go/doc/go1.12.html

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<h2 id="introduction">DRAFT RELEASE NOTES - Introduction to Go 1.12</h2>

<p>
  <strong>
    Go 1.12 is not yet released. These are work-in-progress
    release notes. Go 1.12 is expected to be released in February 2019.
  </strong>
</p>

<p>
  The latest Go release, version 1.12, arrives six months after <a href="go1.11">Go 1.11</a>.
  Most of its changes are in TODO.
  As always, the release maintains the Go 1 <a href="/doc/go1compat.html">promise of compatibility</a>.
  We expect almost all Go programs to continue to compile and run as before.
</p>

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

<p>
  There are no changes to the language specification.
</p>

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

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

<p>
  Go 1.12 is the last release that is supported on FreeBSD 10.x, which has
  already reached end-of-life. Go 1.13 will require FreeBSD 11.2+ or FreeBSD
  12.0+.
</p>

<h3 id="darwin">Darwin</h3>

<p>
Go 1.12 is the last release that will run on macOS 10.10 Yosemite.
Go 1.13 will require macOS 10.11 El Capitan or later.
</p>

<p><!-- CL 141639 -->
  <code>libSystem</code> is now used when making syscalls on Darwin, ensuring forward-compatibility
  with future versions of macOS.
</p>

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

<p>
  TODO: status of ARM32 port?
</p>

<h3 id="aix">AIX</h3>

<p>
  TODO: status of AIX port?
</p>

<h3 id="hurd">Hurd</h3>

<p><!-- CL 146023 -->
  <code>hurd</code> is now a recognized value for <code>GOOS</code>, reserved
  for the GNU/Hurd system for use with <code>gccgo</code>.
</p>

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

<h3 id="vet"><code>go tool vet</code> no longer supported</h3>

<p>
  The <code>go vet</code> command has been rewritten to serve as the
  base for a range of different source code analysis tools. See
  the <a href="https://godoc.org/golang.org/x/tools/go/analysis">golang.org/x/tools/go/analysis</a>
  package for details. A side-effect is that <code>go tool vet</code>
  is no longer supported. External tools that use <code>go tool
  vet</code> must be changed to use <code>go
  vet</code>. Using <code>go vet</code> instead of <code>go tool
  vet</code> should work with all supported versions of Go.
</p>

<h3 id="gocache">Build cache requirement</h3>

<p>
  The <a href="/cmd/go/#hdr-Build_and_test_caching">build cache</a> is now
  required as a step toward eliminating
  <code>$GOPATH/pkg</code>. Setting the environment variable
  <code>GOCACHE=off</code> will cause <code>go</code> commands that write to the
  cache to fail.
</p>

<h3 id="binary-only">Binary-only packages</h3>

<p>
  Go 1.12 is the last release that will support binary-only packages.
</p>

<h3 id="modules">Modules</h3>

<p><!-- CL 148517 -->
  When <code>GO111MODULE</code> is set to <code>on</code>, the <code>go</code>
  command now supports module-aware operations outside of a module directory,
  provided that those operations do not need to resolve import paths relative to
  the current directory or explicitly edit the <code>go.mod</code> file.
  Commands such as <code>go</code> <code>get</code>,
  <code>go</code> <code>list</code>, and
  <code>go</code> <code>mod</code> <code>download</code> behave as if in a
  module with initially-empty requirements.
  In this mode, <code>go</code> <code>env</code> <code>GOMOD</code> reports
  the system's null device (<code>/dev/null</code> or <code>NUL</code>).
</p>

<p><!-- CL 146382 -->
  <code>go</code> commands that download and extract modules are now safe to
  invoke concurrently.
  The module cache (<code>GOPATH/pkg/mod</code>) must reside in a filesystem that
  supports file locking.
</p>

<p><!-- CL 147282, 147281 -->
  The <code>go</code> directive in a <code>go.mod</code> file now indicates the
  version of the language used by the files within that module, and
  <code>go</code> <code>mod</code> <code>tidy</code> sets it to the
  current release (<code>go</code> <code>1.12</code>) if no existing
  version is present.
  If the <code>go</code> directive for a module specifies a
  version <em>newer</em> than the toolchain in use, the <code>go</code> command
  will attempt to build the packages regardless, and will note the mismatch only if
  that build fails.
</p>

