2 "Title": "Go 1.21 Release Notes",
7 NOTE: In this document and others in this directory, the convention is to
8 set fixed-width phrases with non-fixed-width spaces, as in
9 <code>hello</code> <code>world</code>.
10 Do not send CLs removing the interior tags from such phrases.
14 main ul li { margin: 0.5em 0; }
17 <h2 id="introduction">DRAFT RELEASE NOTES — Introduction to Go 1.21</h2>
21 Go 1.21 is not yet released. These are work-in-progress
22 release notes. Go 1.21 is expected to be released in August 2023.
27 The latest Go release, version 1.21, arrives six months after <a href="/doc/go1.20">Go 1.20</a>.
28 Most of its changes are in the implementation of the toolchain, runtime, and libraries.
29 As always, the release maintains the Go 1 <a href="/doc/go1compat">promise of compatibility</a>;
30 in fact, Go 1.21 <a href="#godebug">improves upon that promise</a>.
31 We expect almost all Go programs to continue to compile and run as before.
34 <p><!-- https://go.dev/issue/57631 -->
35 Go 1.21 introduces a small change to the numbering of releases.
36 In the past, we used Go 1.<i>N</i> to refer to both the overall Go language version and release family
37 as well as the first release in that family.
38 Starting in Go 1.21, the first release is now Go 1.<i>N</i>.0.
39 Today we are releasing both the Go 1.21 language and its initial implementation, the Go 1.21.0 release.
40 These notes refer to “Go 1.21”; tools like <code>go</code> <code>version</code> will report “<code>go1.21.0</code>”
41 (until you upgrade to Go 1.21.1).
42 See “<a href="/doc/toolchain#versions">Go versions</a>” in the “Go Toolchains” documentation for details
43 about the new version numbering.
46 <h2 id="language">Changes to the language</h2>
49 Go 1.21 adds three new built-ins to the language.
52 <li><!-- https://go.dev/issue/59488 -->
53 The new functions <code>min</code> and <code>max</code> compute the
54 smallest (or largest, for <code>max</code>) value of a fixed number
56 See the language spec for
57 <a href="https://tip.golang.org/ref/spec#Min_and_max">details</a>.
59 <li><!-- https://go.dev/issue/56351 -->
60 The new function <code>clear</code> deletes all elements from a
61 map or zeroes all elements of a slice.
62 See the language spec for
63 <a href="https://tip.golang.org/ref/spec#Clear">details</a>.
68 <p><!-- https://go.dev/issue/57411 -->
69 Package initialization order is now specified more precisely. The
73 Sort all packages by import path.
75 <li>Repeat until the list of packages is empty:
78 Find the first package in the list for which all imports are
82 Initialize that package and remove it from the list.
87 This may change the behavior of some programs that rely on a
88 specific initialization ordering that was not expressed by explicit
89 imports. The behavior of such programs was not well defined by the
90 spec in past releases. The new rule provides an unambiguous definition.
94 Multiple improvements that increase the power and precision of type inference have been made.
97 <li><!-- https://go.dev/issue/59338 -->
98 A (possibly partially instantiated generic) function may now be called with arguments that are
99 themselves (possibly partially instantiated) generic functions.
100 The compiler will attempt to infer the missing type arguments of the callee (as before) and,
101 for each argument that is a generic function that is not fully instantiated,
102 its missing type arguments (new).
103 Typical use cases are calls to generic functions operating on containers
104 (such as <a href="/pkg/slices#IndexFunc">slices.IndexFunc</a>) where a function argument
105 may also be generic, and where the type argument of the called function and its arguments
106 are inferred from the container type.
107 More generally, a generic function may now be used without explicit instantiation when
108 it is assigned to a variable or returned as a result value if the type arguments can
109 be inferred from the assignment.
111 <li><!-- https://go.dev/issue/60353, https://go.dev/issue/57192, https://go.dev/issue/52397, https://go.dev/issue/41176 -->
112 Type inference now also considers methods when a value is assigned to an interface:
113 type arguments for type parameters used in method signatures may be inferred from
114 the corresponding parameter types of matching methods.
116 <li><!-- https://go.dev/issue/51593 https://go.dev/issue/39661 -->
117 Similarly, since a type argument must implement all the methods of its corresponding constraint,
118 the methods of the type argument and constraint are matched which may lead to the inference of
119 additional type arguments.
121 <li><!-- https://go.dev/issue/58671 -->
122 If multiple untyped constant arguments of different kinds (such as an untyped int and
123 an untyped floating-point constant) are passed to parameters with the same (not otherwise
124 specified) type parameter type, instead of an error, now type inference determines the
125 type using the same approach as an operator with untyped constant operands.
126 This change brings the types inferred from untyped constant arguments in line with the
127 types of constant expressions.
129 <li><!-- https://go.dev/issue/59750 -->
130 Type inference is now precise when matching corresponding types in assignments:
131 component types (such as the elements of slices, or the parameter types in function signatures)
132 must be identical (given suitable type arguments) to match, otherwise inference fails.
133 This change produces more accurate error messages:
134 where in the past type inference may have succeeded incorrectly and lead to an invalid assignment,
135 the compiler now reports an inference error if two types can't possibly match.
139 <p><!-- https://go.dev/issue/58650 -->
140 More generally, the description of
141 <a href="https://tip.golang.org/ref/spec#Type_inference">type inference</a>
142 in the language spec has been clarified.
143 Together, all these changes make type inference more powerful and inference failures less surprising.
146 <!-- https://go.dev/issue/57969 -->
148 Go 1.21 includes a preview of a language change we are considering for a future version of Go:
149 making for loop variables per-iteration instead of per-loop, to avoid accidental sharing bugs.
150 For details about how to try that language change, see <a href="https://go.dev/wiki/LoopvarExperiment">the LoopvarExperiment wiki page</a>.
153 <h2 id="tools">Tools</h2>
155 Go 1.21 adds improved support for backwards compatibility and forwards compatibility
159 <p><!-- https://go.dev/issue/56986 -->
160 To improve backwards compatibility, Go 1.21 formalizes
161 Go's use of the GODEBUG environment variable to control
162 the default behavior for changes that are non-breaking according to the
163 <a href="/doc/go1compat">compatibility policy</a>
164 but nonetheless may cause existing programs to break.
165 (For example, programs that depend on buggy behavior may break
166 when a bug is fixed, but bug fixes are not considered breaking changes.)
167 When Go must make this kind of behavior change,
168 it now chooses between the old and new behavior based on the
169 <code>go</code> line in the workspace's <code>go.work</code> file
170 or else the main module's <code>go.mod</code> file.
171 Upgrading to a new Go toolchain but leaving the <code>go</code> line
172 set to its original (older) Go version preserves the behavior of the older
174 With this compatibility support, the latest Go toolchain should always
175 be the best, most secure, implementation of an older version of Go.
