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.
26 <h2 id="language">Changes to the language</h2>
29 Go 1.21 adds three new built-ins to the language.
32 <li><!-- https://go.dev/issue/59488 -->
33 The new functions <code>min</code> and <code>max</code> compute the
34 smallest (or largest, for <code>max</code>) value of a fixed number
36 See the language spec for
37 <a href="https://tip.golang.org/ref/spec#Min_and_max">details</a>.
39 <li><!-- https://go.dev/issue/56351 -->
40 The new function <code>clear</code> deletes all elements from a
41 map or zeroes all elements of a slice.
42 See the language spec for
43 <a href="https://tip.golang.org/ref/spec#Clear">details</a>.
48 <p><!-- https://go.dev/issue/57411 -->
49 Package initialization order is now specified more precisely. The
53 Sort all packages by import path.
55 <li>Repeat until the list of packages is empty:
58 Find the first package in the list for which all imports are
62 Initialize that package and remove it from the list.
67 This may change the behavior of some programs that rely on a
68 specific initialization ordering that was not expressed by explicit
69 imports. The behavior of such programs was not well defined by the
70 spec in past releases. The new rule provides an unambiguous definition.
74 Multiple improvements that increase the power and precision of type inference have been made.
77 <li><!-- https://go.dev/issue/59338 -->
78 A (possibly partially instantiated generic) function may now be called with arguments that are
79 themselves (possibly partially instantiated) generic functions.
80 The compiler will attempt to infer the missing type arguments of the callee (as before) and,
81 for each argument that is a generic function that is not fully instantiated,
82 its missing type arguments (new).
83 Typical use cases are calls to generic functions operating on containers
84 (such as <a href="/pkg/slices#IndexFunc">slices.IndexFunc</a>) where a function argument
85 may also be generic, and where the type argument of the called function and its arguments
86 are inferred from the container type.
87 More generally, a generic function may now be used without explicit instantiation when
88 it is assigned to a variable or returned as a result value if the type arguments can
89 be inferred from the assignment.
91 <li><!-- https://go.dev/issue/60353, https://go.dev/issue/57192, https://go.dev/issue/52397, https://go.dev/issue/41176 -->
92 Type inference now also considers methods when a value is assigned to an interface:
93 type arguments for type parameters used in method signatures may be inferred from
94 the corresponding parameter types of matching methods.
96 <li><!-- https://go.dev/issue/51593 https://go.dev/issue/39661 -->
97 Similarly, since a type argument must implement all the methods of its corresponding constraint,
98 the methods of the type argument and constraint are matched which may lead to the inference of
99 additional type arguments.
101 <li><!-- https://go.dev/issue/58671 -->
102 If multiple untyped constant arguments of different kinds (such as an untyped int and
103 an untyped floating-point constant) are passed to parameters with the same (not otherwise
104 specified) type parameter type, instead of an error, now type inference determines the
105 type using the same approach as an operator with untyped constant operands.
106 This change brings the types inferred from untyped constant arguments in line with the
107 types of constant expressions.
109 <li><!-- https://go.dev/issue/59750 -->
110 Type inference is now precise when matching corresponding types in assignments:
111 component types (such as the the elements of slices, or the parameter types in function signatures)
112 must be identical (given suitable type arguments) to match, otherwise inference fails.
113 This change produces more accurate error messages:
114 where in the past type inference may have succeeded incorrectly and lead to an invalid assignment,
115 the compiler now reports an inference error if two types can't possibly match.
119 <p><!-- https://go.dev/issue/58650 -->
120 More generally, the description of
121 <a href="https://tip.golang.org/ref/spec#Type_inference">type inference</a>
122 in the language spec has been clarified.
123 Together, all these changes make type inference more powerful and inference failures less surprising.
126 <p><!-- https://go.dev/issue/56986 -->
127 TODO: <a href="https://go.dev/issue/56986">https://go.dev/issue/56986</a>: extended backwards compatibility for Go
130 <!-- https://go.dev/issue/57969 -->
132 <!-- add GOEXPERIMENT=loopvar -->
137 TODO: complete this section
140 <h2 id="ports">Ports</h2>
144 <!-- cmd/dist: default to GOARM=7 on all non-arm systems -->
147 <h3 id="wasm">WebAssembly</h3>
149 <p><!-- https://go.dev/issue/38248, https://go.dev/issue/59149, CL 489255 -->
150 The new <code>go:wasmimport</code> directive can now be used in Go programs
151 to import functions from the WebAssembly host.
154 <!-- https://go.dev/issue/56100 -->
156 The Go scheduler now interacts much more efficiently with the
157 JavaScript event loop, especially in applications that block
158 frequently on asynchronous events.
162 <h3 id="wasip1">WebAssembly System Interface</h3>
164 <p><!-- https://go.dev/issue/58141 -->
165 Go 1.21 adds an experimental port to the <a href="https://wasi.dev/">
166 WebAssembly System Interface (WASI)</a>, Preview 1
167 (<code>GOOS=wasip1</code>, <code>GOARCH=wasm</code>).
