2 "Title": "Go 1.21 Release Notes",
7 NOTE: In this document and others in this directory, the convention is to
8 set fixed-width phrases with non-fixed-width spaces, as in
9 <code>hello</code> <code>world</code>.
10 Do not send CLs removing the interior tags from such phrases.
14 main ul li { margin: 0.5em 0; }
17 <h2 id="introduction">DRAFT RELEASE NOTES — Introduction to Go 1.21</h2>
21 Go 1.21 is not yet released. These are work-in-progress
22 release notes. Go 1.21 is expected to be released in August 2023.
27 The latest Go release, version 1.21, arrives six months after <a href="/doc/go1.20">Go 1.20</a>.
28 Most of its changes are in the implementation of the toolchain, runtime, and libraries.
29 As always, the release maintains the Go 1 <a href="/doc/go1compat">promise of compatibility</a>;
30 in fact, Go 1.21 <a href="#godebug">improves upon that promise</a>.
31 We expect almost all Go programs to continue to compile and run as before.
34 <p><!-- https://go.dev/issue/57631 -->
35 Go 1.21 introduces a small change to the numbering of releases.
36 In the past, we used Go 1.<i>N</i> to refer to both the overall Go language version and release family
37 as well as the first release in that family.
38 Starting in Go 1.21, the first release is now Go 1.<i>N</i>.0.
39 Today we are releasing both the Go 1.21 language and its initial implementation, the Go 1.21.0 release.
40 These notes refer to “Go 1.21”; tools like <code>go</code> <code>version</code> will report “<code>go1.21.0</code>”
41 (until you upgrade to Go 1.21.1).
42 See “<a href="/doc/toolchain#versions">Go versions</a>” in the “Go Toolchains” documentation for details
43 about the new version numbering.
46 <h2 id="language">Changes to the language</h2>
49 Go 1.21 adds three new built-ins to the language.
52 <li><!-- https://go.dev/issue/59488 -->
53 The new functions <code>min</code> and <code>max</code> compute the
54 smallest (or largest, for <code>max</code>) value of a fixed number
56 See the language spec for
57 <a href="https://tip.golang.org/ref/spec#Min_and_max">details</a>.
59 <li><!-- https://go.dev/issue/56351 -->
60 The new function <code>clear</code> deletes all elements from a
61 map or zeroes all elements of a slice.
62 See the language spec for
63 <a href="https://tip.golang.org/ref/spec#Clear">details</a>.
68 <p><!-- https://go.dev/issue/57411 -->
69 Package initialization order is now specified more precisely. The
73 Sort all packages by import path.
75 <li>Repeat until the list of packages is empty:
78 Find the first package in the list for which all imports are
82 Initialize that package and remove it from the list.
87 This may change the behavior of some programs that rely on a
88 specific initialization ordering that was not expressed by explicit
89 imports. The behavior of such programs was not well defined by the
90 spec in past releases. The new rule provides an unambiguous definition.
94 Multiple improvements that increase the power and precision of type inference have been made.
97 <li><!-- https://go.dev/issue/59338 -->
98 A (possibly partially instantiated generic) function may now be called with arguments that are
99 themselves (possibly partially instantiated) generic functions.
100 The compiler will attempt to infer the missing type arguments of the callee (as before) and,
101 for each argument that is a generic function that is not fully instantiated,
102 its missing type arguments (new).
103 Typical use cases are calls to generic functions operating on containers
104 (such as <a href="/pkg/slices#IndexFunc">slices.IndexFunc</a>) where a function argument
105 may also be generic, and where the type argument of the called function and its arguments
106 are inferred from the container type.
107 More generally, a generic function may now be used without explicit instantiation when
108 it is assigned to a variable or returned as a result value if the type arguments can
109 be inferred from the assignment.
111 <li><!-- https://go.dev/issue/60353, https://go.dev/issue/57192, https://go.dev/issue/52397, https://go.dev/issue/41176 -->
112 Type inference now also considers methods when a value is assigned to an interface:
113 type arguments for type parameters used in method signatures may be inferred from
114 the corresponding parameter types of matching methods.
116 <li><!-- https://go.dev/issue/51593 https://go.dev/issue/39661 -->
117 Similarly, since a type argument must implement all the methods of its corresponding constraint,
118 the methods of the type argument and constraint are matched which may lead to the inference of
119 additional type arguments.
121 <li><!-- https://go.dev/issue/58671 -->
122 If multiple untyped constant arguments of different kinds (such as an untyped int and
123 an untyped floating-point constant) are passed to parameters with the same (not otherwise
124 specified) type parameter type, instead of an error, now type inference determines the
125 type using the same approach as an operator with untyped constant operands.
126 This change brings the types inferred from untyped constant arguments in line with the
127 types of constant expressions.
129 <li><!-- https://go.dev/issue/59750 -->
130 Type inference is now precise when matching corresponding types in assignments:
131 component types (such as the elements of slices, or the parameter types in function signatures)
132 must be identical (given suitable type arguments) to match, otherwise inference fails.
133 This change produces more accurate error messages:
134 where in the past type inference may have succeeded incorrectly and lead to an invalid assignment,
135 the compiler now reports an inference error if two types can't possibly match.
139 <p><!-- https://go.dev/issue/58650 -->
140 More generally, the description of
141 <a href="https://tip.golang.org/ref/spec#Type_inference">type inference</a>
142 in the language spec has been clarified.
143 Together, all these changes make type inference more powerful and inference failures less surprising.
146 <!-- https://go.dev/issue/57969 -->
148 Go 1.21 includes a preview of a language change we are considering for a future version of Go:
149 making for loop variables per-iteration instead of per-loop, to avoid accidental sharing bugs.
150 For details about how to try that language change, see <a href="https://go.dev/wiki/LoopvarExperiment">the LoopvarExperiment wiki page</a>.
153 <h2 id="tools">Tools</h2>
155 Go 1.21 adds improved support for backwards compatibility and forwards compatibility
159 <p><!-- https://go.dev/issue/56986 -->
160 To improve backwards compatibility, Go 1.21 formalizes
161 Go's use of the GODEBUG environment variable to control
162 the default behavior for changes that are non-breaking according to the
163 <a href="/doc/go1compat">compatibility policy</a>
164 but nonetheless may cause existing programs to break.
165 (For example, programs that depend on buggy behavior may break
166 when a bug is fixed, but bug fixes are not considered breaking changes.)
167 When Go must make this kind of behavior change,
168 it now chooses between the old and new behavior based on the
169 <code>go</code> line in the workspace's <code>go.work</code> file
170 or else the main module's <code>go.mod</code> file.
171 Upgrading to a new Go toolchain but leaving the <code>go</code> line
172 set to its original (older) Go version preserves the behavior of the older
174 With this compatibility support, the latest Go toolchain should always
175 be the best, most secure, implementation of an older version of Go.
176 See “<a href="/doc/godebug">Go, Backwards Compatibility, and GODEBUG</a>” for details.
179 <p><!-- https://go.dev/issue/57001 -->
180 To improve forwards compatibility, Go 1.21 now reads the <code>go</code> line
181 in a <code>go.work</code> or <code>go.mod</code> file as a strict
182 minimum requirement: <code>go</code> <code>1.21.0</code> means
183 that the workspace or module cannot be used with Go 1.20 or with Go 1.21rc1.