176 See “<a href="/doc/godebug">Go, Backwards Compatibility, and GODEBUG</a>” for details.
179 <p><!-- https://go.dev/issue/57001 -->
180 To improve forwards compatibility, Go 1.21 now reads the <code>go</code> line
181 in a <code>go.work</code> or <code>go.mod</code> file as a strict
182 minimum requirement: <code>go</code> <code>1.21.0</code> means
183 that the workspace or module cannot be used with Go 1.20 or with Go 1.21rc1.
184 This allows projects that depend on fixes made in later versions of Go
185 to ensure that they are not used with earlier versions.
186 It also gives better error reporting for projects that make use of new Go features:
187 when the problem is that a newer Go version is needed,
188 that problem is reported clearly, instead of attempting to build the code
189 and instead printing errors about unresolved imports or syntax errors.
193 To make these new stricter version requirements easier to manage,
194 the <code>go</code> command can now invoke not just the toolchain
195 bundled in its own release but also other Go toolchain versions found in the PATH
196 or downloaded on demand.
197 If a <code>go.mod</code> or <code>go.work</code> <code>go</code> line
198 declares a minimum requirement on a newer version of Go, the <code>go</code>
199 command will find and run that version automatically.
200 The new <code>toolchain</code> directive sets a suggested minimum toolchain to use,
201 which may be newer than the strict <code>go</code> minimum.
202 See “<a href="/doc/toolchain">Go Toolchains</a>” for details.
205 <h3 id="go-command">Go command</h3>
207 <p><!-- https://go.dev/issue/58099, CL 474236 -->
208 The <code>-pgo</code> build flag now defaults to <code>-pgo=auto</code>,
209 and the restriction of specifying a single main package on the command
210 line is now removed. If a file named <code>default.pgo</code> is present
211 in the main package's directory, the <code>go</code> command will use
212 it to enable profile-guided optimization for building the corresponding
217 The <code>-C</code> <code>dir</code> flag must now be the first
218 flag on the command-line when used.
221 <p><!-- https://go.dev/issue/37708, CL 463837 -->
222 The new <code>go</code> <code>test</code> option
223 <code>-fullpath</code> prints full path names in test log messages,
224 rather than just base names.
227 <p><!-- https://go.dev/issue/15513, CL 466397 -->
228 The <code>go</code> <code>test</code> <code>-c</code> flag now
229 supports writing test binaries for multiple packages, each to
230 <code>pkg.test</code> where <code>pkg</code> is the package name.
231 It is an error if more than one test package being compiled has a given package name.]
234 <p><!-- https://go.dev/issue/15513, CL 466397 -->
235 The <code>go</code> <code>test</code> <code>-o</code> flag now
236 accepts a directory argument, in which case test binaries are written to that
237 directory instead of the current directory.
240 <h3 id="cgo">Cgo</h3>
242 <p><!-- CL 490819 -->
243 In files that <code>import "C"</code>, the Go toolchain now
244 correctly reports errors for attempts to declare Go methods on C types.
247 <h2 id="runtime-changes">Runtime</h2>
249 <p><!-- https://go.dev/issue/7181 -->
250 When printing very deep stacks, the runtime now prints the first 50
251 (innermost) frames followed by the bottom 50 (outermost) frames,
252 rather than just printing the first 100 frames. This makes it easier
253 to see how deeply recursive stacks started, and is especially
254 valuable for debugging stack overflows.
257 <p><!-- https://go.dev/issue/59960 -->
258 On Linux platforms that support transparent huge pages, the Go runtime
259 now manages which parts of the heap may be backed by huge pages more
260 explicitly. This leads to better utilization of memory: small heaps
261 should see less memory used (up to 50% in pathological cases) while
262 large heaps should see fewer broken huge pages for dense parts of the
263 heap, improving CPU usage and latency by up to 1%.
266 <p><!-- https://go.dev/issue/57069, https://go.dev/issue/56966 -->
267 As a result of runtime-internal garbage collection tuning,
268 applications may see up to a 40% reduction in application tail latency
269 and a small decrease in memory use. Some applications may also observe
270 a small loss in throughput.
272 The memory use decrease should be proportional to the loss in
273 throughput, such that the previous release's throughput/memory
274 tradeoff may be recovered (with little change to latency) by
275 increasing <code>GOGC</code> and/or <code>GOMEMLIMIT</code> slightly.
278 <p><!-- https://go.dev/issue/51676 -->
279 Calls from C to Go on threads created in C require some setup to prepare for
280 Go execution. On Unix platforms, this setup is now preserved across multiple
281 calls from the same thread. This significantly reduces the overhead of
282 subsequent C to Go calls from ~1-3 microseconds per call to ~100-200
283 nanoseconds per call.
286 <h2 id="compiler">Compiler</h2>
289 Profile-guide optimization (PGO), added as a preview in Go 1.20, is now ready
290 for general use. PGO enables additional optimizations on code identified as
291 hot by profiles of production workloads. As mentioned in the
292 <a href="#go-command">Go command section</a>, PGO is enabled by default for
293 binaries that contain a <code>default.pgo</code> profile in the main
294 package directory. Performance improvements vary depending on application
295 behavior, with most programs from a representative set of Go programs seeing
296 between 2 and 7% improvement from enabling PGO. See the
297 <a href="/doc/pgo">PGO user guide</a> for detailed documentation.
300 <!-- https://go.dev/issue/59959 -->
302 PGO builds can now devirtualize some interface method calls, adding a
303 concrete call to the most common callee. This enables further optimization,
304 such as inlining the callee.
309 Go 1.21 improves build speed by up to 6%, largely thanks to building the
310 compiler itself with PGO.
313 <h2 id="assembler">Assembler</h2>
315 <!-- https://go.dev/issue/58378 -->
317 On amd64, frameless nosplit assembly functions are no longer automatically marked as <code>NOFRAME</code>.
318 Instead, the <code>NOFRAME</code> attribute must be explicitly specified if desired,
319 which is already the behavior on other architectures supporting frame pointers.
320 With this, the runtime now maintains the frame pointers for stack transitions.
325 The verifier that checks for incorrect uses of <code>R15</code> when dynamic linking on amd64 has been improved.
328 <h2 id="linker">Linker</h2>
330 <p><!-- https://go.dev/issue/57302, CL 461749, CL 457455 -->
331 On windows/amd64, the linker (with help from the compiler) now emits
332 SEH unwinding data by default, which improves the integration
333 of Go applications with Windows debuggers and other tools.
336 <!-- CL 463395, CL 461315 -->
338 In Go 1.21 the linker (with help from the compiler) is now capable of
339 deleting dead (unreferenced) global map variables, if the number of
340 entries in the variable initializer is sufficiently large, and if the
341 initializer expressions are side-effect free.
