go/doc/go1.16.html

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<h2 id="introduction">DRAFT RELEASE NOTES — Introduction to Go 1.16</h2>

<p>
  <strong>
    Go 1.16 is not yet released. These are work-in-progress
    release notes. Go 1.16 is expected to be released in February 2021.
  </strong>
</p>

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

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

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

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

<p><!-- golang.org/issue/38485, golang.org/issue/41385, CL 266373, more CLs -->
  Go 1.16 adds support of 64-bit ARM architecture on macOS (also known as
  Apple Silicon) with <code>GOOS=darwin</code>, <code>GOARCH=arm64</code>.
  Like the <code>darwin/amd64</code> port, the <code>darwin/arm64</code>
  port supports cgo, internal and external linking, <code>c-archive</code>,
  <code>c-shared</code>, and <code>pie</code> build modes, and the race
  detector.
</p>

<p><!-- CL 254740 -->
  The iOS port, which was previously <code>darwin/arm64</code>, has
  been renamed to <code>ios/arm64</code>. <code>GOOS=ios</code>
  implies the
  <code>darwin</code> build tag, just as <code>GOOS=android</code>
  implies the <code>linux</code> build tag. This change should be
  transparent to anyone using gomobile to build iOS apps.
</p>

<p><!-- golang.org/issue/42100, CL 263798 -->
  Go 1.16 adds an <code>ios/amd64</code> port, which targets the iOS
  simulator running on AMD64-based macOS. Previously this was
  unofficially supported through <code>darwin/amd64</code> with
  the <code>ios</code> build tag set.
</p>

<p><!-- golang.org/issue/23011 -->
  Go 1.16 is the last release that will run on macOS 10.12 Sierra.
  Go 1.17 will require macOS 10.13 High Sierra or later.
</p>

<h3 id="netbsd">NetBSD</h3>

<p><!-- golang.org/issue/30824 -->
  Go now supports the 64-bit ARM architecture on NetBSD (the
  <code>netbsd/arm64</code> port).
</p>

<h3 id="openbsd">OpenBSD</h3>

<p><!-- golang.org/issue/40995 -->
  Go now supports the MIPS64 architecture on OpenBSD
  (the <code>openbsd/mips64</code> port). This port does not yet
  support cgo.
</p>

<h3 id="386">386</h3>

<p><!-- golang.org/issue/40255, golang.org/issue/41848, CL 258957, and CL 260017 -->
  As <a href="go1.15#386">announced</a> in the Go 1.15 release notes,
  Go 1.16 drops support for x87 mode compilation (<code>GO386=387</code>).
  Support for non-SSE2 processors is now available using soft float
  mode (<code>GO386=softfloat</code>).
  Users running on non-SSE2 processors should replace <code>GO386=387</code>
  with <code>GO386=softfloat</code>.
</p>

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

<p><!-- golang.org/issue/36641, CL 267317 -->
  The <code>linux/riscv64</code> port now supports cgo and
  <code>-buildmode=pie</code>. This release also includes performance
  optimizations and code generation improvements for RISC-V.
</p>

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

<p>
  TODO
</p>

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

<h4 id="modules">Modules</h4>

<p><!-- golang.org/issue/41330 -->
  Module-aware mode is enabled by default, regardless of whether a
  <code>go.mod</code> file is present in the current working directory or a
  parent directory. More precisely, the <code>GO111MODULE</code> environment
  variable now defaults to <code>on</code>. To switch to the previous behavior,
  set <code>GO111MODULE</code> to <code>auto</code>.
</p>

<p><!-- golang.org/issue/40728 -->
  Build commands like <code>go</code> <code>build</code> and <code>go</code>
  <code>test</code> no longer modify <code>go.mod</code> and <code>go.sum</code>
  by default. Instead, they report an error if a module requirement or checksum
  needs to be added or updated (as if the <code>-mod=readonly</code> flag were
  used). Module requirements and sums may be adjusted with <code>go</code>
  <code>mod</code> <code>tidy</code> or <code>go</code> <code>get</code>.
</p>

<p><!-- golang.org/issue/40276 -->
  <code>go</code> <code>install</code> now accepts arguments with
  version suffixes (for example, <code>go</code> <code>install</code>
  <code>example.com/cmd@v1.0.0</code>). This causes <code>go</code>
  <code>install</code> to build and install packages in module-aware mode,
  ignoring the <code>go.mod</code> file in the current directory or any parent
  directory, if there is one. This is useful for installing executables without
  affecting the dependencies of the main module.
</p>

<p><!-- golang.org/issue/40276 -->
  <code>go</code> <code>install</code>, with or without a version suffix (as
  described above), is now the recommended way to build and install packages in
  module mode. <code>go</code> <code>get</code> should be used with the
  <code>-d</code> flag to adjust the current module's dependencies without
  building packages, and use of <code>go</code> <code>get</code> to build and
  install packages is deprecated. In a future release, the <code>-d</code> flag
  will always be enabled.
</p>

