doc/go1.18.html

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<h2 id="introduction">DRAFT RELEASE NOTES — Introduction to Go 1.18</h2>

<p>
  <strong>
    Go 1.18 is not yet released. These are work-in-progress
    release notes. Go 1.18 is expected to be released in February 2022.
  </strong>
</p>

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

<h3 id="generics">Generics</h3>

<p><!-- https://golang.org/issue/43651, https://golang.org/issue/45346 -->
  Go 1.18 includes an implementation of generic features as described by the
  <a href="https://go.googlesource.com/proposal/+/refs/heads/master/design/43651-type-parameters.md">Type
    Parameters Proposal</a>.
  This includes major - but fully backward-compatible - changes to the language.
  The following is a list of the most visible changes. For a more comprehensive overview, see the
  <a href="https://go.googlesource.com/proposal/+/refs/heads/master/design/43651-type-parameters.md">proposal</a>.
  For details see the <a href="https://golang.org/ref/spec">language spec</a>.
</p>
<ul>
  <li>
    The syntax for
    <a href="https://golang.org/ref/spec#Function_declarations">Function</a> and
    <a href="https://golang.org/ref/spec#Type_declarations">type declarations</a>
    now accepts
    <a href="https://golang.org/ref/spec#Type_parameters">type parameters</a>.
  </li>
  <li>
    Parameterized functions and types can be instantiated by following them with a list of
    type arguments in square brackets.
  </li>
  <li>
    The new token <code>~</code> has been added to the set of
    <a href="https://golang.org/ref/spec#Operators_and_punctuation">operators and punctuation</a>.
  </li>
  <li>
    The syntax for
    <a href="https://golang.org/ref/spec#Interface_types">Interface types</a>
    now permits the embedding of arbitrary types (not just type names of interfaces)
    as well as union and <code>~T</code> type elements. Such interfaces may only be used
    as type constraints.
    An interface now defines a set of types as well as a set of methods.
  </li>
  <li>
    The new
    <a href="https://golang.org/ref/spec#Predeclared_identifiers">predeclared identifier</a>
    <code>any</code> is an alias for the empty interface. It may be used instead of
    <code>interface{}</code>.
  </li>
  <li>
    The new
    <a href="https://golang.org/ref/spec#Predeclared_identifiers">predeclared identifier</a>
    <code>comparable</code> is an interface the denotes the set of all types which can be
    compared using <code>==</code> or <code>!=</code>. It may only be used as (or embedded in)
    a type constraint.
  </li>
</ul>

<p>
  The current generics implementation has the following limitations:
  <ul>
    <li><!-- https://golang.org/issue/47631 -->
      The Go compiler cannot currently handle type declarations inside generic functions
      or methods. We hope to provide support for this feature in Go 1.19.
    </li>
  </ul>
</p>

<h3 id="bug_fixes">Bug fixes</h3>

<p>
  The Go 1.18 compiler now correctly reports <code>declared but not used</code> errors
  for variables that are set inside a function literal but are never used. Before Go 1.18,
  the compiler did not report an error in such cases. This fixes long-outstanding compiler
  issue <a href="https://golang.org/issue/8560">#8560</a>. As a result of this change,
  (possibly incorrect) programs may not compile anymore. The necessary fix is
  straightforward: fix the program if it was in fact incorrect, or use the offending
  variable, for instance by assigning it to the blank identifier <code>_</code>.
  Since <code>go vet</code> always pointed out this error, the number of affected
  programs is likely very small.
</p>

<p>
  The Go 1.18 compiler now reports an overflow when passing a rune constant expression
  such as <code>'1' << 32</code> as an argument to the predeclared functions
  <code>print</code> and <code>println</code>, consistent with the behavior of
  user-defined functions. Before Go 1.18, the compiler did not report an error
  in such cases but silently accepted such constant arguments if they fit into an
  <code>int64</code>. As a result of this change, (possibly incorrect) programs
  may not compile anymore. The necessary fix is straightforward: fix the program if it
  was in fact incorrect, or explicitly convert the offending argument to the correct type.
  Since <code>go vet</code> always pointed out this error, the number of affected
  programs is likely very small.
</p>

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

<h3 id="amd64">AMD64</h3>

<p><!-- CL 349595 -->
  Go 1.18 introduces the new <code>GOAMD64</code> environment variable which selects
  a version of the AMD64 architecture. Allowed values are <code>v1</code>,
  <code>v2</code>, <code>v3</code>, or <code>v4</code>. Each higher level requires,
  and takes advantage of, additional processor features.  A detailed description of the
  versions is <a href="https://en.wikipedia.org/wiki/X86-64#Microarchitecture_levels">here</a>.
</p>
<p>
  The <code>GOAMD64</code> environment variable defaults to <code>v1</code>.
</p>