344 <h2 id="library">Core library</h2>
346 <h3 id="slog">New log/slog package</h3>
348 <p><!-- https://go.dev/issue/59060, https://go.dev/issue/59141, https://go.dev/issue/59204, https://go.dev/issue/59280,
349 https://go.dev/issue/59282, https://go.dev/issue/59339, https://go.dev/issue/59345,
350 CL 477295, CL 484096, CL 486376, CL 486415, CL 487855 -->
351 The new <a href="/pkg/log/slog">log/slog</a> package provides structured logging with levels.
352 Structured logging emits key-value pairs
353 to enable fast, accurate processing of large amounts of log data.
354 The package supports integration with popular log analysis tools and services.
357 <h3 id="slogtest">New testing/slogtest package</h3>
359 <p><!-- CL 487895 -->
360 The new <a href="/pkg/testing/slogtest">testing/slogtest</a> package can help
361 to validate <a href="/pkg/log/slog#Handler">slog.Handler</a> implementations.
364 <h3 id="slices">New slices package</h3>
367 <!-- https://go.dev/issue/45955, https://go.dev/issue/54768 -->
368 <!-- https://go.dev/issue/57348, https://go.dev/issue/57433 -->
369 <!-- https://go.dev/issue/58565, https://go.dev/issue/60091 -->
370 <!-- CL 467417, CL 468855, CL 483175, CL 496078, CL 498175 -->
371 The new <a href="/pkg/slices">slices</a> package provides many common
372 operations on slices, using generic functions that work with slices
376 <h3 id="maps">New maps package</h3>
378 <p><!-- https://go.dev/issue/57436, CL 464343 -->
379 The new <a href="/pkg/maps/">maps</a> package provides several
380 common operations on maps, using generic functions that work with
381 maps of any key or element type.
384 <h3 id="cmp">New cmp package</h3>
386 <p><!-- https://go.dev/issue/59488, CL 496356 -->
387 The new <a href="/pkg/cmp/">cmp</a> package defines the type
388 constraint <a href="/pkg/cmp/#Ordered"><code>Ordered</code></a> and
389 two new generic functions
390 <a href="/pkg/cmp/#Less"><code>Less</code></a>
391 and <a href="/pkg/cmp/#Compare"><code>Compare</code></a> that are
392 useful with <a href="/ref/spec/#Comparison_operators">ordered
396 <h3 id="minor_library_changes">Minor changes to the library</h3>
399 As always, there are various minor changes and updates to the library,
400 made with the Go 1 <a href="/doc/go1compat">promise of compatibility</a>
402 There are also various performance improvements, not enumerated here.
405 <dl id="archive/tar"><dt><a href="/pkg/archive/tar/">archive/tar</a></dt>
407 <p><!-- https://go.dev/issue/54451, CL 491175 -->
408 The implementation of the
409 <a href="/pkg/io/fs/#FileInfo"><code>io/fs.FileInfo</code></a>
410 interface returned by
411 <a href="/pkg/archive/tar/#Header.FileInfo"><code>Header.FileInfo</code></a>
412 now implements a <code>String</code> method that calls
413 <a href="/pkg/io/fs/#FormatFileInfo"><code>io/fs.FormatFileInfo</code></a>.
416 </dl><!-- archive/tar -->
418 <dl id="archive/zip"><dt><a href="/pkg/archive/zip/">archive/zip</a></dt>
420 <p><!-- https://go.dev/issue/54451, CL 491175 -->
421 The implementation of the
422 <a href="/pkg/io/fs/#FileInfo"><code>io/fs.FileInfo</code></a>
423 interface returned by
424 <a href="/pkg/archive/zip/#FileHeader.FileInfo"><code>FileHeader.FileInfo</code></a>
425 now implements a <code>String</code> method that calls
426 <a href="/pkg/io/fs/#FormatFileInfo"><code>io/fs.FormatFileInfo</code></a>.
429 <p><!-- https://go.dev/issue/54451, CL 491175 -->
430 The implementation of the
431 <a href="/pkg/io/fs/#DirEntry"><code>io/fs.DirEntry</code></a>
432 interface returned by the
433 <a href="/pkg/io/fs/#ReadDirFile.ReadDir"><code>io/fs.ReadDirFile.ReadDir</code></a>
435 <a href="/pkg/io/fs/#File"><code>io/fs.File</code></a>
437 <a href="/pkg/archive/zip/#Reader.Open"><code>Reader.Open</code></a>
438 now implements a <code>String</code> method that calls
439 <a href="/pkg/io/fs/#FormatDirEntry"><code>io/fs.FormatDirEntry</code></a>.
442 </dl><!-- archive/zip -->
444 <dl id="bytes"><dt><a href="/pkg/bytes/">bytes</a></dt>
446 <p><!-- https://go.dev/issue/53685, CL 474635 -->
447 The <a href="/pkg/bytes/#Buffer"><code>Buffer</code></a> type
449 <a href="/pkg/bytes/#Buffer.Available"><code>Available</code></a>
450 and <a href="/pkg/bytes/#AvailableBuffer"><code>AvailableBuffer</code></a>.
451 These may be used along with the
452 <a href="/pkg/bytes/#Buffer.Write"><code>Write</code></a>
453 method to append directly to the <code>Buffer</code>.
458 <dl id="context"><dt><a href="/pkg/context/">context</a></dt>
460 <p><!-- https://go.dev/issue/40221, CL 479918 -->
461 The new <a href="/pkg/context/#WithoutCancel"><code>WithoutCancel</code></a>
462 function returns a copy of a context that is not canceled when the original
465 <p><!-- https://go.dev/issue/56661, CL 449318 -->
466 The new <a href="/pkg/context/#WithDeadlineCause"><code>WithDeadlineCause</code></a>
467 and <a href="/pkg/context/#WithTimeoutCause"><code>WithTimeoutCause</code></a>
468 functions provide a way to set a context cancellation cause when a deadline or
469 timer expires. The cause may be retrieved with the
470 <a href="/pkg/context/#Cause"><code>Cause</code></a> function.
472 <p><!-- https://go.dev/issue/57928, CL 482695 -->
473 The new <a href="/pkg/context/#AfterFunc"><code>AfterFunc</code></a>
474 function registers a function to run after a context has been cancelled.
477 <p><!-- CL 455455 -->
478 An optimization means that the results of calling
479 <a href="/pkg/context/#Background"><code>Background</code></a>
480 and <a href="/pkg/context/#TODO"><code>TODO</code></a> and
481 converting them to a shared type can be considered equal.
482 In previous releases they were always different. Comparing
483 <a href="/pkg/context/#Context"><code>Context</code></a> values
484 for equality has never been well-defined, so this is not
485 considered to be an incompatible change.
489 <dl id="crypto/elliptic"><dt><a href="/pkg/crypto/elliptic/">crypto/elliptic</a></dt>
491 <p><!-- CL 459977 -->
492 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>.
