go/doc/go1.21.html

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<h2 id="introduction">DRAFT RELEASE NOTES — Introduction to Go 1.21</h2>

<p>
  <strong>
    Go 1.21 is not yet released. These are work-in-progress
    release notes. Go 1.21 is expected to be released in August 2023.
  </strong>
</p>

<h2 id="language">Changes to the language</h2>

<p>
  Go 1.21 adds three new built-ins to the language.

  <ul>
    <li><!-- https://go.dev/issue/59488 -->
      The new functions <code>min</code> and <code>max</code> compute the
      smallest (or largest, for <code>max</code>) value of a fixed number
      of given arguments.
      See the language spec for
      <a href="https://tip.golang.org/ref/spec#Min_and_max">details</a>.
    </li>
    <li><!-- https://go.dev/issue/56351 -->
      The new function <code>clear</code> deletes all elements from a
      map or zeroes all elements of a slice.
      See the language spec for
      <a href="https://tip.golang.org/ref/spec#Clear">details</a>.
    </li>
  </ul>
</p>

<p><!-- https://go.dev/issue/57411 -->
  Package initialization order is now specified more precisely. The
  new algorithm is:
  <ul>
    <li>
      Sort all packages by import path.
    </li>
    <li>Repeat until the list of packages is empty:
      <ul>
	<li>
	  Find the first package in the list for which all imports are
	  already initialized.
	</li>
	<li>
	  Initialize that package and remove it from the list.
	</li>
      </ul>
    </li>
  </ul>
  This may change the behavior of some programs that rely on a
  specific initialization ordering that was not expressed by explicit
  imports. The behavior of such programs was not well defined by the
  spec in past releases. The new rule provides an unambiguous definition.
</p>

<p><!-- https://go.dev/issue/59338 -->
  TODO: <a href="https://go.dev/issue/59338">https://go.dev/issue/59338</a>: infer type arguments from assignments of generic functions (reverse type inference)
</p>

<p><!-- https://go.dev/issue/56986 -->
  TODO: <a href="https://go.dev/issue/56986">https://go.dev/issue/56986</a>: extended backwards compatibility for Go
</p>

<!-- https://go.dev/issue/57969 -->
<p>
  <!-- add GOEXPERIMENT=loopvar -->
</p>


<p>
  TODO: complete this section
</p>

<h2 id="ports">Ports</h2>

<!-- CL 470695 -->
<p>
  <!-- cmd/dist: default to GOARM=7 on all non-arm systems -->
</p>

<!-- https://go.dev/issue/38248 -->
<p>
  <!-- replace CallImport with go:wasmimport directive -->
</p>

<h3 id="wasm">WebAssembly</h3>

<!-- https://go.dev/issue/59149 -->
<p>
  <!-- allow the use of go:wasmimport globally -->
</p>

<!-- CL 489255 -->
<p>
  <!-- cmd/compile: remove go:wasmimport restriction -->
</p>

<!-- https://go.dev/issue/56100 -->
<p>
  The Go scheduler now interacts much more efficiently with the
  JavaScript event loop, especially in applications that block
  frequently on asynchronous events.
</p>


<h3 id="wasip1">WebAssembly System Interface</h3>

<p><!-- https://go.dev/issue/58141 -->
  Go 1.21 adds an experimental port to the <a href="https://wasi.dev/">
  WebAssembly System Interface (WASI)</a>, Preview 1
  (<code>GOOS=wasip1</code>, <code>GOARCH=wasm</code>).
</p>

<p>
  As a result of the addition of the new <code>GOOS</code> value
  "<code>wasip1</code>", Go files named <code>*_wasip1.go</code>
  will now be <a href="/pkg/go/build/#hdr-Build_Constraints">ignored
  by Go tools</a> except when that <code>GOOS</code> value is being
  used.
  If you have existing filenames matching that pattern, you will
  need to rename them.
</p>

<h2 id="tools">Tools</h2>

<!-- https://go.dev/issue/15513 -->
<p>
  <!-- support compiling all tests without running -->
</p>

<h3 id="go-command">Go command</h3>

<p><!-- https://go.dev/issue/58099, CL 474236 -->
  The <code>-pgo</code> build flag now defaults to <code>-pgo=auto</code>,
  and the restriction of specifying a single main package on the command
  line is now removed. If a file named <code>default.pgo</code> is present
  in the main package's directory, the <code>go</code> command will use
  it to enable profile-guided optimization for building the corresponding
  program.
</p>

<p><!-- https://go.dev/issue/37708, CL 463837 -->
  The new <code>go</code> <code>test</code> option
  <code>-fullpath</code> prints full path names in test log messages,
  rather than just base names.
</p>

<!-- CL 466397 -->
<p>
  <!-- cmd/go: make go test build multiple executables; The go test command now supports using the -c flag with multiple packages. -->
</p>

<h2 id="runtime-changes">Runtime</h2>

<p>
  TODO: complete this section, or delete if not needed
</p>

<p><!-- https://go.dev/issue/7181 -->
  When printing very deep stacks, the runtime now prints the first 50
  (innermost) frames followed by the bottom 50 (outermost) frames,
  rather than just printing the first 100 frames. This makes it easier
  to see how deeply recursive stacks started, and is especially
  valuable for debugging stack overflows.
</p>

<p><!-- https://go.dev/issue/59960 -->
  On Linux platforms that support transparent huge pages, the Go runtime
  now manages which parts of the heap may be backed by huge pages more
  explicitly. This leads to better utilization of memory: small heaps
  should see less memory used (up to 50% in pathological cases) while
  large heaps should see fewer broken huge pages for dense parts of the
  heap, improving CPU usage and latency by up to 1%.
</p>

<p><!-- https://go.dev/issue/57069, https://go.dev/issue/56966 -->
  As a result of runtime-internal garbage collection tuning,
  applications may see up to a 40% reduction in application tail latency
  and a small decrease in memory use. Some applications may also observe
  a small loss in throughput.