171 As a result of the addition of the new <code>GOOS</code> value
172 "<code>wasip1</code>", Go files named <code>*_wasip1.go</code>
173 will now be <a href="/pkg/go/build/#hdr-Build_Constraints">ignored
174 by Go tools</a> except when that <code>GOOS</code> value is being
176 If you have existing filenames matching that pattern, you will
180 <h2 id="tools">Tools</h2>
182 <!-- https://go.dev/issue/15513 -->
184 <!-- support compiling all tests without running -->
187 <h3 id="go-command">Go command</h3>
189 <p><!-- https://go.dev/issue/58099, CL 474236 -->
190 The <code>-pgo</code> build flag now defaults to <code>-pgo=auto</code>,
191 and the restriction of specifying a single main package on the command
192 line is now removed. If a file named <code>default.pgo</code> is present
193 in the main package's directory, the <code>go</code> command will use
194 it to enable profile-guided optimization for building the corresponding
198 <p><!-- https://go.dev/issue/37708, CL 463837 -->
199 The new <code>go</code> <code>test</code> option
200 <code>-fullpath</code> prints full path names in test log messages,
201 rather than just base names.
206 <!-- cmd/go: make go test build multiple executables; The go test command now supports using the -c flag with multiple packages. -->
209 <h3 id="cgo">Cgo</h3>
211 <p><!-- CL 490819 -->
212 In files that <code>import "C"</code>, the Go toolchain now
213 correctly reports errors for attempts to declare Go methods on C types.
216 <h2 id="runtime-changes">Runtime</h2>
219 TODO: complete this section, or delete if not needed
222 <p><!-- https://go.dev/issue/7181 -->
223 When printing very deep stacks, the runtime now prints the first 50
224 (innermost) frames followed by the bottom 50 (outermost) frames,
225 rather than just printing the first 100 frames. This makes it easier
226 to see how deeply recursive stacks started, and is especially
227 valuable for debugging stack overflows.
230 <p><!-- https://go.dev/issue/59960 -->
231 On Linux platforms that support transparent huge pages, the Go runtime
232 now manages which parts of the heap may be backed by huge pages more
233 explicitly. This leads to better utilization of memory: small heaps
234 should see less memory used (up to 50% in pathological cases) while
235 large heaps should see fewer broken huge pages for dense parts of the
236 heap, improving CPU usage and latency by up to 1%.
239 <p><!-- https://go.dev/issue/57069, https://go.dev/issue/56966 -->
240 As a result of runtime-internal garbage collection tuning,
241 applications may see up to a 40% reduction in application tail latency
242 and a small decrease in memory use. Some applications may also observe
243 a small loss in throughput.
245 The memory use decrease should be proportional to the loss in
246 throughput, such that the previous release's throughput/memory
247 tradeoff may be recovered (with little change to latency) by
248 increasing <code>GOGC</code> and/or <code>GOMEMLIMIT</code> slightly.
251 <p><!-- https://go.dev/issue/51676 -->
252 Calls from C to Go on threads created in C require some setup to prepare for
253 Go execution. On Unix platforms, this setup is now preserved across multiple
254 calls from the same thread. This significantly reduces the overhead of
255 subsequent C to Go calls from ~1-3 microseconds per call to ~100-200
256 nanoseconds per call.
259 <h2 id="compiler">Compiler</h2>
262 Profile-guide optimization (PGO), added as a preview in Go 1.20, is now ready
263 for general use. PGO enables additional optimizations on code identified as
264 hot by profiles of production workloads. As mentioned in the
265 <a href="#go-command">Go command section</a>, PGO is enabled by default for
266 binaries that contain a <code>default.pgo</code> profile in the main
267 package directory. Performance improvements vary depending on application
268 behavior, with most programs from a representative set of Go programs seeing
269 between 2 and 7% improvement from enabling PGO. See the
270 <a href="/doc/pgo">PGO user guide</a> for detailed documentation.
273 <!-- https://go.dev/issue/59959 -->
275 PGO builds can now devirtualize some interface method calls, adding a
276 concrete call to the most common callee. This enables further optimization,
277 such as inlining the callee.
281 TODO: complete this section, or delete if not needed
284 <h2 id="assembler">Assembler</h2>
288 The verifier that checks for incorrect uses of <code>R15</code> when dynamic linking on amd64 has been improved.
291 <h2 id="linker">Linker</h2>
293 <p><!-- https://go.dev/issue/57302, CL 461749 -->
294 On Windows AMD64, the linker (with help from the compiler) now emits
295 SEH unwinding data by default, which improves the integration
296 of Go applications with Windows debuggers and other tools.
301 <!-- cmd/link: generate .xdata PE section -->
303 <!-- CL 463395, CL 461315 -->
305 In Go 1.21 the linker (with help from the compiler) is now capable of
306 deleting dead (unreferenced) global map variables, if the number of
307 entries in the variable initializer is sufficiently large, and if the
308 initializer expressions are side-effect free.
