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 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.
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, CL 459976 -->
448 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.
451 </dl><!-- crypto/rsa -->
453 <dl id="crypto/sha256"><dt><a href="/pkg/crypto/sha256/">crypto/sha256</a></dt>
455 <p><!-- https://go.dev/issue/50543, CL 408795 -->
456 SHA-224 and SHA-256 operations now use native instructions when available when <code>GOOS=amd64</code>, providing a performance improvement on the order of 3-4x.
459 </dl><!-- crypto/sha256 -->
461 <dl id="crypto/x509"><dt><a href="/pkg/crypto/x509/">crypto/x509</a></dt>
463 <p><!-- https://go.dev/issue/53573, CL 468875 -->
464 <a href="/pkg/crypto/x509/#RevocationList.RevokedCertificates"><code>RevocationList.RevokedCertificates</code></a> has been deprecated and replaced with the new <a href="/pkg/crypto/x509/#RevocationList.Entries"><code>RevocationList.Entries</code></a> field, which is a slice of <a href="/pkg/crypto/x509/#RevocationListEntry"><code>RevocationListEntry</code></a>. <a href="/pkg/crypto/x509/#RevocationListEntry"><code>RevocationListEntry</code></a> contains all of the fields in <a href="/pkg/crypto/x509/#pkix.RevokedCertificate"><code>pkix.RevokedCertificate</code></a>, as well as the revocation reason code.
467 </dl><!-- crypto/x509 -->
469 <dl id="debug/elf"><dt><a href="/pkg/debug/elf/">debug/elf</a></dt>
471 <p><!-- https://go.dev/issue/56892, CL 452617 -->
473 <a href="https:/pkg/debug/elf/#File.DynValue"><code>File.DynValue</code></a>
474 method may be used to retrieve the numeric values listed with a
478 <p><!-- https://go.dev/issue/56887, CL 452496 -->
479 The constant flags permitted in a <code>DT_FLAGS_1</code>
480 dynamic tag are now defined with type
481 <a href="/pkg/debug/elf/#DynFlag1"><code>DynFlag1</code></a>. These
482 tags have names starting with <code>DF_1</code>.
485 <p><!-- CL 473256 -->
486 The package now defines the constant
487 <a href="/pkg/debug/elf/#COMPRESS_ZSTD"><code>COMPRESS_ZSTD</code></a>.
490 </dl><!-- debug/elf -->
492 <dl id="debug/pe"><dt><a href="/pkg/debug/pe/">debug/pe</a></dt>
494 <p><!-- CL 488475 -->
495 Attempts to read from a section containing uninitialized data
497 <a href="/pkg/debug/pe/#Section.Data"><code>Section.Data</code></a>
498 or the reader returned by <a href="/pkg/debug/pe/#Section.Open"><code>Section.Open</code></a>
502 </dl><!-- debug/pe -->
504 <dl id="embed"><dt><a href="/pkg/embed/">embed</a></dt>
506 <p><!-- https://go.dev/issue/57803, CL 483235 -->
507 The <a href="/pkg/io/fs/#File"><code>io/fs.File</code></a>
509 <a href="/pkg/embed/#FS.Open"><code>FS.Open</code></a> now
510 has a <code>ReadAt</code> method that
511 implements <a href="/pkg/io/#ReaderAt"><code>io.ReaderAt</code></a>.
514 <p><!-- https://go.dev/issue/54451, CL 491175 -->
515 Calling <code><a href="/pkg/embed/FS.Open">FS.Open</a>.<a href="/pkg/io/fs/#File.Stat">Stat</a></code>
516 will return a type that now implements a <code>String</code>
518 <a href="/pkg/io/fs/#FormatFileInfo"><code>io/fs.FormatFileInfo</code></a>.
523 <dl id="errors"><dt><a href="/pkg/errors/">errors</a></dt>
525 <p><!-- https://go.dev/issue/41198, CL 473935 -->
527 <a href="/pkg/errors/#ErrUnsupported"><code>ErrUnsupported</code></a>
528 error provides a standardized way to indicate that a requested
529 operation may not be performed because it is unsupported.
