Go 1.23 Release Notes

Introduction to Go 1.23

The latest Go release, version 1.23, arrives six months after Go 1.22. Most of its changes are in the implementation of the toolchain, runtime, and libraries. As always, the release maintains the Go 1 promise of compatibility. We expect almost all Go programs to continue to compile and run as before.

Changes to the language

The “range” clause in a “for-range” loop now accepts iterator functions of the following types

func(func() bool)
func(func(K) bool)
func(func(K, V) bool)

as range expressions. Calls of the iterator argument function produce the iteration values for the “for-range” loop. For details see the iter package documentation, the language spec, and the Range over Function Types blog post. For motivation see the 2022 “range-over-func” discussion.

Go 1.23 includes preview support for generic type aliases. Building the toolchain with GOEXPERIMENT=aliastypeparams enables this feature within a package. (Using generic alias types across package boundaries is not yet supported.)

Tools

Telemetry

Starting in Go 1.23, the Go toolchain can collect usage and breakage statistics that help the Go team understand how the Go toolchain is used and how well it is working. We refer to these statistics as Go telemetry.

Go telemetry is an opt-in system, controlled by the go telemetry command. By default, the toolchain programs collect statistics in counter files that can be inspected locally but are otherwise unused (go telemetry local).

To help us keep Go working well and understand Go usage, please consider opting in to Go telemetry by running go telemetry on. In that mode, anonymous counter reports are uploaded to telemetry.go.dev weekly, where they are aggregated into graphs and also made available for download by any Go contributors or users wanting to analyze the data. See “Go Telemetry” for more details about the Go Telemetry system.

Go command

Setting the GOROOT_FINAL environment variable no longer has an effect (#62047). Distributions that install the go command to a location other than $GOROOT/bin/go should install a symlink instead of relocating or copying the go binary.

The new go env -changed flag causes the command to print only those settings whose effective value differs from the default value that would be obtained in an empty environment with no prior uses of the -w flag.

The new go mod tidy -diff flag causes the command not to modify the files but instead print the necessary changes as a unified diff. It exits with a non-zero code if updates are needed.

The go list -m -json command now includes new Sum and GoModSum fields. This is similar to the existing behavior of the go mod download -json command.

The new godebug directive in go.mod and go.work declares a GODEBUG setting to apply for the work module or workspace in use.

Vet

The go vet subcommand now includes the stdversion analyzer, which flags references to symbols that are too new for the version of Go in effect in the referring file. (The effective version is determined by the go directive in the file’s enclosing go.mod file, and by any //go:build constraints in the file.)

For example, it will report a diagnostic for a reference to the reflect.TypeFor function (introduced in go1.22) from a file in a module whose go.mod file specifies go 1.21.

Cgo

cmd/cgo supports the new -ldflags flag for passing flags to the C linker. The go command uses it automatically, avoiding “argument list too long” errors with a very large CGO_LDFLAGS.

Trace

The trace tool now better tolerates partially broken traces by attempting to recover what trace data it can. This functionality is particularly helpful when viewing a trace that was collected during a program crash, since the trace data leading up to the crash will now be recoverable under most circumstances.

Runtime

The traceback printed by the runtime after an unhandled panic or other fatal error now indents the second and subsequent lines of the error message (for example, the argument to panic) by a single tab, so that it can be unambiguously distinguished from the stack trace of the first goroutine. See #64590 for discussion.

Compiler

The build time overhead to building with Profile Guided Optimization has been reduced significantly. Previously, large builds could see 100%+ build time increase from enabling PGO. In Go 1.23, overhead should be in the single digit percentages.

The compiler in Go 1.23 can now overlap the stack frame slots of local variables accessed in disjoint regions of a function, which reduces stack usage for Go applications.

For 386 and amd64, the compiler will use information from PGO to align certain hot blocks in loops. This improves performance an additional 1-1.5% at a cost of an additional 0.1% text and binary size. This is currently only implemented on 386 and amd64 because it has not shown an improvement on other platforms. Hot block alignment can be disabled with -gcflags=[<packages>=]-d=alignhot=0.

Linker

The linker now disallows using a //go:linkname directive to refer to internal symbols in the standard library (including the runtime) that are not marked with //go:linkname on their definitions. Similarly, the linker disallows references to such symbols from assembly code. For backward compatibility, existing usages of //go:linkname found in a large open-source code corpus remain supported. Any new references to standard library internal symbols will be disallowed.

A linker command line flag -checklinkname=0 can be used to disable this check, for debugging and experimenting purposes.

When building a dynamically linked ELF binary (including PIE binary), the new -bindnow flag enables immediate function binding.

