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 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>
323 The verifier that checks for incorrect uses of <code>R15</code> when dynamic linking on amd64 has been improved.
326 <h2 id="linker">Linker</h2>
328 <p><!-- https://go.dev/issue/57302, CL 461749 -->
329 On Windows AMD64, the linker (with help from the compiler) now emits
330 SEH unwinding data by default, which improves the integration
331 of Go applications with Windows debuggers and other tools.
336 <!-- cmd/link: generate .xdata PE section -->
338 <!-- CL 463395, CL 461315 -->
340 In Go 1.21 the linker (with help from the compiler) is now capable of
341 deleting dead (unreferenced) global map variables, if the number of
342 entries in the variable initializer is sufficiently large, and if the
343 initializer expressions are side-effect free.
346 TODO: complete this section, or delete if not needed
349 <h2 id="library">Core library</h2>
351 <h3 id="slog">New log/slog package</h3>
353 <p><!-- https://go.dev/issue/59060, https://go.dev/issue/59141, https://go.dev/issue/59204, https://go.dev/issue/59280,
354 https://go.dev/issue/59282, https://go.dev/issue/59339, https://go.dev/issue/59345,
355 CL 477295, CL 484096, CL 486376, CL 486415, CL 487855 -->
356 The new <a href="/pkg/log/slog">log/slog</a> package provides structured logging with levels.
357 Structured logging emits key-value pairs
358 to enable fast, accurate processing of large amounts of log data.
359 The package supports integration with popular log analysis tools and services.
362 <h3 id="slogtest">New testing/slogtest package</h3>
364 <p><!-- CL 487895 -->
365 The new <a href="/pkg/testing/slogtest">testing/slogtest</a> package can help
366 to validate <a href="/pkg/log/slog#Handler">slog.Handler</a> implementations.
369 <h3 id="slices">New slices package</h3>
372 <!-- https://go.dev/issue/45955, https://go.dev/issue/54768 -->
373 <!-- https://go.dev/issue/57348, https://go.dev/issue/57433 -->
374 <!-- https://go.dev/issue/58565, https://go.dev/issue/60091 -->
375 <!-- CL 467417, CL 468855, CL 483175, CL 496078, CL 498175 -->
376 The new <a href="/pkg/slices">slices</a> package provides many common
377 operations on slices, using generic functions that work with slices
381 <h3 id="maps">New maps package</h3>
383 <p><!-- https://go.dev/issue/57436, CL 464343 -->
384 The new <a href="/pkg/maps/">maps</a> package provides several
385 common operations on maps, using generic functions that work with
386 maps of any key or element type.
389 <p><!-- https://go.dev/issue/59488, CL 469356 -->
390 The new <a href="/pkg/cmp/">cmp</a> package defines the type
391 constraint <a href="/pkg/cmp/#Ordered"><code>Ordered</code></a> and
392 two new generic functions
393 <a href="/pkg/cmp/#Less"><code>Less</code></a>
394 and <a href="/pkg/cmp/#Compare"><code>Compare</code></a> that are
395 useful with <a href="/ref/spec/#Comparison_operators">ordered
399 <h3 id="minor_library_changes">Minor changes to the library</h3>
402 As always, there are various minor changes and updates to the library,
403 made with the Go 1 <a href="/doc/go1compat">promise of compatibility</a>
405 There are also various performance improvements, not enumerated here.
409 TODO: complete this section
412 <dl id="archive/tar"><dt><a href="/pkg/archive/tar/">archive/tar</a></dt>
414 <p><!-- https://go.dev/issue/54451, CL 491175 -->
415 The implementation of the
416 <a href="/pkg/io/fs/#FileInfo"><code>io/fs.FileInfo</code></a>
417 interface returned by
418 <a href="/pkg/archive/tar/#Header.FileInfo"><code>Header.FileInfo</code></a>
419 now implements a <code>String</code> method that calls
420 <a href="/pkg/io/fs/#FormatFileInfo"><code>io/fs.FormatFileInfo</code></a>.
423 </dl><!-- archive/tar -->
425 <dl id="archive/zip"><dt><a href="/pkg/archive/zip/">archive/zip</a></dt>
427 <p><!-- https://go.dev/issue/54451, CL 491175 -->
428 The implementation of the
429 <a href="/pkg/io/fs/#FileInfo"><code>io/fs.FileInfo</code></a>
430 interface returned by
431 <a href="/pkg/archive/zip/#FileHeader.FileInfo"><code>FileHeader.FileInfo</code></a>
432 now implements a <code>String</code> method that calls
433 <a href="/pkg/io/fs/#FormatFileInfo"><code>io/fs.FormatFileInfo</code></a>.
