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 <!-- https://go.dev/issue/38248 -->
149 <!-- replace CallImport with go:wasmimport directive -->
152 <h3 id="wasm">WebAssembly</h3>
154 <!-- https://go.dev/issue/59149 -->
156 <!-- allow the use of go:wasmimport globally -->
161 <!-- cmd/compile: remove go:wasmimport restriction -->
164 <!-- https://go.dev/issue/56100 -->
166 The Go scheduler now interacts much more efficiently with the
167 JavaScript event loop, especially in applications that block
168 frequently on asynchronous events.
172 <h3 id="wasip1">WebAssembly System Interface</h3>
174 <p><!-- https://go.dev/issue/58141 -->
175 Go 1.21 adds an experimental port to the <a href="https://wasi.dev/">
176 WebAssembly System Interface (WASI)</a>, Preview 1
177 (<code>GOOS=wasip1</code>, <code>GOARCH=wasm</code>).
181 As a result of the addition of the new <code>GOOS</code> value
182 "<code>wasip1</code>", Go files named <code>*_wasip1.go</code>
183 will now be <a href="/pkg/go/build/#hdr-Build_Constraints">ignored
184 by Go tools</a> except when that <code>GOOS</code> value is being
186 If you have existing filenames matching that pattern, you will
190 <h2 id="tools">Tools</h2>
192 <!-- https://go.dev/issue/15513 -->
194 <!-- support compiling all tests without running -->
197 <h3 id="go-command">Go command</h3>
199 <p><!-- https://go.dev/issue/58099, CL 474236 -->
200 The <code>-pgo</code> build flag now defaults to <code>-pgo=auto</code>,
201 and the restriction of specifying a single main package on the command
202 line is now removed. If a file named <code>default.pgo</code> is present
203 in the main package's directory, the <code>go</code> command will use
204 it to enable profile-guided optimization for building the corresponding
208 <p><!-- https://go.dev/issue/37708, CL 463837 -->
209 The new <code>go</code> <code>test</code> option
210 <code>-fullpath</code> prints full path names in test log messages,
211 rather than just base names.
216 <!-- cmd/go: make go test build multiple executables; The go test command now supports using the -c flag with multiple packages. -->
219 <h2 id="runtime-changes">Runtime</h2>
222 TODO: complete this section, or delete if not needed
225 <p><!-- https://go.dev/issue/7181 -->
226 When printing very deep stacks, the runtime now prints the first 50
227 (innermost) frames followed by the bottom 50 (outermost) frames,
228 rather than just printing the first 100 frames. This makes it easier
229 to see how deeply recursive stacks started, and is especially
230 valuable for debugging stack overflows.
233 <p><!-- https://go.dev/issue/59960 -->
234 On Linux platforms that support transparent huge pages, the Go runtime
235 now manages which parts of the heap may be backed by huge pages more
236 explicitly. This leads to better utilization of memory: small heaps
237 should see less memory used (up to 50% in pathological cases) while
238 large heaps should see fewer broken huge pages for dense parts of the
239 heap, improving CPU usage and latency by up to 1%.
242 <p><!-- https://go.dev/issue/57069, https://go.dev/issue/56966 -->
243 As a result of runtime-internal garbage collection tuning,
244 applications may see up to a 40% reduction in application tail latency
245 and a small decrease in memory use. Some applications may also observe
246 a small loss in throughput.
248 The memory use decrease should be proportional to the loss in
249 throughput, such that the previous release's throughput/memory
250 tradeoff may be recovered (with little change to latency) by
251 increasing <code>GOGC</code> and/or <code>GOMEMLIMIT</code> slightly.
254 <p><!-- https://go.dev/issue/51676 -->
255 Calls from C to Go on threads created in C require some setup to prepare for
256 Go execution. On Unix platforms, this setup is now preserved across multiple
257 calls from the same thread. This significantly reduces the overhead of
258 subsequent C to Go calls from ~1-3 microseconds per call to ~100-200
259 nanoseconds per call.
