[test] stdlib/StringTraps: Add some coverage for foreign traps in UTF-8/16 views

test/stdlib/StringTraps.swift

@@ -10,6 +10,9 @@
 // UNSUPPORTED: OS=wasi
 
 import StdlibUnittest
+#if _runtime(_ObjC)
+import Foundation // For NSString
+#endif
 
 let testSuiteSuffix = _isDebugAssertConfiguration() ? "_debug" : "_release"
 
@@ -131,7 +134,7 @@ StringTraps.test("UTF16ViewSubscript/endIndex")
     { _isFastAssertConfiguration() },
     reason: "this trap is not guaranteed to happen in -Ounchecked"))
   .code {
-  var s = "abc"
+  let s = "abc"
   var i = s.utf16.startIndex
   i = s.utf16.index(after: i)
   i = s.utf16.index(after: i)
@@ -260,5 +263,95 @@ StringTraps.test("UnicodeScalarView index(after:) trap on i > endIndex")
   i = s.unicodeScalars.index(after: i)
 }
 
+StringTraps.test("UnicodeScalarView index(before:) trap on i > endIndex")
+.skip(
+  .custom({ _isFastAssertConfiguration() },
+  reason: "trap is not guaranteed to happen in -Ounchecked"))
+.code {
+  guard #available(SwiftStdlib 5.7, *) else { return }
+
+  let long = "abcd"
+  var i = long.unicodeScalars.endIndex
+
+  let s = "abc"
+  expectCrashLater()
+  i = s.unicodeScalars.index(before: i)
+}
+
+#if _runtime(_ObjC)
+StringTraps.test("UTF8View foreign index(after:) trap on i > endIndex")
+.skip(
+  .custom({ _isFastAssertConfiguration() },
+  reason: "trap is not guaranteed to happen in -Ounchecked"))
+.code {
+  guard #available(SwiftStdlib 5.7, *) else { return }
+
+  let long = "🐘 This is a quite large string, with lots of data"
+  let short = ("🐭 I'm much smaller" as NSString) as String
+
+  var i = long.utf8.endIndex
+  expectCrashLater()
+  // Note: we expect that `short` will be UTF-16 encoded here -- this trap only
+  // happens on the foreign path. For native/shared strings, the UTF-8 view's
+  // `index(after:)` is essentially doing a simple `i + 1`, like Array does.
+  i = short.utf8.index(after: i)
+}
+#endif
+
+#if _runtime(_ObjC)
+StringTraps.test("UTF8View foreign index(before:) trap on i > endIndex")
+.skip(
+  .custom({ _isFastAssertConfiguration() },
+  reason: "trap is not guaranteed to happen in -Ounchecked"))
+.code {
+  guard #available(SwiftStdlib 5.7, *) else { return }
+
+  let long = "🐘 This is a quite large string, with lots of data"
+  let short = ("🐭 I'm much smaller" as NSString) as String
+
+  var i = long.utf8.endIndex
+  expectCrashLater()
+  // Note: we expect that `short` will be UTF-16 encoded here -- this trap only
+  // happens on the foreign path. For native/shared strings, the UTF-8 view's
+  // `index(before:)` is essentially doing a simple `i - 1`, like Array does.
+  // (Following the unconditional i != startIndex check.)
+  i = short.utf8.index(before: i)
+}
+#endif
+
+#if _runtime(_ObjC)
+StringTraps.test("UTF8View foreign index(after:) trap on i == endIndex")
+.skip(
+  .custom({ _isFastAssertConfiguration() },
+  reason: "trap is not guaranteed to happen in -Ounchecked"))
+.code {
+  guard #available(SwiftStdlib 5.7, *) else { return }
+
+  let s = ("🦧 The Librarian" as NSString) as String
+
+  var i = s.utf8.endIndex
+  expectCrashLater()
+  // Note: we expect that `short` will be UTF-16 encoded here -- this trap only
+  // happens on the foreign path. For native/shared strings, the UTF-8 view's
+  // `index(after:)` is essentially doing a simple `i + 1`, like Array does.
+  i = s.utf8.index(after: i)
+}
+#endif
+
+#if _runtime(_ObjC)
+StringTraps.test("UTF8View foreign index(before:) trap on i == startIndex")
+.skip(
+  .custom({ _isFastAssertConfiguration() },
+  reason: "trap is not guaranteed to happen in -Ounchecked"))
+.code {
+  guard #available(SwiftStdlib 5.7, *) else { return }
+
+  let s = ("🦧 The Librarian" as NSString) as String
+  var i = s.utf8.startIndex
+  expectCrashLater()
+  i = s.utf8.index(before: i)
+}
+#endif
+
 runAllTests()