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>.
154 TODO: complete this section
157 <h2 id="tools">Tools</h2>
159 Go 1.21 adds improved support for backwards compatibility and forwards compatibility
163 <p><!-- https://go.dev/issue/56986 -->
164 To improve backwards compatibility, Go 1.21 formalizes
165 Go's use of the GODEBUG environment variable to control
166 the default behavior for changes that are non-breaking according to the
167 <a href="/doc/go1compat">compatibility policy</a>
168 but nonetheless may cause existing programs to break.
169 (For example, programs that depend on buggy behavior may break
170 when a bug is fixed, but bug fixes are not considered breaking changes.)
171 When Go must make this kind of behavior change,
172 it now chooses between the old and new behavior based on the
173 <code>go</code> line in the workspace's <code>go.work</code> file
174 or else the main module's <code>go.mod</code> file.
175 Upgrading to a new Go toolchain but leaving the <code>go</code> line
176 set to its original (older) Go version preserves the behavior of the older
178 With this compatibility support, the latest Go toolchain should always
179 be the best, most secure, implementation of an older version of Go.
180 See “<a href="/doc/godebug">Go, Backwards Compatibility, and GODEBUG</a>” for details.
183 <p><!-- https://go.dev/issue/57001 -->
184 To improve forwards compatibility, Go 1.21 now reads the <code>go</code> line
185 in a <code>go.work</code> or <code>go.mod</code> file as a strict
186 minimum requirement: <code>go</code> <code>1.21.0</code> means
187 that the workspace or module cannot be used with Go 1.20 or with Go 1.21rc1.
188 This allows projects that depend on fixes made in later versions of Go
189 to ensure that they are not used with earlier versions.
190 It also gives better error reporting for projects that make use of new Go features:
191 when the problem is that a newer Go version is needed,
192 that problem is reported clearly, instead of attempting to build the code
193 and instead printing errors about unresolved imports or syntax errors.
197 To make these new stricter version requirements easier to manage,
198 the <code>go</code> command can now invoke not just the toolchain
199 bundled in its own release but also other Go toolchain versions found in the PATH
200 or downloaded on demand.
201 If a <code>go.mod</code> or <code>go.work</code> <code>go</code> line
202 declares a minimum requirement on a newer version of Go, the <code>go</code>
203 command will find and run that version automatically.
204 The new <code>toolchain</code> directive sets a suggested minimum toolchain to use,
205 which may be newer than the strict <code>go</code> minimum.
206 See “<a href="/doc/toolchain">Go Toolchains</a>” for details.
209 <h3 id="go-command">Go command</h3>
211 <p><!-- https://go.dev/issue/58099, CL 474236 -->
212 The <code>-pgo</code> build flag now defaults to <code>-pgo=auto</code>,
213 and the restriction of specifying a single main package on the command
214 line is now removed. If a file named <code>default.pgo</code> is present
215 in the main package's directory, the <code>go</code> command will use
216 it to enable profile-guided optimization for building the corresponding
221 The <code>-C</code> <code>dir</code> flag must now be the first
222 flag on the command-line when used.
225 <p><!-- https://go.dev/issue/37708, CL 463837 -->
226 The new <code>go</code> <code>test</code> option
227 <code>-fullpath</code> prints full path names in test log messages,
228 rather than just base names.
231 <p><!-- https://go.dev/issue/15513, CL 466397 -->
232 The <code>go</code> <code>test</code> <code>-c</code> flag now
233 supports writing test binaries for multiple packages, each to
234 <code>pkg.test</code> where <code>pkg</code> is the package name.
235 It is an error if more than one test package being compiled has a given package name.]
238 <p><!-- https://go.dev/issue/15513, CL 466397 -->
239 The <code>go</code> <code>test</code> <code>-o</code> flag now
240 accepts a directory argument, in which case test binaries are written to that
241 directory instead of the current directory.
244 <h3 id="cgo">Cgo</h3>
246 <p><!-- CL 490819 -->
247 In files that <code>import "C"</code>, the Go toolchain now
248 correctly reports errors for attempts to declare Go methods on C types.
251 <h2 id="runtime-changes">Runtime</h2>
254 TODO: complete this section, or delete if not needed
257 <p><!-- https://go.dev/issue/7181 -->
258 When printing very deep stacks, the runtime now prints the first 50
259 (innermost) frames followed by the bottom 50 (outermost) frames,
260 rather than just printing the first 100 frames. This makes it easier
261 to see how deeply recursive stacks started, and is especially
262 valuable for debugging stack overflows.
265 <p><!-- https://go.dev/issue/59960 -->
266 On Linux platforms that support transparent huge pages, the Go runtime
267 now manages which parts of the heap may be backed by huge pages more
268 explicitly. This leads to better utilization of memory: small heaps
269 should see less memory used (up to 50% in pathological cases) while
270 large heaps should see fewer broken huge pages for dense parts of the
271 heap, improving CPU usage and latency by up to 1%.
274 <p><!-- https://go.dev/issue/57069, https://go.dev/issue/56966 -->
275 As a result of runtime-internal garbage collection tuning,
276 applications may see up to a 40% reduction in application tail latency
277 and a small decrease in memory use. Some applications may also observe
278 a small loss in throughput.
280 The memory use decrease should be proportional to the loss in
281 throughput, such that the previous release's throughput/memory
282 tradeoff may be recovered (with little change to latency) by
283 increasing <code>GOGC</code> and/or <code>GOMEMLIMIT</code> slightly.
286 <p><!-- https://go.dev/issue/51676 -->
287 Calls from C to Go on threads created in C require some setup to prepare for
288 Go execution. On Unix platforms, this setup is now preserved across multiple
289 calls from the same thread. This significantly reduces the overhead of
290 subsequent C to Go calls from ~1-3 microseconds per call to ~100-200
291 nanoseconds per call.