262 <h2 id="compiler">Compiler</h2>
266 <!-- cmd/cgo: reject attempts to declare methods on C types -->
271 <!-- cmd/compile: enable deadcode of unreferenced large global maps -->
275 TODO: complete this section, or delete if not needed
278 <h2 id="assembler">Assembler</h2>
282 The verifier that checks for incorrect uses of <code>R15</code> when dynamic linking on amd64 has been improved.
285 <h2 id="linker">Linker</h2>
287 <p><!-- https://go.dev/issue/57302, CL 461749 -->
288 On Windows AMD64, the linker (with help from the compiler) now emits
289 SEH unwinding data by default, which improves the integration
290 of Go applications with Windows debuggers and other tools.
295 <!-- cmd/link: generate .xdata PE section -->
299 In Go 1.21 the linker (with help from the compiler) is now capable of
300 deleting dead (unreferenced) global map variables, if the number of
301 entries in the variable initializer is sufficiently large, and if the
302 initializer expressions are side-effect free.
305 TODO: complete this section, or delete if not needed
308 <h2 id="library">Core library</h2>
310 <h3 id="slog">New log/slog package</h3>
312 <p><!-- https://go.dev/issue/59060, https://go.dev/issue/59141, https://go.dev/issue/59204, https://go.dev/issue/59280,
313 https://go.dev/issue/59282, https://go.dev/issue/59339, https://go.dev/issue/59345,
314 CL 477295, CL 484096, CL 486376, CL 486415, CL 487855 -->
315 The new <a href="/pkg/log/slog">log/slog</a> package provides structured logging with levels.
316 Structured logging emits key-value pairs
317 to enable fast, accurate processing of large amounts of log data.
318 The package supports integration with popular log analysis tools and services.
321 <h3 id="slogtest">New testing/slogtest package</h3>
323 <p><!-- CL 487895 -->
324 The new <a href="/pkg/testing/slogtest">testing/slogtest</a> package can help
325 to validate <a href="/pkg/log/slog#Handler">slog.Handler<a/> implementations.
328 <h3 id="slices">New slices package</h3>
330 <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 -->
331 The new <a href="/pkg/slices">slices</a> package provides many common
332 operations on slices, using generic functions that work with slices
336 <h3 id="maps">New maps package</h3>
338 <p><!-- https://go.dev/issue/57436, CL 464343 -->
339 The new <a href="/pkg/maps/">maps</a> package provides several
340 common operations on maps, using generic functions that work with
341 maps of any key or element type.
344 <h3 id="minor_library_changes">Minor changes to the library</h3>
347 As always, there are various minor changes and updates to the library,
348 made with the Go 1 <a href="/doc/go1compat">promise of compatibility</a>
350 There are also various performance improvements, not enumerated here.
354 TODO: complete this section
357 <dl id="archive/tar"><dt><a href="/pkg/archive/tar/">archive/tar</a></dt>
359 <p><!-- https://go.dev/issue/54451, CL 491175 -->
360 The implementation of the
361 <a href="/pkg/io/fs/#FileInfo"><code>io/fs.FileInfo</code></a>
362 interface returned by
363 <a href="/pkg/archive/tar/#Header.FileInfo"><code>Header.FileInfo</code></a>
364 now implements a <code>String</code> method that calls
365 <a href="/pkg/io/fs/#FormatFileInfo"><code>io/fs.FormatFileInfo</code></a>.
368 </dl><!-- archive/tar -->
370 <dl id="archive/zip"><dt><a href="/pkg/archive/zip/">archive/zip</a></dt>
372 <p><!-- https://go.dev/issue/54451, CL 491175 -->
373 The implementation of the
374 <a href="/pkg/io/fs/#FileInfo"><code>io/fs.FileInfo</code></a>
375 interface returned by
376 <a href="/pkg/archive/zip/#FileHeader.FileInfo"><code>FileHeader.FileInfo</code></a>
377 now implements a <code>String</code> method that calls
378 <a href="/pkg/io/fs/#FormatFileInfo"><code>io/fs.FormatFileInfo</code></a>.