<p><!-- golang.org/issue/24031 -->
  <code>retract</code> directives may now be used in a <code>go.mod</code> file
  to indicate that certain published versions of the module should not be used
  by other modules. A module author may retract a version after a severe problem
  is discovered or if the version was published unintentionally.<br>
</p>

<p><!-- golang.org/issue/26603 -->
  The <code>go</code> <code>mod</code> <code>vendor</code>
  and <code>go</code> <code>mod</code> <code>tidy</code> subcommands now accept
  the <code>-e</code> flag, which instructs them to proceed despite errors in
  resolving missing packages.
</p>

<p><!-- golang.org/issue/36465 -->
  The <code>go</code> command now ignores requirements on module versions
  excluded by <code>exclude</code> directives in the main module. Previously,
  the <code>go</code> command used the next version higher than an excluded
  version, but that version could change over time, resulting in
  non-reproducible builds.
</p>

<h4 id="embed">Embedding Files</h4>

<p>
  The <code>go</code> command now supports including
  static files and file trees as part of the final executable,
  using the new <code>//go:embed</code> directive.
  See the documentation for the new
  <a href="/pkg/embed/"><code>embed</code></a>
  package for details.
</p>

<h4 id="go-test"><code>go</code> <code>test</code></h4>

<p><!-- golang.org/issue/29062 -->
  When using <code>go</code> <code>test</code>, a test that
  calls <code>os.Exit(0)</code> during execution of a test function
  will now be considered to fail.
  This will help catch cases in which a test calls code that calls
  <code>os.Exit(0)</code> and thereby stops running all future tests.
  If a <code>TestMain</code> function calls <code>os.Exit(0)</code>
  that is still considered to be a passing test.
</p>

<p><!-- golang.org/issue/39484 -->
  <code>go</code> <code>test</code> reports an error when the <code>-c</code>
  or <code>-i</code> flags are used together with unknown flags. Normally,
  unknown flags are passed to tests, but when <code>-c</code> or <code>-i</code>
  are used, tests are not run.
</p>

<h4 id="go-get"><code>go</code> <code>get</code></h4>

<p><!-- golang.org/issue/37519 -->
  The <code>go</code> <code>get</code> <code>-insecure</code> flag is
  deprecated and will be removed in a future version. This flag permits
  fetching from repositories and resolving custom domains using insecure
  schemes such as HTTP, and also bypasses module sum validation using the
  checksum database. To permit the use of insecure schemes, use the
  <code>GOINSECURE</code> environment variable instead. To bypass module
  sum validation, use <code>GOPRIVATE</code> or <code>GONOSUMDB</code>.
  See <code>go</code> <code>help</code> <code>environment</code> for details.
</p>

<p><!-- golang.org/cl/263267 -->
  <code>go</code> <code>get</code> <code>example.com/mod@patch</code> now
  requires that some version of <code>example.com/mod</code> already be
  required by the main module.
  (However, <code>go</code> <code>get</code> <code>-u=patch</code> continues
  to patch even newly-added dependencies.)
</p>

<h4 id="govcs"><code>GOVCS</code> environment variable</h4>

<p><!-- golang.org/issue/266420 -->
  <code>GOVCS</code> is a new environment variable that limits which version
  control tools the <code>go</code> command may use to download source code.
  This mitigates security issues with tools that are typically used in trusted,
  authenticated environments. By default, <code>git</code> and <code>hg</code>
  may be used to download code from any repository. <code>svn</code>,
  <code>bzr</code>, and <code>fossil</code> may only be used to download code
  from repositories with module paths or package paths matching patterns in
  the <code>GOPRIVATE</code> environment variable. See
  <a href="/cmd/go/#hdr-Controlling_version_control_with_GOVCS"><code>go</code>
  <code>help</code> <code>vcs</code></a> for details.
</p>

<h4 id="all-pattern">The <code>all</code> pattern</h4>

<p><!-- golang.org/cl/240623 -->
  When the main module's <code>go.mod</code> file
  declares <code>go</code> <code>1.16</code> or higher, the <code>all</code>
  package pattern now matches only those packages that are transitively imported
  by a package or test found in the main module. (Packages imported by <em>tests
  of</em> packages imported by the main module are no longer included.) This is
  the same set of packages retained
  by <code>go</code> <code>mod</code> <code>vendor</code> since Go 1.11.
</p>

<h4 id="toolexec">The <code>-toolexec</code> build flag</h4>

<p><!-- golang.org/cl/263357 -->
  When the <code>-toolexec</code> build flag is specified to use a program when
  invoking toolchain programs like compile or asm, the environment variable
  <code>TOOLEXEC_IMPORTPATH</code> is now set to the import path of the package
  being built.
</p>