311 TODO: complete this section, or delete if not needed
314 <h2 id="library">Core library</h2>
316 <h3 id="slog">New log/slog package</h3>
318 <p><!-- https://go.dev/issue/59060, https://go.dev/issue/59141, https://go.dev/issue/59204, https://go.dev/issue/59280,
319 https://go.dev/issue/59282, https://go.dev/issue/59339, https://go.dev/issue/59345,
320 CL 477295, CL 484096, CL 486376, CL 486415, CL 487855 -->
321 The new <a href="/pkg/log/slog">log/slog</a> package provides structured logging with levels.
322 Structured logging emits key-value pairs
323 to enable fast, accurate processing of large amounts of log data.
324 The package supports integration with popular log analysis tools and services.
327 <h3 id="slogtest">New testing/slogtest package</h3>
329 <p><!-- CL 487895 -->
330 The new <a href="/pkg/testing/slogtest">testing/slogtest</a> package can help
331 to validate <a href="/pkg/log/slog#Handler">slog.Handler</a> implementations.
334 <h3 id="slices">New slices package</h3>
336 <p><!-- https://go.dev/issue/45955, https://go.dev/issue/54768, https://go.dev/issue/57348, https://go.dev/issue/57433, CL 467417, CL 483175 -->
337 The new <a href="/pkg/slices">slices</a> package provides many common
338 operations on slices, using generic functions that work with slices
342 <h3 id="maps">New maps package</h3>
344 <p><!-- https://go.dev/issue/57436, CL 464343 -->
345 The new <a href="/pkg/maps/">maps</a> package provides several
346 common operations on maps, using generic functions that work with
347 maps of any key or element type.
350 <h3 id="minor_library_changes">Minor changes to the library</h3>
353 As always, there are various minor changes and updates to the library,
354 made with the Go 1 <a href="/doc/go1compat">promise of compatibility</a>
356 There are also various performance improvements, not enumerated here.
360 TODO: complete this section
363 <dl id="archive/tar"><dt><a href="/pkg/archive/tar/">archive/tar</a></dt>
365 <p><!-- https://go.dev/issue/54451, CL 491175 -->
366 The implementation of the
367 <a href="/pkg/io/fs/#FileInfo"><code>io/fs.FileInfo</code></a>
368 interface returned by
369 <a href="/pkg/archive/tar/#Header.FileInfo"><code>Header.FileInfo</code></a>
370 now implements a <code>String</code> method that calls
371 <a href="/pkg/io/fs/#FormatFileInfo"><code>io/fs.FormatFileInfo</code></a>.
374 </dl><!-- archive/tar -->
376 <dl id="archive/zip"><dt><a href="/pkg/archive/zip/">archive/zip</a></dt>
378 <p><!-- https://go.dev/issue/54451, CL 491175 -->
379 The implementation of the
380 <a href="/pkg/io/fs/#FileInfo"><code>io/fs.FileInfo</code></a>
381 interface returned by
382 <a href="/pkg/archive/zip/#FileHeader.FileInfo"><code>FileHeader.FileInfo</code></a>
383 now implements a <code>String</code> method that calls
384 <a href="/pkg/io/fs/#FormatFileInfo"><code>io/fs.FormatFileInfo</code></a>.
387 <p><!-- https://go.dev/issue/54451, CL 491175 -->
388 The implementation of the
389 <a href="/pkg/io/fs/#DirEntry"><code>io/fs.DirEntry</code></a>
390 interface returned by the
391 <a href="/pkg/io/fs/#ReadDirFile.ReadDir"><code>io/fs.ReadDirFile.ReadDir</code></a>
393 <a href="/pkg/io/fs/#File"><code>io/fs.File</code></a>
395 <a href="/pkg/archive/zip/#Reader.Open"><code>Reader.Open</code></a>
396 now implements a <code>String</code> method that calls
397 <a href="/pkg/io/fs/#FormatDirEntry"><code>io/fs.FormatDirEntry</code></a>.
400 </dl><!-- archive/zip -->
402 <dl id="bytes"><dt><a href="/pkg/bytes/">bytes</a></dt>
404 <p><!-- https://go.dev/issue/53685, CL 474635 -->
405 The <a href="/pkg/bytes/#Buffer"><code>Buffer</code></a> type
407 <a href="/pkg/bytes/#Buffer.Available"><code>Available</code></a>
408 and <a href="/pkg/bytes/#AvailableBuffer"><code>AvailableBuffer</code></a>.
409 These may be used along with the
410 <a href="/pkg/bytes/#Buffer.Write"><code>Write</code></a>
411 method to append directly to the <code>Buffer</code>.
416 <dl id="context"><dt><a href="/pkg/context/">context</a></dt>
418 <p><!-- https://go.dev/issue/40221, CL 479918 -->
419 The new <a href="/pkg/context/#WithoutCancel"><code>WithoutCancel</code></a>
420 function returns a copy of a context that is not canceled when the original
423 <p><!-- https://go.dev/issue/56661, CL 449318 -->
424 The new <a href="/pkg/context/#WithDeadlineCause"><code>WithDeadlineCause</code></a>
425 and <a href="/pkg/context/#WithTimeoutCause"><code>WithTimeoutCause</code></a>
426 functions provide a way to set a context cancellation cause when a deadline or
427 timer expires. The cause may be retrieved with the
428 <a href="/pkg/context/#Cause"><code>Cause</code></a> function.