381 <p><!-- https://go.dev/issue/54451, CL 491175 -->
382 The implementation of the
383 <a href="/pkg/io/fs/#DirEntry"><code>io/fs.DirEntry</code></a>
384 interface returned by the
385 <a href="/pkg/io/fs/#ReadDirFile.ReadDir"><code>io/fs.ReadDirFile.ReadDir</code></a>
387 <a href="/pkg/io/fs/#File"><code>io/fs.File</code></a>
389 <a href="/pkg/archive/zip/#Reader.Open"><code>Reader.Open</code></a>
390 now implements a <code>String</code> method that calls
391 <a href="/pkg/io/fs/#FormatDirEntry"><code>io/fs.FormatDirEntry</code></a>.
394 </dl><!-- archive/zip -->
396 <dl id="bytes"><dt><a href="/pkg/bytes/">bytes</a></dt>
398 <p><!-- https://go.dev/issue/53685, CL 474635 -->
399 The <a href="/pkg/bytes/#Buffer"><code>Buffer</code></a> type
401 <a href="/pkg/bytes/#Buffer.Available"><code>Available</code></a>
402 and <a href="/pkg/bytes/#AvailableBuffer"><code>AvailableBuffer</code></a>.
403 These may be used along with the
404 <a href="/pkg/bytes/#Buffer.Write"><code>Write</code></a>
405 method to append directly to the <code>Buffer</code>.
410 <dl id="context"><dt><a href="/pkg/context/">context</a></dt>
412 <p><!-- https://go.dev/issue/40221, CL 479918 -->
413 The new <a href="/pkg/context/#WithoutCancel"><code>WithoutCancel</code></a>
414 function returns a copy of a context that is not canceled when the original
417 <p><!-- https://go.dev/issue/56661, CL 449318 -->
418 The new <a href="/pkg/context/#WithDeadlineCause"><code>WithDeadlineCause</code></a>
419 and <a href="/pkg/context/#WithTimeoutCause"><code>WithTimeoutCause</code></a>
420 functions provide a way to set a context cancellation cause when a deadline or
421 timer expires. The cause may be retrieved with the
422 <a href="/pkg/context/#Cause"><code>Cause</code></a> function.
424 <p><!-- https://go.dev/issue/57928, CL 482695 -->
425 The new <a href="/pkg/context/#AfterFunc"><code>AfterFunc</code></a>
426 function registers a function to run after a context has been cancelled.
431 <dl id="crypto/elliptic"><dt><a href="/pkg/crypto/elliptic/">crypto/elliptic</a></dt>
433 <p><!-- CL 459977 -->
434 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
437 </dl><!-- crypto/elliptic -->
439 <dl id="crypto/rsa"><dt><a href="/pkg/crypto/rsa/">crypto/rsa</a></dt>
441 <p><!-- https://go.dev/issue/56921 -->
442 TODO: <a href="https://go.dev/issue/56921">https://go.dev/issue/56921</a>: deprecate GenerateMultiPrimeKey and PrecomputedValues.CRTValues
445 <p><!-- CL 459976 -->
446 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
449 </dl><!-- crypto/rsa -->
451 <dl id="crypto/sha1"><dt><a href="/pkg/crypto/sha1/">crypto/sha1</a></dt>
453 <p><!-- CL 483815 -->
454 TODO: <a href="https://go.dev/cl/483815">https://go.dev/cl/483815</a>: crypto/sha1: add WriteString and WriteByte method
457 </dl><!-- crypto/sha1 -->
459 <dl id="crypto/sha256"><dt><a href="/pkg/crypto/sha256/">crypto/sha256</a></dt>
461 <p><!-- https://go.dev/issue/50543 -->
462 TODO: <a href="https://go.dev/issue/50543">https://go.dev/issue/50543</a>: add native SHA256 instruction implementation for AMD64
465 <p><!-- CL 408795 -->
466 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.