530 For example, a call to
531 <a href="/pkg/os/#Link"><code>os.Link</code></a> when using a
532 file system that does not support hard links.
537 <dl id="flag"><dt><a href="/pkg/flag/">flag</a></dt>
539 <p><!-- https://go.dev/issue/53747, CL 476015 -->
540 The new <a href="/pkg/flag/#BoolFunc"><code>BoolFunc</code></a>
542 <a href="/pkg/flag/#FlagSet.BoolFunc"><code>FlagSet.BoolFunc</code></a>
543 method define a flag that does not require an argument and calls
544 a function when the flag is used. This is similar to
545 <a href="/pkg/flag/#Func"><code>Func</code></a> but for a
549 <p><!-- CL 480215 -->
551 (via <a href="/pkg/flag/#Bool"><code>Bool</code></a>,
552 <a href="/pkg/flag/#BoolVar"><code>BoolVar</code></a>,
553 <a href="/pkg/flag/#Int"><code>Int</code></a>,
554 <a href="/pkg/flag/#IntVar"><code>IntVar</code></a>, etc.)
555 will panic if <a href="/pkg/flag/#Set"><code>Set</code></a> has
556 already been called on a flag with the same name. This change is
557 intended to detect cases where <a href="#language">changes in
558 initialization order</a> cause flag operations to occur in a
559 different order than expected. In many cases the fix to this
560 problem is to introduce a explicit package dependence to
561 correctly order the definition before any
562 <a href="/pkg/flag/#Set"><code>Set</code></a> operations.
567 <dl id="go/ast"><dt><a href="/pkg/go/ast/">go/ast</a></dt>
569 <p><!-- https://go.dev/issue/28089, CL 487935 -->
570 The new <a href="/pkg/go/ast/#IsGenerated"><code>IsGenerated</code></a> predicate
571 reports whether a file syntax tree contains the
572 <a href="https://go.dev/s/generatedcode">special comment</a>
573 that conventionally indicates that the file was generated by a tool.
578 <p><!-- https://go.dev/issue/59033, CL 476276 -->
580 <a href="/pkg/go/ast/#File.GoVersion"><code>File.GoVersion</code></a>
581 field records the minimum Go version required by
582 any <code>//go:build</code> or <code>// +build</code>
588 <dl id="go/build"><dt><a href="/pkg/go/build/">go/build</a></dt>
590 <p><!-- https://go.dev/issue/56986, CL 453603 -->
591 The package now parses build directives (comments that start
592 with <code>//go:</code>) in file headers (before
593 the <code>package</code> declaration). These directives are
595 <a href="/pkg/go/build#Package"><code>Package</code></a> fields
596 <a href="/pkg/go/build#Package.Directives"><code>Directives</code></a>,
597 <a href="/pkg/go/build#Package.TestDirectives"><code>TestDirectives</code></a>,
599 <a href="/pkg/go/build#Package.XTestDirectives"><code>XTestDirectives</code></a>.
602 </dl><!-- go/build -->
604 <dl id="go/build/constraint"><dt><a href="/pkg/go/build/constraint/">go/build/constraint</a></dt>
606 <p><!-- https://go.dev/issue/59033, CL 476275 -->
608 <a href="/pkg/go/build/constraint/#GoVersion"><code>GoVersion</code></a>
609 function returns the minimum Go version implied by a build
613 </dl><!-- go/build/constraint -->
615 <dl id="go/token"><dt><a href="/pkg/go/token/">go/token</a></dt>
617 <p><!-- https://go.dev/issue/57708, CL 464515 -->
618 The new <a href="/pkg/go/token/#File.Lines"><code>File.Lines</code></a> method
619 returns the file's line-number table in the same form as accepted by
620 <code>File.SetLines</code>.
623 </dl><!-- go/token -->
625 <dl id="hash/maphash"><dt><a href="/pkg/hash/maphash/">hash/maphash</a></dt>
627 <p><!-- https://go.dev/issue/47342, CL 468795 -->
628 The <code>hash/maphash</code> package now has a pure Go implementation, selectable with the <code>purego</code> build tag.