<p><!-- CL 152739 -->
  When an import cannot be resolved using the active modules,
  the <code>go</code> command will now try to use the modules mentioned in the
  main module's <code>replace</code> directives before consulting the module
  cache and the usual network sources.
  If a matching replacement is found but the <code>replace</code> directive does
  not specify a version, the <code>go</code> command uses a pseudo-version
  derived from the zero <code>time.Time</code> (such
  as <code>v0.0.0-00010101000000-000000000000</code>).
</p>

<h3 id="compiler">Compiler toolchain</h3>

<p><!-- CL 134155, 134156 -->
  The compiler's live variable analysis has improved. This may mean that
  finalizers will be executed sooner in this release than in previous
  releases. If that is a problem, consider the appropriate addition of a
  <a href="/pkg/runtime/#KeepAlive"><code>runtime.KeepAlive</code></a> call.
</p>

<p><!-- CL 147361 -->
  More functions are now eligible for inlining by default, including
  functions that do nothing but call another function.
  This extra inlining makes it additionally important to use
  <a href="/pkg/runtime/#CallersFrames"><code>runtime.CallersFrames</code></a>
  instead of iterating over the result of
  <a href="/pkg/runtime/#Callers"><code>runtime.Callers</code></a> directly.
<pre>
// Old code which no longer works correctly (it will miss inlined call frames).
var pcs [10]uintptr
n := runtime.Callers(1, pcs[:])
for _, pc := range pcs[:n] {
	f := runtime.FuncForPC(pc)
	if f != nil {
		fmt.Println(f.Name())
	}
}
</pre>
<pre>
// New code which will work correctly.
var pcs [10]uintptr
n := runtime.Callers(1, pcs[:])
frames := runtime.CallersFrames(pcs[:n])
for {
	frame, more := frames.Next()
	fmt.Println(frame.Function)
	if !more {
		break
	}
}
</pre>
</p>

<p><!-- CL 144340 -->
  The compiler now accepts a <code>-lang</code> flag to set the Go language
  version to use. For example, <code>-lang=go1.8</code> causes the compiler to
  emit an error if the program uses type aliases, which were added in Go 1.9.
  Language changes made before Go 1.12 are not consistently enforced.
</p>

<h3 id="godoc">Godoc</h3>

<p>
  In Go 1.12, <code>godoc</code> no longer has a command-line interface and
  is only a web server. Users should use <code>go</code> <code>doc</code>
  for command-line help output instead.
</p>

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

<p>
  All of the changes to the standard library are minor.
</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>

<!-- CL 147218: https://golang.org/cl/147218: cmd/asm: rename R18 to R18_PLATFORM on ARM64 -->
<!-- CL 116275: https://golang.org/cl/116275: cmd/compile: avoid string allocations when map key is struct or array literal -->
<!-- CL 142717: https://golang.org/cl/142717: cmd/compile: remove obsolete "safe" mode -->
<!-- CL 146058: https://golang.org/cl/146058: It is invalid to convert a nil unsafe.Pointer to uintptr and back, with arithmetic.: cmd/compile: assume unsafe pointer arithmetic generates non-nil results -->
<!-- CL 141977: https://golang.org/cl/141977: cmd/doc: add -all flag to print all documentation for package -->
<!-- CL 146898: https://golang.org/cl/146898: cmd/link, runtime: add initial cgo support for ppc64 -->
<!-- CL 60790: https://golang.org/cl/60790: The trace tool now supports plotting mutator utilization curves, including cross-references to the execution trace. These are useful for analyzing the impact of the garbage collector on application latency and throughput.: cmd/trace: add minimum mutator utilization (MMU) plot -->
<!-- CL 115677: https://golang.org/cl/115677: cmd/vet: check embedded field tags too -->
<dl id="bufio"><dt><a href="/pkg/bufio/">bufio</a></dt>
  <dd>
    <p><!-- CL 149297 -->
      <code>Reader</code>'s <a href="/pkg/bufio/#Reader.UnreadRune"><code>UnreadRune</code></a> and
      <a href="/pkg/bufio/#Reader.UnreadByte"><code>UnreadByte</code></a> methods will now return an error
      if they are called after <a href="/pkg/bufio/#Reader.Peek"><code>Peek</code></a>.
    </p>

