Apply unused return value checker

build.gradle.kts

@@ -22,6 +22,10 @@ tasks.test {
 }
 kotlin {
     jvmToolchain(23)
+    compilerOptions {
+        allWarningsAsErrors.set(true)
+        freeCompilerArgs.add("-Xreturn-value-checker=full")
+    }
 }
 
 tasks.withType<org.jetbrains.dokka.gradle.DokkaTask>().configureEach {

src/main/kotlin/kodvent/collections/MapExtensions.kt

@@ -36,6 +36,7 @@ public fun <T> MutableMap<T, Int>.increment(key: T) {
  * @sample samples.IncrementAndDecrementSamples.decrementBasicUsage
  * @sample samples.IncrementAndDecrementSamples.decrementInventoryManagement
  */
+@IgnorableReturnValue
 public fun <T> MutableMap<T, Int>.decrement(key: T): Boolean {
     val count = this[key] ?: return false
     if (count > 1) {

src/main/kotlin/kodvent/datastructures/DisjointSetUnion.kt

@@ -87,6 +87,7 @@ public class DisjointSetUnion(size: Int) {
      * @sample samples.DisjointSetUnionSamples.basicUsage
      * @sample samples.DisjointSetUnionSamples.detectingCycles
      */
+    @IgnorableReturnValue
     public fun union(x: Int, y: Int): Boolean {
         val rootX = find(x)
         val rootY = find(y)
@@ -159,7 +160,9 @@ public class DisjointSetUnion(size: Int) {
             throw IndexOutOfBoundsException("Element ($x) is out of disjoint set bounds: [0, ${parent.size})")
         }
 
-        parent.indices.forEach { find(it) }
+        for (i in parent.indices) {
+            parent[i] = find(i)
+        }
 
         val oldRoot = parent[x]
         val elementsInSet = parent.indices.filter { parent[it] == oldRoot }

src/test/kotlin/kodvent/datastructures/DisjointSetUnionTest.kt

@@ -494,8 +494,9 @@ class DisjointSetUnionTest {
         dsu.union(5, 6)
 
         // Trigger path compression by finding a deep element
-        dsu.find(6)
-        dsu.find(0)
+        val root6BeforeIsolate = dsu.find(6)
+        val root0BeforeIsolate = dsu.find(0)
+        assertEquals(root6BeforeIsolate, root0BeforeIsolate, "All elements should share the same root before isolate(3)")
 
         // Now isolate element 3
         dsu.isolate(3)