495 </dl><!-- crypto/elliptic -->
497 <dl id="crypto/rand"><dt><a href="/pkg/crypto/rand/">crypto/rand</a></dt>
499 <p><!-- CL 463123 -->
500 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.
503 </dl><!-- crypto/rand -->
505 <dl id="crypto/rsa"><dt><a href="/pkg/crypto/rsa/">crypto/rsa</a></dt>
507 <p><!-- CL 471259, CL 492935 -->
508 The performance of private RSA operations (decryption and signing) is now better than Go 1.19 for <code>GOARCH=amd64</code> and <code>GOARCH=arm64</code>. It had regressed in Go 1.20.
511 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.
513 <p><!-- https://go.dev/issue/56921, CL 459976 -->
514 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.
517 </dl><!-- crypto/rsa -->
519 <!-- CL 483815 reverted -->
521 <dl id="crypto/sha256"><dt><a href="/pkg/crypto/sha256/">crypto/sha256</a></dt>
523 <p><!-- https://go.dev/issue/50543, CL 408795 -->
524 SHA-224 and SHA-256 operations now use native instructions when available when <code>GOARCH=amd64</code>, providing a performance improvement on the order of 3-4x.
527 </dl><!-- crypto/sha256 -->
529 <!-- CL 481478 reverted -->
530 <!-- CL 483816 reverted -->
532 <dl id="crypto/tls"><dt><a href="/pkg/crypto/tls/">crypto/tls</a></dt>
534 <p><!-- https://go.dev/issue/60105, CL 496818, CL 496820, CL 496822, CL 496821, CL 501675 -->
535 Applications can now control the content of session tickets.
538 The new <a href="/pkg/crypto/tls/#SessionState"><code>SessionState</code></a> type
539 describes a resumable session.
542 The <a href="/pkg/crypto/tls/#SessionState.Bytes"><code>SessionState.Bytes</code></a>
543 method and <a href="/pkg/crypto/tls/#ParseSessionState"><code>ParseSessionState</code></a>
544 function serialize and deserialize a <code>SessionState</code>.
547 The <a href="/pkg/crypto/tls/#Config.WrapSession"><code>Config.WrapSession</code></a> and
548 <a href="/pkg/crypto/tls/#Config.UnwrapSession"><code>Config.UnwrapSession</code></a>
549 hooks convert a <code>SessionState</code> to and from a ticket.
552 The <a href="/pkg/crypto/tls/#Config.EncryptTicket"><code>Config.EncryptTicket</code></a>
553 and <a href="/pkg/crypto/tls/#Config.DecryptTicket"><code>Config.DecryptTicket</code></a>
554 methods provide a default implementation of <code>WrapSession</code> and
555 <code>UnwrapSession</code>.
558 The <a href="/pkg/crypto/tls/#ClientSessionState.ResumptionState"><code>ClientSessionState.ResumptionState</code></a> method and
559 <a href="/pkg/crypto/tls/#NewResumptionState"><code>NewResumptionState</code></a> function
560 may be used by a <code>ClientSessionCache</code> implementation to store and
566 <p><!-- CL 497376 -->
567 The package now supports the extended master secret extension (RFC 7627),
568 and enables it by default. Additionally, the deprecation of
569 <a href="/pkg/crypto/tls/#ConnectionState.TLSUnique"><code>ConnectionState.TLSUnique</code></a>
570 has been reverted, and it is populated when a connection which uses
571 extended master secret is resumed. Session tickets produced by
572 Go pre-1.21 are not interoperable with Go 1.21, meaning connections
573 resumed across versions will fall back to full handshakes.
576 <p><!-- https://go.dev/issue/44886, https://go.dev/issue/60107, CL 493655, CL 496995 -->
577 The new <a href="/pkg/crypto/tls/#QUICConn"><code>QUICConn</code></a> type
578 provides support for QUIC implementations. Note that this is not itself
579 a QUIC implementation.
582 <p><!-- https://go.dev/issue/46308, CL 497377 -->
583 The new <a href="/pkg/crypto/tls/#VersionName"></code>VersionName</code></a> function
584 returns the name for a TLS version number.
587 </dl><!-- crypto/tls -->
589 <dl id="crypto/x509"><dt><a href="/pkg/crypto/x509/">crypto/x509</a></dt>
591 <p><!-- https://go.dev/issue/53573, CL 468875 -->
592 <a href="/pkg/crypto/x509/#RevocationList.RevokedCertificates"><code>RevocationList.RevokedCertificates</code></a> has been deprecated and replaced with the new <a href="/pkg/crypto/x509/#RevocationList.RevokedCertificateEntries"><code>RevokedCertificateEntries</code></a> field, which is a slice of <a href="/pkg/crypto/x509/#RevocationListEntry"><code>RevocationListEntry</code></a>. <a href="/pkg/crypto/x509/#RevocationListEntry"><code>RevocationListEntry</code></a> contains all of the fields in <a href="/pkg/crypto/x509/pkix#RevokedCertificate"><code>pkix.RevokedCertificate</code></a>, as well as the revocation reason code.
595 </dl><!-- crypto/x509 -->
597 <dl id="debug/elf"><dt><a href="/pkg/debug/elf/">debug/elf</a></dt>
599 <p><!-- https://go.dev/issue/56892, CL 452617 -->
601 <a href="/pkg/debug/elf/#File.DynValue"><code>File.DynValue</code></a>
602 method may be used to retrieve the numeric values listed with a
606 <p><!-- https://go.dev/issue/56887, CL 452496 -->
607 The constant flags permitted in a <code>DT_FLAGS_1</code>
608 dynamic tag are now defined with type
609 <a href="/pkg/debug/elf/#DynFlag1"><code>DynFlag1</code></a>. These
610 tags have names starting with <code>DF_1</code>.
613 <p><!-- CL 473256 -->
614 The package now defines the constant
615 <a href="/pkg/debug/elf/#COMPRESS_ZSTD"><code>COMPRESS_ZSTD</code></a>.
618 <p><!-- https://go.dev/issue/60348, CL 496918 -->
619 The package now defines the constant
620 <a href="/pkg/debug/elf/#R_PPC64_REL24_P9NOTOC"><code>R_PPC64_REL24_P9NOTOC</code></a>.
623 </dl><!-- debug/elf -->
625 <dl id="debug/pe"><dt><a href="/pkg/debug/pe/">debug/pe</a></dt>
627 <p><!-- CL 488475 -->
628 Attempts to read from a section containing uninitialized data
630 <a href="/pkg/debug/pe/#Section.Data"><code>Section.Data</code></a>
631 or the reader returned by <a href="/pkg/debug/pe/#Section.Open"><code>Section.Open</code></a>
635 </dl><!-- debug/pe -->
637 <dl id="embed"><dt><a href="/pkg/embed/">embed</a></dt>
639 <p><!-- https://go.dev/issue/57803, CL 483235 -->
640 The <a href="/pkg/io/fs/#File"><code>io/fs.File</code></a>
642 <a href="/pkg/embed/#FS.Open"><code>FS.Open</code></a> now
643 has a <code>ReadAt</code> method that
644 implements <a href="/pkg/io/#ReaderAt"><code>io.ReaderAt</code></a>.
