test: 100% mutation kill rate, cargo-mutants v27, #[mutants::skip] (0.3.2)

89 mutants tested: 59 caught, 30 unviable (type system rejects them),
0 missed. Added mutants dev-dep for #[mutants::skip] on legacy-msrv
dead code. Upgraded cargo-mutants to v27.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.cargo/mutants.toml

@@ -5,18 +5,13 @@ test_tool = "nextest"
 # are sufficient to catch mutants. Full proptests run separately via just test-unit.
 additional_cargo_test_args = ["-E", "not test(proptests)"]
 
-# Excluded mutants (false positives only):
+# False positives excluded:
 #
-# - legacy-msrv const_copy: dead code under default features, only compiled
-#   with --features legacy-msrv. Tested separately.
+# - `> 0` → `>= 0` or `< 0` on usize: tautological (usize always >= 0)
+# - `< 254` → `<= 254` in max_run: both branches yield 254 at the boundary
 #
-# - usize `> 0` → `>= 0` or `< 0`: tautological on usize (always >= 0).
-#
-# - `< 254` → `<= 254` in max_run: both branches yield 254 when remaining
-#   is exactly 254, so the mutation is a semantic no-op.
+# The legacy-msrv const_copy is excluded via #[mutants::skip] in source.
 exclude_re = [
-    "in const_copy",
-    "replace const_copy with \\(\\)",
     "replace > with < in encode",
     "replace > with >= in encode",
     "replace < with <= in encode",

CHANGELOG.md

@@ -1,14 +1,15 @@
 # Changelog
 
-## 0.3.2 — 2026-03-31
+## 0.3.2 — 2026-04-01
 
 ### Testing
 
-- **Mutation testing passes clean** — 88 mutants tested, 59 caught, 0 missed.
-  Added targeted tests for 254-byte block boundary encoding and exact-fit
-  decode buffer sizing that close gaps found by `cargo-mutants`.
-- **111 tests** (was 106): added block-boundary interop, exact-fit decode zeros,
-  and too-small decode zeros tests.
+- **Mutation testing: 100% kill rate** — 89 mutants tested, 59 caught, 30
+  unviable (type system rejects them), 0 missed. Uses `cargo-mutants` v27
+  with nextest. Added `mutants` crate for `#[mutants::skip]` annotations.
+- **111 tests** (was 106): added block-boundary interop verification against
+  corncobs, exact-fit and too-small decode buffer sizing for the batch zero
+  path.
 
 ## 0.3.1 — 2026-03-31
 

Cargo.toml

@@ -27,6 +27,7 @@ all-features = true
 cobs = "0.5"
 corncobs = "0.1"
 iai-callgrind = "0.16"
+mutants = "0.0.4"
 proptest = "1"
 rand = "0.10"
 

README.md

@@ -295,8 +295,8 @@ affected — it uses a byte loop instead of `copy_from_slice`/memcpy.
   structural/algebraic properties, and boundary-region behavior
 - **Fuzz targets** — 3 `cargo-fuzz` harnesses (decode, round-trip, small-dest)
   for continuous coverage of edge cases
-- **Mutation testing** — `cargo-mutants` verifies that the test suite catches
-  injected bugs in every reachable code path
+- **Mutation testing** — `cargo-mutants` with 100% kill rate (59/59 viable
+  mutants caught; 30 more rejected by the type system)
 - **Boundary tests** — exhaustive coverage of 254/255-byte block boundaries,
   multi-block splits, buffer-exact fits, and off-by-one dest sizes
 - **Compile-time verification** — `const` assertions that validate encode

src/lib.rs

@@ -167,6 +167,7 @@ const fn const_copy(dest: &mut [u8], di: usize, src: &[u8], n: usize) {
 }
 
 /// Fallback const copy for toolchains older than Rust 1.93.
+#[mutants::skip] // dead code under default features; tested via --features legacy-msrv
 #[inline(always)]
 #[cfg(feature = "legacy-msrv")]
 const fn const_copy(dest: &mut [u8], di: usize, src: &[u8], n: usize) {