<h3 id="riscv">RISC-V</h3>

<p><!-- golang.org/issue/47100, CL 334872 -->
  The 64-bit RISC-V architecture on Linux (the <code>linux/riscv64</code> port)
  now supports the <code>c-archive</code> and <code>c-shared</code> build modes.
</p>

<h3 id="windows">Windows</h3>

<p><!-- https://golang.org/issue/49759 -->
  The <code>windows/arm</code> and <code>windows/arm64</code> ports now support
  non-cooperative preemption, bringing that capability to all four Windows
  ports, which should hopefully address subtle bugs encountered when calling
  into Win32 functions that block for extended periods of time.
</p>

<h3 id="ios">iOS</h3>

<p><!-- golang.org/issue/48076, golang.org/issue/49616 -->
  On iOS (the <code>ios/arm64</code> port)
  and iOS simulator running on AMD64-based macOS (the <code>ios/amd64</code> port),
  Go 1.18 now requires iOS 12 or later; support for previous versions has been discontinued.
</p>

<h3 id="freebsd">FreeBSD</h3>

<p>
  Go 1.18 is the last release that is supported on FreeBSD 11.x, which has
  already reached end-of-life. Go 1.19 will require FreeBSD 12.2+ or FreeBSD
  13.0+.
  FreeBSD 13.0+ will require a kernel with the COMPAT_FREEBSD12 option set (this is the default).
</p>

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

<h3 id="fuzzing">Fuzzing</h3>

<p>
  Go 1.18 includes an implementation of fuzzing as described by
  <a href="https://golang.org/issue/44551">the fuzzing proposal</a>.
</p>

<p>
  See the <a href="https://go.dev/doc/fuzz">fuzzing landing page</a> to get
  started.
</p>

<p>
  Please be aware that fuzzing can consume a lot of memory and may impact your
  machine’s performance while it runs. Also be aware that the fuzzing engine
  writes values that expand test coverage to a fuzz cache directory within
  <code>$GOCACHE/fuzz</code> while it runs. There is currently no limit to the
  number of files or total bytes that may be written to the fuzz cache, so it
  may occupy a large amount of storage (possibly several GBs).
</p>

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

<p><!-- golang.org/issue/43684 -->
  <code>go</code> <code>get</code> no longer builds or installs packages in
  module-aware mode. <code>go</code> <code>get</code> is now dedicated to
  adjusting dependencies in <code>go.mod</code>. Effectively, the
  <code>-d</code> flag is always enabled. To install the latest version
  of an executable outside the context of the current module, use
  <a href="https://golang.org/ref/mod#go-install"><code>go</code>
  <code>install</code> <code>example.com/cmd@latest</code></a>. Any
  <a href="https://golang.org/ref/mod#version-queries">version query</a>
  may be used instead of <code>latest</code>. This form of <code>go</code>
  <code>install</code> was added in Go 1.16, so projects supporting older
  versions may need to provide install instructions for both <code>go</code>
  <code>install</code> and <code>go</code> <code>get</code>. <code>go</code>
  <code>get</code> now reports an error when used outside a module, since there
  is no <code>go.mod</code> file to update. In GOPATH mode (with
  <code>GO111MODULE=off</code>), <code>go</code> <code>get</code> still builds
  and installs packages, as before.
</p>

<p><!-- golang.org/issue/37475 -->
  The <code>go</code> command now embeds version control information in
  binaries including the currently checked-out revision, commit time, and a
  flag indicating whether edited or untracked files are present. Version
  control information is embedded if the <code>go</code> command is invoked in
  a directory within a Git, Mercurial, Fossil, or Bazaar repository, and the
  <code>main</code> package and its containing main module are in the same
  repository. This information may be omitted using the flag
  <code>-buildvcs=false</code>.
</p>

<p><!-- golang.org/issue/37475 -->
  Additionally, the <code>go</code> command embeds information about the build
  including build and tool tags (set with <code>-tags</code>), compiler,
  assembler, and linker flags (like <code>-gcflags</code>), whether cgo was
  enabled, and if it was, the values of the cgo environment variables
  (like <code>CGO_CFLAGS</code>). This information may be omitted using the
  flag <code>-buildinfo=false</code>. Both VCS and build information may be
  read together with module information using <code>go</code>
  <code>version</code> <code>-m</code> <code>file</code> or
  <code>runtime/debug.ReadBuildInfo</code> (for the currently running binary)
  or the new <a href="#debug/buildinfo"><code>debug/buildinfo</code></a>
  package.
</p>

