# Xo 0.23 release notes


## Introduction

Xo 0.23 makes Xo's own backend the default on x86-64 hosts, keeps build caches across compiler rebuilds and shares them
across programs, runs `--race` on arm64 with no clang, cuts HTTP tail
latency under CPU-bound handlers from about 86 ms to about 3 ms at p99,
and adds the first batch of a standard library modeled on Go's.

## Changes to the language

- Field wire names, `field as "key": T`, and case rules for derived
  encodings
  ([decision 0277](https://github.com/xo-lang/xo/blob/main/knowledgebase/decisions/0277-field-wire-names.md)).
- A newtype encodes to JSON as its underlying value on every backend
  ([decision 0097](https://github.com/xo-lang/xo/blob/main/knowledgebase/decisions/0097-newtype-value.md)).
- An interface value holding a handle or a function equals nothing.
- `ServerConn` for accepted connections
  ([decision 0280](https://github.com/xo-lang/xo/blob/main/knowledgebase/decisions/0280-connection-effects-by-origin.md)).

## Tools

- `--race` on the arm64 backend: Xo's own linker loads a TSan runtime
  shipped with Xo, so no clang is needed
  ([decision 0273](https://github.com/xo-lang/xo/blob/main/knowledgebase/decisions/0273-native-race-prebuilt-tsan.md)).
- One name resolver behind `xo def`, `refs`, `doc`, `symbols` and
  `query`, covering std too, with text output at a terminal; `xo vet`
  runs on std
  ([decision 0289](https://github.com/xo-lang/xo/blob/main/knowledgebase/decisions/0289-agent-tools-names-and-output.md)).
- x86-64 native binaries carry DWARF line tables and variables on Linux
  and macOS. `XO_TRACEBACK=locals` shows the faulting frame's values
  and inlined callees' variables.

## Compiler

- Native is the default backend on x86-64 hosts too (macOS, Linux and
  Windows): `xo build` uses Xo's own x86-64 backend, with no C
  toolchain, and `--backend=llvm` still selects LLVM
  ([decision 0270](https://github.com/xo-lang/xo/blob/main/knowledgebase/decisions/0270-amd64-native-default.md)).
- Build caches are keyed by the code generation sources instead of
  `xo`'s binary, so a rebuilt `xo` keeps them: rebuilding `xo` after a
  driver-only change takes about 41% of a cold build
  ([decision 0275](https://github.com/xo-lang/xo/blob/main/knowledgebase/decisions/0275-cache-keys-by-codegen-inputs.md)).
  Standard library function bodies are shared across programs
  ([decision 0271](https://github.com/xo-lang/xo/blob/main/knowledgebase/decisions/0271-parallel-module-front-end.md)).
- x86-64: immediates, load sinking and branch on compare (RSA signing
  6% fewer instructions, other compute 2 to 8%).

## Runtime

- A parked root task hands its next task to an idle worker, so an
  accept loop is no longer stuck behind a running handler. With 8
  CPU-bound handlers on Linux arm64, p99 from dial to response went
  from 86 ms to 2.4 to 3.1 ms (Go: 7.5 to 12.5 ms)
  ([decision 0286](https://github.com/xo-lang/xo/blob/main/knowledgebase/decisions/0286-scheduler-stealing-preemption.md)).

## Standard library

The standard library now follows Go's for scope, code and tests, with
Xo's own API
([decision 0276](https://github.com/xo-lang/xo/blob/main/knowledgebase/decisions/0276-std-follows-go-std.md)).
New in this version:

- `std/url`, base64, base32, binary encodings, and hashes with HMAC
  ([decision 0279](https://github.com/xo-lang/xo/blob/main/knowledgebase/decisions/0279-std-encodings-url-hashes.md)).
- MIME, multipart, UUID and mail
  ([decision 0282](https://github.com/xo-lang/xo/blob/main/knowledgebase/decisions/0282-mime-multipart-uuid-mail.md)).
- Public key signatures and keys
  ([decision 0285](https://github.com/xo-lang/xo/blob/main/knowledgebase/decisions/0285-crypto-public-key-signatures.md)).
- Number parsing in any base, Go's math results bit for bit on every
  backend, and `std/netip`
  ([decision 0287](https://github.com/xo-lang/xo/blob/main/knowledgebase/decisions/0287-strconv-math-netip.md)).
- A TLS client on an existing connection
  ([decision 0281](https://github.com/xo-lang/xo/blob/main/knowledgebase/decisions/0281-tls-client-on-a-conn.md)).
- The `std/sql` driver interface.
- A change log per module in
  [std/changes](https://github.com/xo-lang/xo/tree/main/std/changes).