still respects
`RUST_BACKTRACE` / `RUST_LIB_BACKTRACE` at runtime, so symbol
resolution stays opt-in at runtime even when compiled in.

`tsoracle-client`, `tsoracle-consensus`, `tsoracle-server`, and
`tsoracle-driver-file` each expose a `bt` feature that forwards to
`tsoracle-core/bt`. The `tsoracle` binary aggregates them so a single
`--features bt` lights up the whole stack.

First instrumented variant is `ServerError::WatchPanic`, whose
semantics ("watch task panicked — programming bug") fit backtrace
capture cleanly. `tsoracle-openraft-toolkit` is deliberately not
wired in: it has no direct `tsoracle-core` dependency and no current
variant that wants a backtrace, so opting in would mean adding a
fresh crate-edge for no current consumer.

* test(core): cover Bt Display Captured branch

Coverage runs use `--all-features`, so the `#[cfg(not(feature = "bt"))]`
arms are excluded entirely from instrumentation. The remaining gap was
the `BacktraceStatus::Captured` branch of `Display::fmt`, which only
fires when `RUST_BACKTRACE` is set — and CI does not set it.

Add a test that constructs `Bt` directly from `Backtrace::force_capture`
to drive the `write!` branch unconditionally, independent of env vars.

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