436 <p><!-- https://go.dev/issue/54451, CL 491175 -->
437 The implementation of the
438 <a href="/pkg/io/fs/#DirEntry"><code>io/fs.DirEntry</code></a>
439 interface returned by the
440 <a href="/pkg/io/fs/#ReadDirFile.ReadDir"><code>io/fs.ReadDirFile.ReadDir</code></a>
442 <a href="/pkg/io/fs/#File"><code>io/fs.File</code></a>
444 <a href="/pkg/archive/zip/#Reader.Open"><code>Reader.Open</code></a>
445 now implements a <code>String</code> method that calls
446 <a href="/pkg/io/fs/#FormatDirEntry"><code>io/fs.FormatDirEntry</code></a>.
449 </dl><!-- archive/zip -->
451 <dl id="bytes"><dt><a href="/pkg/bytes/">bytes</a></dt>
453 <p><!-- https://go.dev/issue/53685, CL 474635 -->
454 The <a href="/pkg/bytes/#Buffer"><code>Buffer</code></a> type
456 <a href="/pkg/bytes/#Buffer.Available"><code>Available</code></a>
457 and <a href="/pkg/bytes/#AvailableBuffer"><code>AvailableBuffer</code></a>.
458 These may be used along with the
459 <a href="/pkg/bytes/#Buffer.Write"><code>Write</code></a>
460 method to append directly to the <code>Buffer</code>.
465 <dl id="context"><dt><a href="/pkg/context/">context</a></dt>
467 <p><!-- https://go.dev/issue/40221, CL 479918 -->
468 The new <a href="/pkg/context/#WithoutCancel"><code>WithoutCancel</code></a>
469 function returns a copy of a context that is not canceled when the original
472 <p><!-- https://go.dev/issue/56661, CL 449318 -->
473 The new <a href="/pkg/context/#WithDeadlineCause"><code>WithDeadlineCause</code></a>
474 and <a href="/pkg/context/#WithTimeoutCause"><code>WithTimeoutCause</code></a>
475 functions provide a way to set a context cancellation cause when a deadline or
476 timer expires. The cause may be retrieved with the
477 <a href="/pkg/context/#Cause"><code>Cause</code></a> function.
479 <p><!-- https://go.dev/issue/57928, CL 482695 -->
480 The new <a href="/pkg/context/#AfterFunc"><code>AfterFunc</code></a>
481 function registers a function to run after a context has been cancelled.
486 <dl id="crypto/elliptic"><dt><a href="/pkg/crypto/elliptic/">crypto/elliptic</a></dt>
488 <p><!-- CL 459977 -->
489 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.
492 </dl><!-- crypto/elliptic -->
494 <dl id="crypto/rsa"><dt><a href="/pkg/crypto/rsa/">crypto/rsa</a></dt>
496 <p><!-- https://go.dev/issue/56921, CL 459976 -->
497 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.
500 </dl><!-- crypto/rsa -->
502 <!-- CL 483815 reverted -->
504 <dl id="crypto/sha256"><dt><a href="/pkg/crypto/sha256/">crypto/sha256</a></dt>
506 <p><!-- https://go.dev/issue/50543, CL 408795 -->
507 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.
510 </dl><!-- crypto/sha256 -->
512 <!-- CL 481478 reverted -->
513 <!-- CL 483816 reverted -->
515 <dl id="crypto/tls"><dt><a href="/pkg/crypto/tls/">crypto/tls</a></dt>
517 <p><!-- https://go.dev/issue/46308 -->
518 TODO: <a href="https://go.dev/issue/46308">https://go.dev/issue/46308</a>: add VersionName function to return a string version of the TLS Version
521 <p><!-- https://go.dev/issue/60107 -->
522 TODO: <a href="https://go.dev/issue/60107">https://go.dev/issue/60107</a>: QUIC 0-RTT APIs
525 <p><!-- CL 493655 -->
526 TODO: <a href="https://go.dev/cl/493655">https://go.dev/cl/493655</a>: crypto/tls: support QUIC as a transport; modified api/next/44886.txt
529 <p><!-- CL 496818 -->
530 TODO: <a href="https://go.dev/cl/496818">https://go.dev/cl/496818</a>: crypto/tls: add SessionState and use it on the server side; modified api/next/60105.txt
533 <p><!-- CL 496820 -->
534 TODO: <a href="https://go.dev/cl/496820">https://go.dev/cl/496820</a>: crypto/tls: add ClientSessionState.ResumptionState and NewResumptionState; modified api/next/60105.txt
537 <p><!-- CL 496821 -->
538 TODO: <a href="https://go.dev/cl/496821">https://go.dev/cl/496821</a>: crypto/tls: add WrapSession and UnwrapSession; modified api/next/60105.txt
541 <p><!-- CL 496822 -->
542 TODO: <a href="https://go.dev/cl/496822">https://go.dev/cl/496822</a>: crypto/tls: add SessionState.Extra; modified api/next/60105.txt
545 <p><!-- CL 496995 -->
546 TODO: <a href="https://go.dev/cl/496995">https://go.dev/cl/496995</a>: crypto/tls: add QUIC 0-RTT APIs; modified api/next/44886.txt, api/next/60107.txt
549 <p><!-- CL 497376 -->
550 TODO: <a href="https://go.dev/cl/497376">https://go.dev/cl/497376</a>: crypto/tls: implement Extended Master Secret; modified api/except.txt
553 <p><!-- CL 497377 -->
554 TODO: <a href="https://go.dev/cl/497377">https://go.dev/cl/497377</a>: crypto/tls: add VersionName; modified api/next/46308.txt
557 </dl><!-- crypto/tls -->
559 <dl id="crypto/x509"><dt><a href="/pkg/crypto/x509/">crypto/x509</a></dt>
561 <p><!-- https://go.dev/issue/53573, CL 468875 -->
562 <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.