<p><!-- https://golang.org/issue/44435 -->
  If the main module's <code>go.mod</code> file
  specifies <a href="/ref/mod#go-mod-file-go"><code>go</code> <code>1.17</code></a>
  or higher, <code>go</code> <code>mod</code> <code>download</code> without
  arguments now downloads source code for only the modules
  explicitly <a href="/ref/mod#go-mod-file-require">required</a> in the main
  module's <code>go.mod</code> file. (In a <code>go</code> <code>1.17</code> or
  higher module, that set already includes all dependencies needed to build the
  packages and tests in the main module.)
  To also download source code for transitive dependencies, use
  <code>go</code> <code>mod</code> <code>download</code> <code>all</code>.
</p>

<p><!-- https://golang.org/issue/47327 -->
  The <code>go</code> <code>mod</code> <code>vendor</code> subcommand now
  supports a <code>-o</code> flag to set the output directory.
  (Other <code>go</code> commands still read from the <code>vendor</code>
  directory at the module root when loading packages
  with <code>-mod=vendor</code>, so the main use for this flag is for
  third-party tools that need to collect package source code.)
</p>

<p><!-- CL 298612 -->
  The <code>go</code> <code>build</code> command and related commands
  now support an <code>-asan</code> flag that enables interoperation
  with C (or C++) code compiled with the address sanitizer (C compiler
  option <code>-fsanitize=address</code>).
</p>

<h3 id="gofmt"><code>gofmt</code></h3>

<p><!-- https://golang.org/issue/43566 -->
  <code>gofmt</code> now reads and formats input files concurrently, with a
  memory limit proportional to <code>GOMAXPROCS</code>. On a machine with
  multiple CPUs, <code>gofmt</code> should now be significantly faster.
</p>

<h3 id="vet"><code>vet</code></h3>

<h4 id="vet-generics">Updates for Generics</h4>

<p><!-- https://golang.org/issue/48704 -->
	The <code>vet</code> tool is updated to support generic code. In most cases,
	it reports an error in generic code whenever it would report an error in the
	equivalent non-generic code after substituting for type parameters with a
	type from their
	<a href="https://golang.org/ref/spec#Interface_types">type set</a>.

	For example, <code>vet</code> reports a format error in
	<pre>func Print[T ~int|~string](t T) {
	fmt.Printf("%d", t)
}</pre>
	because it would report a format error in the non-generic equivalent of
	<code>Print[string]</code>:
	<pre>func PrintString(x string) {
	fmt.Printf("%d", x)
}</pre>
</p>

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

<p><!-- https://golang.org/issue/44167 -->
  The garbage collector now includes non-heap sources of garbage collector work
  (e.g., stack scanning) when determining how frequently to run. As a result,
  garbage collector overhead is more predictable when these sources are
  significant. For most applications these changes will be negligible; however,
  some Go applications may now use less memory and spend more time on garbage
  collection, or vice versa, than before. The intended workaround is to tweak
  <code>GOGC</code> where necessary.
</p>

<p><!-- CL 358675, CL 353975, CL 353974 -->
  The runtime now returns memory to the operating system more efficiently and has
  been tuned to work more aggressively as a result.
</p>

<p><!-- CL 352057, https://golang.org/issue/45728 -->
  Go 1.17 generally improved the formatting of arguments in stack traces,
  but could print inaccurate values for arguments passed in registers.
  This is improved in Go 1.18 by printing a question mark (<code>?</code>)
  after each value that may be inaccurate.
</p>

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

<p><!-- https://golang.org/issue/40724 -->
  Go 1.17 <a href="go1.17#compiler">implemented</a> a new way of passing
  function arguments and results using registers instead of the stack
  on 64-bit x86 architecture on selected operating systems.
  Go 1.18 expands the supported platforms to include 64-bit ARM (<code>GOARCH=arm64</code>),
  big- and little-endian 64-bit PowerPC (<code>GOARCH=ppc64</code>, <code>ppc64le</code>),
  as well as 64-bit x86 architecture (<code>GOARCH=amd64</code>)
  on all operating systems.
  On 64-bit ARM and 64-bit PowerPC systems, benchmarking shows
  typical performance improvements of 10% or more.
</p>

<p>
  As <a href="go1.17#compiler">mentioned</a> in the Go 1.17 release notes,
  this change does not affect the functionality of any safe Go code and
  is designed to have no impact on most assembly code. See the
  <a href="go1.17#compiler">Go 1.17 release notes</a> for more details.
</p>

<p><!-- CL 355497, CL 356869 -->
  The compiler now can inline functions that contain range loops or
  labeled for loops.
</p>

<p><!-- CL 298611 -->
  The new compiler <code>-asan</code> option supports the
  new <code>go</code> command <code>-asan</code> option.
</p>

<p> <!-- https://github.com/golang/go/issues/49569 -->
  Because of changes in the compiler related to supporting generics, the
  Go 1.18 compile speed can be roughly 15% slower than the Go 1.17 compile speed.
  The execution time of the compiled code is not affected.  We
  intend to improve the speed of the compiler in Go 1.19.
</p>