430 <p><!-- https://go.dev/issue/57928, CL 482695 -->
431 The new <a href="/pkg/context/#AfterFunc"><code>AfterFunc</code></a>
432 function registers a function to run after a context has been cancelled.
437 <dl id="crypto/elliptic"><dt><a href="/pkg/crypto/elliptic/">crypto/elliptic</a></dt>
439 <p><!-- CL 459977 -->
440 TODO: <a href="https://go.dev/cl/459977">https://go.dev/cl/459977</a>: crypto/elliptic: deprecate unsafe APIs; modified api/next/34648.txt, api/next/52221.txt
443 </dl><!-- crypto/elliptic -->
445 <dl id="crypto/rsa"><dt><a href="/pkg/crypto/rsa/">crypto/rsa</a></dt>
447 <p><!-- https://go.dev/issue/56921 -->
448 TODO: <a href="https://go.dev/issue/56921">https://go.dev/issue/56921</a>: deprecate GenerateMultiPrimeKey and PrecomputedValues.CRTValues
451 <p><!-- CL 459976 -->
452 TODO: <a href="https://go.dev/cl/459976">https://go.dev/cl/459976</a>: crypto/rsa: deprecate multiprime RSA support; modified api/next/56921.txt
455 </dl><!-- crypto/rsa -->
457 <!-- CL 483815 reverted -->
458 <!-- CL 481478 reverted -->
459 <!-- CL 483816 reverted -->
461 <dl id="crypto/sha256"><dt><a href="/pkg/crypto/sha256/">crypto/sha256</a></dt>
463 <p><!-- https://go.dev/issue/50543 -->
464 TODO: <a href="https://go.dev/issue/50543">https://go.dev/issue/50543</a>: add native SHA256 instruction implementation for AMD64
467 <p><!-- CL 408795 -->
468 TODO: <a href="https://go.dev/cl/408795">https://go.dev/cl/408795</a>: crypto/sha256: add sha-ni implementation; crypto/sha256 uses Intel SHA extensions on new enough x86 processors, making it 3-4X faster.
471 </dl><!-- crypto/sha256 -->
473 <dl id="crypto/x509"><dt><a href="/pkg/crypto/x509/">crypto/x509</a></dt>
475 <p><!-- https://go.dev/issue/53573 -->
476 TODO: <a href="https://go.dev/issue/53573">https://go.dev/issue/53573</a>: surface ReasonCode inside x509.RevocationList entries
479 <p><!-- CL 468875 -->
480 TODO: <a href="https://go.dev/cl/468875">https://go.dev/cl/468875</a>: crypto/x509: surface ReasonCode in RevocationList API; modified api/next/53573.txt
483 </dl><!-- crypto/x509 -->
485 <dl id="debug/elf"><dt><a href="/pkg/debug/elf/">debug/elf</a></dt>
487 <p><!-- https://go.dev/issue/56892, CL 452617 -->
489 <a href="https:/pkg/debug/elf/#File.DynValue"><code>File.DynValue</code></a>
490 method may be used to retrieve the numeric values listed with a
494 <p><!-- https://go.dev/issue/56887, CL 452496 -->
495 The constant flags permitted in a <code>DT_FLAGS_1</code>
496 dynamic tag are now defined with type
497 <a href="/pkg/debug/elf/#DynFlag1"><code>DynFlag1</code></a>. These
498 tags have names starting with <code>DF_1</code>.
501 <p><!-- CL 473256 -->
502 The package now defines the constant
503 <a href="/pkg/debug/elf/#COMPRESS_ZSTD"><code>COMPRESS_ZSTD</code></a>.
506 </dl><!-- debug/elf -->
508 <dl id="debug/pe"><dt><a href="/pkg/debug/pe/">debug/pe</a></dt>
510 <p><!-- CL 488475 -->
511 Attempts to read from a section containing uninitialized data
513 <a href="/pkg/debug/pe/#Section.Data"><code>Section.Data</code></a>
514 or the reader returned by <a href="/pkg/debug/pe/#Section.Open"><code>Section.Open</code></a>
518 </dl><!-- debug/pe -->
520 <dl id="embed"><dt><a href="/pkg/embed/">embed</a></dt>
522 <p><!-- https://go.dev/issue/57803, CL 483235 -->
523 The <a href="/pkg/io/fs/#File"><code>io/fs.File</code></a>
525 <a href="/pkg/embed/#FS.Open"><code>FS.Open</code></a> now
526 has a <code>ReadAt</code> method that
527 implements <a href="/pkg/io/#ReaderAt"><code>io.ReaderAt</code></a>.
530 <p><!-- https://go.dev/issue/54451, CL 491175 -->
531 Calling <code><a href="/pkg/embed/FS.Open">FS.Open</a>.<a href="/pkg/io/fs/#File.Stat">Stat</a></code>
532 will return a type that now implements a <code>String</code>
534 <a href="/pkg/io/fs/#FormatFileInfo"><code>io/fs.FormatFileInfo</code></a>.
539 <dl id="errors"><dt><a href="/pkg/errors/">errors</a></dt>
541 <p><!-- https://go.dev/issue/41198, CL 473935 -->
543 <a href="/pkg/errors/#ErrUnsupported"><code>ErrUnsupported</code></a>
544 error provides a standardized way to indicate that a requested
545 operation may not be performed because it is unsupported.