<h4 id="i-flag">The <code>-i</code> build flag</h4>

<p><!-- golang.org/issue/41696 -->
  The <code>-i</code> flag accepted by <code>go</code> <code>build</code>,
  <code>go</code> <code>install</code>, and <code>go</code> <code>test</code> is
  now deprecated. The <code>-i</code> flag instructs the <code>go</code> command
  to install packages imported by packages named on the command line. Since
  the build cache was introduced in Go 1.10, the <code>-i</code> flag no longer
  has a significant effect on build times, and it causes errors when the install
  directory is not writable.
</p>

<h4 id="list-buildid">The <code>list</code> command</h4>

<p><!-- golang.org/cl/263542 -->
  When the <code>-export</code> flag is specified, the <code>BuildID</code>
  field is now set to the build ID of the compiled package. This is equivalent
  to running <code>go</code> <code>tool</code> <code>buildid</code> on
  <code>go</code> <code>list</code> <code>-exported</code> <code>-f</code> <code>{{.Export}</code>,
  but without the extra step.
</p>

<h3 id="cgo">Cgo</h3>

<p><!-- CL 252378 -->
  The <a href="/cmd/cgo">cgo</a> tool will no longer try to translate
  C struct bitfields into Go struct fields, even if their size can be
  represented in Go. The order in which C bitfields appear in memory
  is implementation dependent, so in some cases the cgo tool produced
  results that were silently incorrect.
</p>

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

<p>
  TODO

  <!-- CL 235677: https://golang.org/cl/235677: cmd/vet: bring in pass to catch invalid uses of testing.T in goroutines -->
</p>

<p><!-- CL 248686, CL 276372 -->
  The vet tool now warns about amd64 assembly that clobbers the BP
  register (the frame pointer) without saving and restoring it,
  contrary to the calling convention. Code that doesn't preserve the
  BP register must be modified to either not use BP at all or preserve
  BP by saving and restoring it. An easy way to preserve BP is to set
  the frame size to a nonzero value, which causes the generated
  prologue and epilogue to preserve the BP register for you.
  See <a href="https://golang.org/cl/248260">CL 248260</a> for example
  fixes.
</p>

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

<p>
  The new <a href="/pkg/runtime/metrics/"><code>runtime/metrics</code></a> package
  introduces a stable interface for reading
  implementation-defined metrics from the Go runtime.
  It supersedes existing functions like
  <a href="/pkg/runtime/#ReadMemStats"><code>runtime.ReadMemStats</code></a>
  and
  <a href="/pkg/runtime/debug/#GCStats"><code>debug.GCStats</code></a>
  and is significantly more general and efficient.
  See the package documentation for more details.
</p>

<p><!-- CL 254659 -->
  Setting the <code>GODEBUG</code> environment variable
  to <code>inittrace=1</code> now causes the runtime to emit a single
  line to standard error for each package <code>init</code>,
  summarizing its execution time and memory allocation. This trace can
  be used to find bottlenecks or regressions in Go startup
  performance.
  The <a href="/pkg/runtime/#hdr-Environment_Variables"><code>GODEBUG</code><
  documentation</a> describes the format.
</p>

<p><!-- CL 267100 -->
  On Linux, the runtime now defaults to releasing memory to the
  operating system promptly (using <code>MADV_DONTNEED</code>), rather
  than lazily when the operating system is under memory pressure
  (using <code>MADV_FREE</code>). This means process-level memory
  statistics like RSS will more accurately reflect the amount of
  physical memory being used by Go processes. Systems that are
  currently using <code>GODEBUG=madvdontneed=1</code> to improve
  memory monitoring behavior no longer need to set this environment
  variable.
</p>

<p><!-- CL 220419, CL 271987 -->
  Go 1.16 fixes a discrepancy between the race detector and
  the <a href="/ref/mem">Go memory model</a>. The race detector now
  more precisely follows the channel synchronization rules of the
  memory model. As a result, the detector may now report races it
  previously missed.
</p>

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

<p><!-- CL 256459, CL 264837, CL 266203, CL 256460 -->
  The compiler can now inline functions with
  non-labeled <code>for</code> loops, method values, and type
  switches. The inliner can also detect more indirect calls where
  inlining is possible.
</p>

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

<p><!-- CL 248197 -->
  This release includes additional improvements to the Go linker,
  reducing linker resource usage (both time and memory) and improving
  code robustness/maintainability. These changes form the second half
  of a two-release project to
  <a href="https://golang.org/s/better-linker">modernize the Go
  linker</a>.
</p>

<p>
  The linker changes in 1.16 extend the 1.15 improvements to all
  supported architecture/OS combinations (the 1.15 performance improvements
  were primarily focused on <code>ELF</code>-based OSes and
  <code>amd64</code> architectures).  For a representative set of
  large Go programs, linking is 20-25% faster than 1.15 and requires
  5-15% less memory on average for <code>linux/amd64</code>, with larger
  improvements for other architectures and OSes. Most binaries are
  also smaller as a result of more aggressive symbol pruning.
</p>