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

<p><!-- CL 298610 -->
  The new linker <code>-asan</code> option supports the
  new <code>go</code> command <code>-asan</code> option.
</p>

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

<h3 id="constraints">New <code>constraints</code> package</h3>

<p><!-- CL 349709 -->
  The new <a href="/pkg/constraints/"><code>constraints</code></a> package
  defines a set of useful constraints that can be used with type parameters of
  generic functions.
</p>

<h3 id="netip">New <code>net/netip</code> package</h3>

<p>
  The new <a href="/pkg/net/netip/"><code>net/netip</code></a>
  package defines a new IP address type, <a href="/pkg/net/netip/#Addr"><code>Addr</code></a>.
  Compared to the existing
  <a href="/pkg/net/#IP"><code>net.IP</code></a> type, the <code>netip.Addr</code> type takes less
  memory, is immutable, and is comparable so it supports <code>==</code>
  and can be used as a map key.
</p>
<p>
  In addition to <code>Addr</code>, the package defines
  <a href="/pkg/net/netip/#AddrPort"><code>AddrPort</code></a>, representing
  an IP and port, and
  <a href="/pkg/net/netip/#Prefix"><code>Prefix</code></a>, representing
  a network CIDR prefix.
</p>
<p>
  The <a href="/pkg/net/"><code>net</code></a> package includes new
  methods that parallel existing methods, but
  return <code>netip.AddrPort</code> instead of the
  heavier-weight <a href="/pkg/net/#IP"><code>net.IP</code></a> or
  <a href="/pkg/net/#UDPAddr"<code>*net.UDPAddr</code></a> types.
  The <code>net</code> package also now includes functions and methods
  to convert between the existing
  <a href="/pkg/net/#TCPAddr"><code>TCPAddr</code>/<a href="/pkg/net/#UDPAddr"><code>UDPAddr</code>
  types and <code>netip.AddrPort</code>.
</p>

<h3>TODO</h3>

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

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

<dl id="bufio"><dt><a href="/pkg/bufio/">bufio</a></dt>
  <dd>
    <p><!-- CL 345569 -->
      The new <a href="/pkg/bufio#Writer.AvailableBuffer"><code>Writer.AvailableBuffer</code></a>
      method returns an empty buffer with a possibly non-empty capacity for use
      with append-like APIs. After appending, the buffer can be provided to a
			succeeding <code>Write</code> call and possibly avoid any copying.
    </p>

    <p><!-- CL 345570 -->
      The methods <a href="/pkg/bufio#Reader.Reset"><code>Reader.Reset</code></a> and
      <a href="/pkg/bufio#Writer.Reset"><code>Writer.Reset</code></a>
      now use the default buffer size when called on objects with a
      <code>nil</code> buffer.
    </p>
  </dd>
</dl><!-- bufio -->

<dl id="bytes"><dt><a href="/pkg/bytes/">bytes</a></dt>
  <dd>
    <p><!-- CL 351710 -->
      The new <a href="/pkg/bytes/#Cut"><code>Cut</code></a> function
      slices a <code>[]byte</code> around a separator. It can replace
      and simplify many common uses of
      <a href="/pkg/bytes/#Index"><code>Index</code></a>,
      <a href="/pkg/bytes/#IndexByte"><code>IndexByte</code></a>,
      <a href="/pkg/bytes/#IndexRune"><code>IndexRune</code></a>,
      and <a href="/pkg/bytes/#SplitN"><code>SplitN</code></a>.
    </p>

    <p><!-- CL 323318, CL 332771 -->
      <a href="/pkg/bytes/#Trim"><code>Trim</code></a>, <a href="/pkg/bytes/#TrimLeft"><code>TrimLeft</code></a>,
      and <a href="/pkg/bytes/#TrimRight"><code>TrimRight</code></a> are now allocation free and, especially for 
      small ASCII cutsets, up to 10 times faster.
    </p>

    <p><!-- CL 359485 -->
      The <a href="/pkg/bytes/#Title"><code>Title</code></a> function is now deprecated. It doesn't
      handle Unicode punctuation and language-specific capitalization rules, and is superseded by the
      <a href="https://golang.org/x/text/cases">golang.org/x/text/cases</a> package.
    </p>
  </dd>
</dl><!-- bytes -->

<dl id="crypto/tls"><dt><a href="/pkg/crypto/tls/">crypto/tls</a></dt>
  <dd>
    <p><!-- CL 325250 -->
      The new <a href="/pkg/crypto/tls/#Conn.NetConn"><code>Conn.NetConn</code></a>
      method allows access to the underlying
      <a href="/pkg/net#Conn"><code>net.Conn</code></a>.
    </p>
  </dd>
</dl><!-- crypto/tls -->

