[mivinci]

Summary

• New xpp::io::Error in libx++/xpp/io/error.{h,cpp} modeled after std::io::Error. Single uintptr_t of storage, four variants packed into the two low tag bits:
  • SimpleMessage — pointer to a static (kind, msg), zero allocation
  • Custom — heap-allocated (kind, len, char[]), one allocation
  • Os — i32 errno + kind() derived via errno_to_kind
  • Simple — bare ErrorKind
• 26-variant ErrorKind mirrors std::io::ErrorKind plus ResolveFailed / NoAddress.
• Option<io::Error> niche specialization keeps it at one pointer.
• Drop the old SocketError enum. Net failure paths now carry the underlying errno (so callers can distinguish ECONNREFUSED from EHOSTUNREACH, which the old enum lumped together).

Why

SocketError's 8 categories collapsed every connect / bind / accept failure into one of CreateFailed/BindFailed/ConnectFailed. Tokio uses std::io::Error everywhere because that single type carries the OS errno, the cross-platform kind, and an optional message all in 8 bytes — debuggability and unification in one shot.

Static guarantees

static_assert(sizeof(io::Error) == sizeof(void *));            // 8 bytes
static_assert(sizeof(Option<io::Error>) == sizeof(void *));    // niche preserved
static_assert(!std::is_copy_constructible<io::Error>::value);  // move-only

Allocation table

Constructor	Allocs
io::Error(ErrorKind) (implicit)	0
io::Error::from_errno(int)	0
io::Error::from_static(SimpleMessage*)	0
io::Error::with_message(Kind, str)	1

Test plan

• build/xpp_test: 671 pass (was 649; +22 IoErrorTest).
• build-cxx20/xpp_test: 702 pass.
• build-cxx20/xpp_coro_test: 31 pass.
• New IoErrorTest (22 cases): size invariants, every variant constructor, errno mapping (POSIX subset), Option niche, move semantics, to_string golden output, long messages, edge cases (from_errno(0), negative, null message).
• Existing TCP/UDP/DNS tests adapted — EXPECT_EQ(err, SocketError::X) → EXPECT_EQ(err.kind(), io::ErrorKind::Y).

Notable behavior changes

• TcpListener::bind, TcpStream::connect, UdpSocket::bind/connect, accept, set_nodelay/nodelay now return Result<…, io::Error> instead of Result<…, SocketError>.
• Errno-bearing failures (bind, connect, accept) carry the original code: err.raw_os_error() returns Some(EADDRINUSE), err.kind() is ErrorKind::AddrInUse.
• socket.h's SocketError enum and socket_error_message() helper are deleted.

🤖 Generated with Claude Code