<p><!-- CL 255259 -->
  On Windows, <code>go build -buildmode=c-shared</code> now generates Windows
  ASLR DLLs by default. ASLR can be disabled with <code>--ldflags=-aslr=false</code>.
</p>

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

<h3 id="library-embed">Embedded Files</h3>

<p>
  The new <a href="/pkg/embed/"><code>embed</code></a> package
  provides access to files embedded in the program during compilation
  using the new <a href="#embed"><code>//go:embed</code> directive</a>.
</p>

<h3 id="fs">File Systems</h3>

<p>
  The new <a href="/pkg/io/fs/"><code>io/fs</code></a> package
  defines an abstraction for read-only trees of files,
  the <a href="/pkg/io/fs/#FS"><code>fs.FS</code></a> interface,
  and the standard library packages have
  been adapted to make use of the interface as appropriate.
</p>

<p>
  On the producer side of the interface,
  the new <a href="/pkg/embed/#FS">embed.FS</code></a> type
  implements <code>fs.FS</code>, as does
  <a href="/pkg/archive/zip/#Reader"><code>zip.Reader</code></a>.
  The new <a href="/pkg/os/#DirFS"><code>os.DirFS</code></a> function
  provides an implementation of <code>fs.FS</code> backed by a tree
  of operating system files.
</p>

<p>
  On the consumer side,
  the new <a href="/pkg/net/http/#FS"><code>http.FS</code></a>
  function converts an <code>fs.FS</code> to an
  <a href="/pkg/net/http/#Handler"><code>http.Handler</code></a>.
  Also, the <a href="/pkg/html/template/"><code>html/template</code></a>
  and <a href="/pkg/text/template/"><code>text/template</code></a>
  packages’ <a href="/pkg/html/template/#ParseFS"><code>ParseFS</code></a>
  functions and methods read templates from an <code>fs.FS</code>.
</p>

<p>
  For testing code that implements <code>fs.FS</code>,
  the new <a href="/pkg/testing/fstest/"><code>testing/fstest</code></a>
  package provides a <a href="/pkg/testing/fstest/#TestFS"><code>TestFS</code></a>
  function that checks for and reports common mistakes.
  It also provides a simple in-memory file system implementation,
  <a href="/pkg/testing/fstest/#MapFS"><code>MapFS</code></a>,
  which can be useful for testing code that accepts <code>fs.FS</code>
  implementations.
</p>

<p>
  TODO: when the "Minor changes to the library" section is close to completion,
  decide if any changes are worth factoring out and highlighting in "Core library"
</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, resolve TODOs below, add missing entries
</p>

<dl id="crypto/dsa"><dt><a href="/pkg/crypto/dsa/">crypto/dsa</a></dt>
  <dd>
    <p><!-- CL 257939 -->
      The <a href="/pkg/crypto/dsa/"><code>crypto/dsa</code></a> package is now deprecated.
      See <a href="https://golang.org/issue/40337">issue #40337</a>.
    </p>
  </dd>
</dl><!-- crypto/dsa -->

<dl id="crypto/hmac"><dt><a href="/pkg/crypto/hmac/">crypto/hmac</a></dt>
  <dd>
    <p><!-- CL 261960 -->
      <a href="/pkg/crypto/hmac/#New"><code>New</code></a> will now panic if
      separate calls to the hash generation function fail to return new values.
      Previously, the behavior was undefined and invalid outputs were sometimes
      generated.
    </p>
  </dd>
</dl><!-- crypto/hmac -->

<dl id="crypto/tls"><dt><a href="/pkg/crypto/tls/">crypto/tls</a></dt>
  <dd>
    <p><!-- CL 256897 -->
      I/O operations on closing or closed TLS connections can now be detected
      using the new <a href="/pkg/net/#ErrClosed"><code>net.ErrClosed</code></a>
      error. A typical use would be <code>errors.Is(err, net.ErrClosed)</code>.
    </p>

    <p><!-- CL 266037 -->
      A default write deadline is now set in
      <a href="/pkg/crypto/tls/#Conn.Close"><code>Conn.Close</code></a>
      before sending the "close notify" alert, in order to prevent blocking
      indefinitely.
    </p>

    <p><!-- CL 246338 -->
      The new <a href="/pkg/crypto/tls#Conn.HandshakeContext"><code>Conn.HandshakeContext</code></a>
      method allows cancellation of an in-progress handshake. The provided
      context is accessible through the new
      <a href="/pkg/crypto/tls#ClientHelloInfo.Context"><code>ClientHelloInfo.Context</code></a>
      and <a href="/pkg/crypto/tls#CertificateRequestInfo.Context">
      <code>CertificateRequestInfo.Context</code></a> methods. Canceling the
      context after the handshake has finished has no effect.
    </p>