<dl id="debug/buildinfo"><dt><a href="/pkg/debug/buildinfo">debug/buildinfo</a></dt>
  <dd>
    <p><!-- golang.org/issue/39301 -->
      This new package provides access to module versions, version control
      information, and build flags embedded in executable files built by
      the <code>go</code> command. The same information is also available via
      <a href="/pkg/runtime/debug#ReadBuildInfo"><code>runtime/debug.ReadBuildInfo</code></a>
      for the currently running binary and via <code>go</code>
      <code>version</code> <code>-m</code> on the command line.
    </p>
  </dd>
</dl>

<dl id="go/ast"><dt><a href="/pkg/go/ast/">go/ast</a></dt>
  <dd>
    <p><!-- https://golang.org/issue/47781, CL 325689, CL 327149, CL 348375, CL 348609 -->
      Per the proposal
      <a href="https://go.googlesource.com/proposal/+/master/design/47781-parameterized-go-ast.md">
        Additions to go/ast and go/token to support parameterized functions and types
      </a>
      the following additions are made to the <a href="/pkg/go/ast"><code>go/ast</code></a> package:
      <ul>
        <li>
          the <a href="/pkg/go/ast/#FuncType"><code>FuncType</code></a>
          and <a href="/pkg/go/ast/#TypeSpec"><code>TypeSpec</code></a>
          nodes have a new field <code>TypeParams</code> to hold type parameters, if any.
        </li>
        <li>
          The new expression node <a href="/pkg/go/ast/#IndexListExpr"><code>IndexListExpr</code></a>
          represents index expressions with multiple indices, used for function and type instantiations
          with more than one explicit type argument.
        </li>
      </ul>
    </p>
  </dd>
</dl>

<dl id="go/constant"><dt><a href="/pkg/go/constant/">go/constant</a></dt>
  <dd>
    <p><!-- https://golang.org/issue/46211, CL 320491 -->
      The new <a href="/pkg/go/constant/#Kind.String"><code>Kind.String</code></a>
      method returns a human-readable name for the receiver kind.
    </p>
  </dd>
</dl>

<dl id="go/token"><dt><a href="/pkg/go/token/">go/token</a></dt>
  <dd>
    <p><!-- https://golang.org/issue/47781, CL 324992 -->
      The new constant <a href="/pkg/go/token/#TILDE"><code>TILDE</code></a>
      represents the <code>~</code> token per the proposal
      <a href="https://go.googlesource.com/proposal/+/master/design/47781-parameterized-go-ast.md">
        Additions to go/ast and go/token to support parameterized functions and types
      </a>.
    </p>
  </dd>
</dl>

<dl id="go/types"><dt><a href="/pkg/go/types/">go/types</a></dt>
  <dd>
		<p><!-- https://golang.org/issue/46648 -->
			The new <a href="/pkg/go/types/#Config.GoVersion"><code>Config.GoVersion</code></a>
			field sets the accepted Go language version.
		</p>