184 This allows projects that depend on fixes made in later versions of Go
185 to ensure that they are not used with earlier versions.
186 It also gives better error reporting for projects that make use of new Go features:
187 when the problem is that a newer Go version is needed,
188 that problem is reported clearly, instead of attempting to build the code
189 and instead printing errors about unresolved imports or syntax errors.
193 To make these new stricter version requirements easier to manage,
194 the <code>go</code> command can now invoke not just the toolchain
195 bundled in its own release but also other Go toolchain versions found in the PATH
196 or downloaded on demand.
197 If a <code>go.mod</code> or <code>go.work</code> <code>go</code> line
198 declares a minimum requirement on a newer version of Go, the <code>go</code>
199 command will find and run that version automatically.
200 The new <code>toolchain</code> directive sets a suggested minimum toolchain to use,
201 which may be newer than the strict <code>go</code> minimum.
202 See “<a href="/doc/toolchain">Go Toolchains</a>” for details.
205 <h3 id="go-command">Go command</h3>
207 <p><!-- https://go.dev/issue/58099, CL 474236 -->
208 The <code>-pgo</code> build flag now defaults to <code>-pgo=auto</code>,
209 and the restriction of specifying a single main package on the command
210 line is now removed. If a file named <code>default.pgo</code> is present
211 in the main package's directory, the <code>go</code> command will use
212 it to enable profile-guided optimization for building the corresponding
217 The <code>-C</code> <code>dir</code> flag must now be the first
218 flag on the command-line when used.
221 <p><!-- https://go.dev/issue/37708, CL 463837 -->
222 The new <code>go</code> <code>test</code> option
223 <code>-fullpath</code> prints full path names in test log messages,
224 rather than just base names.
227 <p><!-- https://go.dev/issue/15513, CL 466397 -->
228 The <code>go</code> <code>test</code> <code>-c</code> flag now
229 supports writing test binaries for multiple packages, each to
230 <code>pkg.test</code> where <code>pkg</code> is the package name.
231 It is an error if more than one test package being compiled has a given package name.]
234 <p><!-- https://go.dev/issue/15513, CL 466397 -->
235 The <code>go</code> <code>test</code> <code>-o</code> flag now
236 accepts a directory argument, in which case test binaries are written to that
237 directory instead of the current directory.
240 <h3 id="cgo">Cgo</h3>
242 <p><!-- CL 490819 -->
243 In files that <code>import "C"</code>, the Go toolchain now
244 correctly reports errors for attempts to declare Go methods on C types.
247 <h2 id="runtime-changes">Runtime</h2>
249 <p><!-- https://go.dev/issue/7181 -->
250 When printing very deep stacks, the runtime now prints the first 50
251 (innermost) frames followed by the bottom 50 (outermost) frames,
252 rather than just printing the first 100 frames. This makes it easier
253 to see how deeply recursive stacks started, and is especially
254 valuable for debugging stack overflows.
257 <p><!-- https://go.dev/issue/59960 -->
258 On Linux platforms that support transparent huge pages, the Go runtime
259 now manages which parts of the heap may be backed by huge pages more
260 explicitly. This leads to better utilization of memory: small heaps
261 should see less memory used (up to 50% in pathological cases) while
262 large heaps should see fewer broken huge pages for dense parts of the
263 heap, improving CPU usage and latency by up to 1%.
266 <p><!-- https://go.dev/issue/57069, https://go.dev/issue/56966 -->
267 As a result of runtime-internal garbage collection tuning,
268 applications may see up to a 40% reduction in application tail latency
269 and a small decrease in memory use. Some applications may also observe
270 a small loss in throughput.
272 The memory use decrease should be proportional to the loss in
273 throughput, such that the previous release's throughput/memory
274 tradeoff may be recovered (with little change to latency) by
275 increasing <code>GOGC</code> and/or <code>GOMEMLIMIT</code> slightly.
278 <p><!-- https://go.dev/issue/51676 -->
279 Calls from C to Go on threads created in C require some setup to prepare for
280 Go execution. On Unix platforms, this setup is now preserved across multiple
281 calls from the same thread. This significantly reduces the overhead of
282 subsequent C to Go calls from ~1-3 microseconds per call to ~100-200
283 nanoseconds per call.
286 <h2 id="compiler">Compiler</h2>
289 Profile-guide optimization (PGO), added as a preview in Go 1.20, is now ready
290 for general use. PGO enables additional optimizations on code identified as
291 hot by profiles of production workloads. As mentioned in the
292 <a href="#go-command">Go command section</a>, PGO is enabled by default for
293 binaries that contain a <code>default.pgo</code> profile in the main
294 package directory. Performance improvements vary depending on application
295 behavior, with most programs from a representative set of Go programs seeing
296 between 2 and 7% improvement from enabling PGO. See the
297 <a href="/doc/pgo">PGO user guide</a> for detailed documentation.
300 <!-- https://go.dev/issue/59959 -->
302 PGO builds can now devirtualize some interface method calls, adding a
303 concrete call to the most common callee. This enables further optimization,
304 such as inlining the callee.
309 Go 1.21 improves build speed by up to 6%, largely thanks to building the
310 compiler itself with PGO.
313 <h2 id="assembler">Assembler</h2>
315 <!-- https://go.dev/issue/58378 -->
317 On amd64, frameless nosplit assembly functions are no longer automatically marked as <code>NOFRAME</code>.
318 Instead, the <code>NOFRAME</code> attribute must be explicitly specified if desired,
319 which is already the behavior on other architectures supporting frame pointers.
320 With this, the runtime now maintains the frame pointers for stack transitions.
325 The verifier that checks for incorrect uses of <code>R15</code> when dynamic linking on amd64 has been improved.
328 <h2 id="linker">Linker</h2>
330 <p><!-- https://go.dev/issue/57302, CL 461749, CL 457455 -->
331 On windows/amd64, the linker (with help from the compiler) now emits
332 SEH unwinding data by default, which improves the integration
333 of Go applications with Windows debuggers and other tools.
336 <!-- CL 463395, CL 461315 -->
338 In Go 1.21 the linker (with help from the compiler) is now capable of
339 deleting dead (unreferenced) global map variables, if the number of
340 entries in the variable initializer is sufficiently large, and if the
341 initializer expressions are side-effect free.
344 <h2 id="library">Core library</h2>
346 <h3 id="slog">New log/slog package</h3>
348 <p><!-- https://go.dev/issue/59060, https://go.dev/issue/59141, https://go.dev/issue/59204, https://go.dev/issue/59280,
349 https://go.dev/issue/59282, https://go.dev/issue/59339, https://go.dev/issue/59345, https://go.dev/issue/61200,
350 CL 477295, CL 484096, CL 486376, CL 486415, CL 487855, CL 508195 -->
351 The new <a href="/pkg/log/slog">log/slog</a> package provides structured logging with levels.
352 Structured logging emits key-value pairs
353 to enable fast, accurate processing of large amounts of log data.
354 The package supports integration with popular log analysis tools and services.
357 <h3 id="slogtest">New testing/slogtest package</h3>
359 <p><!-- CL 487895 -->
360 The new <a href="/pkg/testing/slogtest">testing/slogtest</a> package can help
361 to validate <a href="/pkg/log/slog#Handler">slog.Handler</a> implementations.