469 <p><!-- CL 481478 -->
470 TODO: <a href="https://go.dev/cl/481478">https://go.dev/cl/481478</a>: crypto/sha256: add WriteString and WriteByte method
473 </dl><!-- crypto/sha256 -->
475 <dl id="crypto/sha512"><dt><a href="/pkg/crypto/sha512/">crypto/sha512</a></dt>
477 <p><!-- CL 483816 -->
478 TODO: <a href="https://go.dev/cl/483816">https://go.dev/cl/483816</a>: crypto/sha512: add WriteString and WriteByte method
481 </dl><!-- crypto/sha512 -->
483 <dl id="crypto/x509"><dt><a href="/pkg/crypto/x509/">crypto/x509</a></dt>
485 <p><!-- https://go.dev/issue/53573 -->
486 TODO: <a href="https://go.dev/issue/53573">https://go.dev/issue/53573</a>: surface ReasonCode inside x509.RevocationList entries
489 <p><!-- CL 468875 -->
490 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
493 </dl><!-- crypto/x509 -->
495 <dl id="debug/elf"><dt><a href="/pkg/debug/elf/">debug/elf</a></dt>
497 <p><!-- https://go.dev/issue/56892, CL 452617 -->
499 <a href="https:/pkg/debug/elf/#File.DynValue"><code>File.DynValue</code></a>
500 method may be used to retrieve the numeric values listed with a
504 <p><!-- https://go.dev/issue/56887, CL 452496 -->
505 The constant flags permitted in a <code>DT_FLAGS_1</code>
506 dynamic tag are now defined with type
507 <a href="/pkg/debug/elf/#DynFlag1"><code>DynFlag1</code></a>. These
508 tags have names starting with <code>DF_1</code>.
511 <p><!-- CL 473256 -->
512 The package now defines the constant
513 <a href="/pkg/debug/elf/#COMPRESS_ZSTD"><code>COMPRESS_ZSTD</code></a>.
516 </dl><!-- debug/elf -->
518 <dl id="debug/pe"><dt><a href="/pkg/debug/pe/">debug/pe</a></dt>
520 <p><!-- CL 488475 -->
521 Attempts to read from a section containing uninitialized data
523 <a href="/pkg/debug/pe/#Section.Data"><code>Section.Data</code></a>
524 or the reader returned by <a href="/pkg/debug/pe/#Section.Open"><code>Section.Open</code></a>
528 </dl><!-- debug/pe -->
530 <dl id="embed"><dt><a href="/pkg/embed/">embed</a></dt>
532 <p><!-- https://go.dev/issue/57803, CL 483235 -->
533 The <a href="/pkg/io/fs/#File"><code>io/fs.File</code></a>
535 <a href="/pkg/embed/#FS.Open"><code>FS.Open</code></a> now
536 has a <code>ReadAt</code> method that
537 implements <a href="/pkg/io/#ReaderAt"><code>io.ReaderAt</code></a>.
540 <p><!-- https://go.dev/issue/54451, CL 491175 -->
541 Calling <code><a href="/pkg/embed/FS.Open">FS.Open</a>.<a href="/pkg/io/fs/#File.Stat">Stat</a></code>
542 will return a type that now implements a <code>String</code>
544 <a href="/pkg/io/fs/#FormatFileInfo"><code>io/fs.FormatFileInfo</code></a>.
549 <dl id="errors"><dt><a href="/pkg/errors/">errors</a></dt>
551 <p><!-- https://go.dev/issue/41198, CL 473935 -->
553 <a href="/pkg/errors/#ErrUnsupported"><code>ErrUnsupported</code></a>
554 error provides a standardized way to indicate that a requested
555 operation may not be performed because it is unsupported.