    <p><!-- https://golang.org/issue/47916 -->
      Per the proposal
      <a href="https://go.googlesource.com/proposal/+/master/design/47916-parameterized-go-types.md">
        Additions to go/types to support type parameters
      </a>
      the following additions are made to the <a href="/pkg/go/types"><code>go/types</code></a> package:
    </p>
    <ul>
      <li>
        The new type
        <a href="/pkg/go/types/#TypeParam"><code>TypeParam</code></a>, factory function
        <a href="/pkg/go/types/#NewTypeParam"><code>NewTypeParam</code></a>,
        and associated methods are added to represent a type parameter.
      </li>
      <li>
        The new type
        <a href="/pkg/go/types/#TypeParamList"><code>TypeParamList</code></a> holds a list of
        type parameters.
      </li>
      <li>
        The new type
        <a href="/pkg/go/types/#TypeList"><code>TypeList</code></a> holds a list of types.
      </li>
      <li>
        The new factory function
        <a href="/pkg/go/types/#NewSignatureType"><code>NewSignatureType</code></a> allocates a
        <a href="/pkg/go/types/#Signature"><code>Signature</code></a> with
        (receiver or function) type parameters.
        To access those type parameters, the <code>Signature</code> type has two new methods
        <a href="/pkg/go/types/#Signature.RecvTypeParams"><code>Signature.RecvTypeParams</code></a> and
        <a href="/pkg/go/types/#Signature.TypeParams"><code>Signature.TypeParams</code></a>.
      </li>
      <li>
        <a href="/pkg/go/types/#Named"><code>Named</code></a> types have four new methods:
        <a href="/pkg/go/types/#Named.Origin"><code>Named.Origin</code></a> to get the original
        parameterized types of instantiated types,
        <a href="/pkg/go/types/#Named.TypeArgs"><code>Named.TypeArgs</code></a> and
        <a href="/pkg/go/types/#Named.TypeParams"><code>Named.TypeParams</code></a> to get the
        type arguments or type parameters of an instantiated or parameterized type, and
        <a href="/pkg/go/types/#Named.TypeParams"><code>Named.SetTypeParams</code></a> to set the
        type parameters (for instance, when importing a named type where allocation of the named
        type and setting of type parameters cannot be done simultaneously due to possible cycles).
      </li>
      <li>
        The <a href="/pkg/go/types/#Interface"><code>Interface</code></a> type has four new methods:
        <a href="/pkg/go/types/#Interface.IsComparable"><code>Interface.IsComparable</code></a> and
        <a href="/pkg/go/types/#Interface.IsMethodSet"><code>Interface.IsMethodSet</code></a> to
        query properties of the type set defined by the interface, and
        <a href="/pkg/go/types/#Interface.MarkImplicit"><code>Interface.MarkImplicit</code></a> and
        <a href="/pkg/go/types/#Interface.IsImplicit"><code>Interface.IsImplicit</code></a> to set
        and test whether the interface is an implicit interface around a type constraint literal.
      </li>
      <li>
        The new types
        <a href="/pkg/go/types/#Union"><code>Union</code></a> and
        <a href="/pkg/go/types/#Term"><code>Term</code></a>, factory functions
        <a href="/pkg/go/types/#NewUnion"><code>NewUnion</code></a> and
        <a href="/pkg/go/types/#NewTerm"><code>NewTerm</code></a>, and associated
        methods are added to represent type sets in interfaces.
      </li>
      <li>
        The new function
        <a href="/pkg/go/types/#Instantiate"><code>Instantiate</code></a>
        instantiates a parameterized type.
      </li>
      <li>
        The new <a href="/pkg/go/types/#Info.Instances"><code>Info.Instances</code></a>
        map records function and type instantiations through the new
        <a href="/pkg/go/types/#Instance"><code>Instance</code></a> type.
      </li>
      <li><!-- CL 342671 -->
        The new type <a href="/pkg/go/types/#ArgumentError"><code>ArgumentError</code></a>
        and associated methods are added to represent an error related to a type argument.
      </li>
      <li><!-- CL 353089 -->
        The new type <a href="/pkg/go/types/#Context"><code>Context</code></a> and factory function
        <a href="/pkg/go/types/#NewContext"><code>NewContext</code></a>
        are added to facilitate sharing of identical type instances across type-checked packages.
      </li>
    </ul>
  </dd>
</dl>

<dl id="image/draw"><dt><a href="/pkg/image/draw/">image/draw</a></dt>
  <dd>
    <p><!-- CL 340049 -->
      The <code>Draw</code> and <code>DrawMask</code> fallback implementations
      (used when the arguments are not the most common image types) are now
      faster when those arguments implement the optional
      <a href="/pkg/image/draw/#RGBA64Image"><code>draw.RGBA64Image</code></a>
      and <a href="/pkg/image/#RGBA64Image"><code>image.RGBA64Image</code></a>
      interfaces that were added in Go 1.17.
    </p>
  </dd>
</dl><!-- image/draw -->

<dl id="net"><dt><a href="/pkg/net/">net</a></dt>
  <dd>
    <p><!-- CL 340261 -->
      <a href="/pkg/net#Error"><code>net.Error.Temporary</code></a> has been deprecated.
    </p>
  </dd>
</dl><!-- net -->

<dl id="net/http"><dt><a href="/pkg/net/http/">net/http</a></dt>
  <dd>
    <p><!-- CL 330852 -->
      On WebAssembly targets, the <code>Dial</code>, <code>DialContext</code>,
      <code>DialTLS</code> and <code>DialTLSContext</code> method fields in
      <a href="/pkg/net/http/#Transport"><code>Transport</code></a>
      will now be correctly used, if specified, for making HTTP requests.
    </p>

    <p><!-- CL 338590 -->
      The new
      <a href="/pkg/net/http#Cookie.Valid"><code>Cookie.Valid</code></a>
      method reports whether the cookie is valid.
    </p>
  </dd>
</dl><!-- net/http -->

<dl id="os/user"><dt><a href="/pkg/os/user/">os/user</a></dt>
  <dd>
    <p><!-- CL 330753 -->
      <a href="/pkg/os/user#User.GroupIds"><code>User.GroupIds</code></a>.
      now uses a Go native implementation when cgo is not available.
    </p>
  </dd>
</dl><!-- os/user -->