294 <h2 id="compiler">Compiler</h2>
297 Profile-guide optimization (PGO), added as a preview in Go 1.20, is now ready
298 for general use. PGO enables additional optimizations on code identified as
299 hot by profiles of production workloads. As mentioned in the
300 <a href="#go-command">Go command section</a>, PGO is enabled by default for
301 binaries that contain a <code>default.pgo</code> profile in the main
302 package directory. Performance improvements vary depending on application
303 behavior, with most programs from a representative set of Go programs seeing
304 between 2 and 7% improvement from enabling PGO. See the
305 <a href="/doc/pgo">PGO user guide</a> for detailed documentation.
308 <!-- https://go.dev/issue/59959 -->
310 PGO builds can now devirtualize some interface method calls, adding a
311 concrete call to the most common callee. This enables further optimization,
312 such as inlining the callee.
316 TODO: complete this section, or delete if not needed
319 <h2 id="assembler">Assembler</h2>
321 <!-- https://go.dev/issue/58378 -->
323 On amd64, frameless nosplit assembly functions are no longer automatically marked as <code>NOFRAME</code>.
324 Instead, the <code>NOFRAME</code> attribute must be explicitly specified if desired,
325 which is already the behavior on other architectures supporting frame pointers.
326 With this, the runtime now maintains the frame pointers for stack transitions.
331 The verifier that checks for incorrect uses of <code>R15</code> when dynamic linking on amd64 has been improved.
334 <h2 id="linker">Linker</h2>
336 <p><!-- https://go.dev/issue/57302, CL 461749 -->
337 On Windows AMD64, the linker (with help from the compiler) now emits
338 SEH unwinding data by default, which improves the integration
339 of Go applications with Windows debuggers and other tools.
344 <!-- cmd/link: generate .xdata PE section -->
346 <!-- CL 463395, CL 461315 -->
348 In Go 1.21 the linker (with help from the compiler) is now capable of
349 deleting dead (unreferenced) global map variables, if the number of
350 entries in the variable initializer is sufficiently large, and if the
351 initializer expressions are side-effect free.
354 TODO: complete this section, or delete if not needed
357 <h2 id="library">Core library</h2>
359 <h3 id="slog">New log/slog package</h3>
361 <p><!-- https://go.dev/issue/59060, https://go.dev/issue/59141, https://go.dev/issue/59204, https://go.dev/issue/59280,
362 https://go.dev/issue/59282, https://go.dev/issue/59339, https://go.dev/issue/59345,
363 CL 477295, CL 484096, CL 486376, CL 486415, CL 487855 -->
364 The new <a href="/pkg/log/slog">log/slog</a> package provides structured logging with levels.
365 Structured logging emits key-value pairs
366 to enable fast, accurate processing of large amounts of log data.
367 The package supports integration with popular log analysis tools and services.
370 <h3 id="slogtest">New testing/slogtest package</h3>
372 <p><!-- CL 487895 -->
373 The new <a href="/pkg/testing/slogtest">testing/slogtest</a> package can help
374 to validate <a href="/pkg/log/slog#Handler">slog.Handler</a> implementations.
377 <h3 id="slices">New slices package</h3>
380 <!-- https://go.dev/issue/45955, https://go.dev/issue/54768 -->
381 <!-- https://go.dev/issue/57348, https://go.dev/issue/57433 -->
382 <!-- https://go.dev/issue/58565, https://go.dev/issue/60091 -->
383 <!-- CL 467417, CL 468855, CL 483175, CL 496078, CL 498175 -->
384 The new <a href="/pkg/slices">slices</a> package provides many common
385 operations on slices, using generic functions that work with slices
389 <h3 id="maps">New maps package</h3>
391 <p><!-- https://go.dev/issue/57436, CL 464343 -->
392 The new <a href="/pkg/maps/">maps</a> package provides several
393 common operations on maps, using generic functions that work with
394 maps of any key or element type.
397 <h3 id="cmp">New cmp package</h3>
399 <p><!-- https://go.dev/issue/59488, CL 469356 -->
400 The new <a href="/pkg/cmp/">cmp</a> package defines the type
401 constraint <a href="/pkg/cmp/#Ordered"><code>Ordered</code></a> and
402 two new generic functions
403 <a href="/pkg/cmp/#Less"><code>Less</code></a>
404 and <a href="/pkg/cmp/#Compare"><code>Compare</code></a> that are
405 useful with <a href="/ref/spec/#Comparison_operators">ordered
409 <h3 id="minor_library_changes">Minor changes to the library</h3>
412 As always, there are various minor changes and updates to the library,
413 made with the Go 1 <a href="/doc/go1compat">promise of compatibility</a>
415 There are also various performance improvements, not enumerated here.
419 TODO: complete this section
422 <dl id="archive/tar"><dt><a href="/pkg/archive/tar/">archive/tar</a></dt>
424 <p><!-- https://go.dev/issue/54451, CL 491175 -->
425 The implementation of the
426 <a href="/pkg/io/fs/#FileInfo"><code>io/fs.FileInfo</code></a>
427 interface returned by
428 <a href="/pkg/archive/tar/#Header.FileInfo"><code>Header.FileInfo</code></a>
429 now implements a <code>String</code> method that calls
430 <a href="/pkg/io/fs/#FormatFileInfo"><code>io/fs.FormatFileInfo</code></a>.
433 </dl><!-- archive/tar -->
435 <dl id="archive/zip"><dt><a href="/pkg/archive/zip/">archive/zip</a></dt>
437 <p><!-- https://go.dev/issue/54451, CL 491175 -->
438 The implementation of the
439 <a href="/pkg/io/fs/#FileInfo"><code>io/fs.FileInfo</code></a>
440 interface returned by
441 <a href="/pkg/archive/zip/#FileHeader.FileInfo"><code>FileHeader.FileInfo</code></a>
442 now implements a <code>String</code> method that calls
443 <a href="/pkg/io/fs/#FormatFileInfo"><code>io/fs.FormatFileInfo</code></a>.