364 <h3 id="slices">New slices package</h3>
367 <!-- https://go.dev/issue/45955, https://go.dev/issue/54768 -->
368 <!-- https://go.dev/issue/57348, https://go.dev/issue/57433 -->
369 <!-- https://go.dev/issue/58565, https://go.dev/issue/60091 -->
370 <!-- https://go.dev/issue/60546 -->
371 <!-- CL 467417, CL 468855, CL 483175, CL 496078, CL 498175, CL 502955 -->
372 The new <a href="/pkg/slices">slices</a> package provides many common
373 operations on slices, using generic functions that work with slices
377 <h3 id="maps">New maps package</h3>
379 <p><!-- https://go.dev/issue/57436, CL 464343 -->
380 The new <a href="/pkg/maps/">maps</a> package provides several
381 common operations on maps, using generic functions that work with
382 maps of any key or element type.
385 <h3 id="cmp">New cmp package</h3>
387 <p><!-- https://go.dev/issue/59488, CL 496356 -->
388 The new <a href="/pkg/cmp/">cmp</a> package defines the type
389 constraint <a href="/pkg/cmp/#Ordered"><code>Ordered</code></a> and
390 two new generic functions
391 <a href="/pkg/cmp/#Less"><code>Less</code></a>
392 and <a href="/pkg/cmp/#Compare"><code>Compare</code></a> that are
393 useful with <a href="/ref/spec/#Comparison_operators">ordered
397 <h3 id="minor_library_changes">Minor changes to the library</h3>
400 As always, there are various minor changes and updates to the library,
401 made with the Go 1 <a href="/doc/go1compat">promise of compatibility</a>
403 There are also various performance improvements, not enumerated here.
406 <dl id="archive/tar"><dt><a href="/pkg/archive/tar/">archive/tar</a></dt>
408 <p><!-- https://go.dev/issue/54451, CL 491175 -->
409 The implementation of the
410 <a href="/pkg/io/fs/#FileInfo"><code>io/fs.FileInfo</code></a>
411 interface returned by
412 <a href="/pkg/archive/tar/#Header.FileInfo"><code>Header.FileInfo</code></a>
413 now implements a <code>String</code> method that calls
414 <a href="/pkg/io/fs/#FormatFileInfo"><code>io/fs.FormatFileInfo</code></a>.
417 </dl><!-- archive/tar -->
419 <dl id="archive/zip"><dt><a href="/pkg/archive/zip/">archive/zip</a></dt>
421 <p><!-- https://go.dev/issue/54451, CL 491175 -->
422 The implementation of the
423 <a href="/pkg/io/fs/#FileInfo"><code>io/fs.FileInfo</code></a>
424 interface returned by
425 <a href="/pkg/archive/zip/#FileHeader.FileInfo"><code>FileHeader.FileInfo</code></a>
426 now implements a <code>String</code> method that calls
427 <a href="/pkg/io/fs/#FormatFileInfo"><code>io/fs.FormatFileInfo</code></a>.
430 <p><!-- https://go.dev/issue/54451, CL 491175 -->
431 The implementation of the
432 <a href="/pkg/io/fs/#DirEntry"><code>io/fs.DirEntry</code></a>
433 interface returned by the
434 <a href="/pkg/io/fs/#ReadDirFile.ReadDir"><code>io/fs.ReadDirFile.ReadDir</code></a>
436 <a href="/pkg/io/fs/#File"><code>io/fs.File</code></a>
438 <a href="/pkg/archive/zip/#Reader.Open"><code>Reader.Open</code></a>
439 now implements a <code>String</code> method that calls
440 <a href="/pkg/io/fs/#FormatDirEntry"><code>io/fs.FormatDirEntry</code></a>.
443 </dl><!-- archive/zip -->
445 <dl id="bytes"><dt><a href="/pkg/bytes/">bytes</a></dt>
447 <p><!-- https://go.dev/issue/53685, CL 474635 -->
448 The <a href="/pkg/bytes/#Buffer"><code>Buffer</code></a> type
450 <a href="/pkg/bytes/#Buffer.Available"><code>Available</code></a>
451 and <a href="/pkg/bytes/#AvailableBuffer"><code>AvailableBuffer</code></a>.
452 These may be used along with the
453 <a href="/pkg/bytes/#Buffer.Write"><code>Write</code></a>
454 method to append directly to the <code>Buffer</code>.
459 <dl id="context"><dt><a href="/pkg/context/">context</a></dt>
461 <p><!-- https://go.dev/issue/40221, CL 479918 -->
462 The new <a href="/pkg/context/#WithoutCancel"><code>WithoutCancel</code></a>
463 function returns a copy of a context that is not canceled when the original
466 <p><!-- https://go.dev/issue/56661, CL 449318 -->
467 The new <a href="/pkg/context/#WithDeadlineCause"><code>WithDeadlineCause</code></a>
468 and <a href="/pkg/context/#WithTimeoutCause"><code>WithTimeoutCause</code></a>
469 functions provide a way to set a context cancellation cause when a deadline or
470 timer expires. The cause may be retrieved with the
471 <a href="/pkg/context/#Cause"><code>Cause</code></a> function.
473 <p><!-- https://go.dev/issue/57928, CL 482695 -->
474 The new <a href="/pkg/context/#AfterFunc"><code>AfterFunc</code></a>
475 function registers a function to run after a context has been cancelled.
478 <p><!-- CL 455455 -->
479 An optimization means that the results of calling
480 <a href="/pkg/context/#Background"><code>Background</code></a>
481 and <a href="/pkg/context/#TODO"><code>TODO</code></a> and
482 converting them to a shared type can be considered equal.
483 In previous releases they were always different. Comparing
484 <a href="/pkg/context/#Context"><code>Context</code></a> values
485 for equality has never been well-defined, so this is not
486 considered to be an incompatible change.
491 <dl id="crypto/elliptic"><dt><a href="/pkg/crypto/elliptic/">crypto/elliptic</a></dt>
493 <p><!-- CL 459977 -->
494 All of the <a href="/pkg/crypto/elliptic/#Curve"><code>Curve</code></a> methods have been deprecated, along with <a href="/pkg/crypto/elliptic/#GenerateKey"><code>GenerateKey</code></a>, <a href="/pkg/crypto/elliptic/#Marshal"><code>Marshal</code></a>, and <a href="/pkg/crypto/elliptic/#Unmarshal"><code>Unmarshal</code></a>. For ECDH operations, the new <a href="/pkg/crypto/ecdh/"><code>crypto/ecdh</code></a> package should be used instead. For lower-level operations, use third-party modules such as <a href="https://pkg.go.dev/filippo.io/nistec">filippo.io/nistec</a>.
497 </dl><!-- crypto/elliptic -->
499 <dl id="crypto/rand"><dt><a href="/pkg/crypto/rand/">crypto/rand</a></dt>
501 <p><!-- CL 463123 -->
502 The <a href="/pkg/crypto/rand/"><code>crypto/rand</code></a> package now uses the <code>getrandom</code> system call on NetBSD 10.0 and later.
505 </dl><!-- crypto/rand -->
507 <dl id="crypto/rsa"><dt><a href="/pkg/crypto/rsa/">crypto/rsa</a></dt>
509 <p><!-- CL 471259, CL 492935 -->
510 The performance of private RSA operations (decryption and signing) is now better than Go 1.19 for <code>GOARCH=amd64</code> and <code>GOARCH=arm64</code>. It had regressed in Go 1.20.