647 <p><!-- https://go.dev/issue/54451, CL 491175 -->
648 Calling <code><a href="/pkg/embed/FS.Open">FS.Open</a>.<a href="/pkg/io/fs/#File.Stat">Stat</a></code>
649 will return a type that now implements a <code>String</code>
651 <a href="/pkg/io/fs/#FormatFileInfo"><code>io/fs.FormatFileInfo</code></a>.
656 <dl id="errors"><dt><a href="/pkg/errors/">errors</a></dt>
658 <p><!-- https://go.dev/issue/41198, CL 473935 -->
660 <a href="/pkg/errors/#ErrUnsupported"><code>ErrUnsupported</code></a>
661 error provides a standardized way to indicate that a requested
662 operation may not be performed because it is unsupported.
663 For example, a call to
664 <a href="/pkg/os/#Link"><code>os.Link</code></a> when using a
665 file system that does not support hard links.
670 <dl id="flag"><dt><a href="/pkg/flag/">flag</a></dt>
672 <p><!-- https://go.dev/issue/53747, CL 476015 -->
673 The new <a href="/pkg/flag/#BoolFunc"><code>BoolFunc</code></a>
675 <a href="/pkg/flag/#FlagSet.BoolFunc"><code>FlagSet.BoolFunc</code></a>
676 method define a flag that does not require an argument and calls
677 a function when the flag is used. This is similar to
678 <a href="/pkg/flag/#Func"><code>Func</code></a> but for a
682 <p><!-- CL 480215 -->
684 (via <a href="/pkg/flag/#Bool"><code>Bool</code></a>,
685 <a href="/pkg/flag/#BoolVar"><code>BoolVar</code></a>,
686 <a href="/pkg/flag/#Int"><code>Int</code></a>,
687 <a href="/pkg/flag/#IntVar"><code>IntVar</code></a>, etc.)
688 will panic if <a href="/pkg/flag/#Set"><code>Set</code></a> has
689 already been called on a flag with the same name. This change is
690 intended to detect cases where <a href="#language">changes in
691 initialization order</a> cause flag operations to occur in a
692 different order than expected. In many cases the fix to this
693 problem is to introduce a explicit package dependence to
694 correctly order the definition before any
695 <a href="/pkg/flag/#Set"><code>Set</code></a> operations.
700 <dl id="go/ast"><dt><a href="/pkg/go/ast/">go/ast</a></dt>
702 <p><!-- https://go.dev/issue/28089, CL 487935 -->
703 The new <a href="/pkg/go/ast/#IsGenerated"><code>IsGenerated</code></a> predicate
704 reports whether a file syntax tree contains the
705 <a href="https://go.dev/s/generatedcode">special comment</a>
706 that conventionally indicates that the file was generated by a tool.
711 <p><!-- https://go.dev/issue/59033, CL 476276 -->
713 <a href="/pkg/go/ast/#File.GoVersion"><code>File.GoVersion</code></a>
714 field records the minimum Go version required by
715 any <code>//go:build</code> or <code>// +build</code>
721 <dl id="go/build"><dt><a href="/pkg/go/build/">go/build</a></dt>
723 <p><!-- https://go.dev/issue/56986, CL 453603 -->
724 The package now parses build directives (comments that start
725 with <code>//go:</code>) in file headers (before
726 the <code>package</code> declaration). These directives are
728 <a href="/pkg/go/build#Package"><code>Package</code></a> fields
729 <a href="/pkg/go/build#Package.Directives"><code>Directives</code></a>,
730 <a href="/pkg/go/build#Package.TestDirectives"><code>TestDirectives</code></a>,
732 <a href="/pkg/go/build#Package.XTestDirectives"><code>XTestDirectives</code></a>.
735 </dl><!-- go/build -->
737 <dl id="go/build/constraint"><dt><a href="/pkg/go/build/constraint/">go/build/constraint</a></dt>
739 <p><!-- https://go.dev/issue/59033, CL 476275 -->
741 <a href="/pkg/go/build/constraint/#GoVersion"><code>GoVersion</code></a>
742 function returns the minimum Go version implied by a build
746 </dl><!-- go/build/constraint -->
748 <dl id="go/token"><dt><a href="/pkg/go/token/">go/token</a></dt>
750 <p><!-- https://go.dev/issue/57708, CL 464515 -->
751 The new <a href="/pkg/go/token/#File.Lines"><code>File.Lines</code></a> method
752 returns the file's line-number table in the same form as accepted by
753 <code>File.SetLines</code>.
756 </dl><!-- go/token -->
758 <dl id="hash/maphash"><dt><a href="/pkg/hash/maphash/">hash/maphash</a></dt>
760 <p><!-- https://go.dev/issue/47342, CL 468795 -->
761 The <code>hash/maphash</code> package now has a pure Go implementation, selectable with the <code>purego</code> build tag.
764 </dl><!-- hash/maphash -->
766 <dl id="html/template"><dt><a href="/pkg/html/template/">html/template</a></dt>
768 <p><!-- https://go.dev/issue/59584, CL 496395 -->
770 <a href="/pkg/html/template/#ErrJSTemplate"><code>ErrJSTemplate</code></a>
771 is returned when an action appears in a JavaScript template
772 literal. Previously an unexported error was returned.
775 </dl><!-- html/template -->
777 <dl id="io/fs"><dt><a href="/pkg/io/fs/">io/fs</a></dt>
779 <p><!-- https://go.dev/issue/54451, CL 489555 -->
781 <a href="/pkg/io/fs/#FormatFileInfo"><code>FormatFileInfo</code></a>
782 function returns a formatted version of a
783 <a href="/pkg/io/fs/#FileInfo"><code>FileInfo</code></a>.
785 <a href="/pkg/io/fs/#FormatDirEntry"><code>FormatDirEntry</code></a>
786 function returns a formatted version of a
787 <a href="/pkg/io/fs/#FileInfo"><code>DirEntry</code></a>.
788 The implementation of
789 <a href="/pkg/io/fs/#DirEntry"><code>DirEntry</code></a>
791 <a href="/pkg/io/fs/#ReadDir"><code>ReadDir</code></a> now
792 implements a <code>String</code> method that calls
793 <a href="/pkg/io/fs/#FormatDirEntry"><code>FormatDirEntry</code></a>,
794 and the same is true for
795 the <a href="/pkg/io/fs/#DirEntry"><code>DirEntry</code></a>
797 <a href="/pkg/io/fs/#WalkDirFunc"><code>WalkDirFunc</code></a>.