446 <p><!-- https://go.dev/issue/54451, CL 491175 -->
447 The implementation of the
448 <a href="/pkg/io/fs/#DirEntry"><code>io/fs.DirEntry</code></a>
449 interface returned by the
450 <a href="/pkg/io/fs/#ReadDirFile.ReadDir"><code>io/fs.ReadDirFile.ReadDir</code></a>
452 <a href="/pkg/io/fs/#File"><code>io/fs.File</code></a>
454 <a href="/pkg/archive/zip/#Reader.Open"><code>Reader.Open</code></a>
455 now implements a <code>String</code> method that calls
456 <a href="/pkg/io/fs/#FormatDirEntry"><code>io/fs.FormatDirEntry</code></a>.
459 </dl><!-- archive/zip -->
461 <dl id="bytes"><dt><a href="/pkg/bytes/">bytes</a></dt>
463 <p><!-- https://go.dev/issue/53685, CL 474635 -->
464 The <a href="/pkg/bytes/#Buffer"><code>Buffer</code></a> type
466 <a href="/pkg/bytes/#Buffer.Available"><code>Available</code></a>
467 and <a href="/pkg/bytes/#AvailableBuffer"><code>AvailableBuffer</code></a>.
468 These may be used along with the
469 <a href="/pkg/bytes/#Buffer.Write"><code>Write</code></a>
470 method to append directly to the <code>Buffer</code>.
475 <dl id="context"><dt><a href="/pkg/context/">context</a></dt>
477 <p><!-- https://go.dev/issue/40221, CL 479918 -->
478 The new <a href="/pkg/context/#WithoutCancel"><code>WithoutCancel</code></a>
479 function returns a copy of a context that is not canceled when the original
482 <p><!-- https://go.dev/issue/56661, CL 449318 -->
483 The new <a href="/pkg/context/#WithDeadlineCause"><code>WithDeadlineCause</code></a>
484 and <a href="/pkg/context/#WithTimeoutCause"><code>WithTimeoutCause</code></a>
485 functions provide a way to set a context cancellation cause when a deadline or
486 timer expires. The cause may be retrieved with the
487 <a href="/pkg/context/#Cause"><code>Cause</code></a> function.
489 <p><!-- https://go.dev/issue/57928, CL 482695 -->
490 The new <a href="/pkg/context/#AfterFunc"><code>AfterFunc</code></a>
491 function registers a function to run after a context has been cancelled.
496 <dl id="crypto/elliptic"><dt><a href="/pkg/crypto/elliptic/">crypto/elliptic</a></dt>
498 <p><!-- CL 459977 -->
499 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>.
502 </dl><!-- crypto/elliptic -->
504 <dl id="crypto/rand"><dt><a href="/pkg/crypto/rand/">crypto/rand</a></dt>
506 <p><!-- CL 463123 -->
507 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.
510 </dl><!-- crypto/rand -->
512 <dl id="crypto/rsa"><dt><a href="/pkg/crypto/rsa/">crypto/rsa</a></dt>
514 <p><!-- CL 471259, CL 492935 -->
515 The performance of private RSA operations (decryption and signing) is now better than Go 1.19 for <code>GOOS=amd64</code> and <code>GOOS=arm64</code>. It had regressed in Go 1.20.
518 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.
520 <p><!-- https://go.dev/issue/56921, CL 459976 -->
521 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.
524 </dl><!-- crypto/rsa -->
526 <!-- CL 483815 reverted -->
528 <dl id="crypto/sha256"><dt><a href="/pkg/crypto/sha256/">crypto/sha256</a></dt>
530 <p><!-- https://go.dev/issue/50543, CL 408795 -->
531 SHA-224 and SHA-256 operations now use native instructions when available when <code>GOOS=amd64</code>, providing a performance improvement on the order of 3-4x.
534 </dl><!-- crypto/sha256 -->
536 <!-- CL 481478 reverted -->
537 <!-- CL 483816 reverted -->
539 <dl id="crypto/tls"><dt><a href="/pkg/crypto/tls/">crypto/tls</a></dt>
541 <p><!-- https://go.dev/issue/60105 -->
542 Applications can now control the content of session tickets.
545 The new <a href="/pkg/crypto/tls/#SessionState"><code>SessionState</code></a> type
546 describes a resumable session.
549 The <a href="/pkg/crypto/tls/#SessionState.Bytes"><code>SessionState.Bytes</code></a>
550 method and <a href="/pkg/crypto/tls/#ParseSessionState"><code>ParseSessionState</code></a>
551 function serialize and deserialize a <code>SessionState</code>.
554 The <a href="/pkg/crypto/tls/#Config.WrapSession"><code>Config.WrapSession</code></a> and
555 <a href="/pkg/crypto/tls/#Config.UnwrapSession"><code>Config.UnwrapSession</code></a>
556 hooks convert a <code>SessionState</code> to and from a ticket.
559 The <a href="/pkg/crypto/tls/#Config.EncryptTicket"><code>Config.EncryptTicket</code></a>
560 and <a href="/pkg/crypto/tls/#Config.DecryptTicket"><code>Config.DecryptTicket</code></a>
561 methods provide a default implementation of <code>WrapSession</code> and
562 <code>UnwrapSession</code>.