546 For example, a call to
547 <a href="/pkg/os/#Link"><code>os.Link</code></a> when using a
548 file system that does not support hard links.
553 <dl id="flag"><dt><a href="/pkg/flag/">flag</a></dt>
555 <p><!-- https://go.dev/issue/53747, CL 476015 -->
556 The new <a href="/pkg/flag/#BoolFunc"><code>BoolFunc</code></a>
558 <a href="/pkg/flag/#FlagSet.BoolFunc"><code>FlagSet.BoolFunc</code></a>
559 method define a flag that does not require an argument and calls
560 a function when the flag is used. This is similar to
561 <a href="/pkg/flag/#Func"><code>Func</code></a> but for a
565 <p><!-- CL 480215 -->
567 (via <a href="/pkg/flag/#Bool"><code>Bool</code></a>,
568 <a href="/pkg/flag/#BoolVar"><code>BoolVar</code></a>,
569 <a href="/pkg/flag/#Int"><code>Int</code></a>,
570 <a href="/pkg/flag/#IntVar"><code>IntVar</code></a>, etc.)
571 will panic if <a href="/pkg/flag/#Set"><code>Set</code></a> has
572 already been called on a flag with the same name. This change is
573 intended to detect cases where <a href="#language">changes in
574 initialization order</a> cause flag operations to occur in a
575 different order than expected. In many cases the fix to this
576 problem is to introduce a explicit package dependence to
577 correctly order the definition before any
578 <a href="/pkg/flag/#Set"><code>Set</code></a> operations.
583 <dl id="go/ast"><dt><a href="/pkg/go/ast/">go/ast</a></dt>
585 <p><!-- https://go.dev/issue/28089, CL 487935 -->
586 The new <a href="/pkg/go/ast/#IsGenerated"><code>IsGenerated</code></a> predicate
587 reports whether a file syntax tree contains the
588 <a href="https://go.dev/s/generatedcode">special comment</a>
589 that conventionally indicates that the file was generated by a tool.
594 <p><!-- https://go.dev/issue/59033, CL 476276 -->
596 <a href="/pkg/go/ast/#File.GoVersion"><code>File.GoVersion</code></a>
597 field records the minimum Go version required by
598 any <code>//go:build</code> or <code>// +build</code>
604 <dl id="go/build"><dt><a href="/pkg/go/build/">go/build</a></dt>
606 <p><!-- https://go.dev/issue/56986, CL 453603 -->
607 The package now parses build directives (comments that start
608 with <code>//go:</code>) in file headers (before
609 the <code>package</code> declaration). These directives are
611 <a href="/pkg/go/build/Package"><code>Package</code></a> fields
612 <a href="/pkg/go/build/Package.Directives"><code>Directives</code></a>,
613 <a href="/pkg/go/build/Package.TestDirectives"><code>TestDirectives</code></a>,
615 <a href="/pkg/go/build/Package.XTestDirectives"><code>XTestDirectives</code></a>.
618 </dl><!-- go/build -->
620 <dl id="go/build/constraint"><dt><a href="/pkg/go/build/constraint/">go/build/constraint</a></dt>
622 <p><!-- https://go.dev/issue/59033, CL 476275 -->
624 <a href="/pkg/go/build/constraint/#GoVersion"><code>GoVersion</code></a>
625 function returns the minimum Go version implied by a build
629 </dl><!-- go/build/constraint -->
631 <dl id="go/token"><dt><a href="/pkg/go/token/">go/token</a></dt>
633 <p><!-- https://go.dev/issue/57708, CL 464515 -->
634 The new <a href="/pkg/go/token/#File.Lines"><code>File.Lines</code></a> method
635 returns the file's line-number table in the same form as accepted by
636 <code>File.SetLines</code>.
639 </dl><!-- go/token -->
641 <dl id="hash/maphash"><dt><a href="/pkg/hash/maphash/">hash/maphash</a></dt>
643 <p><!-- https://go.dev/issue/47342, CL 468795 -->
644 The <code>hash/maphash</code> package now has a pure Go implementation, selectable with the <code>purego</code> build tag.
647 </dl><!-- hash/maphash -->
649 <dl id="io/fs"><dt><a href="/pkg/io/fs/">io/fs</a></dt>
651 <p><!-- https://go.dev/issue/54451, CL 489555 -->
653 <a href="/pkg/io/fs/#FormatFileInfo"><code>FormatFileInfo</code></a>
654 function returns a formatted version of a
655 <a href="/pkg/io/fs/#FileInfo"><code>FileInfo</code></a>.
657 <a href="/pkg/io/fs/#FormatDirEntry"><code>FormatDirEntry</code></a>
658 function returns a formatted version of a
659 <a href="/pkg/io/fs/#FileInfo"><code>DirEntry</code></a>.
660 The implementation of
661 <a href="/pkg/io/fs/#DirEntry"><code>DirEntry</code></a>
663 <a href="/pkg/io/fs/#ReadDir"><code>ReadDir</code></a> now
664 implements a <code>String</code> method that calls
665 <a href="/pkg/io/fs/#FormatDirEntry"><code>FormatDirEntry</code></a>,
666 and the same is true for
667 the <a href="/pkg/io/fs/#DirEntry"><code>DirEntry</code></a>
669 <a href="/pkg/io/fs/#WalkDirFunc"><code>WalkDirFunc</code></a>.
