DRAFT RELEASE NOTES — Introduction to Go 1.21

Go 1.21 is not yet released. These are work-in-progress release notes. Go 1.21 is expected to be released in August 2023.

Changes to the language

Go 1.21 adds three new built-ins to the language.

Package initialization order is now specified more precisely. The new algorithm is:

This may change the behavior of some programs that rely on a specific initialization ordering that was not expressed by explicit imports. The behavior of such programs was not well defined by the spec in past releases. The new rule provides an unambiguous definition.

Multiple improvements that increase the power and precision of type inference have been made.

More generally, the description of type inference in the language spec has been clarified. Together, all these changes make type inference more powerful and inference failures less surprising.

TODO: https://go.dev/issue/56986: extended backwards compatibility for Go

TODO: complete this section

Ports

WebAssembly

The Go scheduler now interacts much more efficiently with the JavaScript event loop, especially in applications that block frequently on asynchronous events.

WebAssembly System Interface

Go 1.21 adds an experimental port to the WebAssembly System Interface (WASI), Preview 1 (GOOS=wasip1, GOARCH=wasm).

As a result of the addition of the new GOOS value "wasip1", Go files named *_wasip1.go will now be ignored by Go tools except when that GOOS value is being used. If you have existing filenames matching that pattern, you will need to rename them.

Tools

Go command

The -pgo build flag now defaults to -pgo=auto, and the restriction of specifying a single main package on the command line is now removed. If a file named default.pgo is present in the main package's directory, the go command will use it to enable profile-guided optimization for building the corresponding program.

The new go test option -fullpath prints full path names in test log messages, rather than just base names.

Runtime

TODO: complete this section, or delete if not needed

When printing very deep stacks, the runtime now prints the first 50 (innermost) frames followed by the bottom 50 (outermost) frames, rather than just printing the first 100 frames. This makes it easier to see how deeply recursive stacks started, and is especially valuable for debugging stack overflows.

On Linux platforms that support transparent huge pages, the Go runtime now manages which parts of the heap may be backed by huge pages more explicitly. This leads to better utilization of memory: small heaps should see less memory used (up to 50% in pathological cases) while large heaps should see fewer broken huge pages for dense parts of the heap, improving CPU usage and latency by up to 1%.

As a result of runtime-internal garbage collection tuning, applications may see up to a 40% reduction in application tail latency and a small decrease in memory use. Some applications may also observe a small loss in throughput. The memory use decrease should be proportional to the loss in throughput, such that the previous release's throughput/memory tradeoff may be recovered (with little change to latency) by increasing GOGC and/or GOMEMLIMIT slightly.

Calls from C to Go on threads created in C require some setup to prepare for Go execution. On Unix platforms, this setup is now preserved across multiple calls from the same thread. This significantly reduces the overhead of subsequent C to Go calls from ~1-3 microseconds per call to ~100-200 nanoseconds per call.

Compiler

TODO: complete this section, or delete if not needed

Assembler

The verifier that checks for incorrect uses of R15 when dynamic linking on amd64 has been improved.

Linker

On Windows AMD64, the linker (with help from the compiler) now emits SEH unwinding data by default, which improves the integration of Go applications with Windows debuggers and other tools.

In Go 1.21 the linker (with help from the compiler) is now capable of deleting dead (unreferenced) global map variables, if the number of entries in the variable initializer is sufficiently large, and if the initializer expressions are side-effect free.

TODO: complete this section, or delete if not needed

Core library

New log/slog package

The new log/slog package provides structured logging with levels. Structured logging emits key-value pairs to enable fast, accurate processing of large amounts of log data. The package supports integration with popular log analysis tools and services.

New testing/slogtest package

The new testing/slogtest package can help to validate slog.Handler implementations.

New slices package

The new slices package provides many common operations on slices, using generic functions that work with slices of any element type.

New maps package

The new maps package provides several common operations on maps, using generic functions that work with maps of any key or element type.

Minor changes to the library

As always, there are various minor changes and updates to the library, made with the Go 1 promise of compatibility in mind. There are also various performance improvements, not enumerated here.

TODO: complete this section

archive/tar

The implementation of the io/fs.FileInfo interface returned by Header.FileInfo now implements a String method that calls io/fs.FormatFileInfo.

archive/zip

The implementation of the io/fs.FileInfo interface returned by FileHeader.FileInfo now implements a String method that calls io/fs.FormatFileInfo.