802 <!-- https://go.dev/issue/56491 rolled back by https://go.dev/issue/60519 -->
803 <!-- CL 459435 reverted by CL 467255 -->
804 <!-- CL 467515 reverted by CL 499416 -->
806 <dl id="math/big"><dt><a href="/pkg/math/big/">math/big</a></dt>
808 <p><!-- https://go.dev/issue/56984, CL 453115, CL 500116 -->
809 The new <a href="/pkg/math/big/#Int.Float64"><code>Int.Float64</code></a>
810 method returns the nearest floating-point value to a
811 multi-precision integer, along with an indication of any
812 rounding that occurred.
815 </dl><!-- math/big -->
817 <dl id="net"><dt><a href="/pkg/net/">net</a></dt>
820 <!-- https://go.dev/issue/59166, https://go.dev/issue/56539 -->
821 <!-- CL 471136, CL 471137, CL 471140 -->
822 On Linux, the <a href="/pkg/net/">net</a> package can now use
823 Multipath TCP when the kernel supports it. It is not used by
824 default. To use Multipath TCP when available on a client, call
826 <a href="/pkg/net/#Dialer.SetMultipathTCP"><code>Dialer.SetMultipathTCP</code></a>
827 method before calling the
828 <a href="/pkg/net/#Dialer.Dial"><code>Dialer.Dial</code></a> or
829 <a href="/pkg/net/#Dialer.DialContext"><code>Dialer.DialContext</code></a>
830 methods. To use Multipath TCP when available on a server, call
832 <a href="/pkg/net/#ListenConfig.SetMultipathTCP"><code>ListenConfig.SetMultipathTCP</code></a>
833 method before calling the
834 <a href="/pkg/net/#ListenConfig.Listen"><code>ListenConfig.Listen</code></a>
835 method. Specify the network as <code>"tcp"</code> or
836 <code>"tcp4"</code> or <code>"tcp6"</code> as usual. If
837 Multipath TCP is not supported by the kernel or the remote host,
838 the connection will silently fall back to TCP. To test whether a
839 particular connection is using Multipath TCP, use the
840 <a href="/pkg/net/#TCPConn.MultipathTCP"><code>TCPConn.MultipathTCP</code></a>
844 In a future Go release we may enable Multipath TCP by default on
845 systems that support it.
850 <dl id="net/http"><dt><a href="/pkg/net/http/">net/http</a></dt>
852 <p><!-- CL 472636 -->
853 The new <a href="/pkg/net/http#ResponseController.EnableFullDuplex"><code>ResponseController.EnableFullDuplex</code></a>
854 method allows server handlers to concurrently read from an HTTP/1
855 request body while writing the response. Normally, the HTTP/1 server
856 automatically consumes any remaining request body before starting to
857 write the response, to avoid deadlocking clients which attempt to
858 write a complete request before reading the response. The
859 <code>EnableFullDuplex</code> method disables this behavior.
862 <p><!-- https://go.dev/issue/44855, CL 382117 -->
863 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.
866 <p><!-- CL 494122 -->
867 The <a href="/pkg/net/http/">net/http</a> package now supports
868 <a href="/pkg/errors/#ErrUnsupported"><code>errors.ErrUnsupported</code></a>,
869 in that the expression
870 <code>errors.Is(http.ErrNotSupported, errors.ErrUnsupported)</code>
874 </dl><!-- net/http -->
876 <dl id="os"><dt><a href="/pkg/os/">os</a></dt>
878 <p><!-- https://go.dev/issue/32558, CL 219638 -->
879 Programs may now pass an empty <code>time.Time</code> value to
880 the <a href="/pkg/os/#Chtimes"><code>Chtimes</code></a> function
881 to leave either the access time or the modification time unchanged.
884 <p><!-- CL 480135 -->
886 <a href="/pkg/os#File.Chdir"><code>File.Chdir</code></a> method
887 now changes the current directory to the file, rather than
888 always returning an error.
891 <p><!-- CL 495079 -->
892 On Unix systems, if a non-blocking descriptor is passed
893 to <a href="/pkg/os/#NewFile"><code>NewFile</code></a>, calling
894 the <a href="/pkg/os/#File.Fd"><code>File.Fd</code></a> method
895 will now return a non-blocking descriptor. Previously the
896 descriptor was converted to blocking mode.
899 <p><!-- CL 477215 -->
901 <a href="/pkg/os/#Truncate"><code>Truncate</code></a> on a
902 non-existent file used to create an empty file. It now returns
903 an error indicating that the file does not exist.
906 <p><!-- https://go.dev/issue/56899, CL 463219 -->
908 <a href="/pkg/os/#TempDir"><code>TempDir</code></a> now uses
909 GetTempPath2W when available, instead of GetTempPathW. The
910 new behavior is a security hardening measure that prevents
911 temporary files created by processes running as SYSTEM to
912 be accessed by non-SYSTEM processes.
915 <p><!-- CL 493036 -->
916 On Windows the os package now supports working with files whose
917 names, stored as UTF-16, can't be represented as valid UTF-8.
920 <p><!-- CL 463177 -->
921 On Windows <a href="/pkg/os/#Lstat"><code>Lstat</code></a> now resolves
922 symbolic links for paths ending with a path separator, consistent with its
923 behavior on POSIX platforms.
926 <p><!-- https://go.dev/issue/54451, CL 491175 -->
927 The implementation of the
928 <a href="/pkg/io/fs/#DirEntry"><code>io/fs.DirEntry</code></a>
929 interface returned by the
930 <a href="/pkg/os/#ReadDir"><code>ReadDir</code></a> function and
931 the <a href="/pkg/os/#File.ReadDir"><code>File.ReadDir</code></a>
932 method now implements a <code>String</code> method that calls
933 <a href="/pkg/io/fs/#FormatDirEntry"><code>io/fs.FormatDirEntry</code></a>.
936 <p><!-- https://go.dev/issue/53761, CL 416775, CL 498015-->
937 The implementation of the
938 <a href="/pkg/io/fs/#FS"><code>io/fs.FS</code></a> interface returned by
939 the <a href="/pkg/os/#DirFS"><code>DirFS</code></a> function now implements
940 the <a href="/pkg/io/fs/#ReadFileFS"><code>io/fs.ReadFileFS</code></a> and
941 the <a href="/pkg/io/fs/#ReadDirFS"><code>io/fs.ReadDirFS</code></a>
947 <dl id="path/filepath"><dt><a href="/pkg/path/filepath/">path/filepath</a></dt>
950 The implementation of the
951 <a href="/pkg/io/fs/#DirEntry"><code>io/fs.DirEntry</code></a>
952 interface passed to the function argument of
953 <a href="/pkg/path/filepath/#WalkDir"><code>WalkDir</code></a>
954 now implements a <code>String</code> method that calls
955 <a href="/pkg/io/fs/#FormatDirEntry"><code>io/fs.FormatDirEntry</code></a>.