513 Due to the addition of private fields to <a href="/pkg/crypto/rsa/#PrecomputedValues"><code>PrecomputedValues</code></a>, <a href="/pkg/crypto/rsa/#PrivateKey.Precompute"><code>PrivateKey.Precompute</code></a> must be called for optimal performance even if deserializing (for example from JSON) a previously-precomputed private key.
515 <p><!-- https://go.dev/issue/56921, CL 459976 -->
516 The <a href="/pkg/crypto/rsa/#GenerateMultiPrimeKey"><code>GenerateMultiPrimeKey</code></a> function and the <a href="/pkg/crypto/rsa/#PrecomputedValues.CRTValues"><code>PrecomputedValues.CRTValues</code></a> field have been deprecated. <a href="/pkg/crypto/rsa/#PrecomputedValues.CRTValues"><code>PrecomputedValues.CRTValues</code></a> will still be populated when <a href="/pkg/crypto/rsa/#PrivateKey.Precompute"><code>PrivateKey.Precompute</code></a> is called, but the values will not be used during decryption operations.
519 </dl><!-- crypto/rsa -->
521 <!-- CL 483815 reverted -->
523 <dl id="crypto/sha256"><dt><a href="/pkg/crypto/sha256/">crypto/sha256</a></dt>
525 <p><!-- https://go.dev/issue/50543, CL 408795 -->
526 SHA-224 and SHA-256 operations now use native instructions when available when <code>GOARCH=amd64</code>, providing a performance improvement on the order of 3-4x.
529 </dl><!-- crypto/sha256 -->
531 <!-- CL 481478 reverted -->
532 <!-- CL 483816 reverted -->
534 <dl id="crypto/tls"><dt><a href="/pkg/crypto/tls/">crypto/tls</a></dt>
536 <p><!-- https://go.dev/issue/60105, CL 496818, CL 496820, CL 496822, CL 496821, CL 501675 -->
537 Applications can now control the content of session tickets.
540 The new <a href="/pkg/crypto/tls/#SessionState"><code>SessionState</code></a> type
541 describes a resumable session.
544 The <a href="/pkg/crypto/tls/#SessionState.Bytes"><code>SessionState.Bytes</code></a>
545 method and <a href="/pkg/crypto/tls/#ParseSessionState"><code>ParseSessionState</code></a>
546 function serialize and deserialize a <code>SessionState</code>.
549 The <a href="/pkg/crypto/tls/#Config.WrapSession"><code>Config.WrapSession</code></a> and
550 <a href="/pkg/crypto/tls/#Config.UnwrapSession"><code>Config.UnwrapSession</code></a>
551 hooks convert a <code>SessionState</code> to and from a ticket.
554 The <a href="/pkg/crypto/tls/#Config.EncryptTicket"><code>Config.EncryptTicket</code></a>
555 and <a href="/pkg/crypto/tls/#Config.DecryptTicket"><code>Config.DecryptTicket</code></a>
556 methods provide a default implementation of <code>WrapSession</code> and
557 <code>UnwrapSession</code>.
560 The <a href="/pkg/crypto/tls/#ClientSessionState.ResumptionState"><code>ClientSessionState.ResumptionState</code></a> method and
561 <a href="/pkg/crypto/tls/#NewResumptionState"><code>NewResumptionState</code></a> function
562 may be used by a <code>ClientSessionCache</code> implementation to store and
568 <p><!-- CL 497376 -->
569 The package now supports the extended master secret extension (RFC 7627),
570 and enables it by default. Additionally, the deprecation of
571 <a href="/pkg/crypto/tls/#ConnectionState.TLSUnique"><code>ConnectionState.TLSUnique</code></a>
572 has been reverted, and it is populated when a connection which uses
573 extended master secret is resumed. Session tickets produced by
574 Go pre-1.21 are not interoperable with Go 1.21, meaning connections
575 resumed across versions will fall back to full handshakes.
578 <p><!-- https://go.dev/issue/44886, https://go.dev/issue/60107, CL 493655, CL 496995 -->
579 The new <a href="/pkg/crypto/tls/#QUICConn"><code>QUICConn</code></a> type
580 provides support for QUIC implementations. Note that this is not itself
581 a QUIC implementation.
584 <p><!-- https://go.dev/issue/46308, CL 497377 -->
585 The new <a href="/pkg/crypto/tls/#VersionName"><code>VersionName</code></a> function
586 returns the name for a TLS version number.
589 <p><!-- https://go.dev/issue/52113, CL 410496 -->
590 The TLS alert codes sent from the server for client authentication failures have
591 been improved. Prior to Go 1.21, these failures always resulted in a "bad certificate" alert.
592 Starting from Go 1.21, certain failures will result in more appropriate alert codes,
593 as defined by RFC 5246 and RFC 8446:
596 For TLS 1.3 connections, if the server is configured to require client authentication using
597 <a href="/pkg/crypto/tls/#RequireAnyClientCert"></code>RequireAnyClientCert</code></a> or
598 <a href="/pkg/crypto/tls/#RequireAndVerifyClientCert"></code>RequireAndVerifyClientCert</code></a>,
599 and the client does not provide any certificate, the server will now return the "certificate required" alert.
602 If the client provides a certificate that is not signed by the set of trusted certificate authorities
603 configured on the server, the server will return the "unknown certificate authority" alert.
606 If the client provides a certificate that is either expired or not yet valid,
607 the server will return the "expired certificate" alert.
610 In all other scenarios related to client authentication failures, the server still returns "bad certificate".
615 </dl><!-- crypto/tls -->
617 <dl id="crypto/x509"><dt><a href="/pkg/crypto/x509/">crypto/x509</a></dt>
619 <p><!-- https://go.dev/issue/53573, CL 468875 -->
620 <a href="/pkg/crypto/x509/#RevocationList.RevokedCertificates"><code>RevocationList.RevokedCertificates</code></a> has been deprecated and replaced with the new <a href="/pkg/crypto/x509/#RevocationList.RevokedCertificateEntries"><code>RevokedCertificateEntries</code></a> field, which is a slice of <a href="/pkg/crypto/x509/#RevocationListEntry"><code>RevocationListEntry</code></a>. <a href="/pkg/crypto/x509/#RevocationListEntry"><code>RevocationListEntry</code></a> contains all of the fields in <a href="/pkg/crypto/x509/pkix#RevokedCertificate"><code>pkix.RevokedCertificate</code></a>, as well as the revocation reason code.
623 </dl><!-- crypto/x509 -->
625 <dl id="debug/elf"><dt><a href="/pkg/debug/elf/">debug/elf</a></dt>
627 <p><!-- https://go.dev/issue/56892, CL 452617 -->
629 <a href="/pkg/debug/elf/#File.DynValue"><code>File.DynValue</code></a>
630 method may be used to retrieve the numeric values listed with a
634 <p><!-- https://go.dev/issue/56887, CL 452496 -->
635 The constant flags permitted in a <code>DT_FLAGS_1</code>
636 dynamic tag are now defined with type
637 <a href="/pkg/debug/elf/#DynFlag1"><code>DynFlag1</code></a>. These
638 tags have names starting with <code>DF_1</code>.
641 <p><!-- CL 473256 -->
642 The package now defines the constant
643 <a href="/pkg/debug/elf/#COMPRESS_ZSTD"><code>COMPRESS_ZSTD</code></a>.