  The memory use decrease should be proportional to the loss in
  throughput, such that the previous release's throughput/memory
  tradeoff may be recovered (with little change to latency) by
  increasing <code>GOGC</code> and/or <code>GOMEMLIMIT</code> slightly.
</p>

<h2 id="compiler">Compiler</h2>

<!-- CL 490819 -->
<p>
  <!-- cmd/cgo: reject attempts to declare methods on C types -->
</p>

<!-- CL 461315 -->
<p>
  <!-- cmd/compile: enable deadcode of unreferenced large global maps -->
</p>

<p>
  TODO: complete this section, or delete if not needed
</p>

<h2 id="assembler">Assembler</h2>

<!-- CL 476295 -->
<p>
  The verifier that checks for incorrect uses of <code>R15</code> when dynamic linking on amd64 has been improved.
</p>

<h2 id="linker">Linker</h2>

<p><!-- https://go.dev/issue/57302, CL 461749 -->
  On Windows AMD64, the linker (with help from the compiler) now emits
  SEH unwinding data by default, which improves the integration
  of Go applications with Windows debuggers and other tools.
</p>

<!-- CL 457455 -->
<p>
  <!-- cmd/link: generate .xdata PE section -->
</p>
<!-- CL 463395 -->
<p>
  In Go 1.21 the linker (with help from the compiler) is now capable of
  deleting dead (unreferenced) global map variables, if the number of
  entries in the variable initializer is sufficiently large, and if the
  initializer expressions are side-effect free.
</p>
<p>
  TODO: complete this section, or delete if not needed
</p>

<h2 id="library">Core library</h2>

<h3 id="slog">New log/slog package</h3>

<p>
  The new <a href="/pkg/log/slog">log/slog</a> package provides structured logging with levels.
  Structured logging emits key-value pairs
  to enable fast, accurate processing of large amounts of log data.
  The package supports integration with popular log analysis tools and services.
</p>

<h3 id="slices">New slices package</h3>

<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 -->
  The new <a href="/pkg/slices">slices</a> package provides many common
  operations on slices, using generic functions that work with slices
  of any element type.
</p>

<h3 id="maps">New maps package</h3>

<p><!-- https://go.dev/issue/57436, CL 464343 -->
  The new <a href="/pkg/maps/">maps</a> package provides several
  common operations on maps, using generic functions that work with
  maps of any key or element type.
</p>

<h3 id="minor_library_changes">Minor changes to the library</h3>

<p>
  As always, there are various minor changes and updates to the library,
  made with the Go 1 <a href="/doc/go1compat">promise of compatibility</a>
  in mind.
  There are also various performance improvements, not enumerated here.
</p>

<p>
  TODO: complete this section
</p>

<dl id="archive/tar"><dt><a href="/pkg/archive/tar/">archive/tar</a></dt>
  <dd>
    <p><!-- https://go.dev/issue/54451, CL 491175 -->
      The implementation of the
      <a href="/pkg/io/fs/#FileInfo"><code>io/fs.FileInfo</code></a>
      interface returned by
      <a href="/pkg/archive/tar/#Header.FileInfo"><code>Header.FileInfo</code></a>
      now implements a <code>String</code> method that calls
      <a href="/pkg/io/fs/#FormatFileInfo"><code>io/fs.FormatFileInfo</code></a>.
    </p>
  </dd>
</dl><!-- archive/tar -->

<dl id="archive/zip"><dt><a href="/pkg/archive/zip/">archive/zip</a></dt>
  <dd>
    <p><!-- https://go.dev/issue/54451, CL 491175 -->
      The implementation of the
      <a href="/pkg/io/fs/#FileInfo"><code>io/fs.FileInfo</code></a>
      interface returned by
      <a href="/pkg/archive/zip/#FileHeader.FileInfo"><code>FileHeader.FileInfo</code></a>
      now implements a <code>String</code> method that calls
      <a href="/pkg/io/fs/#FormatFileInfo"><code>io/fs.FormatFileInfo</code></a>.
    </p>

    <p><!-- https://go.dev/issue/54451, CL 491175 -->
      The implementation of the
      <a href="/pkg/io/fs/#DirEntry"><code>io/fs.DirEntry</code></a>
      interface returned by the
      <a href="/pkg/io/fs/#ReadDirFile.ReadDir"><code>io/fs.ReadDirFile.ReadDir</code></a>
      method of the
      <a href="/pkg/io/fs/#File"><code>io/fs.File</code></a>
      returned by
      <a href="/pkg/archive/zip/#Reader.Open"><code>Reader.Open</code></a>
      now implements a <code>String</code> method that calls
      <a href="/pkg/io/fs/#FormatDirEntry"><code>io/fs.FormatDirEntry</code></a>.
    </p>
  </dd>
</dl><!-- archive/zip -->