</dl><!-- bufio -->

<dl id="build"><dt><a href="/pkg/build/">build</a></dt>
  <dd>
    <p><!-- CL 61511 -->
      TODO: <a href="https://golang.org/cl/61511">https://golang.org/cl/61511</a>: support frame-pointer for arm64
    </p>

</dl><!-- build -->

<dl id="bytes"><dt><a href="/pkg/bytes/">bytes</a></dt>
  <dd>
    <p><!-- CL 137855 -->
      The new function <a href="/pkg/bytes/#ReplaceAll"><code>ReplaceAll</code></a> returns a copy of
      a byte slice with all non-overlapping instances of a value replaced by another.
    </p>

    <p><!-- CL 145098 -->
      A pointer to a zero-value <a href="/pkg/bytes/#Reader"><code>Reader</code></a> is now
      functionally equivalent to <a href="/pkg/bytes/#NewReader"><code>NewReader</code></a><code>(nil)</code>.
      Prior to Go 1.12, the former could not be used as a substitute for the latter in all cases.
    </p>

</dl><!-- bytes -->

<dl id="cmd,runtime"><dt><a href="/pkg/cmd,runtime/">cmd,runtime</a></dt>
  <dd>
    <p><!-- CL 138675 -->
      TODO: <a href="https://golang.org/cl/138675">https://golang.org/cl/138675</a>: enable race detector on arm64
    </p>

</dl><!-- cmd,runtime -->

<dl id="crypto/rand"><dt><a href="/pkg/crypto/rand/">crypto/rand</a></dt>
  <dd>
    <p><!-- CL 120055 -->
      TODO: <a href="https://golang.org/cl/120055">https://golang.org/cl/120055</a>: use the new getrandom syscall on FreeBSD
    </p>

    <p><!-- CL 139419 -->
      TODO: <a href="https://golang.org/cl/139419">https://golang.org/cl/139419</a>: warn to stderr if blocked 60+ sec on first Reader.Read call
    </p>

</dl><!-- crypto/rand -->

<dl id="crypto/rc4"><dt><a href="/pkg/crypto/rc4/">crypto/rc4</a></dt>
  <dd>
    <p><!-- CL 130397 -->
      TODO: <a href="https://golang.org/cl/130397">https://golang.org/cl/130397</a>: remove assembler implementations
    </p>

</dl><!-- crypto/rc4 -->

<dl id="database/sql"><dt><a href="/pkg/database/sql/">database/sql</a></dt>
  <dd>
    <p><!-- CL 145738 -->
      A query cursor can now be obtained by passing a
      <a href="/pkg/database/sql/#Rows"><code>*Rows</code></a>
      value to the <a href="/pkg/database/sql/#Row.Scan"><code>Row.Scan</code></a> method.
    </p>

</dl><!-- database/sql -->

<dl id="expvar"><dt><a href="/pkg/expvar/">expvar</a></dt>
  <dd>
    <p><!-- CL 139537 -->
      The new <a href="/pkg/expvar/#Map.Delete"><code>Delete</code></a> method allows
      for deletion of key/value pairs from a <a href="/pkg/expvar/#Map"><code>Map</code></a>.
    </p>

</dl><!-- expvar -->

<dl id="fmt"><dt><a href="/pkg/fmt/">fmt</a></dt>
  <dd>
    <p><!-- CL 142737 -->
      Maps are now printed in key-sorted order to ease testing. The ordering rules are:
      <ul>
        <li>When applicable, nil compares low
        <li>ints, floats, and strings order by <
        <li>NaN compares less than non-NaN floats
        <li>bool compares false before true
        <li>Complex compares real, then imaginary
        <li>Pointers compare by machine address
        <li>Channel values compare by machine address
        <li>Structs compare each field in turn
        <li>Arrays compare each element in turn
        <li>Interface values compare first by <code>reflect.Type</code> describing the concrete type
            and then by concrete value as described in the previous rules.
      </ul>
    </p>