646 <p><!-- https://go.dev/issue/60348, CL 496918 -->
647 The package now defines the constant
648 <a href="/pkg/debug/elf/#R_PPC64_REL24_P9NOTOC"><code>R_PPC64_REL24_P9NOTOC</code></a>.
651 </dl><!-- debug/elf -->
653 <dl id="debug/pe"><dt><a href="/pkg/debug/pe/">debug/pe</a></dt>
655 <p><!-- CL 488475 -->
656 Attempts to read from a section containing uninitialized data
658 <a href="/pkg/debug/pe/#Section.Data"><code>Section.Data</code></a>
659 or the reader returned by <a href="/pkg/debug/pe/#Section.Open"><code>Section.Open</code></a>
663 </dl><!-- debug/pe -->
665 <dl id="embed"><dt><a href="/pkg/embed/">embed</a></dt>
667 <p><!-- https://go.dev/issue/57803, CL 483235 -->
668 The <a href="/pkg/io/fs/#File"><code>io/fs.File</code></a>
670 <a href="/pkg/embed/#FS.Open"><code>FS.Open</code></a> now
671 has a <code>ReadAt</code> method that
672 implements <a href="/pkg/io/#ReaderAt"><code>io.ReaderAt</code></a>.
675 <p><!-- https://go.dev/issue/54451, CL 491175 -->
676 Calling <code><a href="/pkg/embed/FS.Open">FS.Open</a>.<a href="/pkg/io/fs/#File.Stat">Stat</a></code>
677 will return a type that now implements a <code>String</code>
679 <a href="/pkg/io/fs/#FormatFileInfo"><code>io/fs.FormatFileInfo</code></a>.
684 <dl id="errors"><dt><a href="/pkg/errors/">errors</a></dt>
686 <p><!-- https://go.dev/issue/41198, CL 473935 -->
688 <a href="/pkg/errors/#ErrUnsupported"><code>ErrUnsupported</code></a>
689 error provides a standardized way to indicate that a requested
690 operation may not be performed because it is unsupported.
691 For example, a call to
692 <a href="/pkg/os/#Link"><code>os.Link</code></a> when using a
693 file system that does not support hard links.
698 <dl id="flag"><dt><a href="/pkg/flag/">flag</a></dt>
700 <p><!-- https://go.dev/issue/53747, CL 476015 -->
701 The new <a href="/pkg/flag/#BoolFunc"><code>BoolFunc</code></a>
703 <a href="/pkg/flag/#FlagSet.BoolFunc"><code>FlagSet.BoolFunc</code></a>
704 method define a flag that does not require an argument and calls
705 a function when the flag is used. This is similar to
706 <a href="/pkg/flag/#Func"><code>Func</code></a> but for a
710 <p><!-- CL 480215 -->
712 (via <a href="/pkg/flag/#Bool"><code>Bool</code></a>,
713 <a href="/pkg/flag/#BoolVar"><code>BoolVar</code></a>,
714 <a href="/pkg/flag/#Int"><code>Int</code></a>,
715 <a href="/pkg/flag/#IntVar"><code>IntVar</code></a>, etc.)
716 will panic if <a href="/pkg/flag/#Set"><code>Set</code></a> has
717 already been called on a flag with the same name. This change is
718 intended to detect cases where <a href="#language">changes in
719 initialization order</a> cause flag operations to occur in a
720 different order than expected. In many cases the fix to this
721 problem is to introduce a explicit package dependence to
722 correctly order the definition before any
723 <a href="/pkg/flag/#Set"><code>Set</code></a> operations.
728 <dl id="go/ast"><dt><a href="/pkg/go/ast/">go/ast</a></dt>
730 <p><!-- https://go.dev/issue/28089, CL 487935 -->
731 The new <a href="/pkg/go/ast/#IsGenerated"><code>IsGenerated</code></a> predicate
732 reports whether a file syntax tree contains the
733 <a href="https://go.dev/s/generatedcode">special comment</a>
734 that conventionally indicates that the file was generated by a tool.
739 <p><!-- https://go.dev/issue/59033, CL 476276 -->
741 <a href="/pkg/go/ast/#File.GoVersion"><code>File.GoVersion</code></a>
742 field records the minimum Go version required by
743 any <code>//go:build</code> or <code>// +build</code>
749 <dl id="go/build"><dt><a href="/pkg/go/build/">go/build</a></dt>
751 <p><!-- https://go.dev/issue/56986, CL 453603 -->
752 The package now parses build directives (comments that start
753 with <code>//go:</code>) in file headers (before
754 the <code>package</code> declaration). These directives are
756 <a href="/pkg/go/build#Package"><code>Package</code></a> fields
757 <a href="/pkg/go/build#Package.Directives"><code>Directives</code></a>,
758 <a href="/pkg/go/build#Package.TestDirectives"><code>TestDirectives</code></a>,
760 <a href="/pkg/go/build#Package.XTestDirectives"><code>XTestDirectives</code></a>.
763 </dl><!-- go/build -->
765 <dl id="go/build/constraint"><dt><a href="/pkg/go/build/constraint/">go/build/constraint</a></dt>
767 <p><!-- https://go.dev/issue/59033, CL 476275 -->
769 <a href="/pkg/go/build/constraint/#GoVersion"><code>GoVersion</code></a>
770 function returns the minimum Go version implied by a build
774 </dl><!-- go/build/constraint -->
776 <dl id="go/token"><dt><a href="/pkg/go/token/">go/token</a></dt>
778 <p><!-- https://go.dev/issue/57708, CL 464515 -->
779 The new <a href="/pkg/go/token/#File.Lines"><code>File.Lines</code></a> method
780 returns the file's line-number table in the same form as accepted by
781 <code>File.SetLines</code>.
784 </dl><!-- go/token -->
786 <dl id="go/types"><dt><a href="/pkg/go/types/">go/types</a></dt>
788 <p><!-- https://go.dev/issue/61175, CL 507975 -->
789 The new <a href="/pkg/go/types/#Package.GoVersion"><code>Package.GoVersion</code></a>
790 method returns the Go language version used to check the package.
793 </dl><!-- go/types -->
795 <dl id="hash/maphash"><dt><a href="/pkg/hash/maphash/">hash/maphash</a></dt>
797 <p><!-- https://go.dev/issue/47342, CL 468795 -->
798 The <code>hash/maphash</code> package now has a pure Go implementation, selectable with the <code>purego</code> build tag.
801 </dl><!-- hash/maphash -->
803 <dl id="html/template"><dt><a href="/pkg/html/template/">html/template</a></dt>
805 <p><!-- https://go.dev/issue/59584, CL 496395 -->
807 <a href="/pkg/html/template/#ErrJSTemplate"><code>ErrJSTemplate</code></a>
808 is returned when an action appears in a JavaScript template
809 literal. Previously an unexported error was returned.
812 </dl><!-- html/template -->
814 <dl id="io/fs"><dt><a href="/pkg/io/fs/">io/fs</a></dt>
816 <p><!-- https://go.dev/issue/54451, CL 489555 -->
818 <a href="/pkg/io/fs/#FormatFileInfo"><code>FormatFileInfo</code></a>
819 function returns a formatted version of a
820 <a href="/pkg/io/fs/#FileInfo"><code>FileInfo</code></a>.
822 <a href="/pkg/io/fs/#FormatDirEntry"><code>FormatDirEntry</code></a>
823 function returns a formatted version of a
824 <a href="/pkg/io/fs/#FileInfo"><code>DirEntry</code></a>.