565 </dl><!-- crypto/x509 -->
567 <dl id="debug/elf"><dt><a href="/pkg/debug/elf/">debug/elf</a></dt>
569 <p><!-- https://go.dev/issue/56892, CL 452617 -->
571 <a href="/pkg/debug/elf/#File.DynValue"><code>File.DynValue</code></a>
572 method may be used to retrieve the numeric values listed with a
576 <p><!-- https://go.dev/issue/56887, CL 452496 -->
577 The constant flags permitted in a <code>DT_FLAGS_1</code>
578 dynamic tag are now defined with type
579 <a href="/pkg/debug/elf/#DynFlag1"><code>DynFlag1</code></a>. These
580 tags have names starting with <code>DF_1</code>.
583 <p><!-- CL 473256 -->
584 The package now defines the constant
585 <a href="/pkg/debug/elf/#COMPRESS_ZSTD"><code>COMPRESS_ZSTD</code></a>.
588 <p><!-- https://go.dev/issue/60348, CL 496918 -->
589 The package now defines the constant
590 <a href="/pkg/debug/elf/#R_PPC64_REL24_P9NOTOC"><code>R_PPC64_REL24_P9NOTOC</code></a>.
593 </dl><!-- debug/elf -->
595 <dl id="debug/pe"><dt><a href="/pkg/debug/pe/">debug/pe</a></dt>
597 <p><!-- CL 488475 -->
598 Attempts to read from a section containing uninitialized data
600 <a href="/pkg/debug/pe/#Section.Data"><code>Section.Data</code></a>
601 or the reader returned by <a href="/pkg/debug/pe/#Section.Open"><code>Section.Open</code></a>
605 </dl><!-- debug/pe -->
607 <dl id="embed"><dt><a href="/pkg/embed/">embed</a></dt>
609 <p><!-- https://go.dev/issue/57803, CL 483235 -->
610 The <a href="/pkg/io/fs/#File"><code>io/fs.File</code></a>
612 <a href="/pkg/embed/#FS.Open"><code>FS.Open</code></a> now
613 has a <code>ReadAt</code> method that
614 implements <a href="/pkg/io/#ReaderAt"><code>io.ReaderAt</code></a>.
617 <p><!-- https://go.dev/issue/54451, CL 491175 -->
618 Calling <code><a href="/pkg/embed/FS.Open">FS.Open</a>.<a href="/pkg/io/fs/#File.Stat">Stat</a></code>
619 will return a type that now implements a <code>String</code>
621 <a href="/pkg/io/fs/#FormatFileInfo"><code>io/fs.FormatFileInfo</code></a>.
626 <dl id="errors"><dt><a href="/pkg/errors/">errors</a></dt>
628 <p><!-- https://go.dev/issue/41198, CL 473935 -->
630 <a href="/pkg/errors/#ErrUnsupported"><code>ErrUnsupported</code></a>
631 error provides a standardized way to indicate that a requested
632 operation may not be performed because it is unsupported.
633 For example, a call to
634 <a href="/pkg/os/#Link"><code>os.Link</code></a> when using a
635 file system that does not support hard links.
640 <dl id="flag"><dt><a href="/pkg/flag/">flag</a></dt>
642 <p><!-- https://go.dev/issue/53747, CL 476015 -->
643 The new <a href="/pkg/flag/#BoolFunc"><code>BoolFunc</code></a>
645 <a href="/pkg/flag/#FlagSet.BoolFunc"><code>FlagSet.BoolFunc</code></a>
646 method define a flag that does not require an argument and calls
647 a function when the flag is used. This is similar to
648 <a href="/pkg/flag/#Func"><code>Func</code></a> but for a
652 <p><!-- CL 480215 -->
654 (via <a href="/pkg/flag/#Bool"><code>Bool</code></a>,
655 <a href="/pkg/flag/#BoolVar"><code>BoolVar</code></a>,
656 <a href="/pkg/flag/#Int"><code>Int</code></a>,
657 <a href="/pkg/flag/#IntVar"><code>IntVar</code></a>, etc.)