565 The <a href="/pkg/crypto/tls/#ClientSessionState.ResumptionState"><code>ClientSessionState.ResumptionState</code></a> method and
566 <a href="/pkg/crypto/tls/#NewResumptionState"><code>NewResumptionState</code></a> function
567 may be used by a <code>ClientSessionCache</code> implementation to store and
573 <p><!-- CL 497376 -->
574 The package now supports the extended master secret extension (RFC 7627),
575 and enables it by default. Additionally, the deprecation of
576 <a href="/pkg/crypto/tls/#ConnectionState.TLSUnique"><code>ConnectionState.TLSUnique</code></a>
577 has been reverted, and it is populated when a connection which uses
578 extended master secret is resumed. Session tickets produced by
579 Go pre-1.21 are not interoperable with Go 1.21, meaning connections
580 resumed across versions will fall back to full handshakes.
583 <p><!-- https://go.dev/issue/44886, https://go.dev/issue/60107 -->
584 The new <a href="/pkg/crypto/tls/#QUICConn"><code>QUICConn</code></a> type
585 provides support for QUIC implementations. Note that this is not itself
586 a QUIC implementation.
589 <p><!-- https://go.dev/issue/46308 -->
590 The new <a href="/pkg/crypto/tls/#VersionName"></code>VersionName</code></a> function
591 returns the name for a TLS version number.
594 </dl><!-- crypto/tls -->
596 <dl id="crypto/x509"><dt><a href="/pkg/crypto/x509/">crypto/x509</a></dt>
598 <p><!-- https://go.dev/issue/53573, CL 468875 -->
599 <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.Entries"><code>RevocationList.Entries</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.
602 </dl><!-- crypto/x509 -->
604 <dl id="debug/elf"><dt><a href="/pkg/debug/elf/">debug/elf</a></dt>
606 <p><!-- https://go.dev/issue/56892, CL 452617 -->
608 <a href="/pkg/debug/elf/#File.DynValue"><code>File.DynValue</code></a>
609 method may be used to retrieve the numeric values listed with a
613 <p><!-- https://go.dev/issue/56887, CL 452496 -->
614 The constant flags permitted in a <code>DT_FLAGS_1</code>
615 dynamic tag are now defined with type
616 <a href="/pkg/debug/elf/#DynFlag1"><code>DynFlag1</code></a>. These
617 tags have names starting with <code>DF_1</code>.
620 <p><!-- CL 473256 -->
621 The package now defines the constant
622 <a href="/pkg/debug/elf/#COMPRESS_ZSTD"><code>COMPRESS_ZSTD</code></a>.
625 <p><!-- https://go.dev/issue/60348, CL 496918 -->
626 The package now defines the constant
627 <a href="/pkg/debug/elf/#R_PPC64_REL24_P9NOTOC"><code>R_PPC64_REL24_P9NOTOC</code></a>.
630 </dl><!-- debug/elf -->
632 <dl id="debug/pe"><dt><a href="/pkg/debug/pe/">debug/pe</a></dt>
634 <p><!-- CL 488475 -->
635 Attempts to read from a section containing uninitialized data
637 <a href="/pkg/debug/pe/#Section.Data"><code>Section.Data</code></a>
638 or the reader returned by <a href="/pkg/debug/pe/#Section.Open"><code>Section.Open</code></a>
642 </dl><!-- debug/pe -->
644 <dl id="embed"><dt><a href="/pkg/embed/">embed</a></dt>
646 <p><!-- https://go.dev/issue/57803, CL 483235 -->
647 The <a href="/pkg/io/fs/#File"><code>io/fs.File</code></a>
649 <a href="/pkg/embed/#FS.Open"><code>FS.Open</code></a> now
650 has a <code>ReadAt</code> method that
651 implements <a href="/pkg/io/#ReaderAt"><code>io.ReaderAt</code></a>.
654 <p><!-- https://go.dev/issue/54451, CL 491175 -->
655 Calling <code><a href="/pkg/embed/FS.Open">FS.Open</a>.<a href="/pkg/io/fs/#File.Stat">Stat</a></code>
656 will return a type that now implements a <code>String</code>
658 <a href="/pkg/io/fs/#FormatFileInfo"><code>io/fs.FormatFileInfo</code></a>.
663 <dl id="errors"><dt><a href="/pkg/errors/">errors</a></dt>
665 <p><!-- https://go.dev/issue/41198, CL 473935 -->
667 <a href="/pkg/errors/#ErrUnsupported"><code>ErrUnsupported</code></a>
668 error provides a standardized way to indicate that a requested
669 operation may not be performed because it is unsupported.
670 For example, a call to
671 <a href="/pkg/os/#Link"><code>os.Link</code></a> when using a
672 file system that does not support hard links.
677 <dl id="flag"><dt><a href="/pkg/flag/">flag</a></dt>
679 <p><!-- https://go.dev/issue/53747, CL 476015 -->
680 The new <a href="/pkg/flag/#BoolFunc"><code>BoolFunc</code></a>
682 <a href="/pkg/flag/#FlagSet.BoolFunc"><code>FlagSet.BoolFunc</code></a>
683 method define a flag that does not require an argument and calls
684 a function when the flag is used. This is similar to
685 <a href="/pkg/flag/#Func"><code>Func</code></a> but for a
689 <p><!-- CL 480215 -->
691 (via <a href="/pkg/flag/#Bool"><code>Bool</code></a>,
692 <a href="/pkg/flag/#BoolVar"><code>BoolVar</code></a>,
693 <a href="/pkg/flag/#Int"><code>Int</code></a>,
694 <a href="/pkg/flag/#IntVar"><code>IntVar</code></a>, etc.)
695 will panic if <a href="/pkg/flag/#Set"><code>Set</code></a> has
696 already been called on a flag with the same name. This change is
697 intended to detect cases where <a href="#language">changes in
698 initialization order</a> cause flag operations to occur in a
699 different order than expected. In many cases the fix to this
700 problem is to introduce a explicit package dependence to
701 correctly order the definition before any
702 <a href="/pkg/flag/#Set"><code>Set</code></a> operations.