631 </dl><!-- hash/maphash -->
633 <dl id="io/fs"><dt><a href="/pkg/io/fs/">io/fs</a></dt>
635 <p><!-- https://go.dev/issue/54451, CL 489555 -->
637 <a href="/pkg/io/fs/#FormatFileInfo"><code>FormatFileInfo</code></a>
638 function returns a formatted version of a
639 <a href="/pkg/io/fs/#FileInfo"><code>FileInfo</code></a>.
641 <a href="/pkg/io/fs/#FormatDirEntry"><code>FormatDirEntry</code></a>
642 function returns a formatted version of a
643 <a href="/pkg/io/fs/#FileInfo"><code>DirEntry</code></a>.
644 The implementation of
645 <a href="/pkg/io/fs/#DirEntry"><code>DirEntry</code></a>
647 <a href="/pkg/io/fs/#ReadDir"><code>ReadDir</code></a> now
648 implements a <code>String</code> method that calls
649 <a href="/pkg/io/fs/#FormatDirEntry"><code>FormatDirEntry</code></a>,
650 and the same is true for
651 the <a href="/pkg/io/fs/#DirEntry"><code>DirEntry</code></a>
653 <a href="/pkg/io/fs/#WalkDirFunc"><code>WalkDirFunc</code></a>.
658 <!-- https://go.dev/issue/56491 rolled back -->
659 <!-- CL 459435 reverted -->
660 <!-- CL 467515 reverted -->
662 <dl id="math/big"><dt><a href="/pkg/math/big/">math/big</a></dt>
664 <p><!-- https://go.dev/issue/56984, CL 453115 -->
665 The new <a href="/pkg/math/big/#Int.Float64"><code>Int.Float64</code></a>
666 method returns the nearest floating-point value to a
667 multi-precision integer, along with an indication of any
668 rounding that occurred.
671 </dl><!-- math/big -->
673 <dl id="net"><dt><a href="/pkg/net/">net</a></dt>
676 <!-- https://go.dev/issue/59166, https://go.dev/issue/56539 -->
677 <!-- CL 471136, CL 471137, CL 471140 -->
678 On Linux, the <a href="/pkg/net/">net</a> package can now use
679 Multipath TCP when the kernel supports it. It is not used by
680 default. To use Multipath TCP when available on a client, call
682 <a href="/pkg/net/#Dialer.SetMultipathTCP"><code>Dialer.SetMultipathTCP</code></a>
683 method before calling the
684 <a href="/pkg/net/#Dialer.Dial"><code>Dialer.Dial</code></a> or
685 <a href="/pkg/net/#Dialer.DialContext"><code>Dialer.DialContext</code></a>
686 methods. To use Multipath TCP when available on a server, call
688 <a href="/pkg/net/#ListenConfig.SetMultipathTCP"><code>ListenConfig.SetMultipathTCP</code></a>
689 method before calling the
690 <a href="/pkg/net/#ListenConfig.Listen"><code>ListenConfig.Listen</code></a>
691 method. Specify the network as <code>"tcp"</code> or
692 <code>"tcp4"</code> or <code>"tcp6"</code> as usual. If
693 Multipath TCP is not supported by the kernel or the remote host,
694 the connection will silently fall back to TCP. To test whether a
695 particular connection is using Multipath TCP, use the
696 <a href="/pkg/net/#TCPConn.MultipathTCP"><code>TCPConn.MultipathTCP</code></a>
700 In a future Go release we may enable Multipath TCP by default on
701 systems that support it.
706 <dl id="net/http"><dt><a href="/pkg/net/http/">net/http</a></dt>
708 <p><!-- https://go.dev/issue/44855, CL 382117 -->
709 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.
712 <p><!-- CL 472636 -->
713 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
716 <p><!-- CL 494122 -->
717 The <a href="/pkg/net/http/">net/http</a> package now supports
718 <a href="/pkg/errors/#ErrUnsupported"><code>errors.ErrUnsupported</code></a>,
719 in that the expression
720 <code>errors.Is(http.ErrNotSupported, errors.ErrUnsupported)</code>
724 </dl><!-- net/http -->
726 <dl id="os"><dt><a href="/pkg/os/">os</a></dt>
728 <p><!-- https://go.dev/issue/32558, CL 219638 -->
729 Programs may now pass an empty <code>time.Time</code> value to
730 the <a href="/pkg/os/#Chtimes"><code>Chtimes</code></a> function
731 to leave either the access time or the modification time unchanged.