658 will panic if <a href="/pkg/flag/#Set"><code>Set</code></a> has
659 already been called on a flag with the same name. This change is
660 intended to detect cases where <a href="#language">changes in
661 initialization order</a> cause flag operations to occur in a
662 different order than expected. In many cases the fix to this
663 problem is to introduce a explicit package dependence to
664 correctly order the definition before any
665 <a href="/pkg/flag/#Set"><code>Set</code></a> operations.
670 <dl id="go/ast"><dt><a href="/pkg/go/ast/">go/ast</a></dt>
672 <p><!-- https://go.dev/issue/28089, CL 487935 -->
673 The new <a href="/pkg/go/ast/#IsGenerated"><code>IsGenerated</code></a> predicate
674 reports whether a file syntax tree contains the
675 <a href="https://go.dev/s/generatedcode">special comment</a>
676 that conventionally indicates that the file was generated by a tool.
681 <p><!-- https://go.dev/issue/59033, CL 476276 -->
683 <a href="/pkg/go/ast/#File.GoVersion"><code>File.GoVersion</code></a>
684 field records the minimum Go version required by
685 any <code>//go:build</code> or <code>// +build</code>
691 <dl id="go/build"><dt><a href="/pkg/go/build/">go/build</a></dt>
693 <p><!-- https://go.dev/issue/56986, CL 453603 -->
694 The package now parses build directives (comments that start
695 with <code>//go:</code>) in file headers (before
696 the <code>package</code> declaration). These directives are
698 <a href="/pkg/go/build#Package"><code>Package</code></a> fields
699 <a href="/pkg/go/build#Package.Directives"><code>Directives</code></a>,
700 <a href="/pkg/go/build#Package.TestDirectives"><code>TestDirectives</code></a>,
702 <a href="/pkg/go/build#Package.XTestDirectives"><code>XTestDirectives</code></a>.
705 </dl><!-- go/build -->
707 <dl id="go/build/constraint"><dt><a href="/pkg/go/build/constraint/">go/build/constraint</a></dt>
709 <p><!-- https://go.dev/issue/59033, CL 476275 -->
711 <a href="/pkg/go/build/constraint/#GoVersion"><code>GoVersion</code></a>
712 function returns the minimum Go version implied by a build
716 </dl><!-- go/build/constraint -->
718 <dl id="go/token"><dt><a href="/pkg/go/token/">go/token</a></dt>
720 <p><!-- https://go.dev/issue/57708, CL 464515 -->
721 The new <a href="/pkg/go/token/#File.Lines"><code>File.Lines</code></a> method
722 returns the file's line-number table in the same form as accepted by
723 <code>File.SetLines</code>.
726 </dl><!-- go/token -->
728 <dl id="hash/maphash"><dt><a href="/pkg/hash/maphash/">hash/maphash</a></dt>
730 <p><!-- https://go.dev/issue/47342, CL 468795 -->
731 The <code>hash/maphash</code> package now has a pure Go implementation, selectable with the <code>purego</code> build tag.
734 </dl><!-- hash/maphash -->
736 <dl id="html/template"><dt><a href="/pkg/html/template/">html/template</a></dt>
738 <p><!-- https://go.dev/issue/59584 -->
739 TODO: <a href="https://go.dev/issue/59584">https://go.dev/issue/59584</a>: expose ErrJSTemplate
742 <p><!-- CL 496395 -->
743 TODO: <a href="https://go.dev/cl/496395">https://go.dev/cl/496395</a>: html/template: expose ErrJSTemplate; modified api/next/59584.txt
746 </dl><!-- html/template -->
748 <dl id="io/fs"><dt><a href="/pkg/io/fs/">io/fs</a></dt>
750 <p><!-- https://go.dev/issue/54451, CL 489555 -->
752 <a href="/pkg/io/fs/#FormatFileInfo"><code>FormatFileInfo</code></a>
753 function returns a formatted version of a
754 <a href="/pkg/io/fs/#FileInfo"><code>FileInfo</code></a>.
756 <a href="/pkg/io/fs/#FormatDirEntry"><code>FormatDirEntry</code></a>
757 function returns a formatted version of a
758 <a href="/pkg/io/fs/#FileInfo"><code>DirEntry</code></a>.
759 The implementation of
760 <a href="/pkg/io/fs/#DirEntry"><code>DirEntry</code></a>
762 <a href="/pkg/io/fs/#ReadDir"><code>ReadDir</code></a> now
763 implements a <code>String</code> method that calls
764 <a href="/pkg/io/fs/#FormatDirEntry"><code>FormatDirEntry</code></a>,
765 and the same is true for
766 the <a href="/pkg/io/fs/#DirEntry"><code>DirEntry</code></a>
768 <a href="/pkg/io/fs/#WalkDirFunc"><code>WalkDirFunc</code></a>.
