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"Title": "Go 1.16 Release Notes",
"Path": "/doc/go1.16"
NOTE: In this document and others in this directory, the convention is to
set fixed-width phrases with non-fixed-width spaces, as in
<code>hello</code> <code>world</code>.
Do not send CLs removing the interior tags from such phrases.
main ul li { margin: 0.5em 0; }
<h2 id="introduction">DRAFT RELEASE NOTES — Introduction to Go 1.16</h2>
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.
<h2 id="language">Changes to the language</h2>
<h2 id="ports">Ports</h2>
<h3 id="darwin">Darwin</h3>
<p><!--,, many 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
The iOS port, which was previously <code>darwin/arm64</code>, is now
moved 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.
<p><!--, CL 263798 -->
The <code>ios/amd64</code> port is added, targetting the iOS simulator
running on AMD64-based macOS.
<h3 id="netbsd">NetBSD</h3>
<p><!-- -->
Go now supports the 64-bit ARM architecture on NetBSD (the
<code>netbsd/arm64</code> port).
<h3 id="386">386</h3>
<p><!--,, 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>.
<h2 id="tools">Tools</h2>
<h3 id="go-command">Go command</h3>
<h4 id="modules">Modules</h4>
<p><!-- -->
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><!-- -->
<code>go</code> <code>install</code> now accepts arguments with
version suffixes (for example, <code>go</code> <code>install</code>
<code></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.<br>
TODO: write and link to section in<br>
TODO: write and link to blog post
<p><!-- -->
<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><!-- -->
<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>
TODO: write and link to section in<br>
TODO: write and link to tutorial or blog post
<p><!-- -->
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.
<h4 id="go-test"><code>go</code> <code>test</code></h4>
<p><!-- -->
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><!-- -->
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 bypassess 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.
<h4 id="go-get"><code>go</code> <code>get</code></h4>
<p><!-- -->
<code>go</code> <code>get</code> <code></code> now
requires that some version of <code></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.)
<h4 id="all-pattern">The <code>all</code> pattern</h4>
<p><!-- -->
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.
<h4 id="toolexec">The <code>-toolexec</code> build flag</h4>
<p><!-- -->
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.
<h4 id="i-flag">The <code>-i</code> build flag</h4>
<p><!-- -->
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.
<h4 id="list-buildid">The <code>list</code> command</h4>
<p><!-- -->
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.
<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.
<h2 id="runtime">Runtime</h2>
<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
<h2 id="compiler">Compiler</h2>
<h2 id="linker">Linker</h2>
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="">modernize the Go
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 20-35% 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.
TODO: update with final numbers later in the release.
<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>.
<h2 id="library">Core library</h2>
<h3 id="crypto/hmac"><a href="/pkg/crypto/hmac">crypto/hmac</a></h3>
<p><!-- CL 261960 -->
<a href="/pkg/crypto/hmac/#New">New</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.
<h3 id="crypto/tls"><a href="/pkg/crypto/tls">crypto/tls</a></h3>
<p><!-- CL 256897 -->
I/O operations on closing or closed TLS connections 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>"tls: use of closed connection"</code>.
<p><!-- CL 266037 -->
A default deadline is set in <a href="/pkg/crypto/tls/#Conn.Close">Close</a>
before sending the close notify alert, in order to prevent blocking
<p><!-- CL 246338 -->
<a href="/pkg/crypto/tls#Conn.HandshakeContext">(*Conn).HandshakeContext</a> was added to
allow the user to control cancellation of an in-progress TLS Handshake.
The context provided is propagated into the
<a href="/pkg/crypto/tls#ClientHelloInfo">ClientHelloInfo</a>
and <a href="/pkg/crypto/tls#CertificateRequestInfo">CertificateRequestInfo</a>
structs and accessible through the new
<a href="/pkg/crypto/tls#ClientHelloInfo.Context">(*ClientHelloInfo).Context</a>
<a href="/pkg/crypto/tls#CertificateRequestInfo.Context">
</a> methods respectively. Canceling the context after the handshake has finished
has no effect.
<p><!-- CL 239748 -->
Clients now ensure that the server selects
<a href="/pkg/crypto/tls/#ConnectionState.NegotiatedProtocol">
an ALPN protocol</a> from
<a href="/pkg/crypto/tls/#Config.NextProtos">
the list advertised by the client</a>.
<p><!-- CL 262857 -->
TLS servers will now prefer other AEAD cipher suites (such as ChaCha20Poly1305)
over AES-GCM cipher suites if either the client or server doesn't have AES hardware
support, unless the application set 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>
or there are no other AEAD cipher suites supported.