958 </dl><!-- path/filepath -->
960 <!-- CL 459455 reverted -->
962 <dl id="reflect"><dt><a href="/pkg/reflect/">reflect</a></dt>
964 <p><!-- CL 408826, CL 413474 -->
965 In Go 1.21, <a href="/pkg/reflect/#ValueOf"><code>ValueOf</code></a>
966 no longer forces its argument to be allocated on the heap, allowing
967 a <code>Value</code>'s content to be allocated on the stack. Most
968 operations on a <code>Value</code> also allow the underlying value
969 to be stack allocated.
972 <p><!-- https://go.dev/issue/55002 -->
973 The new <a href="/pkg/reflect/#Value"><code>Value</code></a>
974 method <a href="/pkg/reflect/#Value.Clear"><code>Value.Clear</code></a>
975 clears the contents of a map or zeros the contents of a slice.
976 This corresponds to the new <code>clear</code> built-in
977 <a href="#language">added to the language</a>.
980 <p><!-- https://go.dev/issue/56906, CL 452762 -->
981 The <a href="/pkg/reflect/#SliceHeader"><code>SliceHeader</code></a>
982 and <a href="/pkg/reflect/#StringHeader"><code>StringHeader</code></a>
983 types are now deprecated. In new code
984 prefer <a href="/pkg/unsafe/#Slice"><code>unsafe.Slice</code></a>,
985 <a href="/pkg/unsafe/#SliceData"><code>unsafe.SliceData</code></a>,
986 <a href="/pkg/unsafe/#String"><code>unsafe.String</code></a>,
987 or <a href="/pkg/unsafe/#StringData"><code>unsafe.StringData</code></a>.
990 </dl><!-- reflect -->
992 <dl id="regexp"><dt><a href="/pkg/regexp/">regexp</a></dt>
994 <p><!-- https://go.dev/issue/46159, CL 479401 -->
995 <a href="/pkg/regexp#Regexp"><code>Regexp</code></a> now defines
996 <a href="/pkg/regexp#Regexp.MarshalText"><code>MarshalText</code></a>
997 and <a href="/pkg/regexp#Regexp.UnmarshalText"><code>UnmarshalText</code></a>
998 methods. These implement
999 <a href="/pkg/encoding#TextMarshaler"><code>encoding.TextMarshaler</code></a>
1001 <a href="/pkg/encoding#TextUnmarshaler"><code>encoding.TextUnmarshaler</code></a>
1002 and will be used by packages such as
1003 <a href="/pkg/encoding/json">encoding/json</a>.
1006 </dl><!-- regexp -->
1008 <dl id="runtime"><dt><a href="/pkg/runtime/">runtime</a></dt>
1010 <p><!-- https://go.dev/issue/38651, CL 435337 -->
1011 Textual stack traces produced by Go programs, such as those
1012 produced when crashing, calling <code>runtime.Stack</code>, or
1013 collecting a goroutine profile with <code>debug=2</code>, now
1014 include the IDs of the goroutines that created each goroutine in
1018 <p><!-- https://go.dev/issue/57441, CL 474915 -->
1019 Crashing Go applications can now opt-in to Windows Error Reporting (WER) by setting the environment variable
1020 <code>GOTRACEBACK=wer</code> or calling <a href="/pkg/runtime/debug/#SetTraceback"><code>debug.SetTraceback("wer")</code></a>
1021 before the crash. Other than enabling WER, the runtime will behave as with <code>GOTRACEBACK=crash</code>.
1022 On non-Windows systems, <code>GOTRACEBACK=wer</code> is ignored.
1025 <p><!-- CL 447778 -->
1026 <code>GODEBUG=cgocheck=2</code>, a thorough checker of cgo pointer passing rules,
1027 is no longer available as a <a href="/pkg/runtime#hdr-Environment_Variables">debug option</a>.
1028 Instead, it is available as an experiment using <code>GOEXPERIMENT=cgocheck2</code>.
1029 In particular this means that this mode has to be selected at build time instead of startup time.
1031 <code>GODEBUG=cgocheck=1</code> is still available (and is still the default).
1034 <p><!-- https://go.dev/issue/46787, CL 367296 -->
1035 A new type <code>Pinner</code> has been added to the runtime
1036 package. <code>Pinner</code>s may be used to "pin" Go memory
1037 such that it may be used more freely by non-Go code. For instance,
1038 passing Go values that reference pinned Go memory to C code is
1039 now allowed. Previously, passing any such nested reference was
1041 <a href="https://pkg.go.dev/cmd/cgo#hdr-Passing_pointers">cgo pointer passing rules.</a>
1043 See <a href="/pkg/runtime#Pinner">the docs</a> for more details.
1046 <!-- CL 472195 no release note needed -->
1048 </dl><!-- runtime -->
1050 <dl id="runtime/metrics"><dt><a href="/pkg/runtime/metrics/">runtime/metrics</a></dt>
1052 <p><!-- https://go.dev/issue/56857, CL 497315 -->
1053 A few previously-internal GC metrics, such as live heap size, are
1056 <code>GOGC</code> and <code>GOMEMLIMIT</code> are also now
1057 available as metrics.
1060 </dl><!-- runtime/metrics -->
1062 <dl id="runtime/trace"><dt><a href="/pkg/runtime/trace/">runtime/trace</a></dt>
1064 <p><!-- https://go.dev/issue/16638 -->
1065 Collecting traces on amd64 and arm64 now incurs a substantially
1066 smaller CPU cost: up to a 10x improvement over the previous release.
1069 <p><!-- CL 494495 -->
1070 Traces now contain explicit stop-the-world events for every reason
1071 the Go runtime might stop-the-world, not just garbage collection.
1074 </dl><!-- runtime/trace -->
1076 <dl id="sync"><dt><a href="/pkg/sync/">sync</a></dt>
1078 <p><!-- https://go.dev/issue/56102, CL 451356 -->
1079 The new <a href="/pkg/sync/#OnceFunc"><code>OnceFunc</code></a>,
1080 <a href="/pkg/sync/#OnceValue"><code>OnceValue</code></a>, and
1081 <a href="/pkg/sync/#OnceValues"><code>OnceValues</code></a>
1082 functions capture a common use of <a href="/pkg/sync/#Once">Once</a> to
1083 lazily initialize a value on first use.
1088 <dl id="syscall"><dt><a href="/pkg/syscall/">syscall</a></dt>
1090 <p><!-- CL 480135 -->
1092 <a href="/pkg/syscall#Fchdir"><code>Fchdir</code></a> function
1093 now changes the current directory to its argument, rather than
1094 always returning an error.