674 <!-- https://go.dev/issue/56491 rolled back -->
675 <!-- CL 459435 reverted -->
676 <!-- CL 467515 reverted -->
678 <dl id="math/big"><dt><a href="/pkg/math/big/">math/big</a></dt>
680 <p><!-- https://go.dev/issue/56984, CL 453115 -->
681 The new <a href="/pkg/math/big/#Int.ToFloat64"><code>Int.ToFloat64</code></a>
682 method returns the nearest floating-point value to a
683 multi-precision integer, along with an indication of any
684 rounding that occurred.
687 </dl><!-- math/big -->
689 <dl id="net"><dt><a href="/pkg/net/">net</a></dt>
692 <!-- https://go.dev/issue/59166, https://go.dev/issue/56539 -->
693 <!-- CL 471136, CL 471137, CL 471140 -->
694 On Linux, the <a href="/pkg/net/">net</a> package can now use
695 Multipath TCP when the kernel supports it. It is not used by
696 default. To use Multipath TCP when available on a client, call
698 <a href="/pkg/net/#Dialer.SetMultipathTCP"><code>Dialer.SetMultipathTCP</code></a>
699 method before calling the
700 <a href="/pkg/net/#Dialer.Dial"><code>Dialer.Dial</code></a> or
701 <a href="/pkg/net/#Dialer.DialContext"><code>Dialer.DialContext</code></a>
702 methods. To use Multipath TCP when available on a server, call
704 <a href="/pkg/net/#ListenConfig.SetMultipathTCP"><code>ListenConfig.SetMultipathTCP</code></a>
705 method before calling the
706 <a href="/pkg/net/#ListenConfig.Listen"><code>ListenConfig.Listen</code></a>
707 method. Specify the network as <code>"tcp"</code> or
708 <code>"tcp4"</code> or <code>"tcp6"</code> as usual. If
709 Multipath TCP is not supported by the kernel or the remote host,
710 the connection will silently fall back to TCP. To test whether a
711 particular connection is using Multipath TCP, use the
712 <a href="/pkg/net/#TCPConn.MultipathTCP"><code>TCPConn.MultipathTCP</code></a>
716 In a future Go release we may enable Multipath TCP by default on
717 systems that support it.
722 <dl id="net/http"><dt><a href="/pkg/net/http/">net/http</a></dt>
724 <p><!-- https://go.dev/issue/44855, CL 382117 -->
725 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.
728 <p><!-- CL 472636 -->
729 TODO: <a href="https://go.dev/cl/472636">https://go.dev/cl/472636</a>: net/http: support full-duplex HTTP/1 responses; modified api/next/57786.txt
732 <p><!-- CL 494122 -->
733 The <a href="/pkg/net/http/">net/http</a> package now supports
734 <a href="/pkg/errors/#ErrUnsupported"><code>errors.ErrUnsupported</code></a>,
735 in that the expression
736 <code>errors.Is(http.ErrNotSupported, errors.ErrUnsupported)</code>
740 </dl><!-- net/http -->
742 <dl id="os"><dt><a href="/pkg/os/">os</a></dt>
744 <p><!-- https://go.dev/issue/32558, CL 219638 -->
745 Programs may now pass an empty <code>time.Time</code> value to
746 the <a href="/pkg/os/#Chtimes"><code>Chtimes</code></a> function
747 to leave either the access time or the modification time unchanged.
750 <p><!-- CL 480135 -->
752 <a href="/pkg/os#File.Chdir"><code>File.Chdir</code></a> method
753 now changes the current directory to the file, rather than
754 always returning an error.
757 <p><!-- CL 477215 -->
759 <a href="/pkg/os/#Truncate"><code>Truncate</code></a> on a
760 non-existent file used to create an empty file. It now returns
761 an error indicating that the file does not exist.
764 <p><!-- CL 493036 -->
765 On Windows the os package now supports working with files whose
766 names, stored as UTF-16, can't be represented as valid UTF-8.
769 <p><!-- https://go.dev/issue/54451, CL 491175 -->
770 The implementation of the
771 <a href="/pkg/io/fs/#DirEntry"><code>io/fs.DirEntry</code></a>
772 interface returned by the
773 <a href="/pkg/os/#ReadDir"><code>ReadDir</code></a> function and
774 the <a href="/pkg/os/#File.ReadDir"><code>File.ReadDir</code></a>
775 method now implements a <code>String</code> method that calls
776 <a href="/pkg/io/fs/#FormatDirEntry"><code>io/fs.FormatDirEntry</code></a>.