The client is assumed not to have AES hardware support if it does not signal a
preference for AES-GCM cipher suites.
<h3 id="crypto/x509"><a href="/pkg/crypto/x509">crypto/x509</a></h3>
<p><!-- CL 235078 -->
<a href="/pkg/crypto/x509/#ParseCertificate">ParseCertificate</a> and
<a href="/pkg/crypto/x509/#CreateCertificate">CreateCertificate</a> both
now enforce string encoding restrictions for the fields <code>DNSNames</code>,
<code>EmailAddresses</code>, and <code>URIs</code>. These fields can only
contain strings with characters within the ASCII range.
<p><!-- CL 259697 -->
<a href="/pkg/crypto/x509/#CreateCertificate">CreateCertificate</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><!-- CL 233163 -->
A number of additional fields have been added to the
<a href="/pkg/crypto/x509/#CertificateRequest">CertificateRequest</a> type.
These fields are now parsed in <a href="/pkg/crypto/x509/#ParseCertificateRequest">ParseCertificateRequest</a>
and marshalled in <a href="/pkg/crypto/x509/#CreateCertificateRequest">CreateCertificateRequest</a>.
<h3 id="encoding/json"><a href="/pkg/encoding/json">encoding/json</a></h3>
<p><!-- CL 263619 -->
The error message for
<a href="/pkg/encoding/json/#SyntaxError">SyntaxError</a>
now begins with "json: ", matching the other errors in the package.
<h3 id="net"><a href="/pkg/net/">net</a></h3>
<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><!-- 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>.
<h3 id="text/template/parse"><a href="/pkg/text/template/parse/">text/template/parse</a></h3>
<p><!-- CL 229398, -->
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.
<!-- text/template/parse -->
<h3 id="unicode"><a href="/pkg/unicode/">unicode</a></h3>
<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="">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.
<h3 id="minor_library_changes">Minor changes to the library</h3>
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.
<dl id="crypto/dsa"><dt><a href="/pkg/crypto/dsa/">crypto/dsa</a></dt>
<p><!-- CL 257939 -->
The <a href="/pkg/crypto/dsa/"><code>crypto/dsa</code></a> package is now deprecated.
See <a href="">issue #40337</a>.
</dl><!-- crypto/dsa -->
<dl id="crypto/x509"><dt><a href="/pkg/crypto/x509/">crypto/x509</a></dt>
<p><!-- CL 257939 -->
DSA signature verification is no longer supported. Note that DSA signature
generation was never supported.
See <a href="">issue #40337</a>.
</dl><!-- crypto/x509 -->
<dl id="encoding/xml"><dt><a href="/pkg/encoding/xml/">encoding/xml</a></dt>
<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
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
</dl><!-- encoding/xml -->
<dl id="net/http"><dt><a href="/pkg/net/http/">net/http</a></dt>
<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><!-- 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><!-- CL 256498, -->
Cookies set with <code>SameSiteDefaultMode</code> now behave according to the current
spec (no attribute is set) instead of generating a SameSite key without a value.
<p><!-- CL 246338 -->
The <a href="/pkg/net/http/"><code>net/http</code></a> package now uses the new
<a href="/pkg/crypto/tls#Conn.HandshakeContext"><code>(*tls.Conn).HandshakeContext</code></a>
with the <a href="/pkg/net/http/#Request"><code>Request</code></a> context
when performing TLS handshakes in the client or server.
</dl><!-- net/http -->
<dl id="runtime/debug"><dt><a href="/pkg/runtime/debug/">runtime/debug</a></dt>
<p><!-- CL 249677 -->
TODO: <a href=""></a>: provide Addr method for errors from SetPanicOnFault
</dl><!-- runtime/debug -->
<dl id="syscall"><dt><a href="/pkg/syscall/">syscall</a></dt>
<p><!-- CL 261917 -->
<a href="/pkg/syscall/#SysProcAttr"><code>SysProcAttr</code></a> on Windows has a new NoInheritHandles field that disables inheriting handles when creating a new process.
</dl><!-- syscall -->
<dl id="strconv"><dt><a href="/pkg/strconv/">strconv</a></dt>
<p><!-- CL 260858 -->
<a href="/pkg/strconv/#ParseFloat"><code>ParseFloat</code></a> now uses
the <a
algorithm</a>, improving performance by up to a factor of 2. This can
also speed up decoding textual formats like <a
</dl><!-- strconv -->