707 <dl id="go/ast"><dt><a href="/pkg/go/ast/">go/ast</a></dt>
709 <p><!-- https://go.dev/issue/28089, CL 487935 -->
710 The new <a href="/pkg/go/ast/#IsGenerated"><code>IsGenerated</code></a> predicate
711 reports whether a file syntax tree contains the
712 <a href="https://go.dev/s/generatedcode">special comment</a>
713 that conventionally indicates that the file was generated by a tool.
718 <p><!-- https://go.dev/issue/59033, CL 476276 -->
720 <a href="/pkg/go/ast/#File.GoVersion"><code>File.GoVersion</code></a>
721 field records the minimum Go version required by
722 any <code>//go:build</code> or <code>// +build</code>
728 <dl id="go/build"><dt><a href="/pkg/go/build/">go/build</a></dt>
730 <p><!-- https://go.dev/issue/56986, CL 453603 -->
731 The package now parses build directives (comments that start
732 with <code>//go:</code>) in file headers (before
733 the <code>package</code> declaration). These directives are
735 <a href="/pkg/go/build#Package"><code>Package</code></a> fields
736 <a href="/pkg/go/build#Package.Directives"><code>Directives</code></a>,
737 <a href="/pkg/go/build#Package.TestDirectives"><code>TestDirectives</code></a>,
739 <a href="/pkg/go/build#Package.XTestDirectives"><code>XTestDirectives</code></a>.
742 </dl><!-- go/build -->
744 <dl id="go/build/constraint"><dt><a href="/pkg/go/build/constraint/">go/build/constraint</a></dt>
746 <p><!-- https://go.dev/issue/59033, CL 476275 -->
748 <a href="/pkg/go/build/constraint/#GoVersion"><code>GoVersion</code></a>
749 function returns the minimum Go version implied by a build
753 </dl><!-- go/build/constraint -->
755 <dl id="go/token"><dt><a href="/pkg/go/token/">go/token</a></dt>
757 <p><!-- https://go.dev/issue/57708, CL 464515 -->
758 The new <a href="/pkg/go/token/#File.Lines"><code>File.Lines</code></a> method
759 returns the file's line-number table in the same form as accepted by
760 <code>File.SetLines</code>.
763 </dl><!-- go/token -->
765 <dl id="hash/maphash"><dt><a href="/pkg/hash/maphash/">hash/maphash</a></dt>
767 <p><!-- https://go.dev/issue/47342, CL 468795 -->
768 The <code>hash/maphash</code> package now has a pure Go implementation, selectable with the <code>purego</code> build tag.
771 </dl><!-- hash/maphash -->
773 <dl id="html/template"><dt><a href="/pkg/html/template/">html/template</a></dt>
775 <p><!-- https://go.dev/issue/59584, CL 496395 -->
777 <a href="/pkg/html/template/#ErrJSTemplate"><code>ErrJSTemplate</code></a>
778 is returned when an action appears in a JavaScript template
779 literal. Previously an unexported error was returned.
782 </dl><!-- html/template -->
784 <dl id="io/fs"><dt><a href="/pkg/io/fs/">io/fs</a></dt>
786 <p><!-- https://go.dev/issue/54451, CL 489555 -->
788 <a href="/pkg/io/fs/#FormatFileInfo"><code>FormatFileInfo</code></a>
789 function returns a formatted version of a
790 <a href="/pkg/io/fs/#FileInfo"><code>FileInfo</code></a>.
792 <a href="/pkg/io/fs/#FormatDirEntry"><code>FormatDirEntry</code></a>
793 function returns a formatted version of a
794 <a href="/pkg/io/fs/#FileInfo"><code>DirEntry</code></a>.
795 The implementation of
796 <a href="/pkg/io/fs/#DirEntry"><code>DirEntry</code></a>
798 <a href="/pkg/io/fs/#ReadDir"><code>ReadDir</code></a> now
799 implements a <code>String</code> method that calls
800 <a href="/pkg/io/fs/#FormatDirEntry"><code>FormatDirEntry</code></a>,
801 and the same is true for
802 the <a href="/pkg/io/fs/#DirEntry"><code>DirEntry</code></a>
804 <a href="/pkg/io/fs/#WalkDirFunc"><code>WalkDirFunc</code></a>.
809 <!-- https://go.dev/issue/56491 rolled back by https://go.dev/issue/60519 -->
810 <!-- CL 459435 reverted by CL 467255 -->
811 <!-- CL 467515 reverted by CL 499416 -->
813 <dl id="math/big"><dt><a href="/pkg/math/big/">math/big</a></dt>
815 <p><!-- https://go.dev/issue/56984, CL 453115, CL 500116 -->
816 The new <a href="/pkg/math/big/#Int.Float64"><code>Int.Float64</code></a>
817 method returns the nearest floating-point value to a
818 multi-precision integer, along with an indication of any
819 rounding that occurred.
822 </dl><!-- math/big -->
824 <dl id="net"><dt><a href="/pkg/net/">net</a></dt>
827 <!-- https://go.dev/issue/59166, https://go.dev/issue/56539 -->
828 <!-- CL 471136, CL 471137, CL 471140 -->
829 On Linux, the <a href="/pkg/net/">net</a> package can now use
830 Multipath TCP when the kernel supports it. It is not used by
831 default. To use Multipath TCP when available on a client, call
833 <a href="/pkg/net/#Dialer.SetMultipathTCP"><code>Dialer.SetMultipathTCP</code></a>
834 method before calling the
835 <a href="/pkg/net/#Dialer.Dial"><code>Dialer.Dial</code></a> or
836 <a href="/pkg/net/#Dialer.DialContext"><code>Dialer.DialContext</code></a>
837 methods. To use Multipath TCP when available on a server, call
839 <a href="/pkg/net/#ListenConfig.SetMultipathTCP"><code>ListenConfig.SetMultipathTCP</code></a>
840 method before calling the
841 <a href="/pkg/net/#ListenConfig.Listen"><code>ListenConfig.Listen</code></a>
842 method. Specify the network as <code>"tcp"</code> or
843 <code>"tcp4"</code> or <code>"tcp6"</code> as usual. If
844 Multipath TCP is not supported by the kernel or the remote host,
845 the connection will silently fall back to TCP. To test whether a
846 particular connection is using Multipath TCP, use the
847 <a href="/pkg/net/#TCPConn.MultipathTCP"><code>TCPConn.MultipathTCP</code></a>
851 In a future Go release we may enable Multipath TCP by default on
852 systems that support it.