1097 <p><!-- https://go.dev/issue/46259, CL 458335 -->
1099 <a href="/pkg/syscall#SysProcAttr"><code>SysProcAttr</code></a>
1100 has a new field <code>Jail</code> that may be used to put the
1101 newly created process in a jailed environment.
1104 <p><!-- CL 493036 -->
1105 On Windows the syscall package now supports working with files whose
1106 names, stored as UTF-16, can't be represented as valid UTF-8.
1107 The <a href="/pkg/syscall#UTF16ToString"><code>UTF16ToString</code></a>
1108 and <a href="/pkg/syscall#UTF16FromString"><code>UTF16FromString</code></a>
1109 functions now convert between UTF-16 data and
1110 <a href="https://simonsapin.github.io/wtf-8/">WTF-8</a> strings.
1111 This is backward compatible as WTF-8 is a superset of the UTF-8
1112 format that was used in earlier releases.
1115 <p><!-- CL 476578, CL 476875, CL 476916 -->
1116 Several error values match the new
1117 <a href="/pkg/errors/#ErrUnsupported"><code>errors.ErrUnsupported</code></a>,
1118 such that <code>errors.Is(err, errors.ErrUnsupported)</code>
1121 <li><code>ENOSYS</code></li>
1122 <li><code>ENOTSUP</code></li>
1123 <li><code>EOPNOTSUPP</code></li>
1124 <li><code>EPLAN9</code> (Plan 9 only)</li>
1125 <li><code>ERROR_CALL_NOT_IMPLEMENTED</code> (Windows only)</li>
1126 <li><code>ERROR_NOT_SUPPORTED</code> (Windows only)</li>
1127 <li><code>EWINDOWS</code> (Windows only)</li>
1131 </dl><!-- syscall -->
1133 <dl id="testing"><dt><a href="/pkg/testing/">testing</a></dt>
1135 <p><!-- https://go.dev/issue/37708, CL 463837 -->
1136 The new <code>-test.fullpath</code> option will print full path
1137 names in test log messages, rather than just base names.
1140 <p><!-- https://go.dev/issue/52600, CL 475496 -->
1141 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>.
1144 </dl><!-- testing -->
1146 <dl id="testing/fstest"><dt><a href="/pkg/testing/fstest/">testing/fstest</a></dt>
1148 <p><!-- https://go.dev/issue/54451, CL 491175 -->
1149 Calling <code><a href="/pkg/testing/fstest/MapFS.Open">Open</a>.<a href="/pkg/io/fs/#File.Stat">Stat</a></code>
1150 will return a type that now implements a <code>String</code>
1152 <a href="/pkg/io/fs/#FormatFileInfo"><code>io/fs.FormatFileInfo</code></a>.
1155 </dl><!-- testing/fstest -->
1157 <dl id="unicode"><dt><a href="/pkg/unicode/">unicode</a></dt>
1159 <p><!-- CL 456837 -->
1160 The <a href="/pkg/unicode/"><code>unicode</code></a> package and
1161 associated support throughout the system has been upgraded to
1162 <a href="https://www.unicode.org/versions/Unicode15.0.0/">Unicode 15.0.0</a>.
1164 </dl><!-- unicode -->
1166 <h2 id="ports">Ports</h2>
1168 <h3 id="darwin">Darwin</h3>
1170 <p><!-- https://go.dev/issue/57125 -->
1171 As <a href="go1.20#darwin">announced</a> in the Go 1.20 release notes,
1172 Go 1.21 requires macOS 10.15 Catalina or later;
1173 support for previous versions has been discontinued.
1176 <h3 id="windows">Windows</h3>
1178 <p><!-- https://go.dev/issue/57003, https://go.dev/issue/57004 -->
1179 As <a href="go1.20#windows">announced</a> in the Go 1.20 release notes,
1180 Go 1.21 requires at least Windows 10 or Windows Server 2016;
1181 support for previous versions has been discontinued.
1186 <!-- cmd/dist: default to GOARM=7 on all non-arm systems -->
1189 <h3 id="wasm">WebAssembly</h3>
1191 <p><!-- https://go.dev/issue/38248, https://go.dev/issue/59149, CL 489255 -->
1192 The new <code>go:wasmimport</code> directive can now be used in Go programs
1193 to import functions from the WebAssembly host.
1196 <!-- https://go.dev/issue/56100 -->
1198 The Go scheduler now interacts much more efficiently with the
1199 JavaScript event loop, especially in applications that block
1200 frequently on asynchronous events.
1204 <h3 id="wasip1">WebAssembly System Interface</h3>
1206 <p><!-- https://go.dev/issue/58141 -->
1207 Go 1.21 adds an experimental port to the <a href="https://wasi.dev/">
1208 WebAssembly System Interface (WASI)</a>, Preview 1
1209 (<code>GOOS=wasip1</code>, <code>GOARCH=wasm</code>).
1213 As a result of the addition of the new <code>GOOS</code> value
1214 "<code>wasip1</code>", Go files named <code>*_wasip1.go</code>
1215 will now be <a href="/pkg/go/build/#hdr-Build_Constraints">ignored
1216 by Go tools</a> except when that <code>GOOS</code> value is being
1218 If you have existing filenames matching that pattern, you will
1219 need to rename them.
1222 <h3 id="PPC64">ppc64/ppc64le</h3>
1224 <p><!-- go.dev/issue/44549 -->
1225 On Linux, <code>GOPPC64=power10</code> now generates PC-relative instructions, prefixed
1226 instructions, and other new Power10 instructions. On AIX, <code>GOPPC64=power10</code>
1227 generates Power10 instructions, but does not generate PC-relative instructions.
1231 When building position-independent binaries for <code>GOPPC64=power10</code>
1232 <code>GOOS=linux</code> <code>GOARCH=ppc64le</code>, users can expect reduced binary
1233 sizes in most cases, in some cases 3.5%. Position-independent binaries are built for
1234 ppc64le with the following <code>-buildmode</code> values:
1235 <code>c-archive</code>, <code>c-shared</code>, <code>shared</code>, <code>pie</code>, <code>plugin</code>.
1239 <!-- proposals for x repos that don't need to be mentioned here but
1240 are picked up by the relnote tool. -->
1241 <!-- https://go.dev/issue/54232 -->
1242 <!-- https://go.dev/issue/57792 -->
1243 <!-- https://go.dev/issue/57906 -->
1244 <!-- https://go.dev/issue/58668 -->
1245 <!-- https://go.dev/issue/59676 -->
1247 <!-- changes to cmd/api that don't need release notes. -->
1248 <!-- CL 469115, CL 469135, CL 499981 -->
1250 <!-- proposals that don't need release enotes. -->
1251 <!-- https://go.dev/issue/10275 -->
1252 <!-- https://go.dev/issue/59719 -->