734 <p><!-- CL 480135 -->
736 <a href="/pkg/os#File.Chdir"><code>File.Chdir</code></a> method
737 now changes the current directory to the file, rather than
738 always returning an error.
741 <p><!-- CL 477215 -->
743 <a href="/pkg/os/#Truncate"><code>Truncate</code></a> on a
744 non-existent file used to create an empty file. It now returns
745 an error indicating that the file does not exist.
748 <p><!-- CL 493036 -->
749 On Windows the os package now supports working with files whose
750 names, stored as UTF-16, can't be represented as valid UTF-8.
753 <p><!-- https://go.dev/issue/54451, CL 491175 -->
754 The implementation of the
755 <a href="/pkg/io/fs/#DirEntry"><code>io/fs.DirEntry</code></a>
756 interface returned by the
757 <a href="/pkg/os/#ReadDir"><code>ReadDir</code></a> function and
758 the <a href="/pkg/os/#File.ReadDir"><code>File.ReadDir</code></a>
759 method now implements a <code>String</code> method that calls
760 <a href="/pkg/io/fs/#FormatDirEntry"><code>io/fs.FormatDirEntry</code></a>.
763 <p><!-- https://go.dev/issue/53761, CL 416775, CL 498015-->
764 The implementation of the
765 <a href="/pkg/io/fs/#FS"><code>io/fs.FS</code></a> interface returned by
766 the <a href="/pkg/os/#DirFS"><code>DirFS</code></a> function now implements
767 the <a href="/pkg/io/fs/#ReadFileFS"><code>io/fs.ReadFileFS</code></a> and
768 the <a href="/pkg/io/fs/#ReadDirFS"><code>io/fs.ReadDirFS</code></a>
774 <dl id="path/filepath"><dt><a href="/pkg/path/filepath/">path/filepath</a></dt>
777 The implementation of the
778 <a href="/pkg/io/fs/#DirEntry"><code>io/fs.DirEntry</code></a>
779 interface passed to the function argument of
780 <a href="/pkg/path/filepath/#WalkDir"><code>WalkDir</code></a>
781 now implements a <code>String</code> method that calls
782 <a href="/pkg/io/fs/#FormatDirEntry"><code>io/fs.FormatDirEntry</code></a>.
785 </dl><!-- path/filepath -->
787 <!-- CL 459455 reverted -->
789 <dl id="reflect"><dt><a href="/pkg/reflect/">reflect</a></dt>
791 <p><!-- CL 408826, CL 413474 -->
792 In Go 1.21, <a href="/pkg/reflect/#ValueOf"><code>ValueOf</code></a>
793 no longer forces its argument to be allocated on the heap, allowing
794 a <code>Value</code>'s content to be allocated on the stack. Most
795 operations on a <code>Value</code> also allow the underlying value
796 to be stack allocated.
799 <p><!-- https://go.dev/issue/55002 -->
800 The new <a href="/pkg/reflect/#Value"><code>Value</code></a>
801 method <a href="/pkg/reflect/#Value.Clear"><code>Value.Clear</code></a>
802 clears the contents of a map or zeros the contents of a slice.
803 This corresponds to the new <code>clear</code> built-in
804 <a href="#language">added to the language</a>.
807 <p><!-- https://go.dev/issue/56906, CL 452762 -->
808 The <a href="/pkg/reflect/#SliceHeader"><code>SliceHeader</code></a>
809 and <a href="/pkg/reflect/#StringHeader"><code>StringHeader</code></a>
810 types are now deprecated. In new code
811 prefer <a href="/pkg/unsafe/#Slice"><code>unsafe.Slice</code></a>,
812 <a href="/pkg/unsafe/#SliceData"><code>unsafe.SliceData</code></a>,
813 <a href="/pkg/unsafe/#String"><code>unsafe.String</code></a>,
814 or <a href="/pkg/unsafe/#StringData"><code>unsafe.StringData</code></a>.