The implementation of the io/fs.DirEntry interface returned by the io/fs.ReadDirFile.ReadDir method of the io/fs.File returned by Reader.Open now implements a String method that calls io/fs.FormatDirEntry.

bytes

The Buffer type has two new methods: Available and AvailableBuffer. These may be used along with the Write method to append directly to the Buffer.

context

The new WithoutCancel function returns a copy of a context that is not canceled when the original context is canceled.

The new WithDeadlineCause and WithTimeoutCause functions provide a way to set a context cancellation cause when a deadline or timer expires. The cause may be retrieved with the Cause function.

The new AfterFunc function registers a function to run after a context has been cancelled.

crypto/elliptic

TODO: https://go.dev/cl/459977: crypto/elliptic: deprecate unsafe APIs; modified api/next/34648.txt, api/next/52221.txt

crypto/rsa

TODO: https://go.dev/issue/56921: deprecate GenerateMultiPrimeKey and PrecomputedValues.CRTValues

TODO: https://go.dev/cl/459976: crypto/rsa: deprecate multiprime RSA support; modified api/next/56921.txt

crypto/sha1

TODO: https://go.dev/cl/483815: crypto/sha1: add WriteString and WriteByte method

crypto/sha256

TODO: https://go.dev/issue/50543: add native SHA256 instruction implementation for AMD64

TODO: https://go.dev/cl/408795: crypto/sha256: add sha-ni implementation; crypto/sha256 uses Intel SHA extensions on new enough x86 processors, making it 3-4X faster.

TODO: https://go.dev/cl/481478: crypto/sha256: add WriteString and WriteByte method

crypto/sha512

TODO: https://go.dev/cl/483816: crypto/sha512: add WriteString and WriteByte method

crypto/x509

TODO: https://go.dev/issue/53573: surface ReasonCode inside x509.RevocationList entries

TODO: https://go.dev/cl/468875: crypto/x509: surface ReasonCode in RevocationList API; modified api/next/53573.txt

debug/elf

The new File.DynValue method may be used to retrieve the numeric values listed with a given dynamic tag.

The constant flags permitted in a DT_FLAGS_1 dynamic tag are now defined with type DynFlag1. These tags have names starting with DF_1.

The package now defines the constant COMPRESS_ZSTD.

debug/pe

Attempts to read from a section containing uninitialized data using Section.Data or the reader returned by Section.Open now return an error.

embed

The io/fs.File returned by FS.Open now has a ReadAt method that implements io.ReaderAt.

Calling FS.Open.Stat will return a type that now implements a String method that calls io/fs.FormatFileInfo.

errors

The new ErrUnsupported error provides a standardized way to indicate that a requested operation may not be performed because it is unsupported. For example, a call to os.Link when using a file system that does not support hard links.

flag

The new BoolFunc function and FlagSet.BoolFunc method define a flag that does not require an argument and calls a function when the flag is used. This is similar to Func but for a boolean flag.

A flag definition (via Bool, BoolVar, Int, IntVar, etc.) will panic if Set has already been called on a flag with the same name.

This change is intended to detect cases where changes in initialization order cause flag operations to occur in a different order than expected. In many cases the fix to this problem is to introduce a explicit package dependence to correctly order the definition before any Set operations.

go/ast

The new IsGenerated predicate reports whether a file syntax tree contains the special comment that conventionally indicates that the file was generated by a tool.

The new File.GoVersion field records the minimum Go version required by any //go:build or // +build directives.

go/build

The package now parses build directives (comments that start with //go:) in file headers (before the package declaration). These directives are available in the new Package fields Directives, TestDirectives, and XTestDirectives.

go/build/constraint

The new GoVersion function returns the minimum Go version implied by a build expression.

go/token

The new File.Lines method returns the file's line-number table in the same form as accepted by File.SetLines.

hash/maphash

The hash/maphash package now has a pure Go implementation, selectable with the purego build tag.

io/fs

The new FormatFileInfo function returns a formatted version of a FileInfo. The new FormatDirEntry function returns a formatted version of a DirEntry. The implementation of DirEntry returned by ReadDir now implements a String method that calls FormatDirEntry, and the same is true for the DirEntry value passed to WalkDirFunc.

math/big

The new Int.ToFloat64 method returns the nearest floating-point value to a multi-precision integer, along with an indication of any rounding that occurred.

net

On Linux, the net package can now use Multipath TCP when the kernel supports it. It is not used by default. To use Multipath TCP when available on a client, call the Dialer.SetMultipathTCP method before calling the Dialer.Dial or Dialer.DialContext methods. To use Multipath TCP when available on a server, call the ListenConfig.SetMultipathTCP method before calling the ListenConfig.Listen method. Specify the network as "tcp" or "tcp4" or "tcp6" as usual. If Multipath TCP is not supported by the kernel or the remote host, the connection will silently fall back to TCP. To test whether a particular connection is using Multipath TCP, use the TCPConn.MultipathTCP method.

