Fix unintentionally-quadratic COW copies on insertion

Sources/Testing/Support/Graph.swift

@@ -151,7 +151,9 @@ struct Graph<K, V> where K: Hashable {
     let result: V?
 
     if let key = keyPath.first {
-      if var child = children[key] {
+      // The `.take()` here ensures we remove the item from the graph before mutating,
+      // preventing this from causing a cascading COW update down the tree.
+      if var child = children[key].take() {
         result = child.insertValue(newValue, at: keyPath.dropFirst(), intermediateValue: intermediateValue)
         children[key] = child
       } else {

Tests/TestingTests/Support/GraphTests.swift

@@ -8,7 +8,7 @@
 // See https://swift.org/CONTRIBUTORS.txt for Swift project authors
 //
 
-@testable import Testing
+@_spi(Experimental) @_spi(ForToolsIntegrationOnly) @testable import Testing
 
 @Suite("Graph<K, V> Tests")
 struct GraphTests {
@@ -549,4 +549,49 @@ struct GraphTests {
     }
     #expect(Set(values) == [123, 124, 456, 457, 13579, 13580, 789, 790, 2468, 2469])
   }
+
+  @Test
+  func `inserting many items with the same prefix is not quadratic`() {
+    // This test verifies that inserting various numbers of elements that
+    // all share a common prefix scales ~linearly.
+
+    var perItemTimes = [Int: Double]()
+    let clock = Test.Clock()
+
+    for itemCount in [500, 1000, 5000, 10000] {
+      var graph = Graph<String, Int?>()
+
+      // Start with a representative target/suite/test prefix
+      let sharedPrefix = ["ProjectTests", "ProjectSuite", "something(_:)"]
+      graph[sharedPrefix] = -1
+
+      let startTime = clock.now
+
+      // Insert many items that all share this prefix
+      for i in 0..<itemCount {
+        var path = sharedPrefix
+        path.append("Argument \(i)")
+        graph[path] = i
+      }
+
+      #expect(graph[sharedPrefix + ["Argument 0"]] == 0)
+      #expect(graph[sharedPrefix + ["Argument \(itemCount - 1)"]] == itemCount - 1)
+
+      let elapsedTime = startTime.duration(to: clock.now) / .seconds(1)
+      perItemTimes[itemCount] = elapsedTime / Double(itemCount)
+    }
+
+    let mean = perItemTimes.values.reduce(0, +) / Double(perItemTimes.count)
+    let variance = perItemTimes.values.reduce(0) { $0 + ($1 - mean) * ($1 - mean) }
+    let standardDeviation = (variance / (Double(perItemTimes.count) - 1)).squareRoot()
+
+    // Standard deviation should be less than 25% of mean
+
+    let description = perItemTimes
+      .sorted { $0.key < $1.key }
+      .map { "\($0)\t\($1)" }
+      .joined(separator: "\n")
+
+    #expect(standardDeviation < mean * 0.25, "Expected standard deviation to be <25% of mean, but was \(standardDeviation) (mean: \(mean))\n\(description)")
+  }
 }