Range tests demo

Author: AlexDenisov

.github/workflows/mull.yml

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+name: Mutation Testing
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+permissions:
+  security-events: write
+
+jobs:
+  mutation-testing:
+    runs-on: ubuntu-latest
+    container:
+      image: ubuntu:26.04
+    steps:
+      - name: Install prerequisites
+        run: apt-get update && apt-get install -y curl ca-certificates git clang-22
+
+      - uses: actions/checkout@v4
+
+      - name: Install mull from nightly
+        run: |
+          curl -1sLf 'https://dl.cloudsmith.io/public/mull-project/mull-nightly/setup.deb.sh' | bash
+          apt-get install -y mull-22
+
+      - name: Build range_tests
+        run: |
+          clang-22 \
+            -fpass-plugin=/usr/lib/mull-ir-frontend-22 \
+            -g -grecord-command-line \
+            main.c -o range_tests
+
+      - name: Run mull
+        run: |
+          mull-runner-22 \
+            --reporters GitHubAnnotations \
+            --reporters SARIF \
+            --report-name mull-report \
+            --allow-surviving \
+            range_tests
+
+      - name: Upload SARIF to Code Scanning
+        uses: github/codeql-action/upload-sarif@v3
+        with:
+          sarif_file: mull-report.sarif

README.md

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+# mull-demo
+
+A minimal demo of [Mull](https://mull-project.com) mutation testing, based on the [Quick Primer](https://mull-project.com/getting-started/primer/).
+
+## What this project does
+
+`main.c` implements a (buggy) `in_range` function that checks whether a value falls within a closed interval `[min, max]`, along with three assertions that serve as its test suite:
+
+```c
+int in_range(int value, int min, int max) {
+  return value >= min && value < max;
+}
+```
+
+The tests cover below-min, mid-range, and above-max cases — achieving 100% line coverage. Despite this, Mull reveals that two mutations survive:
+
+- `>=` changed to `>` (the boundary at `min` is not tested)
+- `<` changed to `<=` (the boundary at `max` is not tested)
+
+This demonstrates a core limitation of coverage-based testing: coverage does not guarantee that the tests actually validate the semantics of the code.
+
+## Running locally
+
+Install mull (see [installation docs](https://mull-project.com/getting-started/installation/)), then:
+
+```bash
+clang-22 \
+  -fpass-plugin=/usr/lib/mull-ir-frontend-22 \
+  -g -grecord-command-line \
+  main.c -o range_tests
+
+mull-runner-22 range_tests
+```

main.c

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+#include <assert.h>
+
+// Returns 1 if value is in the closed interval [min, max], 0 otherwise
+int in_range(int value, int min, int max) {
+  return value >= min && value < max;
+}
+
+int main(void) {
+  assert(in_range(5,  1, 10) == 1);  // mid-range: both conditions true
+  assert(in_range(0,  1, 10) == 0);  // below min: first condition false
+  assert(in_range(15, 1, 10) == 0);  // above max: second condition false
+  return 0;
+}

mull.yml

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+mutators:
+  - cxx_boundary
+  - cxx_comparison