    <p><!-- CL 239748 -->
      Clients now return a handshake error if the server selects
      <a href="/pkg/crypto/tls/#ConnectionState.NegotiatedProtocol">
      an ALPN protocol</a> that was not in
      <a href="/pkg/crypto/tls/#Config.NextProtos">
      the list advertised by the client</a>.
    </p>

    <p><!-- CL 262857 -->
      Servers will now prefer other available AEAD cipher suites (such as ChaCha20Poly1305)
      over AES-GCM cipher suites if either the client or server doesn't have AES hardware
      support, unless both <a href="/pkg/crypto/tls/#Config.PreferServerCipherSuites">
      <code>Config.PreferServerCipherSuites</code></a>
      and <a href="/pkg/crypto/tls/#Config.CipherSuites"><code>Config.CipherSuites</code></a>
      are set. The client is assumed not to have AES hardware support if it does
      not signal a preference for AES-GCM cipher suites.
    </p>

    <p><!-- CL 246637 -->
      <a href="/pkg/crypto/tls/#Config.Clone"><code>Config.Clone</code></a> now
      returns nil if the receiver is nil, rather than panicking.
    </p>
  </dd>
</dl><!-- crypto/tls -->

<dl id="crypto/x509"><dt><a href="/pkg/crypto/x509/">crypto/x509</a></dt>
  <dd>
    <p>
      The <code>GODEBUG=x509ignoreCN=0</code> flag will be removed in Go 1.17.
      It enables the legacy behavior of treating the <code>CommonName</code>
      field on X.509 certificates as a host name when no Subject Alternative
      Names are present.
    </p>

    <p><!-- CL 235078 -->
      <a href="/pkg/crypto/x509/#ParseCertificate"><code>ParseCertificate</code></a> and
      <a href="/pkg/crypto/x509/#CreateCertificate"><code>CreateCertificate</code></a>
      now enforce string encoding restrictions for the <code>DNSNames</code>,
      <code>EmailAddresses</code>, and <code>URIs</code> fields. These fields
      can only contain strings with characters within the ASCII range.
    </p>

    <p><!-- CL 259697 -->
      <a href="/pkg/crypto/x509/#CreateCertificate"><code>CreateCertificate</code></a>
      now verifies the generated certificate's signature using the signer's
      public key. If the signature is invalid, an error is returned, instead of
      a malformed certificate.
    </p>

    <p><!-- CL 233163 -->
      A number of additional fields have been added to the
      <a href="/pkg/crypto/x509/#CertificateRequest"><code>CertificateRequest</code></a> type.
      These fields are now parsed in <a href="/pkg/crypto/x509/#ParseCertificateRequest">
      <code>ParseCertificateRequest</code></a> and marshalled in
      <a href="/pkg/crypto/x509/#CreateCertificateRequest"><code>CreateCertificateRequest</code></a>.
    </p>

    <p><!-- CL 257939 -->
      DSA signature verification is no longer supported. Note that DSA signature
      generation was never supported.
      See <a href="https://golang.org/issue/40337">issue #40337</a>.
    </p>

    <p><!-- CL 257257 -->
      On Windows, <a href="/pkg/crypto/x509/#Certificate.Verify"><code>Certificate.Verify</code></a>
      will now return all certificate chains that are built by the platform
      certificate verifier, instead of just the highest ranked chain.
    </p>

    <p><!-- CL 262343 -->
      The new <a href="/pkg/crypto/x509/#SystemRootsError.Unwrap"><code>SystemRootsError.Unwrap</code></a>
      method allows accessing the <a href="/pkg/crypto/x509/#SystemRootsError.Err"><code>Err</code></a>
      field through the <a href="/pkg/errors"><code>errors</code></a> package functions.
    </p>
  </dd>
</dl><!-- crypto/x509 -->

<dl id="encoding/asn1"><dt><a href="/pkg/encoding/asn1">encoding/asn1</a></dt>
  <dd>
    <p><!-- CL 255881 -->
      <a href="/pkg/encoding/asn1/#Unmarshal"><code>Unmarshal</code></a> and
      <a href="/pkg/encoding/asn1/#UnmarshalWithParams"><code>UnmarshalWithParams</code></a>
      now return an error instead of panicking when the argument is not
      a pointer or is nil. This change matches the behavior of other
      encoding packages such as <a href="/pkg/encoding/json"><code>encoding/json</code></a>.
    </p>
  </dd>
</dl>

<dl id="encoding/json"><dt><a href="/pkg/encoding/json/">encoding/json</a></dt>
  <dd>
    <p><!-- CL 234818 -->
      The <code>json</code> struct field tags understood by
      <a href="/pkg/encoding/json/#Marshal"><code>Marshal</code></a>,
      <a href="/pkg/encoding/json/#Unmarshal"><code>Unmarshal</code></a>,
      and related functionality now permit semicolon characters within
      a JSON object name for a Go struct field.
    </p>
  </dd>
</dl><!-- encoding/json -->

