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.
There are no changes to the language.
Go 1.16 adds support of 64-bit ARM architecture on macOS (also known as
Apple Silicon) with GOOS=darwin, GOARCH=arm64.
Like the darwin/amd64 port, the darwin/arm64
port supports cgo, internal and external linking, c-archive,
c-shared, and pie build modes, and the race
detector.
The iOS port, which was previously darwin/arm64, has
been renamed to ios/arm64. GOOS=ios
implies the
darwin build tag, just as GOOS=android
implies the linux build tag. This change should be
transparent to anyone using gomobile to build iOS apps.
Go 1.16 adds an ios/amd64 port, which targets the iOS
simulator running on AMD64-based macOS. Previously this was
unofficially supported through darwin/amd64 with
the ios build tag set. See also
misc/ios/README for
details about how to build programs for iOS and iOS simulator.
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.
Go now supports the 64-bit ARM architecture on NetBSD (the
netbsd/arm64 port).
Go now supports the MIPS64 architecture on OpenBSD
(the openbsd/mips64 port). This port does not yet
support cgo.
On the 64-bit x86 and 64-bit ARM architectures on OpenBSD (the
openbsd/amd64 and openbsd/arm64 ports), system
calls are now made through libc, instead of directly using
the SYSCALL/SVC instruction. This ensures
forward-compatibility with future versions of OpenBSD. In particular,
OpenBSD 6.9 onwards will require system calls to be made through
libc for non-static Go binaries.
As announced in the Go 1.15 release notes,
Go 1.16 drops support for x87 mode compilation (GO386=387).
Support for non-SSE2 processors is now available using soft float
mode (GO386=softfloat).
Users running on non-SSE2 processors should replace GO386=387
with GO386=softfloat.
The linux/riscv64 port now supports cgo and
-buildmode=pie. This release also includes performance
optimizations and code generation improvements for RISC-V.
Module-aware mode is enabled by default, regardless of whether a
go.mod file is present in the current working directory or a
parent directory. More precisely, the GO111MODULE environment
variable now defaults to on. To switch to the previous behavior,
set GO111MODULE to auto.
Build commands like go build and go
test no longer modify go.mod and go.sum
by default. Instead, they report an error if a module requirement or checksum
needs to be added or updated (as if the -mod=readonly flag were
used). Module requirements and sums may be adjusted with go
mod tidy or go get.
go install now accepts arguments with
version suffixes (for example, go install
example.com/cmd@v1.0.0). This causes go
install to build and install packages in module-aware mode,
ignoring the go.mod 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.
go install, with or without a version suffix (as
described above), is now the recommended way to build and install packages in
module mode. go get should be used with the
-d flag to adjust the current module's dependencies without
building packages, and use of go get to build and
install packages is deprecated. In a future release, the -d flag
will always be enabled.
retract directives may now be used in a go.mod 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.
The go mod vendor
and go mod tidy subcommands now accept
the -e flag, which instructs them to proceed despite errors in
resolving missing packages.
The go command now ignores requirements on module versions
excluded by exclude directives in the main module. Previously,
the go command used the next version higher than an excluded
version, but that version could change over time, resulting in
non-reproducible builds.
In module mode, the go command now disallows import paths that
include non-ASCII characters or path elements with a leading dot character
(.). Module paths with these characters were already disallowed
(see Module paths and versions),
so this change affects only paths within module subdirectories.
The go command now supports including
static files and file trees as part of the final executable,
using the new //go:embed directive.
See the documentation for the new
embed
package for details.
go test
When using go test, a test that
calls os.Exit(0) 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
os.Exit(0) and thereby stops running all future tests.
If a TestMain function calls os.Exit(0)
that is still considered to be a passing test.
go test reports an error when the -c
or -i flags are used together with unknown flags. Normally,
unknown flags are passed to tests, but when -c or -i
are used, tests are not run.
go get
The go get -insecure 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
GOINSECURE environment variable instead. To bypass module
sum validation, use GOPRIVATE or GONOSUMDB.
See go help environment for details.
go get example.com/mod@patch now
requires that some version of example.com/mod already be
required by the main module.
(However, go get -u=patch continues
to patch even newly-added dependencies.)
GOVCS environment variable
GOVCS is a new environment variable that limits which version
control tools the go command may use to download source code.
This mitigates security issues with tools that are typically used in trusted,
authenticated environments. By default, git and hg
may be used to download code from any repository. svn,
bzr, and fossil may only be used to download code
from repositories with module paths or package paths matching patterns in
the GOPRIVATE environment variable. See
go
help vcs for details.
all pattern
When the main module's go.mod file
declares go 1.16 or higher, the all
package pattern now matches only those packages that are transitively imported
by a package or test found in the main module. (Packages imported by tests
of packages imported by the main module are no longer included.) This is
the same set of packages retained
by go mod vendor since Go 1.11.