857 <dl id="net/http"><dt><a href="/pkg/net/http/">net/http</a></dt>
859 <p><!-- CL 472636 -->
860 The new <a href="/pkg/net/http#ResponseController.EnableFullDuplex"><code>ResponseController.EnableFullDuplex</code></a>
861 method allows server handlers to concurrently read from an HTTP/1
862 request body while writing the response. Normally, the HTTP/1 server
863 automatically consumes any remaining request body before starting to
864 write the response, to avoid deadlocking clients which attempt to
865 write a complete request before reading the response. The
866 <code>EnableFullDuplex</code> method disables this behavior.
869 <p><!-- https://go.dev/issue/44855, CL 382117 -->
870 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.
873 <p><!-- CL 494122 -->
874 The <a href="/pkg/net/http/">net/http</a> package now supports
875 <a href="/pkg/errors/#ErrUnsupported"><code>errors.ErrUnsupported</code></a>,
876 in that the expression
877 <code>errors.Is(http.ErrNotSupported, errors.ErrUnsupported)</code>
881 </dl><!-- net/http -->
883 <dl id="os"><dt><a href="/pkg/os/">os</a></dt>
885 <p><!-- https://go.dev/issue/32558, CL 219638 -->
886 Programs may now pass an empty <code>time.Time</code> value to
887 the <a href="/pkg/os/#Chtimes"><code>Chtimes</code></a> function
888 to leave either the access time or the modification time unchanged.
891 <p><!-- CL 480135 -->
893 <a href="/pkg/os#File.Chdir"><code>File.Chdir</code></a> method
894 now changes the current directory to the file, rather than
895 always returning an error.
898 <p><!-- CL 495079 -->
899 On Unix systems, if a non-blocking descriptor is passed
900 to <a href="/pkg/os/#NewFile"><code>NewFile</code></a>, calling
901 the <a href="/pkg/os/#File.Fd"><code>File.Fd</code></a> method
902 will now return a non-blocking descriptor. Previously the
903 descriptor was converted to blocking mode.
906 <p><!-- CL 477215 -->
908 <a href="/pkg/os/#Truncate"><code>Truncate</code></a> on a
909 non-existent file used to create an empty file. It now returns
910 an error indicating that the file does not exist.
913 <p><!-- https://go.dev/issue/56899, CL 463219 -->
915 <a href="/pkg/os/#TempDir"><code>TempDir</code></a> now uses
916 GetTempPath2W when available, instead of GetTempPathW. The
917 new behavior is a security hardening measure that prevents
918 temporary files created by processes running as SYSTEM to
919 be accessed by non-SYSTEM processes.
922 <p><!-- CL 493036 -->
923 On Windows the os package now supports working with files whose
924 names, stored as UTF-16, can't be represented as valid UTF-8.
927 <p><!-- https://go.dev/issue/54451, CL 491175 -->
928 The implementation of the
929 <a href="/pkg/io/fs/#DirEntry"><code>io/fs.DirEntry</code></a>
930 interface returned by the
931 <a href="/pkg/os/#ReadDir"><code>ReadDir</code></a> function and
932 the <a href="/pkg/os/#File.ReadDir"><code>File.ReadDir</code></a>
933 method now implements a <code>String</code> method that calls
934 <a href="/pkg/io/fs/#FormatDirEntry"><code>io/fs.FormatDirEntry</code></a>.
937 <p><!-- https://go.dev/issue/53761, CL 416775, CL 498015-->
938 The implementation of the
939 <a href="/pkg/io/fs/#FS"><code>io/fs.FS</code></a> interface returned by
940 the <a href="/pkg/os/#DirFS"><code>DirFS</code></a> function now implements
941 the <a href="/pkg/io/fs/#ReadFileFS"><code>io/fs.ReadFileFS</code></a> and
942 the <a href="/pkg/io/fs/#ReadDirFS"><code>io/fs.ReadDirFS</code></a>
948 <dl id="path/filepath"><dt><a href="/pkg/path/filepath/">path/filepath</a></dt>
951 The implementation of the
952 <a href="/pkg/io/fs/#DirEntry"><code>io/fs.DirEntry</code></a>
953 interface passed to the function argument of
954 <a href="/pkg/path/filepath/#WalkDir"><code>WalkDir</code></a>
955 now implements a <code>String</code> method that calls
956 <a href="/pkg/io/fs/#FormatDirEntry"><code>io/fs.FormatDirEntry</code></a>.
959 </dl><!-- path/filepath -->
961 <!-- CL 459455 reverted -->
963 <dl id="reflect"><dt><a href="/pkg/reflect/">reflect</a></dt>
965 <p><!-- CL 408826, CL 413474 -->
966 In Go 1.21, <a href="/pkg/reflect/#ValueOf"><code>ValueOf</code></a>
967 no longer forces its argument to be allocated on the heap, allowing
968 a <code>Value</code>'s content to be allocated on the stack. Most
969 operations on a <code>Value</code> also allow the underlying value
970 to be stack allocated.
973 <p><!-- https://go.dev/issue/55002 -->
974 The new <a href="/pkg/reflect/#Value"><code>Value</code></a>
975 method <a href="/pkg/reflect/#Value.Clear"><code>Value.Clear</code></a>
976 clears the contents of a map or zeros the contents of a slice.