773 <!-- https://go.dev/issue/56491 rolled back by https://go.dev/issue/60519 -->
774 <!-- CL 459435 reverted by CL 467255 -->
775 <!-- CL 467515 reverted by CL 499416 -->
777 <dl id="math/big"><dt><a href="/pkg/math/big/">math/big</a></dt>
779 <p><!-- https://go.dev/issue/56984, CL 453115, CL 500116 -->
780 The new <a href="/pkg/math/big/#Int.Float64"><code>Int.Float64</code></a>
781 method returns the nearest floating-point value to a
782 multi-precision integer, along with an indication of any
783 rounding that occurred.
786 </dl><!-- math/big -->
788 <dl id="net"><dt><a href="/pkg/net/">net</a></dt>
791 <!-- https://go.dev/issue/59166, https://go.dev/issue/56539 -->
792 <!-- CL 471136, CL 471137, CL 471140 -->
793 On Linux, the <a href="/pkg/net/">net</a> package can now use
794 Multipath TCP when the kernel supports it. It is not used by
795 default. To use Multipath TCP when available on a client, call
797 <a href="/pkg/net/#Dialer.SetMultipathTCP"><code>Dialer.SetMultipathTCP</code></a>
798 method before calling the
799 <a href="/pkg/net/#Dialer.Dial"><code>Dialer.Dial</code></a> or
800 <a href="/pkg/net/#Dialer.DialContext"><code>Dialer.DialContext</code></a>
801 methods. To use Multipath TCP when available on a server, call
803 <a href="/pkg/net/#ListenConfig.SetMultipathTCP"><code>ListenConfig.SetMultipathTCP</code></a>
804 method before calling the
805 <a href="/pkg/net/#ListenConfig.Listen"><code>ListenConfig.Listen</code></a>
806 method. Specify the network as <code>"tcp"</code> or
807 <code>"tcp4"</code> or <code>"tcp6"</code> as usual. If
808 Multipath TCP is not supported by the kernel or the remote host,
809 the connection will silently fall back to TCP. To test whether a
810 particular connection is using Multipath TCP, use the
811 <a href="/pkg/net/#TCPConn.MultipathTCP"><code>TCPConn.MultipathTCP</code></a>
815 In a future Go release we may enable Multipath TCP by default on
816 systems that support it.
821 <dl id="net/http"><dt><a href="/pkg/net/http/">net/http</a></dt>
823 <p><!-- CL 472636 -->
824 The new <a href="/pkg/net/http#ResponseController.EnableFullDuplex"><code>ResponseController.EnableFullDuplex</code></a>
825 method allows server handlers to concurrently read from an HTTP/1
826 request body while writing the response. Normally, the HTTP/1 server
827 automatically consumes any remaining request body before starting to
828 write the response, to avoid deadlocking clients which attempt to
829 write a complete request before reading the response. The
830 <code>EnableFullDuplex</code> method disables this behavior.
833 <p><!-- https://go.dev/issue/44855, CL 382117 -->
834 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.
837 <p><!-- CL 494122 -->
838 The <a href="/pkg/net/http/">net/http</a> package now supports
839 <a href="/pkg/errors/#ErrUnsupported"><code>errors.ErrUnsupported</code></a>,
840 in that the expression
841 <code>errors.Is(http.ErrNotSupported, errors.ErrUnsupported)</code>
845 </dl><!-- net/http -->
847 <dl id="os"><dt><a href="/pkg/os/">os</a></dt>
849 <p><!-- https://go.dev/issue/32558, CL 219638 -->
850 Programs may now pass an empty <code>time.Time</code> value to
851 the <a href="/pkg/os/#Chtimes"><code>Chtimes</code></a> function
852 to leave either the access time or the modification time unchanged.
855 <p><!-- CL 480135 -->
857 <a href="/pkg/os#File.Chdir"><code>File.Chdir</code></a> method
858 now changes the current directory to the file, rather than
859 always returning an error.
862 <p><!-- CL 495079 -->
863 TODO: <a href="https://go.dev/cl/495079">https://go.dev/cl/495079</a>: os: if descriptor is non-blocking, retain that in Fd method
866 <p><!-- CL 477215 -->
868 <a href="/pkg/os/#Truncate"><code>Truncate</code></a> on a
869 non-existent file used to create an empty file. It now returns
870 an error indicating that the file does not exist.
873 <p><!-- https://go.dev/issue/56899, CL 463219 -->
875 <a href="/pkg/os/#TempDir"><code>TempDir</code></a> now uses
876 GetTempPath2W when available, instead of GetTempPathW. The
877 new behavior is a security hardening measure that prevents
878 temporary files created by processes running as SYSTEM to
879 be accessed by non-SYSTEM processes.