-toolexec build flag
When the -toolexec build flag is specified to use a program when
invoking toolchain programs like compile or asm, the environment variable
TOOLEXEC_IMPORTPATH is now set to the import path of the package
being built.
-i build flag
The -i flag accepted by go build,
go install, and go test is
now deprecated. The -i flag instructs the go command
to install packages imported by packages named on the command line. Since
the build cache was introduced in Go 1.10, the -i flag no longer
has a significant effect on build times, and it causes errors when the install
directory is not writable.
list command
When the -export flag is specified, the BuildID
field is now set to the build ID of the compiled package. This is equivalent
to running go tool buildid on
go list -exported -f {{.Export}},
but without the extra step.
-overlay flag
The -overlay flag specifies a JSON configuration file containing
a set of file path replacements. The -overlay flag may be used
with all build commands and go mod subcommands.
It is primarily intended to be used by editor tooling such as gopls to
understand the effects of unsaved changes to source files. The config file
maps actual file paths to replacement file paths and the go
command and its builds will run as if the actual file paths exist with the
contents given by the replacement file paths, or don't exist if the replacement
file paths are empty.
The cgo 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.
The vet tool now warns about invalid calls to the testing.T
method Fatal from within a goroutine created during the test.
This also warns on calls to Fatalf, FailNow, and
Skip{,f,Now} methods on testing.T tests or
testing.B benchmarks.
Calls to these methods stop the execution of the created goroutine and not
the Test* or Benchmark* function. So these are
required to be called by the goroutine
running the test or benchmark function. For example:
func TestFoo(t *testing.T) {
go func() {
if condition() {
t.Fatal("oops") // This exits the inner func instead of TestFoo.
}
...
}()
}
Code calling t.Fatal (or a similar method) from a created
goroutine should be rewritten to signal the test failure using
t.Error and exit the goroutine early using an alternative
method, such as using a return statement. The previous example
could be rewritten as:
func TestFoo(t *testing.T) {
go func() {
if condition() {
t.Error("oops")
return
}
...
}()
}
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 CL 248260 for example fixes.
The vet tool now warns about incorrectly passing a non-pointer or nil argument to
asn1.Unmarshal.
This is like the existing checks for
encoding/json.Unmarshal
and encoding/xml.Unmarshal.
The new runtime/metrics package
introduces a stable interface for reading
implementation-defined metrics from the Go runtime.
It supersedes existing functions like
runtime.ReadMemStats
and
debug.GCStats
and is significantly more general and efficient.
See the package documentation for more details.
Setting the GODEBUG environment variable
to inittrace=1 now causes the runtime to emit a single
line to standard error for each package init,
summarizing its execution time and memory allocation. This trace can
be used to find bottlenecks or regressions in Go startup
performance.
The GODEBUG
documentation describes the format.
On Linux, the runtime now defaults to releasing memory to the
operating system promptly (using MADV_DONTNEED), rather
than lazily when the operating system is under memory pressure
(using MADV_FREE). 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 GODEBUG=madvdontneed=1 to improve
memory monitoring behavior no longer need to set this environment
variable.
Go 1.16 fixes a discrepancy between the race detector and the Go memory model. 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.
The compiler can now inline functions with
non-labeled for loops, method values, and type
switches. The inliner can also detect more indirect calls where
inlining is possible.
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 modernize the Go linker.
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 ELF-based OSes and
amd64 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 linux/amd64, with larger
improvements for other architectures and OSes. Most binaries are
also smaller as a result of more aggressive symbol pruning.
On Windows, go build -buildmode=c-shared now generates Windows
ASLR DLLs by default. ASLR can be disabled with --ldflags=-aslr=false.
The new embed package
provides access to files embedded in the program during compilation
using the new //go:embed directive.
The new io/fs package
defines the fs.FS interface,
an abstraction for read-only trees of files.
The standard library packages have been adapted to make use
of the interface as appropriate.
On the producer side of the interface,
the new embed.FS type
implements fs.FS, as does
zip.Reader.
The new os.DirFS function
provides an implementation of fs.FS backed by a tree
of operating system files.
On the consumer side,
the new http.FS
function converts an fs.FS to an
http.Handler.
Also, the html/template
and text/template
packages’ ParseFS
functions and methods read templates from an fs.FS.
For testing code that implements fs.FS,
the new testing/fstest
package provides a TestFS
function that checks for and reports common mistakes.
It also provides a simple in-memory file system implementation,
MapFS,
which can be useful for testing code that accepts fs.FS
implementations.
The io/ioutil package has
turned out to be a poorly defined and hard to understand collection
of things. All functionality provided by the package has been moved
to other packages. The io/ioutil package remains and
will continue to work as before, but we encourage new code to use
the new definitions in the io and
os packages.