977 This corresponds to the new <code>clear</code> built-in
978 <a href="#language">added to the language</a>.
981 <p><!-- https://go.dev/issue/56906, CL 452762 -->
982 The <a href="/pkg/reflect/#SliceHeader"><code>SliceHeader</code></a>
983 and <a href="/pkg/reflect/#StringHeader"><code>StringHeader</code></a>
984 types are now deprecated. In new code
985 prefer <a href="/pkg/unsafe/#Slice"><code>unsafe.Slice</code></a>,
986 <a href="/pkg/unsafe/#SliceData"><code>unsafe.SliceData</code></a>,
987 <a href="/pkg/unsafe/#String"><code>unsafe.String</code></a>,
988 or <a href="/pkg/unsafe/#StringData"><code>unsafe.StringData</code></a>.
991 </dl><!-- reflect -->
993 <dl id="regexp"><dt><a href="/pkg/regexp/">regexp</a></dt>
995 <p><!-- https://go.dev/issue/46159, CL 479401 -->
996 <a href="/pkg/regexp#Regexp"><code>Regexp</code></a> now defines
997 <a href="/pkg/regexp#Regexp.MarshalText"><code>MarshalText</code></a>
998 and <a href="/pkg/regexp#Regexp.UnmarshalText"><code>UnmarshalText</code></a>
999 methods. These implement
1000 <a href="/pkg/encoding#TextMarshaler"><code>encoding.TextMarshaler</code></a>
1002 <a href="/pkg/encoding#TextUnmarshaler"><code>encoding.TextUnmarshaler</code></a>
1003 and will be used by packages such as
1004 <a href="/pkg/encoding/json">encoding/json</a>.
1007 </dl><!-- regexp -->
1009 <dl id="runtime"><dt><a href="/pkg/runtime/">runtime</a></dt>
1011 <p><!-- https://go.dev/issue/38651, CL 435337 -->
1012 Textual stack traces produced by Go programs, such as those
1013 produced when crashing, calling <code>runtime.Stack</code>, or
1014 collecting a goroutine profile with <code>debug=2</code>, now
1015 include the IDs of the goroutines that created each goroutine in
1019 <p><!-- https://go.dev/issue/57441, CL 474915 -->
1020 Crashing Go applications can now opt-in to Windows Error Reporting (WER) by setting the environment variable
1021 <code>GOTRACEBACK=wer</code> or calling <a href="/pkg/runtime/debug/#SetTraceback"><code>debug.SetTraceback("wer")</code></a>
1022 before the crash. Other than enabling WER, the runtime will behave as with <code>GOTRACEBACK=crash</code>.
1023 On non-Windows systems, <code>GOTRACEBACK=wer</code> is ignored.
1026 <p><!-- CL 447778 -->
1027 <code>GODEBUG=cgocheck=2</code>, a thorough checker of cgo pointer passing rules,
1028 is no longer available as a <a href="/pkg/runtime#hdr-Environment_Variables">debug option</a>.
1029 Instead, it is available as an experiment using <code>GOEXPERIMENT=cgocheck2</code>.
1030 In particular this means that this mode has to be selected at build time instead of startup time.
1032 <code>GODEBUG=cgocheck=1</code> is still available (and is still the default).
1035 <p><!-- https://go.dev/issue/46787, CL 367296 -->
1036 A new type <code>Pinner</code> has been added to the runtime
1037 package. <code>Pinner</code>s may be used to "pin" Go memory
1038 such that it may be used more freely by non-Go code. For instance,
1039 passing Go values that reference pinned Go memory to C code is
1040 now allowed. Previously, passing any such nested reference was
1042 <a href="https://pkg.go.dev/cmd/cgo#hdr-Passing_pointers">cgo pointer passing rules.</a>
1044 See <a href="/pkg/runtime#Pinner">the docs</a> for more details.
1047 <!-- CL 472195 no release note needed -->
1049 </dl><!-- runtime -->
1051 <dl id="runtime/metrics"><dt><a href="/pkg/runtime/metrics/">runtime/metrics</a></dt>
1053 <p><!-- https://go.dev/issue/56857, CL 497315 -->
1054 A few previously-internal GC metrics, such as live heap size, are
1057 <code>GOGC</code> and <code>GOMEMLIMIT</code> are also now
1058 available as metrics.
1061 </dl><!-- runtime/metrics -->
1063 <dl id="runtime/trace"><dt><a href="/pkg/runtime/trace/">runtime/trace</a></dt>
1065 <p><!-- https://go.dev/issue/16638 -->
1066 Collecting traces on amd64 and arm64 now incurs a substantially
1067 smaller CPU cost: up to a 10x improvement over the previous release.
1070 <p><!-- CL 494495 -->
1071 Traces now contain explicit stop-the-world events for every reason
1072 the Go runtime might stop-the-world, not just garbage collection.
1075 </dl><!-- runtime/trace -->
1077 <dl id="sync"><dt><a href="/pkg/sync/">sync</a></dt>
1079 <p><!-- https://go.dev/issue/56102, CL 451356 -->
1080 The new <a href="/pkg/sync/#OnceFunc"><code>OnceFunc</code></a>,
1081 <a href="/pkg/sync/#OnceValue"><code>OnceValue</code></a>, and
1082 <a href="/pkg/sync/#OnceValues"><code>OnceValues</code></a>
1083 functions capture a common use of <a href="/pkg/sync/#Once">Once</a> to
1084 lazily initialize a value on first use.
1089 <dl id="syscall"><dt><a href="/pkg/syscall/">syscall</a></dt>
1091 <p><!-- CL 480135 -->
1093 <a href="/pkg/syscall#Fchdir"><code>Fchdir</code></a> function
1094 now changes the current directory to its argument, rather than
1095 always returning an error.