<dl id="bytes"><dt><a href="/pkg/bytes/">bytes</a></dt>
  <dd>
    <p><!-- https://go.dev/issue/53685, CL 474635 -->
      The <a href="/pkg/bytes/#Buffer"><code>Buffer</code></a> type
      has two new methods:
      <a href="/pkg/bytes/#Buffer.Available"><code>Available</code></a>
      and <a href="/pkg/bytes/#AvailableBuffer"><code>AvailableBuffer</code></a>.
      These may be used along with the
      <a href="/pkg/bytes/#Buffer.Write"><code>Write</code></a>
      method to append directly to the <code>Buffer</code>.
    </p>
  </dd>
</dl><!-- bytes -->

<dl id="context"><dt><a href="/pkg/context/">context</a></dt>
  <dd>
    <p><!-- https://go.dev/issue/40221, CL 479918 -->
      The new <a href="/pkg/context/#WithoutCancel"><code>WithoutCancel</code></a>
      function returns a copy of a context that is not canceled when the original
      context is canceled.
    </p>
    <p><!-- https://go.dev/issue/56661, CL 449318 -->
      The new <a href="/pkg/context/#WithDeadlineCause"><code>WithDeadlineCause</code></a>
      and <a href="/pkg/context/#WithTimeoutCause"><code>WithTimeoutCause</code></a>
      functions provide a way to set a context cancellation cause when a deadline or
      timer expires. The cause may be retrieved with the
      <a href="/pkg/context/#Cause"><code>Cause</code></a> function.
    </p>
    <p><!-- https://go.dev/issue/57928, CL 482695 -->
      The new <a href="/pkg/context/#AfterFunc"><code>AfterFunc</code></a>
      function registers a function to run after a context has been cancelled.
    </p>
  </dd>
</dl>

<dl id="crypto/elliptic"><dt><a href="/pkg/crypto/elliptic/">crypto/elliptic</a></dt>
  <dd>
    <p><!-- CL 459977 -->
      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
    </p>
  </dd>
</dl><!-- crypto/elliptic -->

<dl id="crypto/rsa"><dt><a href="/pkg/crypto/rsa/">crypto/rsa</a></dt>
  <dd>
    <p><!-- https://go.dev/issue/56921 -->
      TODO: <a href="https://go.dev/issue/56921">https://go.dev/issue/56921</a>: deprecate GenerateMultiPrimeKey and PrecomputedValues.CRTValues
    </p>

    <p><!-- CL 459976 -->
      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
    </p>
  </dd>
</dl><!-- crypto/rsa -->

<dl id="crypto/sha1"><dt><a href="/pkg/crypto/sha1/">crypto/sha1</a></dt>
  <dd>
    <p><!-- CL 483815 -->
      TODO: <a href="https://go.dev/cl/483815">https://go.dev/cl/483815</a>: crypto/sha1: add WriteString and WriteByte method
    </p>
  </dd>
</dl><!-- crypto/sha1 -->

<dl id="crypto/sha256"><dt><a href="/pkg/crypto/sha256/">crypto/sha256</a></dt>
  <dd>
    <p><!-- https://go.dev/issue/50543 -->
      TODO: <a href="https://go.dev/issue/50543">https://go.dev/issue/50543</a>: add native SHA256 instruction implementation for AMD64
    </p>

    <p><!-- CL 408795 -->
      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.
    </p>

    <p><!-- CL 481478 -->
      TODO: <a href="https://go.dev/cl/481478">https://go.dev/cl/481478</a>: crypto/sha256: add WriteString and WriteByte method
    </p>
  </dd>
</dl><!-- crypto/sha256 -->

<dl id="crypto/sha512"><dt><a href="/pkg/crypto/sha512/">crypto/sha512</a></dt>
  <dd>
    <p><!-- CL 483816 -->
      TODO: <a href="https://go.dev/cl/483816">https://go.dev/cl/483816</a>: crypto/sha512: add WriteString and WriteByte method
    </p>
  </dd>
</dl><!-- crypto/sha512 -->

<dl id="crypto/x509"><dt><a href="/pkg/crypto/x509/">crypto/x509</a></dt>
  <dd>
    <p><!-- https://go.dev/issue/53573 -->
      TODO: <a href="https://go.dev/issue/53573">https://go.dev/issue/53573</a>: surface ReasonCode inside x509.RevocationList entries
    </p>

    <p><!-- CL 468875 -->
      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
    </p>
  </dd>
</dl><!-- crypto/x509 -->

<dl id="debug/elf"><dt><a href="/pkg/debug/elf/">debug/elf</a></dt>
  <dd>
    <p><!-- https://go.dev/issue/56892, CL 452617 -->
      The new
      <a href="https:/pkg/debug/elf/#File.DynValue"><code>File.DynValue</code></a>
      method may be used to retrieve the numeric values listed with a
      given dynamic tag.
    </p>

