Rust test failures — error[E____] / thread panicked

Log signatures

error[E0308]: mismatched types
thread 'tests::it_works' panicked
test result: FAILED
assertion `left == right` failed

What’s happening

Two very different failures share this class: error[E____] is a compile error — your tests never ran (Rust error codes are exceptionally well documented; rustc --explain E0308 gives you a worked example). thread '...' panicked + test result: FAILED is a runtime test failure — an assert!/assert_eq! miss, an explicit panic!, or an unwrap() on None/Err inside test or code under test. The thread name in the panic line is the test name — that's your rerun target.

Fix playbook

1. 1Compile errors first: cargo test won't run anything until the crate compiles. Fix the first error[E____]; later errors are often cascade.
2. 2Runtime failure: rerun just that test with output visible: cargo test test_name -- --nocapture --exact.
3. 3unwrap() panics in code under test: the fix is usually error propagation (?) at the panicking call, not in the test.
4. 4Set RUST_BACKTRACE=1 in CI's env permanently — a panic without a backtrace wastes one full round-trip per failure.
5. 5Rerun the crate's tests (cargo test -p <crate>) before the workspace.