Guard against testing with older stdlibs

Replace a couple of `#if os(Linux)` checks with
a check to see if we have a newer stdlib
available. This lets us emit an expected failure
in the case where we're testing on an older
stdlib.

Author: hamishknight

Package.swift

@@ -75,15 +75,17 @@ let package = Package(
             name: "RegexBuilder",
             dependencies: ["_StringProcessing", "_RegexParser"],
             swiftSettings: publicStdlibSettings),
+        .target(name: "TestSupport",
+                swiftSettings: [availabilityDefinition]),
         .testTarget(
             name: "RegexTests",
-            dependencies: ["_StringProcessing"],
+            dependencies: ["_StringProcessing", "TestSupport"],
             swiftSettings: [
                 .unsafeFlags(["-Xfrontend", "-disable-availability-checking"]),
             ]),
         .testTarget(
             name: "RegexBuilderTests",
-            dependencies: ["_StringProcessing", "RegexBuilder"],
+            dependencies: ["_StringProcessing", "RegexBuilder", "TestSupport"],
             swiftSettings: [
                 .unsafeFlags(["-Xfrontend", "-disable-availability-checking"])
             ]),

Sources/TestSupport/TestSupport.swift

@@ -0,0 +1,27 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2022 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+//
+//===----------------------------------------------------------------------===//
+
+import XCTest
+
+// We need to split this out of the test files, as it needs to be compiled
+// *without* `-disable-availability-checking` to ensure the #available check is
+// not compiled into a no-op.
+
+/// Guards certain tests to make sure we have a new stdlib available.
+public func ensureNewStdlib(
+  file: StaticString = #file, line: UInt = #line
+) -> Bool {
+  guard #available(SwiftStdlib 5.7, *) else {
+    XCTExpectFailure { XCTFail("Unsupported stdlib", file: file, line: line) }
+    return false
+  }
+  return true
+}

Tests/RegexTests/MatchTests.swift

@@ -12,6 +12,7 @@
 import XCTest
 @testable import _RegexParser
 @testable import _StringProcessing
+import TestSupport
 
 struct MatchError: Error {
   var message: String
@@ -1003,6 +1004,9 @@ extension RegexTests {
   }
 
   func testMatchAnchors() throws {
+    // Must have new stdlib for character class ranges and word boundaries.
+    guard ensureNewStdlib() else { return }
+
     // MARK: Anchors
     firstMatchTests(
       #"^\d+"#,
@@ -1051,8 +1055,6 @@ extension RegexTests {
       (" 123\n456\n", nil),
       ("123 456", "456"))
 
-    // FIXME: Keep this until _wordIndex and friends are
-#if os(Linux)
     firstMatchTests(
       #"\d+\b"#,
       ("123", "123"),
@@ -1070,7 +1072,6 @@ extension RegexTests {
       ("123", "23"),
       (" 123", "23"),
       ("123 456", "23"))
-#endif
 
     // TODO: \G and \K
     do {
@@ -1101,9 +1102,10 @@ extension RegexTests {
       ("Sol Cafe", nil), xfail: true)
   }
 
-  // FIXME: Keep this until _wordIndex and friends are
-#if os(Linux)
   func testLevel2WordBoundaries() {
+    // Must have new stdlib for character class ranges and word boundaries.
+    guard ensureNewStdlib() else { return }
+
     // MARK: Level 2 Word Boundaries
     firstMatchTest(#"\b😊\b"#, input: "🔥😊👍", match: "😊")
     firstMatchTest(#"\b👨🏽\b"#, input: "👩🏻👶🏿👨🏽🧑🏾👩🏼", match: "👨🏽")
@@ -1119,8 +1121,7 @@ extension RegexTests {
     firstMatchTest(#"can\B\'\Bt"#, input: "I can't do that.", match: "can't")
     firstMatchTest(#"\b÷\b"#, input: "3 ÷ 3 = 1", match: "÷")
   }
-#endif
-  
+
   func testMatchGroups() {
     // MARK: Groups
 
@@ -1345,6 +1346,9 @@ extension RegexTests {
   }
 
   func testMatchExamples() {
+    // Must have new stdlib for character class ranges and word boundaries.
+    guard ensureNewStdlib() else { return }
+
     // Backreferences
     matchTest(
       #"(sens|respons)e and \1ibility"#,
@@ -1394,8 +1398,6 @@ extension RegexTests {
       xfail: true
     )
 
-    // FIXME: Keep this until _wordIndex and friends are
-#if os(Linux)
     // HTML tags
     matchTest(
       #"<([a-zA-Z][a-zA-Z0-9]*)\b[^>]*>.*?</\1>"#,
@@ -1413,7 +1415,6 @@ extension RegexTests {
       ("pass me the the kettle", ["the"]),
       ("this doesn't have any", nil)
     )
-#endif
 
     // Floats
     flatCaptureTest(
@@ -1493,6 +1494,9 @@ extension RegexTests {
   }
 
   func testASCIIClasses() {
+    // Must have new stdlib for character class ranges and word boundaries.
+    guard ensureNewStdlib() else { return }
+
     // 'D' ASCII-only digits
     matchTest(
       #"\d+"#,
@@ -1521,8 +1525,6 @@ extension RegexTests {
       ("aeiou", true),
       ("åe\u{301}ïôú", false))
 
-    // FIXME: Keep this until _wordIndex and friends are
-#if os(Linux)
     matchTest(
       #"abcd\b.+"#,
       ("abcd ef", true),
@@ -1538,7 +1540,6 @@ extension RegexTests {
       ("abcd ef", true),
       ("abcdef", false),
       ("abcdéf", false))
-#endif
 
     // 'S' ASCII-only spaces
     matchTest(