    <p><!-- CL 129777 -->
      When printing maps, non-reflexive key values like <code>NaN</code> were previously
      displayed as <code>&lt;nil&gt;</code>. As of this release, the correct values are printed.
    </p>

</dl><!-- fmt -->

<dl id="go/doc"><dt><a href="/pkg/go/doc/">go/doc</a></dt>
  <dd>
    <p><!-- CL 140958 -->
      To address some outstanding issues in <a href="/cmd/doc/"><code>cmd/doc</code></a>,
      this package has a new <a href="/pkg/go/doc/#Mode"><code>Mode</code></a> bit,
      <code>PreserveAST</code>, which controls whether AST data is cleared.
    </p>

</dl><!-- go/doc -->

<dl id="go/token"><dt><a href="/pkg/go/token/">go/token</a></dt>
  <dd>
    <p><!-- CL 134075 -->
      The <a href="/pkg/go/token#File"><code>File</code></a> type has a new
      <a href="/pkg/go/token#File.LineStart"><code>LineStart</code></a> field,
      which returns the position of the start of a given line. This is especially useful
      in programs that occassionally handle non-Go files, such as assembly, but wish to use
      the <code>token.Pos</code> mechanism to identify file positions.
    </p>

</dl><!-- go/token -->

<dl id="image"><dt><a href="/pkg/image/">image</a></dt>
  <dd>
    <p><!-- CL 118755 -->
      The <a href="/pkg/image/#RegisterFormat"><code>RegisterFormat</code></a> function is now safe for concurrent use.
    </p>

</dl><!-- image -->

<dl id="image/png"><dt><a href="/pkg/image/png/">image/png</a></dt>
  <dd>
    <p><!-- CL 134235 -->
      Paletted images with fewer than 16 colors now encode to smaller outputs.
    </p>

</dl><!-- image/png -->

<dl id="internal/cpu"><dt><a href="/pkg/internal/cpu/">internal/cpu</a></dt>
  <dd>
    <p><!-- CL 149578 -->
      TODO: <a href="https://golang.org/cl/149578">https://golang.org/cl/149578</a>: move GODEBUGCPU options into GODEBUG
    </p>

</dl><!-- internal/cpu -->

<dl id="internal/poll"><dt><a href="/pkg/internal/poll/">internal/poll</a></dt>
  <dd>
    <p><!-- CL 130676 -->
      TODO: <a href="https://golang.org/cl/130676">https://golang.org/cl/130676</a>: use F_FULLFSYNC fcntl for FD.Fsync on OS X
    </p>

</dl><!-- internal/poll -->

<dl id="io"><dt><a href="/pkg/io/">io</a></dt>
  <dd>
    <p><!-- CL 139457 -->
      The new <a href="/pkg/io#StringWriter"><code>StringWriter</code></a> interface wraps the
      <a href="/pkg/io/#WriteString"><code>WriteString</code></a> function.
    </p>

</dl><!-- io -->

<dl id="lib/time"><dt><a href="/pkg/lib/time/">lib/time</a></dt>
  <dd>
    <p><!-- CL 151299 -->
    The time zone database in <code>$GOROOT/lib/time/zoneinfo.zip</code> has been updated to version 2018g. Note that this ZIP file is only used if a time zone database is not provided by the operating system.
    </p>

</dl><!-- lib/time -->

<dl id="math/bits"><dt><a href="/pkg/math/bits/">math/bits</a></dt>
  <dd>
    <p><!-- CL 123157 -->
    New extended precision operations <a href="/pkg/math/bits/#Add"><code>Add</code></a>, <a href="/pkg/math/bits/#Sub"><code>Sub</code></a>, <a href="/pkg/math/bits/#Mul"><code>Mul</code></a>, and <a href="/pkg/math/bits/#Div"><code>Div</code></a> are available in <code>int</code>, <code>int32</code>, and <code>int64</code> versions.
    </p>