882 <p><!-- CL 493036 -->
883 On Windows the os package now supports working with files whose
884 names, stored as UTF-16, can't be represented as valid UTF-8.
887 <p><!-- https://go.dev/issue/54451, CL 491175 -->
888 The implementation of the
889 <a href="/pkg/io/fs/#DirEntry"><code>io/fs.DirEntry</code></a>
890 interface returned by the
891 <a href="/pkg/os/#ReadDir"><code>ReadDir</code></a> function and
892 the <a href="/pkg/os/#File.ReadDir"><code>File.ReadDir</code></a>
893 method now implements a <code>String</code> method that calls
894 <a href="/pkg/io/fs/#FormatDirEntry"><code>io/fs.FormatDirEntry</code></a>.
897 <p><!-- https://go.dev/issue/53761, CL 416775, CL 498015-->
898 The implementation of the
899 <a href="/pkg/io/fs/#FS"><code>io/fs.FS</code></a> interface returned by
900 the <a href="/pkg/os/#DirFS"><code>DirFS</code></a> function now implements
901 the <a href="/pkg/io/fs/#ReadFileFS"><code>io/fs.ReadFileFS</code></a> and
902 the <a href="/pkg/io/fs/#ReadDirFS"><code>io/fs.ReadDirFS</code></a>
908 <dl id="path/filepath"><dt><a href="/pkg/path/filepath/">path/filepath</a></dt>
911 The implementation of the
912 <a href="/pkg/io/fs/#DirEntry"><code>io/fs.DirEntry</code></a>
913 interface passed to the function argument of
914 <a href="/pkg/path/filepath/#WalkDir"><code>WalkDir</code></a>
915 now implements a <code>String</code> method that calls
916 <a href="/pkg/io/fs/#FormatDirEntry"><code>io/fs.FormatDirEntry</code></a>.
919 </dl><!-- path/filepath -->
921 <!-- CL 459455 reverted -->
923 <dl id="reflect"><dt><a href="/pkg/reflect/">reflect</a></dt>
925 <p><!-- CL 408826, CL 413474 -->
926 In Go 1.21, <a href="/pkg/reflect/#ValueOf"><code>ValueOf</code></a>
927 no longer forces its argument to be allocated on the heap, allowing
928 a <code>Value</code>'s content to be allocated on the stack. Most
929 operations on a <code>Value</code> also allow the underlying value
930 to be stack allocated.
933 <p><!-- https://go.dev/issue/55002 -->
934 The new <a href="/pkg/reflect/#Value"><code>Value</code></a>
935 method <a href="/pkg/reflect/#Value.Clear"><code>Value.Clear</code></a>
936 clears the contents of a map or zeros the contents of a slice.
937 This corresponds to the new <code>clear</code> built-in
938 <a href="#language">added to the language</a>.
941 <p><!-- https://go.dev/issue/56906, CL 452762 -->
942 The <a href="/pkg/reflect/#SliceHeader"><code>SliceHeader</code></a>
943 and <a href="/pkg/reflect/#StringHeader"><code>StringHeader</code></a>
944 types are now deprecated. In new code
945 prefer <a href="/pkg/unsafe/#Slice"><code>unsafe.Slice</code></a>,
946 <a href="/pkg/unsafe/#SliceData"><code>unsafe.SliceData</code></a>,
947 <a href="/pkg/unsafe/#String"><code>unsafe.String</code></a>,
948 or <a href="/pkg/unsafe/#StringData"><code>unsafe.StringData</code></a>.
951 </dl><!-- reflect -->
953 <dl id="regexp"><dt><a href="/pkg/regexp/">regexp</a></dt>
955 <p><!-- https://go.dev/issue/46159, CL 479401 -->
956 <a href="/pkg/regexp#Regexp"><code>Regexp</code></a> now defines
957 <a href="/pkg/regexp#Regexp.MarshalText"><code>MarshalText</code></a>
958 and <a href="/pkg/regexp#Regexp.UnmarshalText"><code>UnmarshalText</code></a>
959 methods. These implement
960 <a href="/pkg/encoding#TextMarshaler"><code>encoding.TextMarshaler</code></a>
962 <a href="/pkg/encoding#TextUnmarshaler"><code>encoding.TextUnmarshaler</code></a>
963 and will be used by packages such as
964 <a href="/pkg/encoding/json">encoding/json</a>.
969 <dl id="runtime"><dt><a href="/pkg/runtime/">runtime</a></dt>
971 <p><!-- https://go.dev/issue/38651, CL 435337 -->
972 Textual stack traces produced by Go programs, such as those
973 produced when crashing, calling <code>runtime.Stack</code>, or
974 collecting a goroutine profile with <code>debug=2</code>, now
975 include the IDs of the goroutines that created each goroutine in
979 <p><!-- https://go.dev/issue/57441, CL 474915 -->
980 Crashing Go applications can now opt-in to Windows Error Reporting (WER) by setting the environment variable
981 <code>GOTRACEBACK=wer</code> or calling <a href="/pkg/runtime/debug/#SetTraceback"><code>debug.SetTraceback("wer")</code></a>
982 before the crash. Other than enabling WER, the runtime will behave as with <code>GOTRACEBACK=crash</code>.