817 </dl><!-- reflect -->
819 <dl id="regexp"><dt><a href="/pkg/regexp/">regexp</a></dt>
821 <p><!-- https://go.dev/issue/46159, CL 479401 -->
822 <a href="/pkg/regexp#Regexp"><code>Regexp</code></a> now defines
823 <a href="/pkg/regexp#Regexp.MarshalText"><code>MarshalText</code></a>
824 and <a href="/pkg/regexp#Regexp.UnmarshalText"><code>UnmarshalText</code></a>
825 methods. These implement
826 <a href="/pkg/encoding#TextMarshaler"><code>encoding.TextMarshaler</code></a>
828 <a href="/pkg/encoding#TextUnmarshaler"><code>encoding.TextUnmarshaler</code></a>
829 and will be used by packages such as
830 <a href="/pkg/encoding/json">encoding/json</a>.
835 <dl id="runtime"><dt><a href="/pkg/runtime/">runtime</a></dt>
837 <p><!-- https://go.dev/issue/38651 -->
838 TODO: <a href="https://go.dev/issue/38651">https://go.dev/issue/38651</a>: add 'created by goroutine number' to stack traces
841 <p><!-- https://go.dev/issue/57441, CL 474915 -->
842 Crashing Go applications can now opt-in to Windows Error Reporting (WER) by setting the environment variable
843 <code>GOTRACEBACK=wer</code> or calling <a href="/pkg/runtime/debug/#SetTraceback"><code>debug.SetTraceback("wer")</code></a>
844 before the crash. Other than enabling WER, the runtime will behave as with <code>GOTRACEBACK=crash</code>.
845 On non-Windows systems, <code>GOTRACEBACK=wer</code> is ignored.
848 <p><!-- CL 447778 -->
849 <code>GODEBUG=cgocheck=2</code>, a thorough checker of cgo pointer passing rules,
850 is no longer available as a <a href="/pkg/runtime#hdr-Environment_Variables">debug option</a>.
851 Instead, it is available as an experiment using <code>GOEXPERIMENT=cgocheck2</code>.
852 In particular this means that this mode has to be selected at build time instead of startup time.
854 <code>GODEBUG=cgocheck=1</code> is still available (and is still the default).
857 <p><!-- https://go.dev/issue/46787 -->
858 A new type <code>Pinner</code> has been added to the runtime
859 package. <code>Pinner</code>s may be used to "pin" Go memory
860 such that it may be used more freely by non-Go code. For instance,
861 passing Go values that reference pinned Go memory to C code is
862 now allowed. Previously, passing any such nested reference was
864 <a href="https://pkg.go.dev/cmd/cgo#hdr-Passing_pointers">cgo pointer passing rules.</a>
866 See <a href="/pkg/runtime#Pinner">the docs</a> for more details.
869 </dl><!-- runtime -->
871 <dl id="runtime/trace"><dt><a href="/pkg/runtime/trace/">runtime/trace</a></dt>
873 <p><!-- https://go.dev/issue/16638 -->
874 Collecting traces on amd64 and arm64 now incurs a substantially
875 smaller CPU cost: up to a 10x improvement over the previous release.
878 <p><!-- CL 494495 -->
879 Traces now contain explicit stop-the-world events for every reason
880 the Go runtime might stop-the-world, not just garbage collection.
883 </dl><!-- runtime/trace -->
885 <dl id="runtime/metrics"><dt><a href="/pkg/runtime/metrics/">runtime/metrics</a></dt>
887 <p><!-- https://go.dev/issue/56857 -->
888 A few previously-internal GC metrics, such as live heap size, are
891 <code>GOGC</code> and <code>GOMEMLIMIT</code> are also now
892 available as metrics.