<dl id="encoding/xml"><dt><a href="/pkg/encoding/xml/">encoding/xml</a></dt>
  <dd>
    <p><!-- CL 264024 -->
      The encoder has always taken care to avoid using namespace prefixes
      beginning with <code>xml</code>, which are reserved by the XML
      specification.
      Now, following the specification more closely, that check is
      case-insensitive, so that prefixes beginning
      with <code>XML</code>, <code>XmL</code>, and so on are also
      avoided.
    </p>
  </dd>
</dl><!-- encoding/xml -->

<dl id="flag"><dt><a href="/pkg/flag/">flag</a></dt>
  <dd>
    <p><!-- CL 240014 -->
      The new <a href="/pkg/flag/#Func"><code>Func</code></a> function
      allows registering a flag implemented by calling a function,
      as a lighter-weight alternative to implementing the
      <a href="/pkg/flag/#Value"><code>Value</code></a> interface.
    </p>
  </dd>
</dl><!-- flag -->

<dl id="io"><dt><a href="/pkg/io/">io</a></dt>
  <dd>
    <p><!-- CL 261577 -->
      The package now defines a
      <a href="/pkg/io/#ReadSeekCloser"><code>ReadSeekCloser</code></a> interface.
    </p>
  </dd>
</dl><!-- io -->

<dl id="log"><dt><a href="/pkg/log/">log</a></dt>
  <dd>
    <p><!-- CL 264460 -->
      The new <a href="/pkg/log/#Default"><code>Default</code></a> function
      provides access to the default <a href="/pkg/log/#Logger"><code>Logger</code></a>.
    </p>
  </dd>
</dl><!-- log -->

<dl id="log/syslog"><dt><a href="/pkg/log/syslog/">log/syslog</a></dt>
  <dd>
    <p><!-- CL 264297 -->
      The <a href="/pkg/log/syslog/#Writer"><code>Writer</code></a>
      now uses the local message format
      (omitting the host name and using a shorter time stamp)
      when logging to custom Unix domain sockets,
      matching the format already used for the default log socket.
    </p>
  </dd>
</dl><!-- log/syslog -->

<dl id="mime/multipart"><dt><a href="/pkg/mime/multipart/">mime/multipart</a></dt>
  <dd>
    <p><!-- CL 247477 -->
      The <a href="/pkg/mime/multipart/#Reader"><code>Reader</code></a>'s
      <a href="/pkg/mime/multipart/#Reader.ReadForm"><code>ReadForm</code></a>
      method no longer rejects form data
      when passed the maximum int64 value as a limit.
    </p>
  </dd>
</dl><!-- mime/multipart -->

<dl id="net"><dt><a href="/pkg/net/">net</a></dt>
  <dd>
    <p><!-- CL 250357 -->
      The case of I/O on a closed network connection, or I/O on a network
      connection that is closed before any of the I/O completes, can now
      be detected using the new <a href="/pkg/net/#ErrClosed">ErrClosed</a> error.
      A typical use would be <code>errors.Is(err, net.ErrClosed)</code>.
      In earlier releases the only way to reliably detect this case was to
      match the string returned by the <code>Error</code> method
      with <code>"use of closed network connection"</code>.
    </p>

    <p><!-- CL 255898 -->
      In previous Go releases the default TCP listener backlog size on Linux systems,
      set by <code>/proc/sys/net/core/somaxconn</code>, was limited to a maximum of <code>65535</code>.
      On Linux kernel version 4.1 and above, the maximum is now <code>4294967295</code>.
    </p>

    <p><!-- CL 238629 -->
      On Linux, host name lookups no longer use DNS before checking
      <code>/etc/hosts</code> when <code>/etc/nsswitch.conf</code>
      is missing; this is common on musl-based systems and makes
      Go programs match the behavior of C programs on those systems.
    </p>
  </dd>
</dl><!-- net -->

<dl id="net/http"><dt><a href="/pkg/net/http/">net/http</a></dt>
  <dd>
    <p><!-- CL 233637 -->
      In the <a href="/pkg/net/http/"><code>net/http</code></a> package, the
      behavior of <a href="/pkg/net/http/#StripPrefix"><code>StripPrefix</code></a>
      has been changed to strip the prefix from the request URL's
      <code>RawPath</code> field in addition to its <code>Path</code> field.
      In past releases, only the <code>Path</code> field was trimmed, and so if the
      request URL contained any escaped characters the URL would be modified to
      have mismatched <code>Path</code> and <code>RawPath</code> fields.
      In Go 1.16, <code>StripPrefix</code> trims both fields.
      If there are escaped characters in the prefix part of the request URL the
      handler serves a 404 instead of its previous behavior of invoking the
      underlying handler with a mismatched <code>Path</code>/<code>RawPath</code> pair.
    </p>

