//! A TPX-style external test runner for Buck2, specialized for Rust tests.
//!
//! Buck2 owns execution and caching. This runner implements buck2's external
//! test protocol: it serves the `TestExecutor` service (receiving one
//! `ExternalRunnerSpec` per target), lists each target's tests via a cacheable
//! `Execute2(Listing)`, then issues one `Execute2(Testing)` per discovered test
//! and reports each result. See `DESIGN.md` for the full design.