825 The implementation of
826 <a href="/pkg/io/fs/#DirEntry"><code>DirEntry</code></a>
828 <a href="/pkg/io/fs/#ReadDir"><code>ReadDir</code></a> now
829 implements a <code>String</code> method that calls
830 <a href="/pkg/io/fs/#FormatDirEntry"><code>FormatDirEntry</code></a>,
831 and the same is true for
832 the <a href="/pkg/io/fs/#DirEntry"><code>DirEntry</code></a>
834 <a href="/pkg/io/fs/#WalkDirFunc"><code>WalkDirFunc</code></a>.
839 <!-- https://go.dev/issue/56491 rolled back by https://go.dev/issue/60519 -->
840 <!-- CL 459435 reverted by CL 467255 -->
841 <!-- CL 467515 reverted by CL 499416 -->
843 <dl id="math/big"><dt><a href="/pkg/math/big/">math/big</a></dt>
845 <p><!-- https://go.dev/issue/56984, CL 453115, CL 500116 -->
846 The new <a href="/pkg/math/big/#Int.Float64"><code>Int.Float64</code></a>
847 method returns the nearest floating-point value to a
848 multi-precision integer, along with an indication of any
849 rounding that occurred.
852 </dl><!-- math/big -->
854 <dl id="net"><dt><a href="/pkg/net/">net</a></dt>
857 <!-- https://go.dev/issue/59166, https://go.dev/issue/56539 -->
858 <!-- CL 471136, CL 471137, CL 471140 -->
859 On Linux, the <a href="/pkg/net/">net</a> package can now use
860 Multipath TCP when the kernel supports it. It is not used by
861 default. To use Multipath TCP when available on a client, call
863 <a href="/pkg/net/#Dialer.SetMultipathTCP"><code>Dialer.SetMultipathTCP</code></a>
864 method before calling the
865 <a href="/pkg/net/#Dialer.Dial"><code>Dialer.Dial</code></a> or
866 <a href="/pkg/net/#Dialer.DialContext"><code>Dialer.DialContext</code></a>
867 methods. To use Multipath TCP when available on a server, call
869 <a href="/pkg/net/#ListenConfig.SetMultipathTCP"><code>ListenConfig.SetMultipathTCP</code></a>
870 method before calling the
871 <a href="/pkg/net/#ListenConfig.Listen"><code>ListenConfig.Listen</code></a>
872 method. Specify the network as <code>"tcp"</code> or
873 <code>"tcp4"</code> or <code>"tcp6"</code> as usual. If
874 Multipath TCP is not supported by the kernel or the remote host,
875 the connection will silently fall back to TCP. To test whether a
876 particular connection is using Multipath TCP, use the
877 <a href="/pkg/net/#TCPConn.MultipathTCP"><code>TCPConn.MultipathTCP</code></a>
881 In a future Go release we may enable Multipath TCP by default on
882 systems that support it.
887 <dl id="net/http"><dt><a href="/pkg/net/http/">net/http</a></dt>
889 <p><!-- CL 472636 -->
890 The new <a href="/pkg/net/http#ResponseController.EnableFullDuplex"><code>ResponseController.EnableFullDuplex</code></a>
891 method allows server handlers to concurrently read from an HTTP/1
892 request body while writing the response. Normally, the HTTP/1 server
893 automatically consumes any remaining request body before starting to
894 write the response, to avoid deadlocking clients which attempt to
895 write a complete request before reading the response. The
896 <code>EnableFullDuplex</code> method disables this behavior.
899 <p><!-- https://go.dev/issue/44855, CL 382117 -->
900 The new <a href="/pkg/net/http/#ErrSchemeMismatch"><code>ErrSchemeMismatch</code></a> error is returned by <a href="/pkg/net/http/#Client"><code>Client</code></a> and <a href="/pkg/net/http/#Transport"><code>Transport</code></a> when the server responds to an HTTPS request with an HTTP response.
903 <p><!-- CL 494122 -->
904 The <a href="/pkg/net/http/">net/http</a> package now supports
905 <a href="/pkg/errors/#ErrUnsupported"><code>errors.ErrUnsupported</code></a>,
906 in that the expression
907 <code>errors.Is(http.ErrNotSupported, errors.ErrUnsupported)</code>
911 </dl><!-- net/http -->
913 <dl id="os"><dt><a href="/pkg/os/">os</a></dt>
915 <p><!-- https://go.dev/issue/32558, CL 219638 -->
916 Programs may now pass an empty <code>time.Time</code> value to
917 the <a href="/pkg/os/#Chtimes"><code>Chtimes</code></a> function
918 to leave either the access time or the modification time unchanged.
921 <p><!-- CL 480135 -->
923 <a href="/pkg/os#File.Chdir"><code>File.Chdir</code></a> method
924 now changes the current directory to the file, rather than
925 always returning an error.
928 <p><!-- CL 495079 -->
929 On Unix systems, if a non-blocking descriptor is passed
930 to <a href="/pkg/os/#NewFile"><code>NewFile</code></a>, calling
931 the <a href="/pkg/os/#File.Fd"><code>File.Fd</code></a> method
932 will now return a non-blocking descriptor. Previously the
933 descriptor was converted to blocking mode.
936 <p><!-- CL 477215 -->
938 <a href="/pkg/os/#Truncate"><code>Truncate</code></a> on a
939 non-existent file used to create an empty file. It now returns
940 an error indicating that the file does not exist.
943 <p><!-- https://go.dev/issue/56899, CL 463219 -->
945 <a href="/pkg/os/#TempDir"><code>TempDir</code></a> now uses
946 GetTempPath2W when available, instead of GetTempPathW. The
947 new behavior is a security hardening measure that prevents
948 temporary files created by processes running as SYSTEM to
949 be accessed by non-SYSTEM processes.
952 <p><!-- CL 493036 -->
953 On Windows the os package now supports working with files whose
954 names, stored as UTF-16, can't be represented as valid UTF-8.
957 <p><!-- CL 463177 -->
958 On Windows <a href="/pkg/os/#Lstat"><code>Lstat</code></a> now resolves
959 symbolic links for paths ending with a path separator, consistent with its
960 behavior on POSIX platforms.
963 <p><!-- https://go.dev/issue/54451, CL 491175 -->
964 The implementation of the
965 <a href="/pkg/io/fs/#DirEntry"><code>io/fs.DirEntry</code></a>
966 interface returned by the
967 <a href="/pkg/os/#ReadDir"><code>ReadDir</code></a> function and
968 the <a href="/pkg/os/#File.ReadDir"><code>File.ReadDir</code></a>
969 method now implements a <code>String</code> method that calls
970 <a href="/pkg/io/fs/#FormatDirEntry"><code>io/fs.FormatDirEntry</code></a>.