556 For example, a call to
557 <a href="/pkg/os/#Link"><code>os.Link</code></a> when using a
558 file system that does not support hard links.
563 <dl id="flag"><dt><a href="/pkg/flag/">flag</a></dt>
565 <p><!-- https://go.dev/issue/53747, CL 476015 -->
566 The new <a href="/pkg/flag/#BoolFunc"><code>BoolFunc</code></a>
568 <a href="/pkg/flag/#FlagSet.BoolFunc"><code>FlagSet.BoolFunc</code></a>
569 method define a flag that does not require an argument and calls
570 a function when the flag is used. This is similar to
571 <a href="/pkg/flag/#Func"><code>Func</code></a> but for a
575 <p><!-- CL 480215 -->
576 A flag definition (via <code>Bool</code>, <code>BoolVar</code>, <code>Int</code>, <code>IntVar</code>, etc.) will panic if <code>Set</code> has already been called on a flag with the same name.
578 This change is intended to detect cases where <a href="#language">changes in initialization order</a> cause flag operations to occur in a different order than expected. In many cases the fix to this problem is to introduce a explicit package dependence to correctly order the definition before any <code>Set</code> 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>
775 method now implements a <code>String</code> method that calls
776 <a href="/pkg/io/fs/#FormatDirEntry"><code>io/fs.FormatDirEntry</code></a>.
781 <dl id="path/filepath"><dt><a href="/pkg/path/filepath/">path/filepath</a></dt>
784 The implementation of the
785 <a href="/pkg/io/fs/#DirEntry"><code>io/fs.DirEntry</code></a>
786 interface passed to the function argument of
787 <a href="/pkg/path/filepath/#WalkDir"><code>WalkDir</code></a>
788 now implements a <code>String</code> method that calls
789 <a href="/pkg/io/fs/#FormatDirEntry"><code>io/fs.FormatDirEntry</code></a>.
792 </dl><!-- path/filepath -->
794 <!-- CL 459455 reverted -->
796 <dl id="reflect"><dt><a href="/pkg/reflect/">reflect</a></dt>
798 <p><!-- CL 408826, CL 413474 -->
799 In Go 1.21, <a href="/pkg/reflect/#ValueOf"><code>ValueOf</code></a>
800 no longer forces its argument to be allocated on the heap, allowing
801 a <code>Value</code>'s content to be allocated on the stack. Most
802 operations on a <code>Value</code> also allow the underlying value
803 to be stack allocated.
806 <p><!-- https://go.dev/issue/55002 -->
807 The new <a href="/pkg/reflect/#Value"><code>Value</code></a>
808 method <a href="/pkg/reflect/#Value.Clear"><code>Value.Clear</code></a>
809 clears the contents of a map or zeros the contents of a slice.
810 This corresponds to the new <code>clear</code> built-in
811 <a href="#language">added to the language</a>.
814 <p><!-- https://go.dev/issue/56906, CL 452762 -->
815 The <a href="/pkg/reflect/#SliceHeader"><code>SliceHeader</code></a>
816 and <a href="/pkg/reflect/#StringHeader"><code>StringHeader</code></a>
817 types are now deprecated. In new code
818 prefer <a href="/pkg/unsafe/#Slice"><code>unsafe.Slice</code></a>,
819 <a href="/pkg/unsafe/#SliceData"><code>unsafe.SliceData</code></a>,
820 <a href="/pkg/unsafe/#String"><code>unsafe.String</code></a>,
821 or <a href="/pkg/unsafe/#StringData"><code>unsafe.StringData</code></a>.