    <p><!-- https://go.dev/issue/56887, CL 452496 -->
      The constant flags permitted in a <code>DT_FLAGS_1</code>
      dynamic tag are now defined with type
      <a href="/pkg/debug/elf/#DynFlag1"><code>DynFlag1</code></a>. These
      tags have names starting with <code>DF_1</code>.
    </p>

    <p><!-- CL 473256 -->
      The package now defines the constant
      <a href="/pkg/debug/elf/#COMPRESS_ZSTD"><code>COMPRESS_ZSTD</code></a>.
    </p>
  </dd>
</dl><!-- debug/elf -->

<dl id="debug/pe"><dt><a href="/pkg/debug/pe/">debug/pe</a></dt>
  <dd>
    <p><!-- CL 488475 -->
      Attempts to read from a section containing uninitialized data
      using
      <a href="/pkg/debug/pe/#Section.Data"><code>Section.Data</code></a>
      or the reader returned by <a href="/pkg/debug/pe/#Section.Open"><code>Section.Open</code></a>
      now return an error.
    </p>
  </dd>
</dl><!-- debug/pe -->

<dl id="embed"><dt><a href="/pkg/embed/">embed</a></dt>
  <dd>
    <p><!-- https://go.dev/issue/57803, CL 483235 -->
      The <a href="/pkg/io/fs/#File"><code>io/fs.File</code></a>
      returned by
      <a href="/pkg/embed/#FS.Open"><code>FS.Open</code></a> now
      has a <code>ReadAt</code> method that
      implements <a href="/pkg/io/#ReaderAt"><code>io.ReaderAt</code></a>.
    </p>

    <p><!-- https://go.dev/issue/54451, CL 491175 -->
      Calling <code><a href="/pkg/embed/FS.Open">FS.Open</a>.<a href="/pkg/io/fs/#File.Stat">Stat</a></code>
      will return a type that now implements a <code>String</code>
      method that calls
      <a href="/pkg/io/fs/#FormatFileInfo"><code>io/fs.FormatFileInfo</code></a>.
    </p>
  </dd>
</dl><!-- embed -->

<dl id="errors"><dt><a href="/pkg/errors/">errors</a></dt>
  <dd>
    <p><!-- https://go.dev/issue/41198, CL 473935 -->
      The new
      <a href="/pkg/errors/#ErrUnsupported"><code>ErrUnsupported</code></a>
      error provides a standardized way to indicate that a requested
      operation may not be performed because it is unsupported.
      For example, a call to
      <a href="/pkg/os/#Link"><code>os.Link</code></a> when using a
      file system that does not support hard links.
    </p>
  </dd>
</dl><!-- errors -->

<dl id="flag"><dt><a href="/pkg/flag/">flag</a></dt>
  <dd>
    <p><!-- https://go.dev/issue/53747, CL 476015 -->
      The new <a href="/pkg/flag/#BoolFunc"><code>BoolFunc</code></a>
      function and
      <a href="/pkg/flag/#FlagSet.BoolFunc"><code>FlagSet.BoolFunc</code></a>
      method define a flag that does not require an argument and calls
      a function when the flag is used. This is similar to
      <a href="/pkg/flag/#Func"><code>Func</code></a> but for a
      boolean flag.
    </p>

    <p><!-- CL 480215 -->
      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.
      <p>
      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.
    </p>
  </dd>
</dl><!-- flag -->

<dl id="go/ast"><dt><a href="/pkg/go/ast/">go/ast</a></dt>
  <dd>
    <p><!-- https://go.dev/issue/28089, CL 487935 -->
      The new <a href="/pkg/go/ast/#IsGenerated"><code>IsGenerated</code></a> predicate
      reports whether a file syntax tree contains the
      <a href="https://go.dev/s/generatedcode">special comment</a>
      that conventionally indicates that the file was generated by a tool.
    </p>
  </dd>

  <dd>
    <p><!-- https://go.dev/issue/59033, CL 476276 -->
      The new
      <a href="/pkg/go/ast/#File.GoVersion"><code>File.GoVersion</code></a>
      field records the minimum Go version required by
      any <code>//go:build</code> or <code>// +build</code>
      directives.
    </p>
  </dd>
</dl><!-- go/ast -->

<dl id="go/build"><dt><a href="/pkg/go/build/">go/build</a></dt>
  <dd>
    <p><!-- https://go.dev/issue/56986, CL 453603 -->
      The package now parses build directives (comments that start
      with <code>//go:</code>) in file headers (before
      the <code>package</code> declaration). These directives are
      available in the new
      <a href="/pkg/go/build/Package"><code>Package</code></a> fields
      <a href="/pkg/go/build/Package.Directives"><code>Directives</code></a>,
      <a href="/pkg/go/build/Package.TestDirectives"><code>TestDirectives</code></a>,
      and
      <a href="/pkg/go/build/Package.XTestDirectives"><code>XTestDirectives</code></a>.
    </p>
  </dd>
</dl><!-- go/build -->

<dl id="go/build/constraint"><dt><a href="/pkg/go/build/constraint/">go/build/constraint</a></dt>
  <dd>
    <p><!-- https://go.dev/issue/59033, CL 476275 -->
      The new
      <a href="/pkg/go/build/constraint/#GoVersion"><code>GoVersion</code></a>
      function returns the minimum Go version implied by a build
      expression.
    </p>
  </dd>
</dl><!-- go/build/constraint -->