895 </dl><!-- runtime/metrics -->
897 <dl id="sync"><dt><a href="/pkg/sync/">sync</a></dt>
899 <p><!-- https://go.dev/issue/56102, CL 451356 -->
900 The new <a href="/pkg/sync/#OnceFunc"><code>OnceFunc</code></a>,
901 <a href="/pkg/sync/#OnceValue"><code>OnceValue</code></a>, and
902 <a href="/pkg/sync/#OnceValues"><code>OnceValues</code></a>
903 functions capture a common use of <a href="/pkg/sync/#Once">Once</a> to
904 lazily initialize a value on first use.
909 <dl id="syscall"><dt><a href="/pkg/syscall/">syscall</a></dt>
911 <p><!-- CL 480135 -->
913 <a href="/pkg/syscall#Fchdir"><code>Fchdir</code></a> function
914 now changes the current directory to its argument, rather than
915 always returning an error.
918 <p><!-- https://go.dev/issue/46259, CL 458335 -->
920 <a href="/pkg/syscall#SysProcAttr"><code>SysProcAttr</code></a>
921 has a new field <code>Jail</code> that may be used to put the
922 newly created process in a jailed environment.
925 <p><!-- CL 493036 -->
926 On Windows the syscall package now supports working with files whose
927 names, stored as UTF-16, can't be represented as valid UTF-8.
928 The <a href="/pkg/syscall#UTF16ToString"><code>UTF16ToString</code></a>
929 and <a href="/pkg/syscall#UTF16FromString"><code>UTF16FromString</code></a>
930 functions now convert between UTF-16 data and
931 <a href="https://simonsapin.github.io/wtf-8/">WTF-8</a> strings.
932 This is backward compatible as WTF-8 is a superset of the UTF-8
933 format that was used in earlier releases.
936 <p><!-- CL 476578, CL 476875, CL 476916 -->
937 Several error values match the new
938 <a href="/pkg/errors/#ErrUnsupported"><code>errors.ErrUnsupported</code></a>,
939 such that <code>errors.Is(err, errors.ErrUnsupported)</code>
942 <li><code>ENOSYS</code></li>
943 <li><code>ENOTSUP</code></li>
944 <li><code>EOPNOTSUPP</code></li>
945 <li><code>EPLAN9</code> (Plan 9 only)</li>
946 <li><code>ERROR_CALL_NOT_IMPLEMENTED</code> (Windows only)</li>
947 <li><code>ERROR_NOT_SUPPORTED</code> (Windows only)</li>
948 <li><code>EWINDOWS</code> (Windows only)</li>
952 </dl><!-- syscall -->
954 <dl id="testing"><dt><a href="/pkg/testing/">testing</a></dt>
956 <p><!-- https://go.dev/issue/37708, CL 463837 -->
957 The new <code>-test.fullpath</code> option will print full path
958 names in test log messages, rather than just base names.
961 <p><!-- https://go.dev/issue/52600, CL 475496 -->
962 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>.
965 </dl><!-- testing -->
967 <dl id="testing/fstest"><dt><a href="/pkg/testing/fstest/">testing/fstest</a></dt>
969 <p><!-- https://go.dev/issue/54451, CL 491175 -->
970 Calling <code><a href="/pkg/testing/fstest/MapFS.Open">Open</a>.<a href="/pkg/io/fs/#File.Stat">Stat</a></code>
971 will return a type that now implements a <code>String</code>
973 <a href="/pkg/io/fs/#FormatFileInfo"><code>io/fs.FormatFileInfo</code></a>.
976 </dl><!-- testing/fstest -->
978 <dl id="unicode"><dt><a href="/pkg/unicode/">unicode</a></dt>
980 <p><!-- CL 456837 -->
981 The <a href="/pkg/unicode/"><code>unicode</code></a> package and
982 associated support throughout the system has been upgraded to
983 <a href="https://www.unicode.org/versions/Unicode15.0.0/">Unicode 15.0.0</a>.
985 </dl><!-- unicode -->
987 <!-- proposals for x repos that don't need to be mentioned here but
988 are picked up by the relnote tool. -->
989 <!-- https://go.dev/issue/54232 -->
990 <!-- https://go.dev/issue/57906 -->
991 <!-- https://go.dev/issue/58668 -->