824 </dl><!-- reflect -->
826 <dl id="regexp"><dt><a href="/pkg/regexp/">regexp</a></dt>
828 <p><!-- https://go.dev/issue/46159, CL 479401 -->
829 <a href="/pkg/regexp#Regexp"><code>Regexp</code></a> now defines
830 <a href="/pkg/regexp#Regexp.MarshalText"><code>MarshalText</code></a>
831 and <a href="/pkg/regexp#Regexp.UnmarshalText"><code>UnmarshalText</code></a>
832 methods. These implement
833 <a href="/pkg/encoding#TextMarshaler"><code>encoding.TextMarshaler</code></a>
835 <a href="/pkg/encoding#TextUnmarshaler"><code>encoding.TextUnmarshaler</code></a>
836 and will be used by packages such as
837 <a href="/pkg/encoding/json">encoding/json</a>.
842 <dl id="runtime"><dt><a href="/pkg/runtime/">runtime</a></dt>
844 <p><!-- https://go.dev/issue/38651 -->
845 TODO: <a href="https://go.dev/issue/38651">https://go.dev/issue/38651</a>: add 'created by goroutine number' to stack traces
848 <p><!-- https://go.dev/issue/57441, CL 474915 -->
849 Crashing Go applications can now opt-in to Windows Error Reporting (WER) by setting the environment variable
850 <code>GOTRACEBACK=wer</code> or calling <a href="/pkg/runtime/debug/#SetTraceback"><code>debug.SetTraceback("wer")</code></a>
851 before the crash. Other than enabling WER, the runtime will behave as with <code>GOTRACEBACK=crash</code>.
852 On non-Windows systems, <code>GOTRACEBACK=wer</code> is ignored.
855 <p><!-- CL 447778 -->
856 <code>GODEBUG=cgocheck=2</code>, a thorough checker of cgo pointer passing rules,
857 is no longer available as a <a href="/pkg/runtime#hdr-Environment_Variables">debug option</a>.
858 Instead, it is available as an experiment using <code>GOEXPERIMENT=cgocheck2</code>.
859 In particular this means that this mode has to be selected at build time instead of startup time.
861 <code>GODEBUG=cgocheck=1</code> is still available (and is still the default).
864 <p><!-- https://go.dev/issue/46787 -->
865 A new type <code>Pinner</code> has been added to the runtime
866 package. <code>Pinner</code>s may be used to "pin" Go memory
867 such that it may be used more freely by non-Go code. For instance,
868 passing Go values that reference pinned Go memory to C code is
869 now allowed. Previously, passing any such nested reference was
871 <a href="https://pkg.go.dev/cmd/cgo#hdr-Passing_pointers">cgo pointer passing rules.</a>
873 See <a href="/pkg/runtime#Pinner">the docs</a> for more details.
876 </dl><!-- runtime -->
878 <dl id="runtime/trace"><dt><a href="/pkg/runtime/trace/">runtime/trace</a></dt>
880 <p><!-- https://go.dev/issue/16638 -->
881 Collecting traces on amd64 and arm64 now incurs a substantially
882 smaller CPU cost: up to a 10x improvement over the previous release.
885 <p><!-- CL 494495 -->
886 Traces now contain explicit stop-the-world events for every reason
887 the Go runtime might stop-the-world, not just garbage collection.
890 </dl><!-- runtime/trace -->
892 <dl id="runtime/metrics"><dt><a href="/pkg/runtime/metrics/">runtime/metrics</a></dt>
894 <p><!-- https://go.dev/issue/56857 -->
895 A few previously-internal GC metrics, such as live heap size, are
898 <code>GOGC</code> and <code>GOMEMLIMIT</code> are also now
899 available as metrics.
902 </dl><!-- runtime/metrics -->
904 <dl id="sync"><dt><a href="/pkg/sync/">sync</a></dt>
906 <p><!-- https://go.dev/issue/56102, CL 451356 -->
907 The new <a href="/pkg/sync/#OnceFunc"><code>OnceFunc</code></a>,
908 <a href="/pkg/sync/#OnceValue"><code>OnceValue</code></a>, and
909 <a href="/pkg/sync/#OnceValues"><code>OnceValues</code></a>
910 functions capture a common use of <a href="/pkg/sync/#Once">Once</a> to
911 lazily initialize a value on first use.