<dl id="go/token"><dt><a href="/pkg/go/token/">go/token</a></dt>
  <dd>
    <p><!-- https://go.dev/issue/57708, CL 464515 -->
      The new <a href="/pkg/go/token/#File.Lines"><code>File.Lines</code></a> method
      returns the file's line-number table in the same form as accepted by
      <code>File.SetLines</code>.
    </p>
  </dd>
</dl><!-- go/token -->

<dl id="hash/maphash"><dt><a href="/pkg/hash/maphash/">hash/maphash</a></dt>
  <dd>
    <p><!-- https://go.dev/issue/47342, CL 468795 -->
      The <code>hash/maphash</code> package now has a pure Go implementation, selectable with the <code>purego</code> build tag.
    </p>
  </dd>
</dl><!-- hash/maphash -->

<dl id="io/fs"><dt><a href="/pkg/io/fs/">io/fs</a></dt>
  <dd>
    <p><!-- https://go.dev/issue/54451, CL 489555 -->
      The new
      <a href="/pkg/io/fs/#FormatFileInfo"><code>FormatFileInfo</code></a>
      function returns a formatted version of a
      <a href="/pkg/io/fs/#FileInfo"><code>FileInfo</code></a>.
      The new
      <a href="/pkg/io/fs/#FormatDirEntry"><code>FormatDirEntry</code></a>
      function returns a formatted version of a
      <a href="/pkg/io/fs/#FileInfo"><code>DirEntry</code></a>.
      The implementation of
      <a href="/pkg/io/fs/#DirEntry"><code>DirEntry</code></a>
      returned by
      <a href="/pkg/io/fs/#ReadDir"><code>ReadDir</code></a> now
      implements a <code>String</code> method that calls
      <a href="/pkg/io/fs/#FormatDirEntry"><code>FormatDirEntry</code></a>,
      and the same is true for
      the <a href="/pkg/io/fs/#DirEntry"><code>DirEntry</code></a>
      value passed to
      <a href="/pkg/io/fs/#WalkDirFunc"><code>WalkDirFunc</code></a>.
    </p>
  </dd>
</dl><!-- io/fs -->

<dl id="log/slog"><dt><a href="/pkg/log/slog/">log/slog</a></dt>
  <dd>
    <p><!-- https://go.dev/issue/59060 -->
      TODO: <a href="https://go.dev/issue/59060">https://go.dev/issue/59060</a>: arg to Record.Attrs should return bool
    </p>

    <p><!-- https://go.dev/issue/59141 -->
      TODO: <a href="https://go.dev/issue/59141">https://go.dev/issue/59141</a>: handle panics in LogValuer.LogValue
    </p>

    <p><!-- https://go.dev/issue/59204 -->
      TODO: <a href="https://go.dev/issue/59204">https://go.dev/issue/59204</a>: change slog.Group signature to ...any
    </p>

    <p><!-- https://go.dev/issue/59280 -->
      TODO: <a href="https://go.dev/issue/59280">https://go.dev/issue/59280</a>: ReplaceAttr gets Source struct instead of file:line
    </p>

    <p><!-- https://go.dev/issue/59282 -->
      TODO: <a href="https://go.dev/issue/59282">https://go.dev/issue/59282</a>: drop only completely empty Attrs
    </p>

    <p><!-- https://go.dev/issue/59339 -->
      TODO: <a href="https://go.dev/issue/59339">https://go.dev/issue/59339</a>: change constructors to NewXXXHandler(io.Writer, *HandlerOptions)
    </p>

    <p><!-- https://go.dev/issue/59345 -->
      TODO: <a href="https://go.dev/issue/59345">https://go.dev/issue/59345</a>: JSONHandler should format everything like json.Marshal
    </p>

    <p><!-- CL 477295 -->
      TODO: <a href="https://go.dev/cl/477295">https://go.dev/cl/477295</a>: log/slog: initial commit; modified api/next/56345.txt
    </p>

    <p><!-- CL 484096 -->
      TODO: <a href="https://go.dev/cl/484096">https://go.dev/cl/484096</a>: log/slog: function argument to Record.Attrs returns bool; modified api/next/59060.txt
    </p>

    <p><!-- CL 486376 -->
      TODO: <a href="https://go.dev/cl/486376">https://go.dev/cl/486376</a>: log/slog: add Source type for source location; modified api/next/59280.txt
    </p>

    <p><!-- CL 486415 -->
      TODO: <a href="https://go.dev/cl/486415">https://go.dev/cl/486415</a>: log/slog: built-in handler constructors take options as a second arg; modified api/next/56345.txt, api/next/59339.txt
    </p>

    <p><!-- CL 487855 -->
      TODO: <a href="https://go.dev/cl/487855">https://go.dev/cl/487855</a>: log/slog: Group takes ...any; modified api/next/56345.txt, api/next/59204.txt
    </p>
  </dd>
</dl><!-- log/slog -->

<dl id="math"><dt><a href="/pkg/math/">math</a></dt>
  <dd>
    <p><!-- https://go.dev/issue/56491 -->
      TODO: <a href="https://go.dev/issue/56491">https://go.dev/issue/56491</a>: add Compare and Compare32
    </p>