973 <p><!-- https://go.dev/issue/53761, CL 416775, CL 498015-->
974 The implementation of the
975 <a href="/pkg/io/fs/#FS"><code>io/fs.FS</code></a> interface returned by
976 the <a href="/pkg/os/#DirFS"><code>DirFS</code></a> function now implements
977 the <a href="/pkg/io/fs/#ReadFileFS"><code>io/fs.ReadFileFS</code></a> and
978 the <a href="/pkg/io/fs/#ReadDirFS"><code>io/fs.ReadDirFS</code></a>
984 <dl id="path/filepath"><dt><a href="/pkg/path/filepath/">path/filepath</a></dt>
987 The implementation of the
988 <a href="/pkg/io/fs/#DirEntry"><code>io/fs.DirEntry</code></a>
989 interface passed to the function argument of
990 <a href="/pkg/path/filepath/#WalkDir"><code>WalkDir</code></a>
991 now implements a <code>String</code> method that calls
992 <a href="/pkg/io/fs/#FormatDirEntry"><code>io/fs.FormatDirEntry</code></a>.
995 </dl><!-- path/filepath -->
997 <!-- CL 459455 reverted -->
999 <dl id="reflect"><dt><a href="/pkg/reflect/">reflect</a></dt>
1001 <p><!-- CL 408826, CL 413474 -->
1002 In Go 1.21, <a href="/pkg/reflect/#ValueOf"><code>ValueOf</code></a>
1003 no longer forces its argument to be allocated on the heap, allowing
1004 a <code>Value</code>'s content to be allocated on the stack. Most
1005 operations on a <code>Value</code> also allow the underlying value
1006 to be stack allocated.
1009 <p><!-- https://go.dev/issue/55002 -->
1010 The new <a href="/pkg/reflect/#Value"><code>Value</code></a>
1011 method <a href="/pkg/reflect/#Value.Clear"><code>Value.Clear</code></a>
1012 clears the contents of a map or zeros the contents of a slice.
1013 This corresponds to the new <code>clear</code> built-in
1014 <a href="#language">added to the language</a>.
1017 <p><!-- https://go.dev/issue/56906, CL 452762 -->
1018 The <a href="/pkg/reflect/#SliceHeader"><code>SliceHeader</code></a>
1019 and <a href="/pkg/reflect/#StringHeader"><code>StringHeader</code></a>
1020 types are now deprecated. In new code
1021 prefer <a href="/pkg/unsafe/#Slice"><code>unsafe.Slice</code></a>,
1022 <a href="/pkg/unsafe/#SliceData"><code>unsafe.SliceData</code></a>,
1023 <a href="/pkg/unsafe/#String"><code>unsafe.String</code></a>,
1024 or <a href="/pkg/unsafe/#StringData"><code>unsafe.StringData</code></a>.
1027 </dl><!-- reflect -->
1029 <dl id="regexp"><dt><a href="/pkg/regexp/">regexp</a></dt>
1031 <p><!-- https://go.dev/issue/46159, CL 479401 -->
1032 <a href="/pkg/regexp#Regexp"><code>Regexp</code></a> now defines
1033 <a href="/pkg/regexp#Regexp.MarshalText"><code>MarshalText</code></a>
1034 and <a href="/pkg/regexp#Regexp.UnmarshalText"><code>UnmarshalText</code></a>
1035 methods. These implement
1036 <a href="/pkg/encoding#TextMarshaler"><code>encoding.TextMarshaler</code></a>
1038 <a href="/pkg/encoding#TextUnmarshaler"><code>encoding.TextUnmarshaler</code></a>
1039 and will be used by packages such as
1040 <a href="/pkg/encoding/json">encoding/json</a>.
1043 </dl><!-- regexp -->
1045 <dl id="runtime"><dt><a href="/pkg/runtime/">runtime</a></dt>
1047 <p><!-- https://go.dev/issue/38651, CL 435337 -->
1048 Textual stack traces produced by Go programs, such as those
1049 produced when crashing, calling <code>runtime.Stack</code>, or
1050 collecting a goroutine profile with <code>debug=2</code>, now
1051 include the IDs of the goroutines that created each goroutine in
1055 <p><!-- https://go.dev/issue/57441, CL 474915 -->
1056 Crashing Go applications can now opt-in to Windows Error Reporting (WER) by setting the environment variable
1057 <code>GOTRACEBACK=wer</code> or calling <a href="/pkg/runtime/debug/#SetTraceback"><code>debug.SetTraceback("wer")</code></a>
1058 before the crash. Other than enabling WER, the runtime will behave as with <code>GOTRACEBACK=crash</code>.
1059 On non-Windows systems, <code>GOTRACEBACK=wer</code> is ignored.
1062 <p><!-- CL 447778 -->
1063 <code>GODEBUG=cgocheck=2</code>, a thorough checker of cgo pointer passing rules,
1064 is no longer available as a <a href="/pkg/runtime#hdr-Environment_Variables">debug option</a>.
1065 Instead, it is available as an experiment using <code>GOEXPERIMENT=cgocheck2</code>.
1066 In particular this means that this mode has to be selected at build time instead of startup time.
1070 <code>GODEBUG=cgocheck=1</code> is still available (and is still the default).
1073 <p><!-- https://go.dev/issue/46787, CL 367296 -->
1074 A new type <code>Pinner</code> has been added to the runtime
1075 package. <code>Pinner</code>s may be used to "pin" Go memory
1076 such that it may be used more freely by non-Go code. For instance,
1077 passing Go values that reference pinned Go memory to C code is
1078 now allowed. Previously, passing any such nested reference was
1080 <a href="https://pkg.go.dev/cmd/cgo#hdr-Passing_pointers">cgo pointer passing rules.</a>
1082 See <a href="/pkg/runtime#Pinner">the docs</a> for more details.
1085 <!-- CL 472195 no release note needed -->
1087 </dl><!-- runtime -->
1089 <dl id="runtime/metrics"><dt><a href="/pkg/runtime/metrics/">runtime/metrics</a></dt>
1091 <p><!-- https://go.dev/issue/56857, CL 497315 -->
1092 A few previously-internal GC metrics, such as live heap size, are
1095 <code>GOGC</code> and <code>GOMEMLIMIT</code> are also now
1096 available as metrics.
1099 </dl><!-- runtime/metrics -->
1101 <dl id="runtime/trace"><dt><a href="/pkg/runtime/trace/">runtime/trace</a></dt>
1103 <p><!-- https://go.dev/issue/16638 -->
1104 Collecting traces on amd64 and arm64 now incurs a substantially
1105 smaller CPU cost: up to a 10x improvement over the previous release.
1108 <p><!-- CL 494495 -->
1109 Traces now contain explicit stop-the-world events for every reason
1110 the Go runtime might stop-the-world, not just garbage collection.
1113 </dl><!-- runtime/trace -->
1115 <dl id="sync"><dt><a href="/pkg/sync/">sync</a></dt>
1117 <p><!-- https://go.dev/issue/56102, CL 451356 -->
1118 The new <a href="/pkg/sync/#OnceFunc"><code>OnceFunc</code></a>,
1119 <a href="/pkg/sync/#OnceValue"><code>OnceValue</code></a>, and
1120 <a href="/pkg/sync/#OnceValues"><code>OnceValues</code></a>
1121 functions capture a common use of <a href="/pkg/sync/#Once">Once</a> to
1122 lazily initialize a value on first use.
1127 <dl id="syscall"><dt><a href="/pkg/syscall/">syscall</a></dt>
1129 <p><!-- CL 480135 -->
1131 <a href="/pkg/syscall#Fchdir"><code>Fchdir</code></a> function
1132 now changes the current directory to its argument, rather than
1133 always returning an error.
1136 <p><!-- https://go.dev/issue/46259, CL 458335 -->
1138 <a href="/pkg/syscall#SysProcAttr"><code>SysProcAttr</code></a>
1139 has a new field <code>Jail</code> that may be used to put the
1140 newly created process in a jailed environment.