1098 <p><!-- https://go.dev/issue/46259, CL 458335 -->
1100 <a href="/pkg/syscall#SysProcAttr"><code>SysProcAttr</code></a>
1101 has a new field <code>Jail</code> that may be used to put the
1102 newly created process in a jailed environment.
1105 <p><!-- CL 493036 -->
1106 On Windows the syscall package now supports working with files whose
1107 names, stored as UTF-16, can't be represented as valid UTF-8.
1108 The <a href="/pkg/syscall#UTF16ToString"><code>UTF16ToString</code></a>
1109 and <a href="/pkg/syscall#UTF16FromString"><code>UTF16FromString</code></a>
1110 functions now convert between UTF-16 data and
1111 <a href="https://simonsapin.github.io/wtf-8/">WTF-8</a> strings.
1112 This is backward compatible as WTF-8 is a superset of the UTF-8
1113 format that was used in earlier releases.
1116 <p><!-- CL 476578, CL 476875, CL 476916 -->
1117 Several error values match the new
1118 <a href="/pkg/errors/#ErrUnsupported"><code>errors.ErrUnsupported</code></a>,
1119 such that <code>errors.Is(err, errors.ErrUnsupported)</code>
1122 <li><code>ENOSYS</code></li>
1123 <li><code>ENOTSUP</code></li>
1124 <li><code>EOPNOTSUPP</code></li>
1125 <li><code>EPLAN9</code> (Plan 9 only)</li>
1126 <li><code>ERROR_CALL_NOT_IMPLEMENTED</code> (Windows only)</li>
1127 <li><code>ERROR_NOT_SUPPORTED</code> (Windows only)</li>
1128 <li><code>EWINDOWS</code> (Windows only)</li>
1132 </dl><!-- syscall -->
1134 <dl id="testing"><dt><a href="/pkg/testing/">testing</a></dt>
1136 <p><!-- https://go.dev/issue/37708, CL 463837 -->
1137 The new <code>-test.fullpath</code> option will print full path
1138 names in test log messages, rather than just base names.
1141 <p><!-- https://go.dev/issue/52600, CL 475496 -->
1142 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>.
1145 </dl><!-- testing -->
1147 <dl id="testing/fstest"><dt><a href="/pkg/testing/fstest/">testing/fstest</a></dt>
1149 <p><!-- https://go.dev/issue/54451, CL 491175 -->
1150 Calling <code><a href="/pkg/testing/fstest/MapFS.Open">Open</a>.<a href="/pkg/io/fs/#File.Stat">Stat</a></code>
1151 will return a type that now implements a <code>String</code>
1153 <a href="/pkg/io/fs/#FormatFileInfo"><code>io/fs.FormatFileInfo</code></a>.
1156 </dl><!-- testing/fstest -->
1158 <dl id="unicode"><dt><a href="/pkg/unicode/">unicode</a></dt>
1160 <p><!-- CL 456837 -->
1161 The <a href="/pkg/unicode/"><code>unicode</code></a> package and
1162 associated support throughout the system has been upgraded to
1163 <a href="https://www.unicode.org/versions/Unicode15.0.0/">Unicode 15.0.0</a>.
1165 </dl><!-- unicode -->
1167 <h2 id="ports">Ports</h2>
1169 <h3 id="darwin">Darwin</h3>
1171 <p><!-- https://go.dev/issue/57125 -->
1172 As <a href="go1.20#darwin">announced</a> in the Go 1.20 release notes,
1173 Go 1.21 requires macOS 10.15 Catalina or later;
1174 support for previous versions has been discontinued.
1177 <h3 id="windows">Windows</h3>
1179 <p><!-- https://go.dev/issue/57003, https://go.dev/issue/57004 -->
1180 As <a href="go1.20#windows">announced</a> in the Go 1.20 release notes,
1181 Go 1.21 requires at least Windows 10 or Windows Server 2016;
1182 support for previous versions has been discontinued.
1187 <!-- cmd/dist: default to GOARM=7 on all non-arm systems -->
1190 <h3 id="wasm">WebAssembly</h3>
1192 <p><!-- https://go.dev/issue/38248, https://go.dev/issue/59149, CL 489255 -->
1193 The new <code>go:wasmimport</code> directive can now be used in Go programs
1194 to import functions from the WebAssembly host.
1197 <!-- https://go.dev/issue/56100 -->
1199 The Go scheduler now interacts much more efficiently with the
1200 JavaScript event loop, especially in applications that block
1201 frequently on asynchronous events.
1205 <h3 id="wasip1">WebAssembly System Interface</h3>
1207 <p><!-- https://go.dev/issue/58141 -->
1208 Go 1.21 adds an experimental port to the <a href="https://wasi.dev/">
1209 WebAssembly System Interface (WASI)</a>, Preview 1
1210 (<code>GOOS=wasip1</code>, <code>GOARCH=wasm</code>).
1214 As a result of the addition of the new <code>GOOS</code> value
1215 "<code>wasip1</code>", Go files named <code>*_wasip1.go</code>
1216 will now be <a href="/pkg/go/build/#hdr-Build_Constraints">ignored
1217 by Go tools</a> except when that <code>GOOS</code> value is being
1219 If you have existing filenames matching that pattern, you will
1220 need to rename them.
1224 <!-- proposals for x repos that don't need to be mentioned here but
1225 are picked up by the relnote tool. -->
1226 <!-- https://go.dev/issue/54232 -->
1227 <!-- https://go.dev/issue/57906 -->
1228 <!-- https://go.dev/issue/58668 -->
1229 <!-- https://go.dev/issue/59676 -->
1231 <!-- changes to cmd/api that don't need release notes. -->
1232 <!-- CL 469115, CL 469135, CL 499981 -->
1234 <!-- proposals that don't need release enotes. -->
1235 <!-- https://go.dev/issue/10275 -->
1236 <!-- https://go.dev/issue/59719 -->