Module io/index

Async I/O traits — the D5 redesign (plans/archive/STD_API_AUDIT.md §D5).

Reader and Writer are the async byte-stream interfaces every I/O source/sink implements: File (std/fs), TcpStream (std/net), and the Stdin/Stdout/Stderr handles in std/io/stdio. All methods run on the single-threaded async event loop and report failure by throwing IoExn (D1 style 1 — everything on the io path throws).

The raw read/write take a caller buffer as *(u8) + length — the same shape TcpStream.read and the sys layer already speak. Raw pointers are not available in safe code, so IMPLEMENTING or calling the raw methods directly needs pragma(Pragma.AllowUnsafe);; safe code uses the pointer-free defaults (read_to_end, read_to_string) and the per-type conveniences on each concrete type.

The generic BufReader(R) / BufWriter(W) wrappers landed in ./bufio.yo, and the std/sys/bufio → std/io move COMPLETED 2026-08-28 (fd-based pair deleted; the compiler's three stdin consumers read through BufReader(Stdin)).

The Dyn(Reader) spelling works too (C17 fixed 2026-08-29; pinned in tests/io/async_traits.test.yo): a Dyn(Reader) dispatches read and the defaults through its vtable and can be the R of BufReader(R). Concrete types carry NO duplicate spellings of the trait operations — File/TcpStream read-everything is read_to_end/read_to_string, the trait defaults (D2: one Rust-shaped name per operation); their write_string/write_bytes conveniences stay because the trait has no counterpart.

Seek(From) joins them as of 2026-09-09, and is the one trait here that is SYNCHRONOUS — moving a position is arithmetic on a handle's own state, not I/O. Implemented by File; see the trait's own doc for why it takes the reference-point type as a parameter and has no rewind.

STILL TO COME: a buffered lines(). The async iterator protocol it was waiting on landed 2026-09-11 (std/async/stream.yo), so the remaining blocker is this trait's own error style: a Stream reports failure in its ITEM and read/read_line THROW, which cannot be caught inside an async body. See plans/backlog/ASYNC_LINES_NEEDS_A_NONTHROWING_READ.md for the try_read proposal.

Stability

unstable — the traits' core signature is the open question. read and write take a caller buffer as *(u8) + length because Yo has no slice type; if one arrives, that parameter shape is what it would replace, and that is a breaking change to every implementor (File, TcpStream, UnixStream, TlsStream, the stdio handles, BufReader/BufWriter, Child's pipe handles) at once. It is also why safe code cannot implement these traits at all today, only consume them through the pointer-free defaults. Two smaller items ride along: read_exact is a BufReader method rather than the Reader default Rust has, and lines() waits on the non-throwing read named above. Seek(From) is newer still — 2026-09-09 — and inside its one-release window. copy, and the read_to_end/read_to_string/write_all defaults, are expected to keep their names and meanings.