<dl id="reflect"><dt><a href="/pkg/reflect/">reflect</a></dt>
  <dd>
    <p><!-- CL 356049, CL 320929 -->
      The new
      <a href="/pkg/reflect/#Value.SetIterKey"><code>Value.SetIterKey</code></a>
      and <a href="/pkg/reflect/#Value.SetIterValue"><code>Value.SetIterValue</code></a>
      methods set a Value using a map iterator as the source. They are equivalent to
      <code>Value.Set(iter.Key())</code> and <code>Value.Set(iter.Value())</code> but
      do fewer allocations.
    </p>

    <p><!-- CL 350691 -->
      The new
      <a href="/pkg/reflect/#Value.UnsafePointer"><code>Value.UnsafePointer</code></a>
      method returns the Value's value as an <a href="/pkg/unsafe/#Pointer"><code>unsafe.Pointer</code></a>.
      This allows callers to migrate from <a href="/pkg/reflect/#Value.UnsafeAddr"><code>Value.UnsafeAddr</code></a>
      and <a href="/pkg/reflect/#Value.Pointer"><code>Value.Pointer</code></a>
      to eliminate the need to perform uintptr to unsafe.Pointer conversions at the callsite (as unsafe.Pointer rules require).
    </p>

    <p><!-- CL 321891 -->
      The new
      <a href="/pkg/reflect/#MapIter.Reset"><code>MapIter.Reset</code></a>
      method changes its receiver to iterate over a
      different map. The use of
      <a href="/pkg/reflect/#MapIter.Reset"><code>MapIter.Reset</code></a>
      allows allocation-free iteration
      over many maps.
    </p>

    <p><!-- CL 352131 -->
      A number of methods (
      <a href="/pkg/reflect#Value.CanInt"><code>Value.CanInt</code></a>,
      <a href="/pkg/reflect#Value.CanUint"><code>Value.CanUint</code></a>,
      <a href="/pkg/reflect#Value.CanFloat"><code>Value.CanFloat</code></a>,
      <a href="/pkg/reflect#Value.CanComplex"><code>Value.CanComplex</code></a>
      )
      have been added to
      <a href="/pkg/reflect#Value"><code>Value</code></a>
      to test if a conversion is safe.
    </p>

    <p><!-- CL 357962 -->
      <a href="/pkg/reflect#Value.FieldByIndexErr"><code>Value.FieldByIndexErr</code></a>
      has been added to avoid the panic that occurs in
      <a href="/pkg/reflect#Value.FieldByIndex"><code>Value.FieldByIndex</code></a>
      when stepping through a nil pointer to an embedded struct.
    </p>

    <p><!-- CL 341333 -->
      <a href="/pkg/reflect#Ptr"><code>reflect.Ptr</code></a> and
      <a href="/pkg/reflect#PtrTo"><code>reflect.PtrTo</code></a>
      have been renamed to
      <a href="/pkg/reflect#Pointer"><code>reflect.Pointer</code></a> and
      <a href="/pkg/reflect#PointerTo"><code>reflect.PointerTo</code></a>,
      respectively, for consistency with the rest of the reflect package.
      The old names will continue to work, but will be deprecated in a
      future Go release.
    </p>
  </dd>
</dl><!-- reflect -->

<dl id="regexp"><dt><a href="/pkg/regexp/">regexp</a></dt>
  <dd>
    <p><!-- CL 354569 -->
      <a href="/pkg/regexp/"><code>regexp</code></a>
      now treats each invalid byte of a UTF-8 string as <code>U+FFFD</code>.
    </p>
  </dd>
</dl><!-- regexp -->

<dl id="strconv"><dt><a href="/pkg/strconv/">strconv</a></dt>
  <dd>
    <p><!-- CL 343877 -->
      <a href="/pkg/strconv/#strconv.Unquote"><code>strconv.Unquote</code></a>
      now rejects Unicode surrogate halves.
    </p>
  </dd>
</dl><!-- strconv -->

<dl id="strings"><dt><a href="/pkg/strings/">strings</a></dt>
  <dd>
    <p><!-- CL 351710 -->
      The new <a href="/pkg/strings/#Cut"><code>Cut</code></a> function
      slices a <code>string</code> around a separator. It can replace
      and simplify many common uses of
      <a href="/pkg/strings/#Index"><code>Index</code></a>,
      <a href="/pkg/strings/#IndexByte"><code>IndexByte</code></a>,
      <a href="/pkg/strings/#IndexRune"><code>IndexRune</code></a>,
      and <a href="/pkg/strings/#SplitN"><code>SplitN</code></a>.
    </p>

    <p><!-- CL 345849 -->
      The new <a href="/pkg/strings/#Clone"><code>Clone</code></a> function copies the input
      <code>string</code> without the returned cloned <code>string</code> referencing
      the input string's memory.
    </p>