983 On non-Windows systems, <code>GOTRACEBACK=wer</code> is ignored.
986 <p><!-- CL 447778 -->
987 <code>GODEBUG=cgocheck=2</code>, a thorough checker of cgo pointer passing rules,
988 is no longer available as a <a href="/pkg/runtime#hdr-Environment_Variables">debug option</a>.
989 Instead, it is available as an experiment using <code>GOEXPERIMENT=cgocheck2</code>.
990 In particular this means that this mode has to be selected at build time instead of startup time.
992 <code>GODEBUG=cgocheck=1</code> is still available (and is still the default).
995 <p><!-- https://go.dev/issue/46787, CL 367296 -->
996 A new type <code>Pinner</code> has been added to the runtime
997 package. <code>Pinner</code>s may be used to "pin" Go memory
998 such that it may be used more freely by non-Go code. For instance,
999 passing Go values that reference pinned Go memory to C code is
1000 now allowed. Previously, passing any such nested reference was
1002 <a href="https://pkg.go.dev/cmd/cgo#hdr-Passing_pointers">cgo pointer passing rules.</a>
1004 See <a href="/pkg/runtime#Pinner">the docs</a> for more details.
1007 <p><!-- CL 472195 -->
1008 TODO: <a href="https://go.dev/cl/472195">https://go.dev/cl/472195</a>: runtime: remove NOFRAME from asmcgocall, systemstack and mcall
1011 </dl><!-- runtime -->
1013 <dl id="runtime/metrics"><dt><a href="/pkg/runtime/metrics/">runtime/metrics</a></dt>
1015 <p><!-- https://go.dev/issue/56857, CL 497315 -->
1016 A few previously-internal GC metrics, such as live heap size, are
1019 <code>GOGC</code> and <code>GOMEMLIMIT</code> are also now
1020 available as metrics.
1023 </dl><!-- runtime/metrics -->
1025 <dl id="runtime/trace"><dt><a href="/pkg/runtime/trace/">runtime/trace</a></dt>
1027 <p><!-- https://go.dev/issue/16638 -->
1028 Collecting traces on amd64 and arm64 now incurs a substantially
1029 smaller CPU cost: up to a 10x improvement over the previous release.
1032 <p><!-- CL 494495 -->
1033 Traces now contain explicit stop-the-world events for every reason
1034 the Go runtime might stop-the-world, not just garbage collection.
1037 </dl><!-- runtime/trace -->
1039 <dl id="sync"><dt><a href="/pkg/sync/">sync</a></dt>
1041 <p><!-- https://go.dev/issue/56102, CL 451356 -->
1042 The new <a href="/pkg/sync/#OnceFunc"><code>OnceFunc</code></a>,
1043 <a href="/pkg/sync/#OnceValue"><code>OnceValue</code></a>, and
1044 <a href="/pkg/sync/#OnceValues"><code>OnceValues</code></a>
1045 functions capture a common use of <a href="/pkg/sync/#Once">Once</a> to
1046 lazily initialize a value on first use.
1051 <dl id="syscall"><dt><a href="/pkg/syscall/">syscall</a></dt>
1053 <p><!-- CL 480135 -->
1055 <a href="/pkg/syscall#Fchdir"><code>Fchdir</code></a> function
1056 now changes the current directory to its argument, rather than
1057 always returning an error.
1060 <p><!-- https://go.dev/issue/46259, CL 458335 -->
1062 <a href="/pkg/syscall#SysProcAttr"><code>SysProcAttr</code></a>
1063 has a new field <code>Jail</code> that may be used to put the
1064 newly created process in a jailed environment.
1067 <p><!-- CL 493036 -->
1068 On Windows the syscall package now supports working with files whose
1069 names, stored as UTF-16, can't be represented as valid UTF-8.
1070 The <a href="/pkg/syscall#UTF16ToString"><code>UTF16ToString</code></a>
1071 and <a href="/pkg/syscall#UTF16FromString"><code>UTF16FromString</code></a>
1072 functions now convert between UTF-16 data and
1073 <a href="https://simonsapin.github.io/wtf-8/">WTF-8</a> strings.
1074 This is backward compatible as WTF-8 is a superset of the UTF-8
1075 format that was used in earlier releases.