In a future Go release we may enable Multipath TCP by default on systems that support it.

net/http

The new ErrSchemeMismatch error is returned by Client and Transport when the server responds to an HTTPS request with an HTTP response.

TODO: https://go.dev/cl/472636: net/http: support full-duplex HTTP/1 responses; modified api/next/57786.txt

The net/http package now supports errors.ErrUnsupported, in that the expression errors.Is(http.ErrNotSupported, errors.ErrUnsupported) will return true.

os

Programs may now pass an empty time.Time value to the Chtimes function to leave either the access time or the modification time unchanged.

On Windows the File.Chdir method now changes the current directory to the file, rather than always returning an error.

On Windows calling Truncate on a non-existent file used to create an empty file. It now returns an error indicating that the file does not exist.

On Windows the os package now supports working with files whose names, stored as UTF-16, can't be represented as valid UTF-8.

The implementation of the io/fs.DirEntry interface returned by the ReadDir function and the File.ReadDir method now implements a String method that calls io/fs.FormatDirEntry.

path/filepath

The implementation of the io/fs.DirEntry interface passed to the function argument of WalkDir now implements a String method that calls io/fs.FormatDirEntry.

reflect

In Go 1.21, ValueOf no longer forces its argument to be allocated on the heap, allowing a Value's content to be allocated on the stack. Most operations on a Value also allow the underlying value to be stack allocated.

The new Value method Value.Clear clears the contents of a map or zeros the contents of a slice. This corresponds to the new clear built-in added to the language.

The SliceHeader and StringHeader types are now deprecated. In new code prefer unsafe.Slice, unsafe.SliceData, unsafe.String, or unsafe.StringData.

regexp

Regexp now defines MarshalText and UnmarshalText methods. These implement encoding.TextMarshaler and encoding.TextUnmarshaler and will be used by packages such as encoding/json.

runtime

TODO: https://go.dev/issue/38651: add 'created by goroutine number' to stack traces

Crashing Go applications can now opt-in to Windows Error Reporting (WER) by setting the environment variable GOTRACEBACK=wer or calling debug.SetTraceback("wer") before the crash. Other than enabling WER, the runtime will behave as with GOTRACEBACK=crash. On non-Windows systems, GOTRACEBACK=wer is ignored.

GODEBUG=cgocheck=2, a thorough checker of cgo pointer passing rules, is no longer available as a debug option. Instead, it is available as an experiment using GOEXPERIMENT=cgocheck2. In particular this means that this mode has to be selected at build time instead of startup time.

GODEBUG=cgocheck=1 is still available (and is still the default).

A new type Pinner has been added to the runtime package. Pinners may be used to "pin" Go memory such that it may be used more freely by non-Go code. For instance, passing Go values that reference pinned Go memory to C code is now allowed. Previously, passing any such nested reference was disallowed by the cgo pointer passing rules. See the docs for more details.

runtime/trace

Collecting traces on amd64 and arm64 now incurs a substantially smaller CPU cost: up to a 10x improvement over the previous release.

Traces now contain explicit stop-the-world events for every reason the Go runtime might stop-the-world, not just garbage collection.

runtime/metrics

A few previously-internal GC metrics, such as live heap size, are now available. GOGC and GOMEMLIMIT are also now available as metrics.

sync

The new OnceFunc, OnceValue, and OnceValues functions capture a common use of Once to lazily initialize a value on first use.

syscall

On Windows the Fchdir function now changes the current directory to its argument, rather than always returning an error.

On FreeBSD SysProcAttr has a new field Jail that may be used to put the newly created process in a jailed environment.

On Windows the syscall package now supports working with files whose names, stored as UTF-16, can't be represented as valid UTF-8. The UTF16ToString and UTF16FromString functions now convert between UTF-16 data and WTF-8 strings. This is backward compatible as WTF-8 is a superset of the UTF-8 format that was used in earlier releases.

Several error values match the new errors.ErrUnsupported, such that errors.Is(err, errors.ErrUnsupported) returns true.

testing

The new -test.fullpath option will print full path names in test log messages, rather than just base names.

The new Testing function reports whether the program is a test created by go test.

testing/fstest

Calling Open.Stat will return a type that now implements a String method that calls io/fs.FormatFileInfo.

unicode

The unicode package and associated support throughout the system has been upgraded to Unicode 15.0.0.