diff --git a/Package.swift b/Package.swift
index 8bf5470..7a90b1b 100644
--- a/Package.swift
+++ b/Package.swift
@@ -19,7 +19,7 @@ let package = Package(
     )
   ],
   dependencies: [
-    .package(url: "https://github.com/swiftlang/swift-syntax", "509.0.0"..<"511.0.0")
+    .package(url: "https://github.com/swiftlang/swift-syntax", "509.0.0"..<"601.0.0-prerelease")
   ],
   targets: [
     .macro(
diff --git a/Tests/SpyableMacroTests/Assertions/AssertBuildResult.swift b/Tests/SpyableMacroTests/Assertions/AssertBuildResult.swift
index 43e4013..8979c8f 100644
--- a/Tests/SpyableMacroTests/Assertions/AssertBuildResult.swift
+++ b/Tests/SpyableMacroTests/Assertions/AssertBuildResult.swift
@@ -1,10 +1,7 @@
 import SwiftSyntax
 import SwiftSyntaxMacrosTestSupport
-import _SwiftSyntaxTestSupport
+import XCTest
 
-/*
- Source: https://github.com/apple/swift-syntax/blob/4db7cb20e63f4dba0b030068687996723e352874/Tests/SwiftSyntaxBuilderTest/Assertions.swift#L19
- */
 func assertBuildResult<T: SyntaxProtocol>(
   _ buildable: T,
   _ expectedResult: String,
@@ -25,3 +22,104 @@ func assertBuildResult<T: SyntaxProtocol>(
     line: line
   )
 }
+
+/*
+ Source: https://github.com/apple/swift-syntax/blob/4db7cb20e63f4dba0b030068687996723e352874/Tests/SwiftSyntaxBuilderTest/Assertions.swift#L19
+ */
+
+/// Asserts that the two strings are equal, providing Unix `diff`-style output if they are not.
+///
+/// - Parameters:
+///   - actual: The actual string.
+///   - expected: The expected string.
+///   - message: An optional description of the failure.
+///   - additionalInfo: Additional information about the failed test case that will be printed after the diff
+///   - file: The file in which failure occurred. Defaults to the file name of the test case in
+///     which this function was called.
+///   - line: The line number on which failure occurred. Defaults to the line number on which this
+///     function was called.
+private func assertStringsEqualWithDiff(
+  _ actual: String,
+  _ expected: String,
+  _ message: String = "",
+  additionalInfo: @autoclosure () -> String? = nil,
+  file: StaticString = #file,
+  line: UInt = #line
+) {
+  if actual == expected {
+    return
+  }
+  failStringsEqualWithDiff(
+    actual,
+    expected,
+    message,
+    additionalInfo: additionalInfo(),
+    file: file,
+    line: line
+  )
+}
+
+/// `XCTFail` with `diff`-style output.
+private func failStringsEqualWithDiff(
+  _ actual: String,
+  _ expected: String,
+  _ message: String = "",
+  additionalInfo: @autoclosure () -> String? = nil,
+  file: StaticString = #file,
+  line: UInt = #line
+) {
+  // Use `CollectionDifference` on supported platforms to get `diff`-like line-based output. On
+  // older platforms, fall back to simple string comparison.
+  if #available(macOS 10.15, *) {
+    let actualLines = actual.components(separatedBy: .newlines)
+    let expectedLines = expected.components(separatedBy: .newlines)
+
+    let difference = actualLines.difference(from: expectedLines)
+
+    var result = ""
+
+    var insertions = [Int: String]()
+    var removals = [Int: String]()
+
+    for change in difference {
+      switch change {
+      case .insert(let offset, let element, _):
+        insertions[offset] = element
+      case .remove(let offset, let element, _):
+        removals[offset] = element
+      }
+    }
+
+    var expectedLine = 0
+    var actualLine = 0
+
+    while expectedLine < expectedLines.count || actualLine < actualLines.count {
+      if let removal = removals[expectedLine] {
+        result += "–\(removal)\n"
+        expectedLine += 1
+      } else if let insertion = insertions[actualLine] {
+        result += "+\(insertion)\n"
+        actualLine += 1
+      } else {
+        result += " \(expectedLines[expectedLine])\n"
+        expectedLine += 1
+        actualLine += 1
+      }
+    }
+
+    let failureMessage = "Actual output (+) differed from expected output (-):\n\(result)"
+    var fullMessage = message.isEmpty ? failureMessage : "\(message) - \(failureMessage)"
+    if let additionalInfo = additionalInfo() {
+      fullMessage = """
+        \(fullMessage)
+        \(additionalInfo)
+        """
+    }
+    XCTFail(fullMessage, file: file, line: line)
+  } else {
+    // Fall back to simple message on platforms that don't support CollectionDifference.
+    let failureMessage = "Actual output differed from expected output:"
+    let fullMessage = message.isEmpty ? failureMessage : "\(message) - \(failureMessage)"
+    XCTFail(fullMessage, file: file, line: line)
+  }
+}