779 <p><!-- https://go.dev/issue/53761, CL 416775, CL 498015-->
780 The implementation of the
781 <a href="/pkg/io/fs/#FS"><code>io/fs.FS</code></a> interface returned by
782 the <a href="/pkg/os/#DirFS"><code>DirFS</code></a> function now implements
783 the <a href="/pkg/io/fs/#ReadFileFS"><code>io/fs.ReadFileFS</code></a> and
784 the <a href="/pkg/io/fs/#ReadDirFS"><code></code>ios/fs.ReadDirFS</a>
790 <dl id="path/filepath"><dt><a href="/pkg/path/filepath/">path/filepath</a></dt>
793 The implementation of the
794 <a href="/pkg/io/fs/#DirEntry"><code>io/fs.DirEntry</code></a>
795 interface passed to the function argument of
796 <a href="/pkg/path/filepath/#WalkDir"><code>WalkDir</code></a>
797 now implements a <code>String</code> method that calls
798 <a href="/pkg/io/fs/#FormatDirEntry"><code>io/fs.FormatDirEntry</code></a>.
801 </dl><!-- path/filepath -->
803 <!-- CL 459455 reverted -->
805 <dl id="reflect"><dt><a href="/pkg/reflect/">reflect</a></dt>
807 <p><!-- CL 408826, CL 413474 -->
808 In Go 1.21, <a href="/pkg/reflect/#ValueOf"><code>ValueOf</code></a>
809 no longer forces its argument to be allocated on the heap, allowing
810 a <code>Value</code>'s content to be allocated on the stack. Most
811 operations on a <code>Value</code> also allow the underlying value
812 to be stack allocated.
815 <p><!-- https://go.dev/issue/55002 -->
816 The new <a href="/pkg/reflect/#Value"><code>Value</code></a>
817 method <a href="/pkg/reflect/#Value.Clear"><code>Value.Clear</code></a>
818 clears the contents of a map or zeros the contents of a slice.
819 This corresponds to the new <code>clear</code> built-in
820 <a href="#language">added to the language</a>.
823 <p><!-- https://go.dev/issue/56906, CL 452762 -->
824 The <a href="/pkg/reflect/#SliceHeader"><code>SliceHeader</code></a>
825 and <a href="/pkg/reflect/#StringHeader"><code>StringHeader</code></a>
826 types are now deprecated. In new code
827 prefer <a href="/pkg/unsafe/#Slice"><code>unsafe.Slice</code></a>,
828 <a href="/pkg/unsafe/#SliceData"><code>unsafe.SliceData</code></a>,
829 <a href="/pkg/unsafe/#String"><code>unsafe.String</code></a>,
830 or <a href="/pkg/unsafe/#StringData"><code>unsafe.StringData</code></a>.
833 </dl><!-- reflect -->
835 <dl id="regexp"><dt><a href="/pkg/regexp/">regexp</a></dt>
837 <p><!-- https://go.dev/issue/46159, CL 479401 -->
838 <a href="/pkg/regexp#Regexp"><code>Regexp</code></a> now defines
839 <a href="/pkg/regexp#Regexp.MarshalText"><code>MarshalText</code></a>
840 and <a href="/pkg/regexp#Regexp.UnmarshalText"><code>UnmarshalText</code></a>
841 methods. These implement
842 <a href="/pkg/encoding#TextMarshaler"><code>encoding.TextMarshaler</code></a>
844 <a href="/pkg/encoding#TextUnmarshaler"><code>encoding.TextUnmarshaler</code></a>
845 and will be used by packages such as
846 <a href="/pkg/encoding/json">encoding/json</a>.
851 <dl id="runtime"><dt><a href="/pkg/runtime/">runtime</a></dt>
853 <p><!-- https://go.dev/issue/38651 -->
854 TODO: <a href="https://go.dev/issue/38651">https://go.dev/issue/38651</a>: add 'created by goroutine number' to stack traces
857 <p><!-- https://go.dev/issue/57441, CL 474915 -->
858 Crashing Go applications can now opt-in to Windows Error Reporting (WER) by setting the environment variable
859 <code>GOTRACEBACK=wer</code> or calling <a href="/pkg/runtime/debug/#SetTraceback"><code>debug.SetTraceback("wer")</code></a>
860 before the crash. Other than enabling WER, the runtime will behave as with <code>GOTRACEBACK=crash</code>.
861 On non-Windows systems, <code>GOTRACEBACK=wer</code> is ignored.
864 <p><!-- CL 447778 -->
865 <code>GODEBUG=cgocheck=2</code>, a thorough checker of cgo pointer passing rules,
866 is no longer available as a <a href="/pkg/runtime#hdr-Environment_Variables">debug option</a>.
867 Instead, it is available as an experiment using <code>GOEXPERIMENT=cgocheck2</code>.
868 In particular this means that this mode has to be selected at build time instead of startup time.
870 <code>GODEBUG=cgocheck=1</code> is still available (and is still the default).
873 <p><!-- https://go.dev/issue/46787 -->
874 A new type <code>Pinner</code> has been added to the runtime
875 package. <code>Pinner</code>s may be used to "pin" Go memory
876 such that it may be used more freely by non-Go code. For instance,
877 passing Go values that reference pinned Go memory to C code is
878 now allowed. Previously, passing any such nested reference was
880 <a href="https://pkg.go.dev/cmd/cgo#hdr-Passing_pointers">cgo pointer passing rules.</a>
882 See <a href="/pkg/runtime#Pinner">the docs</a> for more details.
885 </dl><!-- runtime -->
887 <dl id="runtime/trace"><dt><a href="/pkg/runtime/trace/">runtime/trace</a></dt>
889 <p><!-- https://go.dev/issue/16638 -->
890 Collecting traces on amd64 and arm64 now incurs a substantially
891 smaller CPU cost: up to a 10x improvement over the previous release.