    <p><!-- CL 252497 -->
      The <a href="/pkg/net/http/"><code>net/http</code></a> package now rejects HTTP range requests
      of the form <code>"Range": "bytes=--N"</code> where <code>"-N"</code> is a negative suffix length, for
      example <code>"Range": "bytes=--2"</code>. It now replies with a <code>416 "Range Not Satisfiable"</code> response.
    </p>

    <p><!-- CL 256498, golang.org/issue/36990 -->
      Cookies set with <a href="/pkg/net/http/#SameSiteDefaultMode"><code>SameSiteDefaultMode</code></a>
      now behave according to the current spec (no attribute is set) instead of
      generating a SameSite key without a value.
    </p>

    <p><!-- CL 246338 -->
      The <a href="/pkg/net/http/"><code>net/http</code></a> package now passes the
      <a href="/pkg/net/http/#Request.Context"><code>Request</code> context</a> to
      <a href="/pkg/crypto/tls#Conn.HandshakeContext"><code>tls.Conn.HandshakeContext</code></a>
      when performing TLS handshakes.
    </p>

    <p><!-- CL 250039 -->
      The <a href="/pkg/net/http/#Client">Client</a> now sends
      an explicit <code>Content-Length:</code> <code>0</code>
      header in <code>PATCH</code> requests with empty bodies,
      matching the existing behavior of <code>POST</code> and <code>PUT</code>.
    </p>

    <p><!-- CL 249440 -->
      The <a href="/pkg/net/http/#ProxyFromEnvironment">ProxyFromEnvironment</a> function
      no longer returns the setting of the <code>HTTP_PROXY</code> environment
      variable for <code>https://</code> URLs when <code>HTTPS_PROXY</code> is unset.
    </p>
  </dd>
</dl><!-- net/http -->

<dl id="net/http/httputil"><dt><a href="/pkg/net/http/httputil/">net/http/httputil</a></dt>
  <dd>
    <p><!-- CL 260637 -->
      The <a href="/pkg/net/http/httputil/#ReverseProxy">ReverseProxy</a>
      now flushes buffered data more aggressively when proxying
      streamed responses with unknown body lengths.
    </p>
  </dd>
</dl><!-- net/http/httputil -->

<dl id="net/smtp"><dt><a href="/pkg/net/smtp/">net/smtp</a></dt>
  <dd>
    <p><!-- CL 247257 -->
      The <a href="/pkg/net/smtp/#Client">Client</a>'s
      <a href="/pkg/net/smtp/#Client.Mail"><code>Mail</code></a>
      method now sends the <code>SMTPUTF8</code> directive to
      servers that support it, signaling that addresses are encoded in UTF-8.
    </p>
  </dd>
</dl><!-- net/smtp -->

<dl id="os"><dt><a href="/pkg/os/">os</a></dt>
  <dd>
    <p><!-- CL 242998 -->
      <a href="/pkg/os/#Process.Signal"><code>Process.Signal</code></a> now
      returns <a href="/pkg/os/#ErrProcessDone"><code>ErrProcessDone</code></a>
      instead of the unexported <code>errFinished</code> when the process has
      already finished.
    </p>
  </dd>
</dl><!-- os -->

<dl id="os/signal"><dt><a href="/pkg/os/signal/">os/signal</a></dt>
  <dd>
    <p><!-- CL 219640 -->
      The new
      <a href="/pkg/os/signal/#NotifyContext"><code>NotifyContext</code></a>
      function allows creating contexts that are canceled upon arrival of
      specific signals.
    </p>
  </dd>
</dl><!-- os/signal -->

<dl id="path"><dt><a href="/pkg/path/">path</a></dt>
  <dd>
    <p><!-- CL 264397, golang.org/issues/28614 -->
      The <code>Match</code> and <code>Glob</code> functions now
      return an error if the unmatched part of the pattern has a
      syntax error. Previously, the functions returned early on a failed
      match, and thus did not report any later syntax error in the
      pattern.
    </p>
  </dd>
</dl><!-- path -->

<dl id="path/filepath"><dt><a href="/pkg/path/filepath/">path/filepath</a></dt>
  <dd>
    <p><!-- CL 264397, golang.org/issues/28614 -->
      The <code>Match</code> and <code>Glob</code> functions now
      return an error if the unmatched part of the pattern has a
      syntax error. Previously, the functions returned early on a failed
      match, and thus did not report any later syntax error in the
      pattern.
    </p>
  </dd>
</dl><!-- path/filepath -->