    <p><!-- CL 459435 -->
      TODO: <a href="https://go.dev/cl/459435">https://go.dev/cl/459435</a>: math: add Compare and Compare32; modified api/next/56491.txt
    </p>

    <p><!-- CL 467515 -->
      TODO: <a href="https://go.dev/cl/467515">https://go.dev/cl/467515</a>: math: add Compare and Compare32; modified api/next/56491.txt
    </p>
  </dd>
</dl><!-- math -->

<dl id="math/big"><dt><a href="/pkg/math/big/">math/big</a></dt>
  <dd>
    <p><!-- https://go.dev/issue/56984, CL 453115 -->
      The new <a href="/pkg/math/big/#Int.ToFloat64"><code>Int.ToFloat64</code></a>
      method returns the nearest floating-point value to a
      multi-precision integer, along with an indication of any
      rounding that occurred.
    </p>
  </dd>
</dl><!-- math/big -->

<dl id="net"><dt><a href="/pkg/net/">net</a></dt>
  <dd>
    <p>
      <!-- https://go.dev/issue/59166, https://go.dev/issue/56539 -->
      <!-- CL 471136, CL 471137, CL 471140 -->
      On Linux, the <a href="/pkg/net/">net</a> package can now use
      Multipath TCP when the kernel supports it. It is not used by
      default. To use Multipath TCP when available on a client, call
      the
      <a href="/pkg/net/#Dialer.SetMultipathTCP"><code>Dialer.SetMultipathTCP</code></a>
      method before calling the
      <a href="/pkg/net/#Dialer.Dial"><code>Dialer.Dial</code></a> or
      <a href="/pkg/net/#Dialer.DialContext"><code>Dialer.DialContext</code></a>
      methods. To use Multipath TCP when available on a server, call
      the
      <a href="/pkg/net/#ListenConfig.SetMultipathTCP"><code>ListenConfig.SetMultipathTCP</code></a>
      method before calling the
      <a href="/pkg/net/#ListenConfig.Listen"><code>ListenConfig.Listen</code></a>
      method. Specify the network as <code>"tcp"</code> or
      <code>"tcp4"</code> or <code>"tcp6"</code> as usual. If
      Multipath TCP is not supported by the kernel or the remote host,
      the connection will silently fall back to TCP. To test whether a
      particular connection is using Multipath TCP, use the
      <a href="/pkg/net/#TCPConn.MultipathTCP"><code>TCPConn.MultipathTCP</code></a>
      method.
    </p>
    <p>
      In a future Go release we may enable Multipath TCP by default on
      systems that support it.
    </p>
  </dd>
</dl><!-- net -->

<dl id="net/http"><dt><a href="/pkg/net/http/">net/http</a></dt>
  <dd>
    <p><!-- https://go.dev/issue/44855, CL 382117 -->
      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.
    </p>

    <p><!-- CL 472636 -->
      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
    </p>

    <p><!-- CL 494122 -->
      The <a href="/pkg/net/http/">net/http</a> package now supports
      <a href="/pkg/errors/#ErrUnsupported"><code>errors.ErrUnsupported</code></a>,
      in that the expression
      <code>errors.Is(http.ErrNotSupported, errors.ErrUnsupported)</code>
      will return true.
    </p>
  </dd>
</dl><!-- net/http -->

<dl id="os"><dt><a href="/pkg/os/">os</a></dt>
  <dd>
    <p><!-- https://go.dev/issue/32558, CL 219638 -->
      Programs may now pass an empty <code>time.Time</code> value to
      the <a href="/pkg/os/#Chtimes"><code>Chtimes</code></a> function
      to leave either the access time or the modification time unchanged.
    </p>

    <p><!-- CL 480135 -->
      On Windows the
      <a href="/pkg/os#File.Chdir"><code>File.Chdir</code></a> method
      now changes the current directory to the file, rather than
      always returning an error.
    </p>

    <p><!-- CL 477215 -->
      On Windows calling
      <a href="/pkg/os/#Truncate"><code>Truncate</code></a> on a
      non-existent file used to create an empty file. It now returns
      an error indicating that the file does not exist.
    </p>

    <p><!-- CL 493036 -->
      On Windows the os package now supports working with files whose
      names, stored as UTF-16, can't be represented as valid UTF-8.
    </p>

    <p><!-- https://go.dev/issue/54451, CL 491175 -->
      The implementation of the
      <a href="/pkg/io/fs/#DirEntry"><code>io/fs.DirEntry</code></a>
      interface returned by the
      <a href="/pkg/os/#ReadDir"><code>ReadDir</code></a> function and
      the <a href="/pkg/os/#File.ReadDir"><code>File.ReadDir</code>
      method now implements a <code>String</code> method that calls
      <a href="/pkg/io/fs/#FormatDirEntry"><code>io/fs.FormatDirEntry</code></a>.
    </p>
  </dd>
</dl><!-- os -->

<dl id="path/filepath"><dt><a href="/pkg/path/filepath/">path/filepath</a></dt>
  <dd>
    <p>
      The implementation of the
      <a href="/pkg/io/fs/#DirEntry"><code>io/fs.DirEntry</code></a>
      interface passed to the function argument of
      <a href="/pkg/path/filepath/#WalkDir"><code>WalkDir</code></a>
      now implements a <code>String</code> method that calls
      <a href="/pkg/io/fs/#FormatDirEntry"><code>io/fs.FormatDirEntry</code></a>.
    </p>
  </dd>
</dl><!-- path/filepath -->