1143 <p><!-- CL 493036 -->
1144 On Windows the syscall package now supports working with files whose
1145 names, stored as UTF-16, can't be represented as valid UTF-8.
1146 The <a href="/pkg/syscall#UTF16ToString"><code>UTF16ToString</code></a>
1147 and <a href="/pkg/syscall#UTF16FromString"><code>UTF16FromString</code></a>
1148 functions now convert between UTF-16 data and
1149 <a href="https://simonsapin.github.io/wtf-8/">WTF-8</a> strings.
1150 This is backward compatible as WTF-8 is a superset of the UTF-8
1151 format that was used in earlier releases.
1154 <p><!-- CL 476578, CL 476875, CL 476916 -->
1155 Several error values match the new
1156 <a href="/pkg/errors/#ErrUnsupported"><code>errors.ErrUnsupported</code></a>,
1157 such that <code>errors.Is(err, errors.ErrUnsupported)</code>
1160 <li><code>ENOSYS</code></li>
1161 <li><code>ENOTSUP</code></li>
1162 <li><code>EOPNOTSUPP</code></li>
1163 <li><code>EPLAN9</code> (Plan 9 only)</li>
1164 <li><code>ERROR_CALL_NOT_IMPLEMENTED</code> (Windows only)</li>
1165 <li><code>ERROR_NOT_SUPPORTED</code> (Windows only)</li>
1166 <li><code>EWINDOWS</code> (Windows only)</li>
1170 </dl><!-- syscall -->
1172 <dl id="testing"><dt><a href="/pkg/testing/">testing</a></dt>
1174 <p><!-- https://go.dev/issue/37708, CL 463837 -->
1175 The new <code>-test.fullpath</code> option will print full path
1176 names in test log messages, rather than just base names.
1179 <p><!-- https://go.dev/issue/52600, CL 475496 -->
1180 The new <a href="/pkg/testing/#Testing"><code>Testing</code></a> function reports whether the program is a test created by <code>go</code> <code>test</code>.
1183 </dl><!-- testing -->
1185 <dl id="testing/fstest"><dt><a href="/pkg/testing/fstest/">testing/fstest</a></dt>
1187 <p><!-- https://go.dev/issue/54451, CL 491175 -->
1188 Calling <code><a href="/pkg/testing/fstest/MapFS.Open">Open</a>.<a href="/pkg/io/fs/#File.Stat">Stat</a></code>
1189 will return a type that now implements a <code>String</code>
1191 <a href="/pkg/io/fs/#FormatFileInfo"><code>io/fs.FormatFileInfo</code></a>.
1194 </dl><!-- testing/fstest -->
1196 <dl id="unicode"><dt><a href="/pkg/unicode/">unicode</a></dt>
1198 <p><!-- CL 456837 -->
1199 The <a href="/pkg/unicode/"><code>unicode</code></a> package and
1200 associated support throughout the system has been upgraded to
1201 <a href="https://www.unicode.org/versions/Unicode15.0.0/">Unicode 15.0.0</a>.
1204 </dl><!-- unicode -->
1206 <h2 id="ports">Ports</h2>
1208 <h3 id="darwin">Darwin</h3>
1210 <p><!-- https://go.dev/issue/57125 -->
1211 As <a href="go1.20#darwin">announced</a> in the Go 1.20 release notes,
1212 Go 1.21 requires macOS 10.15 Catalina or later;
1213 support for previous versions has been discontinued.
1216 <h3 id="windows">Windows</h3>
1218 <p><!-- https://go.dev/issue/57003, https://go.dev/issue/57004 -->
1219 As <a href="go1.20#windows">announced</a> in the Go 1.20 release notes,
1220 Go 1.21 requires at least Windows 10 or Windows Server 2016;
1221 support for previous versions has been discontinued.
1226 <!-- cmd/dist: default to GOARM=7 on all non-arm systems -->
1229 <h3 id="wasm">WebAssembly</h3>
1231 <p><!-- https://go.dev/issue/38248, https://go.dev/issue/59149, CL 489255 -->
1232 The new <code>go:wasmimport</code> directive can now be used in Go programs
1233 to import functions from the WebAssembly host.
1236 <!-- https://go.dev/issue/56100 -->
1238 The Go scheduler now interacts much more efficiently with the
1239 JavaScript event loop, especially in applications that block
1240 frequently on asynchronous events.
1244 <h3 id="wasip1">WebAssembly System Interface</h3>
1246 <p><!-- https://go.dev/issue/58141 -->
1247 Go 1.21 adds an experimental port to the <a href="https://wasi.dev/">
1248 WebAssembly System Interface (WASI)</a>, Preview 1
1249 (<code>GOOS=wasip1</code>, <code>GOARCH=wasm</code>).
1253 As a result of the addition of the new <code>GOOS</code> value
1254 "<code>wasip1</code>", Go files named <code>*_wasip1.go</code>
1255 will now be <a href="/pkg/go/build/#hdr-Build_Constraints">ignored
1256 by Go tools</a> except when that <code>GOOS</code> value is being
1258 If you have existing filenames matching that pattern, you will
1259 need to rename them.
1262 <h3 id="PPC64">ppc64/ppc64le</h3>
1264 <p><!-- go.dev/issue/44549 -->
1265 On Linux, <code>GOPPC64=power10</code> now generates PC-relative instructions, prefixed
1266 instructions, and other new Power10 instructions. On AIX, <code>GOPPC64=power10</code>
1267 generates Power10 instructions, but does not generate PC-relative instructions.
1271 When building position-independent binaries for <code>GOPPC64=power10</code>
1272 <code>GOOS=linux</code> <code>GOARCH=ppc64le</code>, users can expect reduced binary
1273 sizes in most cases, in some cases 3.5%. Position-independent binaries are built for
1274 ppc64le with the following <code>-buildmode</code> values:
1275 <code>c-archive</code>, <code>c-shared</code>, <code>shared</code>, <code>pie</code>, <code>plugin</code>.
1278 <h3 id="loong64">loong64</h3>
1280 <p><!-- go.dev/issue/53301, CL 455075, CL 425474, CL 425476, CL 425478, CL 489576 -->
1281 The <code>linux/loong64</code> port now supports <code>-buildmode=c-archive</code>,
1282 <code>-buildmode=c-shared</code> and <code>-buildmode=pie</code>.
1285 <!-- proposals for x repos that don't need to be mentioned here but
1286 are picked up by the relnote tool. -->
1287 <!-- https://go.dev/issue/54232 -->
1288 <!-- https://go.dev/issue/57051 -->
1289 <!-- https://go.dev/issue/57792 -->
1290 <!-- https://go.dev/issue/57906 -->
1291 <!-- https://go.dev/issue/58668 -->
1292 <!-- https://go.dev/issue/59016 -->
1293 <!-- https://go.dev/issue/59676 -->
1294 <!-- https://go.dev/issue/60409 -->
1295 <!-- https://go.dev/issue/61176 -->
1297 <!-- changes to cmd/api that don't need release notes. -->
1298 <!-- CL 469115, CL 469135, CL 499981 -->
1300 <!-- proposals that don't need release notes. -->
1301 <!-- https://go.dev/issue/10275 -->
1302 <!-- https://go.dev/issue/59719 -->