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.
Go 1.21 adds three new built-ins to the language.
min and max compute the
smallest (or largest, for max) value of a fixed number
of given arguments.
See the language spec for
details.
clear deletes all elements from a
map or zeroes all elements of a slice.
See the language spec for
details.
Package initialization order is now specified more precisely. The new algorithm is:
TODO: https://go.dev/issue/59338: infer type arguments from assignments of generic functions (reverse type inference)
TODO: https://go.dev/issue/56986: extended backwards compatibility for Go
TODO: complete this section
The Go scheduler now interacts much more efficiently with the JavaScript event loop, especially in applications that block frequently on asynchronous events.
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.
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.
TODO: https://go.dev/cl/453603: cmd/go, go/build: parse directives in file headers; modified api/next/56986.txt
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.
TODO: complete this section, or delete if not needed
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
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.
The new slices package provides many common operations on slices, using generic functions that work with slices of any element type.
The new maps package provides several common operations on maps, using generic functions that work with maps of any key or element type.
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: https://go.dev/cl/456837: all: upgrade Unicode from 13.0.0 to 15.0.0; modified api/except.txt, api/next/55079.txt
TODO: complete this section
TODO: https://go.dev/issue/53685: add Buffer.Available and Buffer.AvailableBuffer
TODO: https://go.dev/cl/474635: bytes: add Buffer.Available and Buffer.AvailableBuffer; modified api/next/53685.txt
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.
TODO: https://go.dev/cl/459977: crypto/elliptic: deprecate unsafe APIs; modified api/next/34648.txt, api/next/52221.txt
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
TODO: https://go.dev/cl/483815: crypto/sha1: add WriteString and WriteByte method
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
TODO: https://go.dev/cl/483816: crypto/sha512: add WriteString and WriteByte method
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
TODO: https://go.dev/issue/56887: add DT_FLAGS_1 (dynamic flags) constants
TODO: https://go.dev/issue/56892: add (*File).DynValue
TODO: https://go.dev/cl/452496: debug/elf: add DT_FLAGS_1 constants; modified api/next/56887.txt
TODO: https://go.dev/cl/452617: debug/elf: retrieve values for dynamic section tags; modified api/next/56892.txt
TODO: https://go.dev/cl/473256: debug/elf: support zstd compression; modified api/next/55107.txt
TODO: https://go.dev/cl/488475: debug/pe: return error on reading from section with uninitialized data
TODO: https://go.dev/issue/57803: openFile should implement ReaderAt
TODO: https://go.dev/cl/483235: embed: implement openFile.ReadAt
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.
TODO: https://go.dev/issue/53747: add BoolFunc(name, usage string, fn func(string)error)
TODO: https://go.dev/cl/476015: flag: add BoolFunc; FlagSet.BoolFunc; modified api/next/53747.txt
TODO: https://go.dev/cl/480215: flag: panic if a flag is defined after being set
TODO: https://go.dev/issue/54451: standard implementations of FileInfo and DirEntry should implement fmt.Stringer
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.
TODO: https://go.dev/cl/476276: go/ast: add File.GoVersion; modified api/next/59033.txt
TODO: https://go.dev/cl/453603: cmd/go, go/build: parse directives in file headers; modified api/next/56986.txt
TODO: https://go.dev/cl/476275: go/build/constraint: add GoVersion; modified api/next/59033.txt
The new File.Lines method
returns the file's line-number table in the same form as accepted by
File.SetLines.
TODO: https://go.dev/issue/47342: Provide a `purego` implementation
TODO: https://go.dev/cl/468795: hash/maphash: add purego implementation
TODO: https://go.dev/cl/489555: io/fs: add FormatFileInfo and FormatDirEntry functions; modified api/next/54451.txt
TODO: https://go.dev/issue/59060: arg to Record.Attrs should return bool
TODO: https://go.dev/issue/59141: handle panics in LogValuer.LogValue
TODO: https://go.dev/issue/59204: change slog.Group signature to ...any
TODO: https://go.dev/issue/59280: ReplaceAttr gets Source struct instead of file:line
TODO: https://go.dev/issue/59282: drop only completely empty Attrs
TODO: https://go.dev/issue/59339: change constructors to NewXXXHandler(io.Writer, *HandlerOptions)
TODO: https://go.dev/issue/59345: JSONHandler should format everything like json.Marshal
TODO: https://go.dev/cl/477295: log/slog: initial commit; modified api/next/56345.txt
TODO: https://go.dev/cl/484096: log/slog: function argument to Record.Attrs returns bool; modified api/next/59060.txt
TODO: https://go.dev/cl/486376: log/slog: add Source type for source location; modified api/next/59280.txt
TODO: https://go.dev/cl/486415: log/slog: built-in handler constructors take options as a second arg; modified api/next/56345.txt, api/next/59339.txt
TODO: https://go.dev/cl/487855: log/slog: Group takes ...any; modified api/next/56345.txt, api/next/59204.txt
TODO: https://go.dev/issue/56491: add Compare and Compare32
TODO: https://go.dev/cl/459435: math: add Compare and Compare32; modified api/next/56491.txt
TODO: https://go.dev/cl/467515: math: add Compare and Compare32; modified api/next/56491.txt
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.
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.
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.
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.
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 now defines
MarshalText
and UnmarshalText
methods. These implement
encoding.TextMarshaler
and
encoding.TextUnmarshaler
and will be used by packages such as
encoding/json.
TODO: https://go.dev/issue/38651: add 'created by goroutine number' to stack traces
TODO: https://go.dev/issue/57441: use WER for GOTRACEBACK=wer on Windows
TODO: https://go.dev/cl/447778: runtime: reimplement GODEBUG=cgocheck=2 as a GOEXPERIMENT
TODO: https://go.dev/cl/472195: runtime: remove NOFRAME from asmcgocall, systemstack and mcall
TODO: https://go.dev/cl/474915: runtime: support GOTRACEBACK=wer on Windows
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.
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.
A few previously-internal GC metrics, such as live heap size, are
now available.
GOGC and GOMEMLIMIT are also now
available as metrics.
The new OnceFunc,
OnceValue, and
OnceValues
functions capture a common use of Once to
lazily initialize a value on first use.
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.
ENOSYSENOTSUPEOPNOTSUPPEPLAN9 (Plan 9 only)ERROR_CALL_NOT_IMPLEMENTED (Windows only)ERROR_NOT_SUPPORTED (Windows only)EWINDOWS (Windows only)
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.
TODO: https://go.dev/cl/487895: testing/slogtest: tests for slog handlers; modified api/next/56345.txt