1078 <p><!-- CL 476578, CL 476875, CL 476916 -->
1079 Several error values match the new
1080 <a href="/pkg/errors/#ErrUnsupported"><code>errors.ErrUnsupported</code></a>,
1081 such that <code>errors.Is(err, errors.ErrUnsupported)</code>
1084 <li><code>ENOSYS</code></li>
1085 <li><code>ENOTSUP</code></li>
1086 <li><code>EOPNOTSUPP</code></li>
1087 <li><code>EPLAN9</code> (Plan 9 only)</li>
1088 <li><code>ERROR_CALL_NOT_IMPLEMENTED</code> (Windows only)</li>
1089 <li><code>ERROR_NOT_SUPPORTED</code> (Windows only)</li>
1090 <li><code>EWINDOWS</code> (Windows only)</li>
1094 </dl><!-- syscall -->
1096 <dl id="testing"><dt><a href="/pkg/testing/">testing</a></dt>
1098 <p><!-- https://go.dev/issue/37708, CL 463837 -->
1099 The new <code>-test.fullpath</code> option will print full path
1100 names in test log messages, rather than just base names.
1103 <p><!-- https://go.dev/issue/52600, CL 475496 -->
1104 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>.
1107 </dl><!-- testing -->
1109 <dl id="testing/fstest"><dt><a href="/pkg/testing/fstest/">testing/fstest</a></dt>
1111 <p><!-- https://go.dev/issue/54451, CL 491175 -->
1112 Calling <code><a href="/pkg/testing/fstest/MapFS.Open">Open</a>.<a href="/pkg/io/fs/#File.Stat">Stat</a></code>
1113 will return a type that now implements a <code>String</code>
1115 <a href="/pkg/io/fs/#FormatFileInfo"><code>io/fs.FormatFileInfo</code></a>.
1118 </dl><!-- testing/fstest -->
1120 <dl id="unicode"><dt><a href="/pkg/unicode/">unicode</a></dt>
1122 <p><!-- CL 456837 -->
1123 The <a href="/pkg/unicode/"><code>unicode</code></a> package and
1124 associated support throughout the system has been upgraded to
1125 <a href="https://www.unicode.org/versions/Unicode15.0.0/">Unicode 15.0.0</a>.
1127 </dl><!-- unicode -->
1129 <h2 id="ports">Ports</h2>
1131 <h3 id="darwin">Darwin</h3>
1133 <p><!-- https://go.dev/issue/57125 -->
1134 As <a href="go1.20#darwin">announced</a> in the Go 1.20 release notes,
1135 Go 1.21 requires macOS 10.15 Catalina or later;
1136 support for previous versions has been discontinued.
1139 <h3 id="windows">Windows</h3>
1141 <p><!-- https://go.dev/issue/57003, https://go.dev/issue/57004 -->
1142 As <a href="go1.20#windows">announced</a> in the Go 1.20 release notes,
1143 Go 1.21 requires at least Windows 10 or Windows Server 2016;
1144 support for previous versions has been discontinued.
1149 <!-- cmd/dist: default to GOARM=7 on all non-arm systems -->
1152 <h3 id="wasm">WebAssembly</h3>
1154 <p><!-- https://go.dev/issue/38248, https://go.dev/issue/59149, CL 489255 -->
1155 The new <code>go:wasmimport</code> directive can now be used in Go programs
1156 to import functions from the WebAssembly host.
1159 <!-- https://go.dev/issue/56100 -->
1161 The Go scheduler now interacts much more efficiently with the
1162 JavaScript event loop, especially in applications that block
1163 frequently on asynchronous events.
1167 <h3 id="wasip1">WebAssembly System Interface</h3>
1169 <p><!-- https://go.dev/issue/58141 -->
1170 Go 1.21 adds an experimental port to the <a href="https://wasi.dev/">
1171 WebAssembly System Interface (WASI)</a>, Preview 1
1172 (<code>GOOS=wasip1</code>, <code>GOARCH=wasm</code>).
1176 As a result of the addition of the new <code>GOOS</code> value
1177 "<code>wasip1</code>", Go files named <code>*_wasip1.go</code>
1178 will now be <a href="/pkg/go/build/#hdr-Build_Constraints">ignored
1179 by Go tools</a> except when that <code>GOOS</code> value is being
1181 If you have existing filenames matching that pattern, you will
1182 need to rename them.
1186 <!-- proposals for x repos that don't need to be mentioned here but
1187 are picked up by the relnote tool. -->
1188 <!-- https://go.dev/issue/54232 -->
1189 <!-- https://go.dev/issue/57906 -->
1190 <!-- https://go.dev/issue/58668 -->
1191 <!-- https://go.dev/issue/59676 -->
1193 <!-- changes to cmd/api that don't need release notes. -->
1194 <!-- CL 469115, CL 469135, CL 499981 -->
1196 <!-- proposals that don't need release enotes. -->
1197 <!-- https://go.dev/issue/10275 -->
1198 <!-- https://go.dev/issue/59719 -->