916 <dl id="syscall"><dt><a href="/pkg/syscall/">syscall</a></dt>
918 <p><!-- CL 480135 -->
920 <a href="/pkg/syscall#Fchdir"><code>Fchdir</code></a> function
921 now changes the current directory to its argument, rather than
922 always returning an error.
925 <p><!-- https://go.dev/issue/46259, CL 458335 -->
927 <a href="/pkg/syscall#SysProcAttr"><code>SysProcAttr</code></a>
928 has a new field <code>Jail</code> that may be used to put the
929 newly created process in a jailed environment.
932 <p><!-- CL 493036 -->
933 On Windows the syscall package now supports working with files whose
934 names, stored as UTF-16, can't be represented as valid UTF-8.
935 The <a href="/pkg/syscall#UTF16ToString"><code>UTF16ToString</code></a>
936 and <a href="/pkg/syscall#UTF16FromString"><code>UTF16FromString</code></a>
937 functions now convert between UTF-16 data and
938 <a href="https://simonsapin.github.io/wtf-8/">WTF-8</a> strings.
939 This is backward compatible as WTF-8 is a superset of the UTF-8
940 format that was used in earlier releases.
943 <p><!-- CL 476578, CL 476875, CL 476916 -->
944 Several error values match the new
945 <a href="/pkg/errors/#ErrUnsupported"><code>errors.ErrUnsupported</code></a>,
946 such that <code>errors.Is(err, errors.ErrUnsupported)</code>
949 <li><code>ENOSYS</code></li>
950 <li><code>ENOTSUP</code></li>
951 <li><code>EOPNOTSUPP</code></li>
952 <li><code>EPLAN9</code> (Plan 9 only)</li>
953 <li><code>ERROR_CALL_NOT_IMPLEMENTED</code> (Windows only)</li>
954 <li><code>ERROR_NOT_SUPPORTED</code> (Windows only)</li>
955 <li><code>EWINDOWS</code> (Windows only)</li>
959 </dl><!-- syscall -->
961 <dl id="testing"><dt><a href="/pkg/testing/">testing</a></dt>
963 <p><!-- https://go.dev/issue/37708, CL 463837 -->
964 The new <code>-test.fullpath</code> option will print full path
965 names in test log messages, rather than just base names.
968 <p><!-- https://go.dev/issue/52600, CL 475496 -->
969 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>.
972 </dl><!-- testing -->
974 <dl id="testing/fstest"><dt><a href="/pkg/testing/fstest/">testing/fstest</a></dt>
976 <p><!-- https://go.dev/issue/54451, CL 491175 -->
977 Calling <code><a href="/pkg/testing/fstest/MapFS.Open">Open</a>.<a href="/pkg/io/fs/#File.Stat">Stat</a></code>
978 will return a type that now implements a <code>String</code>
980 <a href="/pkg/io/fs/#FormatFileInfo"><code>io/fs.FormatFileInfo</code></a>.
983 </dl><!-- testing/fstest -->
985 <dl id="unicode"><dt><a href="/pkg/unicode/">unicode</a></dt>
987 <p><!-- CL 456837 -->
988 The <a href="/pkg/unicode/"><code>unicode</code></a> package and
989 associated support throughout the system has been upgraded to
990 <a href="https://www.unicode.org/versions/Unicode15.0.0/">Unicode 15.0.0</a>.
992 </dl><!-- unicode -->
994 <!-- proposals for x repos that don't need to be mentioned here but
995 are picked up by the relnote tool. -->
996 <!-- https://go.dev/issue/54232 -->
997 <!-- https://go.dev/issue/57906 -->
998 <!-- https://go.dev/issue/58668 -->