<!-- CL 459455 reverted -->

<dl id="reflect"><dt><a href="/pkg/reflect/">reflect</a></dt>
  <dd>
    <p><!-- CL 408826, CL 413474 -->
      In Go 1.21, <a href="/pkg/reflect/#ValueOf"><code>ValueOf</code></a>
      no longer forces its argument to be allocated on the heap, allowing
      a <code>Value</code>'s content to be allocated on the stack. Most
      operations on a <code>Value</code> also allow the underlying value
      to be stack allocated.
    </p>

    <p><!-- https://go.dev/issue/55002 -->
      The new <a href="/pkg/reflect/#Value"><code>Value</code></a>
      method <a href="/pkg/reflect/#Value.Clear"><code>Value.Clear</code></a>
      clears the contents of a map or zeros the contents of a slice.
      This corresponds to the new <code>clear</code> built-in
      <a href="#language">added to the language</a>.
    </p>

    <p><!-- https://go.dev/issue/56906, CL 452762 -->
      The <a href="/pkg/reflect/#SliceHeader"><code>SliceHeader</code></a>
      and <a href="/pkg/reflect/#StringHeader"><code>StringHeader</code></a>
      types are now deprecated. In new code
      prefer <a href="/pkg/unsafe/#Slice"><code>unsafe.Slice</code></a>,
      <a href="/pkg/unsafe/#SliceData"><code>unsafe.SliceData</code></a>,
      <a href="/pkg/unsafe/#String"><code>unsafe.String</code></a>,
      or <a href="/pkg/unsafe/#StringData"><code>unsafe.StringData</code></a>.
    </p>
  </dd>
</dl><!-- reflect -->

<dl id="regexp"><dt><a href="/pkg/regexp/">regexp</a></dt>
  <dd>
    <p><!-- https://go.dev/issue/46159, CL 479401 -->
      <a href="/pkg/regexp#Regexp"><code>Regexp</code></a> now defines
      <a href="/pkg/regexp#Regexp.MarshalText"><code>MarshalText</code></a>
      and <a href="/pkg/regexp#Regexp.UnmarshalText"><code>UnmarshalText</code></a>
      methods. These implement
      <a href="/pkg/encoding#TextMarshaler"><code>encoding.TextMarshaler</code></a>
      and
      <a href="/pkg/encoding#TextUnmarshaler"><code>encoding.TextUnmarshaler</code></a>
      and will be used by packages such as
      <a href="/pkg/encoding/json">encoding/json</a>.
    </p>
  </dd>
</dl><!-- regexp -->

<dl id="runtime"><dt><a href="/pkg/runtime/">runtime</a></dt>
  <dd>
    <p><!-- https://go.dev/issue/38651 -->
      TODO: <a href="https://go.dev/issue/38651">https://go.dev/issue/38651</a>: add &#39;created by goroutine number&#39; to stack traces
    </p>

    <p><!-- https://go.dev/issue/57441, CL 474915 -->
      Crashing Go applications can now opt-in to Windows Error Reporting (WER) by setting the environment variable
      <code>GOTRACEBACK=wer</code> or calling <a href="/pkg/runtime/debug/#SetTraceback"><code>debug.SetTraceback("wer")</code></a>
      before the crash. Other than enabling WER, the runtime will behave as with <code>GOTRACEBACK=crash</code>.
      On non-Windows systems, <code>GOTRACEBACK=wer</code> is ignored.
    </p>

    <p><!-- CL 447778 -->
      <code>GODEBUG=cgocheck=2</code>, a thorough checker of cgo pointer passing rules,
      is no longer available as a <a href="/pkg/runtime#hdr-Environment_Variables">debug option</a>.
      Instead, it is available as an experiment using <code>GOEXPERIMENT=cgocheck2</code>.
      In particular this means that this mode has to be selected at build time instead of startup time.
    <p>
      <code>GODEBUG=cgocheck=1</code> is still available (and is still the default).
    </p>

    <p><!-- https://go.dev/issue/46787 -->
      A new type <code>Pinner</code> has been added to the runtime
      package. <code>Pinner</code>s may be used to "pin" Go memory
      such that it may be used more freely by non-Go code. For instance,
      passing Go values that reference pinned Go memory to C code is
      now allowed. Previously, passing any such nested reference was
      disallowed by the
      <a href="https://pkg.go.dev/cmd/cgo#hdr-Passing_pointers">cgo pointer passing rules.</a>

      See <a href="/pkg/runtime#Pinner">the docs</a> for more details.
    </p>
  </dd>
</dl><!-- runtime -->

<dl id="runtime/trace"><dt><a href="/pkg/runtime/trace/">runtime/trace</a></dt>
  <dd>
    <p><!-- https://go.dev/issue/16638 -->
      Collecting traces on amd64 and arm64 now incurs a substantially
      smaller CPU cost: up to a 10x improvement over the previous release.
    </p>

    <p><!-- CL 494495 -->
      Traces now contain explicit stop-the-world events for every reason
      the Go runtime might stop-the-world, not just garbage collection.
    </p>
  </dd>
</dl><!-- runtime/trace -->

<dl id="runtime/metrics"><dt><a href="/pkg/runtime/metrics/">runtime/metrics</a></dt>
  <dd>
    <p><!-- https://go.dev/issue/56857 -->
      A few previously-internal GC metrics, such as live heap size, are
      now available.