</dl><!-- math/bits -->

<dl id="net"><dt><a href="/pkg/net/">net</a></dt>
  <dd>
    <p><!-- CL 146659 -->
      The
      <a href="/pkg/net/#Dialer.DualStack"><code>Dialer.DualStack</code></a> setting is now ignored and deprecated;
      RFC 6555 Fast Fallback ("Happy Eyeballs") is now enabled by default. To disable, set
      <a href="/pkg/net/#Dialer.FallbackDelay"><code>Dialer.FallbackDelay</code></a> to a negative value.
    </p>

    <p><!-- CL 107196 -->
      Similarly, TCP keep-alives are now enabled by default if
      <a href="/pkg/net/#Dialer.KeepAlive"><code>Dialer.KeepAlive</code></a> is zero.
      To disable, set it to a negative value.
    </p>

    <p><!-- CL 113997 -->
      On Linux, the <a href="http://man7.org/linux/man-pages/man2/splice.2.html"><code>splice</code> system call</a> is now used when copying from a
      <a href="/pkg/net/#UnixConn"><code>UnixConn</code></a> to a
      <a href="/pkg/net/#TCPConn"><code>TCPConn</code></a>.
    </p>
</dl><!-- net -->

<dl id="net/http"><dt><a href="/pkg/net/http/">net/http</a></dt>
  <dd>
    <p><!-- CL 143177 -->
      The HTTP server now rejects misdirected HTTP requests to HTTPS servers with a plaintext "400 Bad Request" response.
    </p>

    <p><!-- CL 130115 -->
      The new <a href="/pkg/net/http/#Client.CloseIdleConnections"><code>Client.CloseIdleConnections</code></a>
      method calls the <code>Client</code>'s underlying <code>Transport</code>'s <code>CloseIdleConnections</code>
      if it has one.
    </p>

    <p><!-- CL 145398 -->
      The <a href="/pkg/net/http/#Transport"><code>Transport</code></a> no longer rejects HTTP responses which declare
      HTTP Trailers but don't use chunked encoding. Instead, the declared trailers are now just ignored.
    </p>

    <p><!-- CL 152080 --> <!-- CL 151857 -->
      The <a href="/pkg/net/http/#Transport"><code>Transport</code></a> no longer handles <code>MAX_CONCURRENT_STREAMS</code> values
      advertised from HTTP/2 servers as strictly as it did during Go 1.10 and Go 1.11. The default behavior is now back
      to how it was in Go 1.9: each connection to a server can have up to <code>MAX_CONCURRENT_STREAMS</code> requests
      active and then new TCP connections are created as needed. In Go 1.10 and Go 1.11 the <code>http2</code> package
      would block and wait for requests to finish instead of creating new connections.
      To get the stricter behavior back, import the
      <a href="https://godoc.org/golang.org/x/net/http2"><code>golang.org/x/net/http2</code></a> package
      directly and set
      <a href="https://godoc.org/golang.org/x/net/http2#Transport.StrictMaxConcurrentStreams"><code>Transport.StrictMaxConcurrentStreams</code></a> to
      <code>true</code>.
    </p>

</dl><!-- net/http -->

<dl id="net/http/httputil"><dt><a href="/pkg/net/http/httputil/">net/http/httputil</a></dt>
  <dd>
    <p><!-- CL 146437 -->
      The <a href="/pkg/net/http/httputil/#ReverseProxy"><code>ReverseProxy</code></a> now automatically
      proxies WebSocket requests.
    </p>

</dl><!-- net/http/httputil -->

<dl id="os"><dt><a href="/pkg/os/">os</a></dt>
  <dd>
    <p><!-- CL 125443 -->
      The new <a href="/pkg/os/#ProcessState.ExitCode"><code>ProcessState.ExitCode</code></a> method
      returns the process's exit code.
    </p>

    <p><!-- CL 135075 -->
      <code>ModeCharDevice</code> has been added to the <code>ModeType</code> bitmask, allowing for
      <code>ModeDevice | ModeCharDevice</code> to be recovered when masking a
      <a href="/pkg/os/#FileMode"><code>FileMode</code></a> with <code>ModeType</code>.
    </p>

    <p><!-- CL 139418 -->
      The new function <a href="/pkg/os/#UserHomeDir"><code>UserHomeDir</code></a> returns the
      current user's home directory.
    </p>