894 <p><!-- CL 494495 -->
895 Traces now contain explicit stop-the-world events for every reason
896 the Go runtime might stop-the-world, not just garbage collection.
899 </dl><!-- runtime/trace -->
901 <dl id="runtime/metrics"><dt><a href="/pkg/runtime/metrics/">runtime/metrics</a></dt>
903 <p><!-- https://go.dev/issue/56857 -->
904 A few previously-internal GC metrics, such as live heap size, are
907 <code>GOGC</code> and <code>GOMEMLIMIT</code> are also now
908 available as metrics.
911 </dl><!-- runtime/metrics -->
913 <dl id="sync"><dt><a href="/pkg/sync/">sync</a></dt>
915 <p><!-- https://go.dev/issue/56102, CL 451356 -->
916 The new <a href="/pkg/sync/#OnceFunc"><code>OnceFunc</code></a>,
917 <a href="/pkg/sync/#OnceValue"><code>OnceValue</code></a>, and
918 <a href="/pkg/sync/#OnceValues"><code>OnceValues</code></a>
919 functions capture a common use of <a href="/pkg/sync/#Once">Once</a> to
920 lazily initialize a value on first use.
925 <dl id="syscall"><dt><a href="/pkg/syscall/">syscall</a></dt>
927 <p><!-- CL 480135 -->
929 <a href="/pkg/syscall#Fchdir"><code>Fchdir</code></a> function
930 now changes the current directory to its argument, rather than
931 always returning an error.
934 <p><!-- https://go.dev/issue/46259, CL 458335 -->
936 <a href="/pkg/syscall#SysProcAttr"><code>SysProcAttr</code></a>
937 has a new field <code>Jail</code> that may be used to put the
938 newly created process in a jailed environment.
941 <p><!-- CL 493036 -->
942 On Windows the syscall package now supports working with files whose
943 names, stored as UTF-16, can't be represented as valid UTF-8.
944 The <a href="/pkg/syscall#UTF16ToString"><code>UTF16ToString</code></a>
945 and <a href="/pkg/syscall#UTF16FromString"><code>UTF16FromString</code></a>
946 functions now convert between UTF-16 data and
947 <a href="https://simonsapin.github.io/wtf-8/">WTF-8</a> strings.
948 This is backward compatible as WTF-8 is a superset of the UTF-8
949 format that was used in earlier releases.
952 <p><!-- CL 476578, CL 476875, CL 476916 -->
953 Several error values match the new
954 <a href="/pkg/errors/#ErrUnsupported"><code>errors.ErrUnsupported</code></a>,
955 such that <code>errors.Is(err, errors.ErrUnsupported)</code>
958 <li><code>ENOSYS</code></li>
959 <li><code>ENOTSUP</code></li>
960 <li><code>EOPNOTSUPP</code></li>
961 <li><code>EPLAN9</code> (Plan 9 only)</li>
962 <li><code>ERROR_CALL_NOT_IMPLEMENTED</code> (Windows only)</li>
963 <li><code>ERROR_NOT_SUPPORTED</code> (Windows only)</li>
964 <li><code>EWINDOWS</code> (Windows only)</li>
968 </dl><!-- syscall -->
970 <dl id="testing"><dt><a href="/pkg/testing/">testing</a></dt>
972 <p><!-- https://go.dev/issue/37708, CL 463837 -->
973 The new <code>-test.fullpath</code> option will print full path
974 names in test log messages, rather than just base names.
977 <p><!-- https://go.dev/issue/52600, CL 475496 -->
978 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>.
981 </dl><!-- testing -->
983 <dl id="testing/fstest"><dt><a href="/pkg/testing/fstest/">testing/fstest</a></dt>
985 <p><!-- https://go.dev/issue/54451, CL 491175 -->
986 Calling <code><a href="/pkg/testing/fstest/MapFS.Open">Open</a>.<a href="/pkg/io/fs/#File.Stat">Stat</a></code>
987 will return a type that now implements a <code>String</code>
989 <a href="/pkg/io/fs/#FormatFileInfo"><code>io/fs.FormatFileInfo</code></a>.
992 </dl><!-- testing/fstest -->
994 <dl id="unicode"><dt><a href="/pkg/unicode/">unicode</a></dt>
996 <p><!-- CL 456837 -->
997 The <a href="/pkg/unicode/"><code>unicode</code></a> package and
998 associated support throughout the system has been upgraded to
999 <a href="https://www.unicode.org/versions/Unicode15.0.0/">Unicode 15.0.0</a>.
1001 </dl><!-- unicode -->
1003 <!-- proposals for x repos that don't need to be mentioned here but
1004 are picked up by the relnote tool. -->
1005 <!-- https://go.dev/issue/54232 -->
1006 <!-- https://go.dev/issue/57906 -->
1007 <!-- https://go.dev/issue/58668 -->