test: pin sourceCount(hex"00") == 0 explicitly

[thedavidmeister]

Closes #115

Summary

LibBytecode.sourceCount's NatSpec documents that 0x and 0x00 are equivalent, both having 0 sources. The empty case is pinned by testSourceCount0, but the single-byte hex"00" case was only covered incidentally by fuzzing (testSourceCount1 / testSourceCountReference include it in their input domain without naming it). This adds the dedicated, deterministic pin the issue asks for:

function testSourceCountSingleZeroByte() external pure {
    assertEq(LibBytecode.sourceCount(hex"00"), 0);
}

Test-only change; no source or bytecode is touched.

QA

• Baseline: forge test --mc LibBytecodeSourceCountTest green on unmutated main-based branch — 4 passed / 0 failed (2048 fuzz runs each for the two fuzz tests).
• Discriminating test, mutation-validated: mutating the covered line count := byte(0, mload(add(bytecode, 0x20))) → add(1, byte(...)) fails the new test with assertion failed: 1 != 0; restoring the source returns the suite to 4/4 green.
• Mutation matrix for the unit (whole-suite judgment):
  • empty → 0: mutant return 0 → return 1 killed by existing testSourceCount0 + testSourceCountReference (existing coverage validated).
  • first byte is the count: mutant add(1, byte(0, …)) killed by the new testSourceCountSingleZeroByte and testSourceCount1.
  • reads byte 0 specifically: mutant byte(0, …) → byte(1, …) killed by existing testSourceCount1 + testSourceCountReference.
• Adversarial pass: intent derived from the NatSpec oracle (empty → 0; else first byte = count; 0x ≡ 0x00; integrity of sources explicitly NOT checked). Implementation matches on all points; the byte(0, …) read extracts only the first data byte, which exists for any length ≥ 1, and the differential fuzz against LibBytecodeSlow.sourceCountSlow guards the read shape. No candidate bugs; no scope beyond the named gap.
• Oracle independence: the expected value (0) comes from the documented equivalence, not from the code's output.
• Category check: issue test(LibBytecode): pin the documented sourceCount(hex"00") == 0 invariant explicitly #115 asks for exactly this one named assertion; fully covered, hence Closes.
• forge fmt --check clean on the changed file.

Co-Authored-By: Claude noreply@anthropic.com

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📥 Commits

Reviewing files that changed from the base of the PR and between a0974b0 and 36c3b0d.

📒 Files selected for processing (1)

• test/src/lib/bytecode/LibBytecode.sourceCount.t.sol

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch 2026-07-19-issue-115-sourcecount-zero-byte-pin

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[thedavidmeister]

🤖 ai:vetter
Reviewed 36c3b0d: ready — closes #115 — exactly the named deterministic pin the issue asks for; QA block in body with mutation matrix (add-1 mutant killed by the new test), oracle from NatSpec equivalence; test-only
cost 45 — one-line deterministic test pin