Make AutoCloseFd stringifiable (broken by operator bool deletion).

c++/src/kj/io-test.c++

@@ -54,5 +54,13 @@ TEST(Io, WriteVec) {
   EXPECT_STREQ("foobar", buf);
 }
 
+KJ_TEST("stringify AutoCloseFd") {
+  int fds[2];
+  miniposix::pipe(fds);
+  AutoCloseFd in(fds[0]), out(fds[1]);
+
+  KJ_EXPECT(kj::str(in) == kj::str(fds[0]), in, fds[0]);
+}
+
 }  // namespace
 }  // namespace kj

c++/src/kj/io.h

@@ -269,6 +269,11 @@ private:
   UnwindDetector unwindDetector;
 };
 
+inline auto KJ_STRINGIFY(const AutoCloseFd& fd)
+    -> decltype(kj::toCharSequence(implicitCast<int>(fd))) {
+  return kj::toCharSequence(implicitCast<int>(fd));
+}
+
 class FdInputStream: public InputStream {
   // An InputStream wrapping a file descriptor.