Standard library

Timer changes

Go 1.23 makes two significant changes to the implementation of time.Timer and time.Ticker.

First, Timers and Tickers that are no longer referred to by the program become eligible for garbage collection immediately, even if their Stop methods have not been called. Earlier versions of Go did not collect unstopped Timers until after they had fired and never collected unstopped Tickers.

Second, the timer channel associated with a Timer or Ticker is now unbuffered, with capacity 0. The main effect of this change is that Go now guarantees that for any call to a Reset or Stop method, no stale values prepared before that call will be sent or received after the call. Earlier versions of Go used channels with a one-element buffer, making it difficult to use Reset and Stop correctly. A visible effect of this change is that len and cap of timer channels now returns 0 instead of 1, which may affect programs that poll the length to decide whether a receive on the timer channel will succeed. Such code should use a non-blocking receive instead.

These new behaviors are only enabled when the main Go program is in a module with a go.mod go line using Go 1.23.0 or later. When Go 1.23 builds older programs, the old behaviors remain in effect. The new GODEBUG setting asynctimerchan=1 can be used to revert back to asynchronous channel behaviors even when a program names Go 1.23.0 or later in its go.mod file.

New unique package

The new unique package provides facilities for canonicalizing values (like “interning” or “hash-consing”).

Any value of comparable type may be canonicalized with the new Make[T] function, which produces a reference to a canonical copy of the value in the form of a Handle[T]. Two Handle[T] are equal if and only if the values used to produce the handles are equal, allowing programs to deduplicate values and reduce their memory footprint. Comparing two Handle[T] values is efficient, reducing down to a simple pointer comparison.

Iterators

The new iter package provides the basic definitions for working with user-defined iterators.

The slices package adds several functions that work with iterators:

All returns an iterator over slice indexes and values.
Values returns an iterator over slice elements.
Backward returns an iterator that loops over a slice backward.
Collect collects values from an iterator into a new slice.
AppendSeq appends values from an iterator to an existing slice.
Sorted collects values from an iterator into a new slice, and then sorts the slice.
SortedFunc is like Sorted but with a comparison function.
SortedStableFunc is like SortFunc but uses a stable sort algorithm.
Chunk returns an iterator over consecutive sub-slices of up to n elements of a slice.

The maps package adds several functions that work with iterators:

All returns an iterator over key-value pairs from a map.
Keys returns an iterator over keys in a map.
Values returns an iterator over values in a map.
Insert adds the key-value pairs from an iterator to an existing map.
Collect collects key-value pairs from an iterator into a new map and returns it.

New structs package

The new structs package provides types for struct fields that modify properties of the containing struct type such as memory layout.

In this release, the only such type is HostLayout which indicates that a structure with a field of that type has a layout that conforms to host platform expectations. HostLayout should be used in types that are passed to, returned from, or accessed via a pointer passed to/from host APIs. Without this marker, struct layout order is not guaranteed by the language spec, though as of Go 1.23 the host and language layouts happen to match.

Minor changes to the library

`archive/tar`

If the argument to FileInfoHeader implements the new FileInfoNames interface, then the interface methods will be used to set the Uname/Gname of the file header. This allows applications to override the system-dependent Uname/Gname lookup.

`crypto/tls`

The TLS client now supports the Encrypted Client Hello draft specification. This feature can be enabled by setting the Config.EncryptedClientHelloConfigList field to an encoded ECHConfigList for the host that is being connected to.

The QUICConn type used by QUIC implementations includes new events reporting on the state of session resumption, and provides a way for the QUIC layer to add data to session tickets and session cache entries.

3DES cipher suites were removed from the default list used when Config.CipherSuites is nil. The default can be reverted by adding tls3des=1 to the GODEBUG environment variable.

The experimental post-quantum key exchange mechanism X25519Kyber768Draft00 is now enabled by default when Config.CurvePreferences is nil. The default can be reverted by adding tlskyber=0 to the GODEBUG environment variable. This can be useful when dealing with buggy TLS servers that do not handle large records correctly, causing a timeout during the handshake (see TLS post-quantum TL;DR fail).

Go 1.23 changed the behavior of X509KeyPair and LoadX509KeyPair to populate the Certificate.Leaf field of the returned Certificate. The new x509keypairleaf GODEBUG setting is added for this behavior.

`crypto/x509`

CreateCertificateRequest now correctly supports RSA-PSS signature algorithms.

CreateCertificateRequest and CreateRevocationList now verify the generated signature using the signer’s public key. If the signature is invalid, an error is returned. This has been the behavior of CreateCertificate since Go 1.16.

