procoder: Instructions file for GitHub Copilot

Tests in procoder

A test here is not finished when it passes. It is finished when it has been watched to fail.

Every test carries its mutation

Each test has a // proved by: line naming a specific change to the source that must make it fail:

// proved by: made the unmarshal failure return nil — a crashed semgrep
// then reads as a clean SAST pass.
func TestSastUnreadableOutputIsNotRun(t *testing.T) {

Then actually apply that change, build, and run the test. Three discoveries came only from doing it, never from reading the assertion:

• A test exercised a helper directly and never the dispatch, so deleting the call from run() left it passing. It goes through the real entry point now.
• A P-CONTROL test ran procoder format only over files that were already formatted, so the branch that prints a rewritten file never executed — and a mutation making that branch write to disk passed. A mutation must reach the branch; check the fixture can provoke it.
• A mutation that does not compile is not a mutation, and neither is one that produces no diff. Verify it applied.

Where two redundant mechanisms guard one behaviour, no single mutation will fail the test. Say so in the comment and name the pair, rather than claiming a proof that does not hold.

Assertions

• Assert the verdict, not the subject. grep "unformatted <file>", not grep "<file>" — the file's name also appears in "UNCHECKED — csharpier is not installed", which reports the opposite of a finding.
• Never assert on a word that appears either way. Checking for "deny" or "block" in a hook envelope passes on an allow, because "block" sits inside "1 blocking finding(s)". Read the decision field.
• An assertion that cannot fail is worse than none. if before >= after passes exactly when the knob under test did nothing.

Portability

Windows runs go test ./internal/... in CI, and two habits break there:

• exec.LookPath needs an executable extension, so an extensionless shell-script stub is invisible. Skip on Windows like the existing stubs do, or use a real binary.
• os.Chmod(dir, 0o000) does not deny reads on Windows, so a test that provokes an unreadable-directory branch that way never enters it.
• Never use a rooted literal such as /repo as a test repository root: macOS and Linux call it absolute and Windows does not.