<dl id="reflect"><dt><a href="/pkg/reflect/">reflect</a></dt>
  <dd>
    <p><!-- CL 248341, golang.org/issues/40281 -->
      <code>StructTag</code> now allows multiple space-separated keys
      in key:value pairs, as in <code>`json xml:"field1"`</code>
      (equivalent to <code>`json:"field1" xml:"field1"`</code>).
    </p>
  </dd>
</dl><!-- reflect -->

<dl id="runtime/debug"><dt><a href="/pkg/runtime/debug/">runtime/debug</a></dt>
  <dd>
    <p><!-- CL 249677 -->
      The <a href="/pkg/runtime#Error"><code>runtime.Error</code></a> values
      used when <code>SetPanicOnFault</code> is enabled may now have an
      <code>Addr</code> method. If that method exists, it returns the memory
      address that triggered the fault.
    </p>
  </dd>
</dl><!-- runtime/debug -->

<dl id="strconv"><dt><a href="/pkg/strconv/">strconv</a></dt>
  <dd>
    <p><!-- CL 260858 -->
      <a href="/pkg/strconv/#ParseFloat"><code>ParseFloat</code></a> now uses
      the <a
      href="https://nigeltao.github.io/blog/2020/eisel-lemire.html">Eisel-Lemire
      algorithm</a>, improving performance by up to a factor of 2. This can
      also speed up decoding textual formats like <a
      href="/pkg/encoding/json/"><code>encoding/json</code></a>.
    </p>
  </dd>
</dl><!-- strconv -->

<dl id="syscall"><dt><a href="/pkg/syscall/">syscall</a></dt>
  <dd>
    <p><!-- CL 263271 -->
      <a href="/pkg/syscall/?GOOS=windows#NewCallback"><code>NewCallback</code></a>
      and
      <a href="/pkg/syscall/?GOOS=windows#NewCallbackCDecl"><code>NewCallbackCDecl</code></a>
      now correctly support callback functions with multiple
      sub-<code>uintptr</code>-sized arguments in a row. This may
      require changing uses of these functions to eliminate manual
      padding between small arguments.
    </p>

    <p><!-- CL 261917 -->
      <a href="/pkg/syscall/?GOOS=windows#SysProcAttr"><code>SysProcAttr</code></a> on Windows has a new NoInheritHandles field that disables inheriting handles when creating a new process.
    </p>

    <p><!-- CL 269761, golang.org/issue/42584 -->
      <a href="/pkg/syscall/?GOOS=windows#DLLError"><code>DLLError</code></a> on Windows now has an Unwrap function for unwrapping its underlying error.
    </p>

    <p><!-- CL 210639 -->
      On Linux,
      <a href="/pkg/syscall/#Setgid"><code>Setgid</code></a>,
      <a href="/pkg/syscall/#Setuid"><code>Setuid</code></a>,
      and related calls are now implemented.
      Previously, they returned an <code>syscall.EOPNOTSUPP</code> error.
    </p>
  </dd>
</dl><!-- syscall -->

<dl id="text/template"><dt><a href="/pkg/text/template/">text/template</a></dt>
  <dd>
    <p><!-- CL 254257, golang.org/issue/29770 -->
      Newlines characters are now allowed inside action delimiters,
      permitting actions to span multiple lines.
    </p>
  </dd>
</dl><!-- text/template -->

<dl id="text/template/parse"><dt><a href="/pkg/text/template/parse/">text/template/parse</a></dt>
  <dd>
    <p><!-- CL 229398, golang.org/issue/34652 -->
      A new <a href="/pkg/text/template/parse/#CommentNode"><code>CommentNode</code></a>
      was added to the parse tree. The <a href="/pkg/text/template/parse/#Mode"><code>Mode</code></a>
      field in the <code>parse.Tree</code> enables access to it.
    </p>
  </dd>
</dl><!-- text/template/parse -->

<dl id="time/tzdata"><dt><a href="/pkg/time/tzdata/">time/tzdata</a></dt>
  <dd>
    <p><!-- CL 261877 -->
      The slim timezone data format is now used for the timezone database in
      <code>$GOROOT/lib/time/zoneinfo.zip</code> and the embedded copy in this
      package. This reduces the size of the timezone database by about 350 KB.
    </p>
  </dd>
</dl><!-- time/tzdata -->

<dl id="unicode"><dt><a href="/pkg/unicode/">unicode</a></dt>
  <dd>
    <p><!-- CL 248765 -->
      The <a href="/pkg/unicode/"><code>unicode</code></a> package and associated
      support throughout the system has been upgraded from Unicode 12.0.0 to
      <a href="https://www.unicode.org/versions/Unicode13.0.0/">Unicode 13.0.0</a>,
      which adds 5,930 new characters, including four new scripts, and 55 new emoji.
      Unicode 13.0.0 also designates plane 3 (U+30000-U+3FFFF) as the tertiary
      ideographic plane.
    </p>
  </dd>
</dl><!-- unicode -->