The x509sha1 GODEBUG setting will be removed in the next Go major release (Go 1.24). This will mean that crypto/x509 will no longer support verifying signatures on certificates that use SHA-1 based signature algorithms.

The new ParseOID function parses a dot-encoded ASN.1 Object Identifier string. The OID type now implements the encoding.BinaryMarshaler, encoding.BinaryUnmarshaler, encoding.TextMarshaler, encoding.TextUnmarshaler interfaces.

`database/sql`

Errors returned by driver.Valuer implementations are now wrapped for improved error handling during operations like DB.Query, DB.Exec, and DB.QueryRow.

`debug/elf`

The debug/elf package now defines PT_OPENBSD_NOBTCFI. This ProgType is used to disable Branch Tracking Control Flow Integrity (BTCFI) enforcement on OpenBSD binaries.

Now defines the symbol type constants STT_RELC, STT_SRELC, and STT_GNU_IFUNC.

`encoding/binary`

The new Encode and Decode functions are byte slice equivalents to Read and Write. Append allows marshaling multiple data into the same byte slice.

`go/ast`

The new Preorder function returns a convenient iterator over all the nodes of a syntax tree.

`go/types`

The Func type, which represents a function or method symbol, now has a Func.Signature method that returns the function’s type, which is always a Signature.

The Alias type now has an Rhs method that returns the type on the right-hand side of its declaration: given type A = B, the Rhs of A is B. (#66559)

The methods Alias.Origin, Alias.SetTypeParams, Alias.TypeParams, and Alias.TypeArgs have been added. They are needed for generic alias types.

By default, go/types now produces Alias type nodes for type aliases. This behavior can be controlled by the GODEBUG gotypesalias flag. Its default has changed from 0 in Go 1.22 to 1 in Go 1.23.

`math/rand/v2`

The Uint function and Rand.Uint method have been added. They were inadvertently left out of Go 1.22.

The new ChaCha8.Read method implements the io.Reader interface.

`net`

The new type KeepAliveConfig permits fine-tuning the keep-alive options for TCP connections, via a new TCPConn.SetKeepAliveConfig method and new KeepAliveConfig fields for Dialer and ListenConfig.

The DNSError type now wraps errors caused by timeouts or cancellation. For example, errors.Is(someDNSErr, context.DeadlineExceedeed) will now report whether a DNS error was caused by a timeout.

The new GODEBUG setting netedns0=0 disables sending EDNS0 additional headers on DNS requests, as they reportedly break the DNS server on some modems.

`net/http`

Cookie now preserves double quotes surrounding a cookie value. The new Cookie.Quoted field indicates whether the Cookie.Value was originally quoted.

The new Request.CookiesNamed method retrieves all cookies that match the given name.

The new Cookie.Partitioned field identifies cookies with the Partitioned attribute.

The patterns used by ServeMux now allow one or more spaces or tabs after the method name. Previously, only a single space was permitted.

The new ParseCookie function parses a Cookie header value and returns all the cookies which were set in it. Since the same cookie name can appear multiple times the returned Values can contain more than one value for a given key.

The new ParseSetCookie function parses a Set-Cookie header value and returns a cookie. It returns an error on syntax error.

ServeContent, ServeFile, and ServeFileFS now remove the Cache-Control, Content-Encoding, Etag, and Last-Modified headers when serving an error. These headers usually apply to the non-error content, but not to the text of errors.

Middleware which wraps a ResponseWriter and applies on-the-fly encoding, such as Content-Encoding: gzip, will not function after this change. The previous behavior of ServeContent, ServeFile, and ServeFileFS may be restored by setting GODEBUG=httpservecontentkeepheaders=1.

Note that middleware which changes the size of the served content (such as by compressing it) already does not function properly when ServeContent handles a Range request. On-the-fly compression should use the Transfer-Encoding header instead of Content-Encoding.

For inbound requests, the new Request.Pattern field contains the ServeMux pattern (if any) that matched the request. This field is not set when GODEBUG=httpmuxgo121=1 is set.

`net/http/httptest`

The new NewRequestWithContext method creates an incoming request with a context.Context.

`net/netip`

In Go 1.22 and earlier, using reflect.DeepEqual to compare an Addr holding an IPv4 address to one holding the IPv4-mapped IPv6 form of that address incorrectly returned true, even though the Addr values were different when comparing with == or Addr.Compare. This bug is now fixed and all three approaches now report the same result.

`os`

The Stat function now sets the ModeSocket bit for files that are Unix sockets on Windows. These files are identified by having a reparse tag set to IO_REPARSE_TAG_AF_UNIX.