    <p><!-- CL 146020 -->
      <a href="/pkg/os/#RemoveAll"><code>RemoveAll</code></a> now supports paths longer than 4096 characters
      on most Unix systems.
    </p>

</dl><!-- os -->

<dl id="path/filepath"><dt><a href="/pkg/path/filepath/">path/filepath</a></dt>
  <dd>
    <p><!-- CL 145220 -->
      The <a href="/pkg/path/filepath/#IsAbs"><code>IsAbs</code></a> function now returns true when passed
      a reserved filename on Windows such as <code>NUL</code>.
      <a href="https://docs.microsoft.com/en-us/windows/desktop/fileio/naming-a-file#naming-conventions">List of reserved names.</a>
    </p>

</dl><!-- path/filepath -->

<dl id="reflect"><dt><a href="/pkg/reflect/">reflect</a></dt>
  <dd>
    <p><!-- CL 33572 -->
      A new <a href="/pkg/reflect#MapIter"><code>MapIter</code></a> type is
      an iterator for ranging over a map. This type is exposed through the
      <a href="/pkg/reflect#Value"><code>Value</code></a> type's new
      <a href="/pkg/reflect#Value.MapRange"><code>MapRange</code></a> method.
      This follows the same iteration semantics as a range statment, with <code>Next</code>
      to advance the iterator, and <code>Key</code>/<code>Value</code> to access each entry.
    </p>

</dl><!-- reflect -->

<dl id="regexp"><dt><a href="/pkg/regexp/">regexp</a></dt>
  <dd>
    <p><!-- CL 139784 -->
      <a href="/pkg/regexp/#Regexp.Copy"><code>Copy</code></a> is no longer necessary
      to avoid lock contention, so it has been given a partial deprecation comment.
      <a href="/pkg/regexp/#Regexp.Copy"><code>Copy</code></a>
      may still be appropriate if the reason for its use is to make two copies with
      different <a href="/pkg/regexp/#Regexp.Longest"><code>Longest</code></a> settings.
    </p>

</dl><!-- regexp -->

<dl id="runtime"><dt><a href="/pkg/runtime/">runtime</a></dt>
  <dd>
    <p><!-- CL 135395 -->
      On Linux, the Go runtime now releases memory only when the OS is under memory
      pressure. This is more efficient, but means a process's RSS (resident set size)
      won't decrease unless the OS is running out of memory.
    </p>

</dl><!-- runtime -->

<dl id="runtime/debug"><dt><a href="/pkg/runtime/debug/">runtime/debug</a></dt>
  <dd>
    <p><!-- CL 144220 -->
      A new <a href="/pkg/runtime/debug/#BuildInfo"><code>BuildInfo</code></a> type
      exposes the build information read from the running binary, available only in
      binaries built with module support. This includes the main package path, main
      module information, and the module dependencies. This type is given though the
      <a href="/pkg/runtime/debug/#ReadBuildInfo"><code>ReadBuildInfo</code></a> function
      on <a href="/pkg/runtime/debug/#BuildInfo"><code>BuildInfo</code></a>.
    </p>

</dl><!-- runtime/debug -->

<dl id="strings"><dt><a href="/pkg/strings/">strings</a></dt>
  <dd>
    <p><!-- CL 137855 -->
      The new function <a href="/pkg/strings/#ReplaceAll"><code>ReplaceAll</code></a> returns a copy of
      a string with all non-overlapping instances of a value replaced by another.
    </p>

    <p><!-- CL 145098 -->
      A pointer to a zero-value <a href="/pkg/strings/#Reader"><code>Reader</code></a> is now
      functionally equivalent to <a href="/pkg/strings/#NewReader"><code>NewReader</code></a><code>(nil)</code>.
      Prior to Go 1.12, the former could not be used as a substitute for the latter in all cases.
    </p>

    <p><!-- CL 122835 -->
      The new <a href="/pkg/strings/#Builder.Cap"><code>Builder.Cap</code></a> method returns the capacity of the builder's underlying byte slice.
    </p>

