[cxx-interop] Add std.string.withUTF8 method

This new overlay method is a safe alternative to `std.string.__c_strUnsafe()`.

rdar://112765666

Author: egorzhdan

stdlib/public/Cxx/std/String.swift

@@ -168,13 +168,9 @@ extension String {
   ///
   /// - Complexity: O(*n*), where *n* is the number of bytes in the C++ string.
   public init(_ cxxString: std.string) {
-    let buffer = UnsafeBufferPointer<CChar>(
-      start: cxxString.__c_strUnsafe(),
-      count: cxxString.size())
-    self = buffer.withMemoryRebound(to: UInt8.self) {
+    self = cxxString.withUTF8 {
       String(decoding: $0, as: UTF8.self)
     }
-    withExtendedLifetime(cxxString) {}
   }
 
   /// Creates a String having the same content as the given C++ UTF-16 string.
@@ -193,3 +189,17 @@ extension String {
     withExtendedLifetime(cxxU16String) {}
   }
 }
+
+// MARK: Converting a C++ string to a C string
+
+extension std.string {
+  @inlinable
+  public borrowing func withUTF8<Result>(
+    _ body: (UnsafeBufferPointer<UInt8>) throws -> Result
+  ) rethrows -> Result {
+    let buffer = UnsafeBufferPointer<CChar>(
+      start: self.__c_strUnsafe(),
+      count: self.size())
+    return try buffer.withMemoryRebound(to: UInt8.self, body)
+  }
+}

test/Interop/Cxx/stdlib/overlay/std-string-overlay.swift

@@ -256,4 +256,28 @@ StdStringOverlayTestSuite.test("std::string from C string") {
   expectEqual(str, std.string("abc"))
 }
 
+StdStringOverlayTestSuite.test("std::string to UTF-8") {
+  std.string().withUTF8 { ptr in
+    expectEqual(ptr.count, 0)
+  }
+  std.string("abc").withUTF8 { ptr in
+    expectEqual(ptr.count, 3)
+    expectEqual(ptr.baseAddress?.pointee, 97)
+    expectEqual(ptr.baseAddress?.successor().pointee, 98)
+    expectEqual(ptr.baseAddress?.successor().successor().pointee, 99)
+  }
+
+  let bytes: [UInt8] = [0xE1, 0xC1, 0xAC]
+  var str = std.string()
+  for byte in bytes {
+    str.push_back(CChar(bitPattern: byte))
+  }
+  str.withUTF8 { ptr in
+    expectEqual(ptr.count, 3)
+    expectEqual(ptr.baseAddress?.pointee, 0xE1)
+    expectEqual(ptr.baseAddress?.successor().pointee, 0xC1)
+    expectEqual(ptr.baseAddress?.successor().successor().pointee, 0xAC)
+  }
+}
+
 runAllTests()