Reproduce Reduce Regress

Reproduce, Reduce, Regress

When debugging a bug, follow this workflow:

1. Reproduce

Goal: Get a failing test that demonstrates the bug using REAL data.

• Copy the EXACT input that triggers the bug (don't paraphrase or simplify yet)
• Use the EXACT types/structs from the failing code
• Verify the test actually fails with the same error message

2. Reduce

Goal: Find the MINIMAL input that still triggers the bug.

• Create MULTIPLE test variants, don't comment things in/out
• Name them descriptively: test_minimal_one_field, test_with_queries, etc.
• Binary search: remove half the input, see if it still fails
• Keep narrowing until you find the smallest failing case
• Also find a PASSING case that's as close as possible to the failing one

The difference between your minimal failing case and your minimal passing case IS the bug.

3. Regress (Regression Tests)

Goal: Ensure the bug never comes back.

• Keep ALL your test variants - both passing and failing
• The failing ones become regression tests after the fix
• The passing ones document expected behavior
• Name tests after the issue number: test_issue_1356_*

Anti-patterns

❌ Commenting code in/out to test different scenarios ❌ Modifying a single test repeatedly ❌ "Simplifying" input without verifying the bug still reproduces ❌ Deleting test variants after finding the bug ❌ Theorizing about what MIGHT cause the bug before reproducing

Example Structure

// Minimal failing case
#[test]
fn test_issue_1356_fails_without_queries_default() { ... }

// Minimal passing case (shows the workaround)
#[test]
fn test_issue_1356_passes_with_queries_default() { ... }

// Original reproduction from user's code
#[test]
fn test_issue_1356_full_reproduction() { ... }

Source: facet-rs/facet — distributed by TomeVault.