    <p><!-- CL 131495 -->
      The character mapping functions <a href="/pkg/strings/#Map"><code>Map</code></a>,
      <a href="/pkg/strings/#Title"><code>Title</code></a>,
      <a href="/pkg/strings/#ToLower"><code>ToLower</code></a>,
      <a href="/pkg/strings/#ToLowerSpecial"><code>ToLowerSpecial</code></a>,
      <a href="/pkg/strings/#ToTitle"><code>ToTitle</code></a>,
      <a href="/pkg/strings/#ToTitleSpecial"><code>ToTitleSpecial</code></a>,
      <a href="/pkg/strings/#ToUpper"><code>ToUpper</code></a>, and
      <a href="/pkg/strings/#ToUpperSpecial"><code>ToUpperSpecial</code></a>
      now always guarantee to return valid UTF-8. In earlier releases, if the input was invalid UTF-8 but no character replacements
      needed to be applied, these routines incorrectly returned the invalid UTF-8 unmodified.
    </p>

</dl><!-- strings -->

<dl id="syscall"><dt><a href="/pkg/syscall/">syscall</a></dt>
  <dd>
    <p><!-- CL 138595 -->
      64-bit inodes are now supported on FreeBSD 12. Some types have been adjusted accordingly.
    </p>

    <p><!-- CL 125456 -->
      The Unix socket
      (<a href="https://blogs.msdn.microsoft.com/commandline/2017/12/19/af_unix-comes-to-windows/"><code>AF_UNIX</code></a>)
      address family is now supported for compatible versions of Windows.
    </p>

    <p><!-- CL 147117 -->
      The new function  <a href="/pkg/syscall/?GOOS=windows&GOARCH=armd64#Syscall18"><code>Syscall18</code></a>
      has been introduced for Windows, allowing for calls with up to 18 arguments.
    </p>

</dl><!-- syscall -->

<dl id="syscall/js"><dt><a href="/pkg/syscall/js/">syscall/js</a></dt>
  <dd>
    <p><!-- CL 153559 -->
    <p>
      The <code>Callback</code> type and <code>NewCallback</code> function have been renamed;
      they are now called
      <a href="/pkg/syscall/js/?GOOS=js&GOARCH=wasm#Func"><code>Func</code></a> and
      <a href="/pkg/syscall/js/?GOOS=js&GOARCH=wasm#FuncOf"><code>FuncOf</code></a>, respectively.
      This is a breaking change, but WebAssembly support is still experimental
      and not yet subject to the
      <a href="/doc/go1compat">Go 1 compatibility promise</a>. Any code using the
      old name will need to be updated.
    </p>

    <p><!-- CL 141644 -->
      TODO: <a href="https://golang.org/cl/141644">https://golang.org/cl/141644</a>: add Wrapper interface to support external Value wrapper types
    </p>

    <p><!-- CL 143137 -->
      TODO: <a href="https://golang.org/cl/143137">https://golang.org/cl/143137</a>: make zero js.Value represent &#34;undefined&#34;
    </p>

    <p><!-- CL 144384 -->
      TODO: <a href="https://golang.org/cl/144384">https://golang.org/cl/144384</a>: add the Value.Truthy method
    </p>

</dl><!-- syscall/js -->

<dl id="testing"><dt><a href="/pkg/testing/">testing</a></dt>
  <dd>
    <p><!-- CL 139258 -->
    The <a href="/cmd/go/#hdr-Testing_flags"><code>-benchtime</code></a> flag now supports setting an explicit iteration count instead of a time when the value ends with an "<code>x</code>". For example, <code>-benchtime=100x</code> runs the benchmark 100 times.
    </p>

</dl><!-- testing -->

<dl id="text/template"><dt><a href="/pkg/text/template/">text/template</a></dt>
  <dd>
    <p><!-- CL 142217 -->
      When executing a template, long context values are no longer truncated in errors.
    </p>
    <p>
      <code>executing "tmpl" at <.very.deep.context.v...>: map has no entry for key "notpresent"</code>
    </p>
    <p>
      is now
    </p>
    <p>
      <code>executing "tmpl" at <.very.deep.context.value.notpresent>: map has no entry for key "notpresent"</code>
    </p>

</dl><!-- text/template -->