    <p><!-- CL 323318, CL 332771 -->
      <a href="/pkg/strings/#Trim"><code>Trim</code></a>, <a href="/pkg/strings/#TrimLeft"><code>TrimLeft</code></a>,
      and <a href="/pkg/strings/#TrimRight"><code>TrimRight</code></a> are now allocation free and, especially for 
      small ASCII cutsets, up to 10 times faster.
    </p>

    <p><!-- CL 359485 -->
      The <a href="/pkg/strings/#Title"><code>Title</code></a> function is now deprecated. It doesn't
      handle Unicode punctuation and language-specific capitalization rules, and is superseded by the
      <a href="https://golang.org/x/text/cases">golang.org/x/text/cases</a> package.
    </p>
  </dd>
</dl><!-- strings -->

<dl id="sync"><dt><a href="/pkg/sync/">sync</a></dt>
  <dd>
    <p><!-- CL 319769 -->
      The new methods
      <a href="/pkg/sync#Mutex.TryLock"><code>Mutex.TryLock</code></a>,
      <a href="/pkg/sync#RWMutex.TryLock"><code>RWMutex.TryLock</code></a>, and
      <a href="/pkg/sync#RWMutex.TryRLock"><code>RWMutex.TryRLock</code></a>,
      will acquire the lock if it is not currently held.
    </p>
  </dd>
</dl><!-- sync -->

<dl id="syscall"><dt><a href="/pkg/syscall/">syscall</a></dt>
  <dd>
    <p><!-- CL 336550 -->
      The new function <a href="/pkg/syscall/?GOOS=windows#SyscallN"><code>SyscallN</code></a>
      has been introduced for Windows, allowing for calls with arbitrary number
      of arguments. As a result,
      <a href="/pkg/syscall/?GOOS=windows#Syscall"><code>Syscall</code></a>,
      <a href="/pkg/syscall/?GOOS=windows#Syscall6"><code>Syscall6</code></a>,
      <a href="/pkg/syscall/?GOOS=windows#Syscall9"><code>Syscall9</code></a>,
      <a href="/pkg/syscall/?GOOS=windows#Syscall12"><code>Syscall12</code></a>,
      <a href="/pkg/syscall/?GOOS=windows#Syscall15"><code>Syscall15</code></a>, and
      <a href="/pkg/syscall/?GOOS=windows#Syscall18"><code>Syscall18</code></a> are
      deprecated in favor of <a href="/pkg/syscall/?GOOS=windows#SyscallN"><code>SyscallN</code></a>.
    </p>

    <p><!-- CL 355570 -->
      <a href="/pkg/syscall/?GOOS=freebsd#SysProcAttr.Pdeathsig"><code>SysProcAttr.Pdeathsig</code></a>.
      is now supported in FreeBSD.
    </p>
  </dd>
</dl><!-- syscall -->

<dl id="syscall/js"><dt><a href="/pkg/syscall/js/">syscall/js</a></dt>
  <dd>
    <p><!-- CL 356430 -->
      <code>Wrapper</code> interface has been removed.
    </p>
  </dd>
</dl><!-- syscall/js -->

<dl id="testing"><dt><a href="/pkg/testing/">testing</a></dt>
  <dd>
    <p><!-- CL 343883 -->
      The precedence of <code>/</code> in the argument for <code>-run</code> and
      <code>-bench</code> has been increased. <code>A/B|C/D</code> used to be
      treated as <code>A/(B|C)/D</code> and is now treated as
      <code>(A/B)/(C/D)</code>.
    </p>

    <p><!-- CL 356669 -->
      If the <code>-run</code> option does not select any tests, the
      <code>-count</code> option is ignored. This could change the behavior of
      existing tests in the unlikely case that a test changes the set of subtests
      that are run each time the test function itself is run.
    </p>
  </dd>
</dl><!-- testing -->

<dl id="text/template"><dt><a href="/pkg/text/template/">text/template</a></dt>
  <dd>
    <p><!-- CL 321490 -->
      The <code>and</code> function no longer always evaluates all arguments; it
      stops evaluating arguments after the first argument that evaluates to
      false.  Similarly, the <code>or</code> function now stops evaluating
      arguments after the first argument that evaluates to true. This makes a
      difference if any of the arguments is a function call.
    </p>
  </dd>
</dl><!-- text/template -->

<dl id="unicode/utf8"><dt><a href="/pkg/unicode/utf8/">unicode/utf8</a></dt>
  <dd>
    <p><!-- CL 345571 -->
      The <a href="/pkg/unicode/utf8/#AppendRune"><code>AppendRune</code></a> function appends the UTF-8 new
      encoding of a <code>rune</code> to a <code>[]byte</code>.
    </p>
  </dd>
</dl><!-- unicode/utf8 -->