      <code>GOGC</code> and <code>GOMEMLIMIT</code> are also now
      available as metrics.
    </p>
  </dd>
</dl><!-- runtime/metrics -->

<dl id="sync"><dt><a href="/pkg/sync/">sync</a></dt>
  <dd>
    <p><!-- https://go.dev/issue/56102, CL 451356 -->
      The new <a href="/pkg/sync/#OnceFunc"><code>OnceFunc</code></a>,
      <a href="/pkg/sync/#OnceValue"><code>OnceValue</code></a>, and
      <a href="/pkg/sync/#OnceValues"><code>OnceValues</code></a>
      functions capture a common use of <a href="/pkg/sync/#Once">Once</a> to
      lazily initialize a value on first use.
    </p>
  </dd>
</dl>

<dl id="syscall"><dt><a href="/pkg/syscall/">syscall</a></dt>
  <dd>
    <p><!-- CL 480135 -->
      On Windows the
      <a href="/pkg/syscall#Fchdir"><code>Fchdir</code></a> function
      now changes the current directory to its argument, rather than
      always returning an error.
    </p>

    <p><!-- https://go.dev/issue/46259, CL 458335 -->
      On FreeBSD
      <a href="/pkg/syscall#SysProcAttr"><code>SysProcAttr</code></a>
      has a new field <code>Jail</code> that may be used to put the
      newly created process in a jailed environment.
    </p>

    <p><!-- CL 493036 -->
      On Windows the syscall package now supports working with files whose
      names, stored as UTF-16, can't be represented as valid UTF-8.
      The <a href="/pkg/syscall#UTF16ToString"><code>UTF16ToString</code></a>
      and <a href="/pkg/syscall#UTF16FromString"><code>UTF16FromString</code></a>
      functions now convert between UTF-16 data and
      <a href=http://simonsapin.github.io/wtf-8/"">WTF-8</a> strings.
      This is backward compatible as WTF-8 is a superset of the UTF-8
      format that was used in earlier releases.
    </p>

    <p><!-- CL 476578, CL 476875, CL 476916 -->
      Several error values match the new
      <a href="/pkg/errors/#ErrUnsupported"><code>errors.ErrUnsupported</code></a>,
      such that <code>errors.Is(err, errors.ErrUnsupported)</code>
      returns true.
      <ul>
	<li><code>ENOSYS</code></li>
	<li><code>ENOTSUP</code></li>
	<li><code>EOPNOTSUPP</code></li>
	<li><code>EPLAN9</code> (Plan 9 only)</li>
	<li><code>ERROR_CALL_NOT_IMPLEMENTED</code> (Windows only)</li>
	<li><code>ERROR_NOT_SUPPORTED</code> (Windows only)</li>
	<li><code>EWINDOWS</code> (Windows only)</li>
      </ul>
    </p>
  </dd>
</dl><!-- syscall -->

<dl id="testing"><dt><a href="/pkg/testing/">testing</a></dt>
  <dd>
    <p><!-- https://go.dev/issue/37708, CL 463837 -->
      The new <code>-test.fullpath</code> option will print full path
      names in test log messages, rather than just base names.
    </p>

    <p><!-- https://go.dev/issue/52600, CL 475496 -->
      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>.
    </p>
  </dd>
</dl><!-- testing -->

<dl id="testing/fstest"><dt><a href="/pkg/testing/fstest/">testing/fstest</a></dt>
  <dd>
    <p><!-- https://go.dev/issue/54451, CL 491175 -->
      Calling <code><a href="/pkg/testing/fstest/MapFS.Open">Open</a>.<a href="/pkg/io/fs/#File.Stat">Stat</a></code>
      will return a type that now implements a <code>String</code>
      method that calls
      <a href="/pkg/io/fs/#FormatFileInfo"><code>io/fs.FormatFileInfo</code></a>.
    </p>
  </dd>
</dl><!-- testing/fstest -->

<dl id="testing/slogtest"><dt><a href="/pkg/testing/slogtest/">testing/slogtest</a></dt>
  <dd>
    <p><!-- CL 487895 -->
      TODO: <a href="https://go.dev/cl/487895">https://go.dev/cl/487895</a>: testing/slogtest: tests for slog handlers; modified api/next/56345.txt
    </p>
  </dd>
</dl><!-- testing/slogtest -->

<dl id="unicode"><dt><a href="/pkg/unicode/">unicode</a></dt>
  <dd>
    <p><!-- CL 456837 -->
      The <a href="/pkg/unicode/"><code>unicode</code></a> package and
      associated support throughout the system has been upgraded to
      <a href="https://www.unicode.org/versions/Unicode15.0.0/">Unicode 15.0.0</a>.
    </p>
</dl><!-- unicode -->

<!-- proposals for x repos that don't need to be mentioned here but
     are picked up by the relnote tool. -->
<!-- https://go.dev/issue/54232 -->
<!-- https://go.dev/issue/57906 -->
<!-- https://go.dev/issue/58668 -->