On Windows, the mode bits reported by Lstat and Stat for reparse points changed. Mount points no longer have ModeSymlink set, and reparse points that are not symlinks, Unix sockets, or dedup files now always have ModeIrregular set. This behavior is controlled by the winsymlink setting. For Go 1.23, it defaults to winsymlink=1. Previous versions default to winsymlink=0.

The CopyFS function copies an io/fs.FS into the local filesystem.

On Windows, Readlink no longer tries to normalize volumes to drive letters, which was not always even possible. This behavior is controlled by the winreadlinkvolume setting. For Go 1.23, it defaults to winreadlinkvolume=1. Previous versions default to winreadlinkvolume=0.

On Linux with pidfd support (generally Linux v5.4+), Process-related functions and methods use pidfd (rather than PID) internally, eliminating potential mistargeting when a PID is reused by the OS. Pidfd support is fully transparent to a user, except for additional process file descriptors that a process may have.

`path/filepath`

The new Localize function safely converts a slash-separated path into an operating system path.

On Windows, EvalSymlinks no longer evaluates mount points, which was a source of many inconsistencies and bugs. This behavior is controlled by the winsymlink setting. For Go 1.23, it defaults to winsymlink=1. Previous versions default to winsymlink=0.

On Windows, EvalSymlinks no longer tries to normalize volumes to drive letters, which was not always even possible. This behavior is controlled by the winreadlinkvolume setting. For Go 1.23, it defaults to winreadlinkvolume=1. Previous versions default to winreadlinkvolume=0.

`reflect`

The new methods synonymous with the methods of the same name in Value are added to Type:

The new SliceAt function is analogous to NewAt, but for slices.

The Value.Pointer and Value.UnsafePointer methods now support values of kind String.

The new methods Value.Seq and Value.Seq2 return sequences that iterate over the value as though it were used in a for/range loop. The new methods Type.CanSeq and Type.CanSeq2 report whether calling Value.Seq and Value.Seq2, respectively, will succeed without panicking.

`runtime/debug`

The SetCrashOutput function allows the user to specify an alternate file to which the runtime should write its fatal crash report. It may be used to construct an automated reporting mechanism for all unexpected crashes, not just those in goroutines that explicitly use recover.

`runtime/pprof`

The maximum stack depth for alloc, mutex, block, threadcreate and goroutine profiles has been raised from 32 to 128 frames.

`runtime/trace`

The runtime now explicitly flushes trace data when a program crashes due to an uncaught panic. This means that more complete trace data will be available in a trace if the program crashes while tracing is active.

`slices`

The Repeat function returns a new slice that repeats the provided slice the given number of times.

`sync`

The Map.Clear method deletes all the entries, resulting in an empty Map. It is analogous to clear.

`sync/atomic`

The new And and Or operators apply a bitwise AND or OR to the given input, returning the old value.

`syscall`

The syscall package now defines WSAENOPROTOOPT on Windows.

The GetsockoptInt function is now supported on Windows.

`testing/fstest`

TestFS now returns a structured error that can be unwrapped (via method Unwrap() []error). This allows inspecting errors using errors.Is or errors.As.

`text/template`

Templates now support the new “else with” action, which reduces template complexity in some use cases.

`time`

Parse and ParseInLocation now return an error if the time zone offset is out of range.

On Windows, Timer, Ticker, and functions that put the goroutine to sleep, such as Sleep, got their time resolution improved to 0.5ms instead of 15.6ms.

`unicode/utf16`

The RuneLen function returns the number of 16-bit words in the UTF-16 encoding of the rune. It returns -1 if the rune is not a valid value to encode in UTF-16.

Ports

Darwin

As announced in the Go 1.22 release notes, Go 1.23 requires macOS 11 Big Sur or later; support for previous versions has been discontinued.

Linux

Go 1.23 is the last release that requires Linux kernel version 2.6.32 or later. Go 1.24 will require Linux kernel version 3.2 or later.

OpenBSD

Go 1.23 adds experimental support for OpenBSD on 64-bit RISC-V (GOOS=openbsd, GOARCH=riscv64).

ARM64

Go 1.23 introduces a new GOARM64 environment variable, which specifies the minimum target version of the ARM64 architecture at compile time. Allowed values are v8.{0-9} and v9.{0-5}. This may be followed by an option specifying extensions implemented by target hardware. Valid options are ,lse and ,crypto.

The GOARM64 environment variable defaults to v8.0.

RISC-V

Go 1.23 introduces a new GORISCV64 environment variable, which selects the RISC-V user-mode application profile for which to compile. Allowed values are rva20u64 and rva22u64.

The GORISCV64 environment variable defaults to rva20u64.

Wasm

The go_wasip1_wasm_exec script in GOROOT/misc/wasm has dropped support for versions of wasmtime < 14.0.0.