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:
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
The new go:wasmimport directive can now be used in Go programs
to import functions from the WebAssembly host.
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.
In files that import "C", the Go toolchain now
correctly reports errors for attempts to declare Go methods on C types.
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.
Profile-guide optimization (PGO), added as a preview in Go 1.20, is now ready
for general use. PGO enables additional optimizations on code identified as
hot by profiles of production workloads. As mentioned in the
Go command section, PGO is enabled by default for
binaries that contain a default.pgo profile in the main
package directory. Performance improvements vary depending on application
behavior, with most programs from a representative set of Go programs seeing
between 2 and 7% improvement from enabling PGO. See the
PGO user guide for detailed documentation.
PGO builds can now devirtualize some interface method calls, adding a concrete call to the most common callee. This enables further optimization, such as inlining the callee.
TODO: complete this section, or delete if not needed
The verifier that checks for incorrect uses of R15 when dynamic linking on amd64 has been improved.
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
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 testing/slogtest package can help to validate slog.Handler implementations.
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: complete this section
The implementation of the
io/fs.FileInfo
interface returned by
Header.FileInfo
now implements a String method that calls
io/fs.FormatFileInfo.
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.
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.
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.
All of the Curve methods have been deprecated, along with GenerateKey, Marshal, and Unmarshal. For ECDH operations, the new crypto/ecdh package should be used instead.
The GenerateMultiPrimeKey function and the PrecomputedValues.CRTValues field have been deprecated. PrecomputedValues.CRTValues will still be populated when PrivateKey.Precompute is called, but the values will not be used during decryption operations.
SHA-224 and SHA-256 operations now use native instructions when available when GOOS=amd64, providing a performance improvement on the order of 3-4x.
RevocationList.RevokedCertificates has been deprecated and replaced with the new RevocationList.Entries field, which is a slice of RevocationListEntry. RevocationListEntry contains all of the fields in pkix.RevokedCertificate, as well as the revocation reason code.
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.
Attempts to read from a section containing uninitialized data
using
Section.Data
or the reader returned by Section.Open
now return an error.
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.
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.
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.
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.
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.
The new
GoVersion
function returns the minimum Go version implied by a build
expression.
The new File.Lines method
returns the file's line-number table in the same form as accepted by
File.SetLines.
The hash/maphash package now has a pure Go implementation, selectable with the purego build tag.
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.
The new Int.Float64
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.
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.
The implementation of the
io/fs.FS interface returned by
the DirFS function now implements
the io/fs.ReadFileFS and
the io/fs.ReadDirFS
interfaces.
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.
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
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.
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.
Calling Open.Stat
will return a type that now implements a String
method that calls
io/fs.FormatFileInfo.
The unicode package and
associated support throughout the system has been upgraded to
Unicode 15.0.0.