Here is a list of the new locations of the names exported
by io/ioutil:
Discard
=> io.DiscardNopCloser
=> io.NopCloserReadAll
=> io.ReadAllReadDir
=> os.ReadDir
(note: returns a slice of
os.DirEntry
rather than a slice of
fs.FileInfo)
ReadFile
=> os.ReadFileTempDir
=> os.MkdirTempTempFile
=> os.CreateTempWriteFile
=> os.WriteFileAs always, there are various minor changes and updates to the library, made with the Go 1 promise of compatibility in mind.
The new Reader.Open
method implements the fs.FS
interface.
The crypto/dsa package is now deprecated.
See issue #40337.
New 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.
I/O operations on closing or closed TLS connections can now be detected
using the new net.ErrClosed
error. A typical use would be errors.Is(err, net.ErrClosed).
A default write deadline is now set in
Conn.Close
before sending the "close notify" alert, in order to prevent blocking
indefinitely.
Clients now return a handshake error if the server selects an ALPN protocol that was not in the list advertised by the client.
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
Config.PreferServerCipherSuites
and Config.CipherSuites
are set. The client is assumed not to have AES hardware support if it does
not signal a preference for AES-GCM cipher suites.
Config.Clone now
returns nil if the receiver is nil, rather than panicking.
The GODEBUG=x509ignoreCN=0 flag will be removed in Go 1.17.
It enables the legacy behavior of treating the CommonName
field on X.509 certificates as a host name when no Subject Alternative
Names are present.
ParseCertificate and
CreateCertificate
now enforce string encoding restrictions for the DNSNames,
EmailAddresses, and URIs fields. These fields
can only contain strings with characters within the ASCII range.
CreateCertificate
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.
DSA signature verification is no longer supported. Note that DSA signature generation was never supported. See issue #40337.
On Windows, Certificate.Verify
will now return all certificate chains that are built by the platform
certificate verifier, instead of just the highest ranked chain.
The new SystemRootsError.Unwrap
method allows accessing the Err
field through the errors package functions.
On Unix systems, the crypto/x509 package is now more
efficient in how it stores its copy of the system cert pool.
Programs that use only a small number of roots will use around a
half megabyte less memory.
Unmarshal and
UnmarshalWithParams
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 encoding/json.
The json struct field tags understood by
Marshal,
Unmarshal,
and related functionality now permit semicolon characters within
a JSON object name for a Go struct field.
The encoder has always taken care to avoid using namespace prefixes
beginning with xml, which are reserved by the XML
specification.
Now, following the specification more closely, that check is
case-insensitive, so that prefixes beginning
with XML, XmL, and so on are also
avoided.
The new Func function
allows registering a flag implemented by calling a function,
as a lighter-weight alternative to implementing the
Value interface.
The Package
struct has new fields that report information
about //go:embed directives in the package:
EmbedPatterns,
EmbedPatternPos,
TestEmbedPatterns,
TestEmbedPatternPos,
XTestEmbedPatterns,
XTestEmbedPatternPos.
The Package field
IgnoredGoFiles
will no longer include files that start with "_" or ".",
as those files are always ignored.
IgnoredGoFiles is for files ignored because of
build constraints.
The new Package
field IgnoredOtherFiles
has a list of non-Go files ignored because of build constraints.
The new
go/build/constraint
package parses build constraint lines, both the original
// +build syntax and the //go:build
syntax that will be introduced in Go 1.17.
This package exists so that tools built with Go 1.16 will be able
to process Go 1.17 source code.
See https://golang.org/design/draft-gobuild
for details about the build constraint syntaxes and the planned
transition to the //go:build syntax.
Note that //go:build lines are not supported
in Go 1.16 and should not be introduced into Go programs yet.
The new template.ParseFS
function and template.Template.ParseFS
method are like template.ParseGlob
and template.Template.ParseGlob,
but read the templates from an fs.FS.
The package now defines a
ReadSeekCloser interface.
The package now defines
Discard,
NopCloser, and
ReadAll,
to be used instead of the same names in the
io/ioutil package.
The Writer
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.
The Reader's
ReadForm
method no longer rejects form data
when passed the maximum int64 value as a limit.
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 ErrClosed
error. A typical use would be errors.Is(err, net.ErrClosed).
In earlier releases the only way to reliably detect this case was to
match the string returned by the Error method
with "use of closed network connection".
In previous Go releases the default TCP listener backlog size on Linux systems,
set by /proc/sys/net/core/somaxconn, was limited to a maximum of 65535.
On Linux kernel version 4.1 and above, the maximum is now 4294967295.
On Linux, host name lookups no longer use DNS before checking
/etc/hosts when /etc/nsswitch.conf
is missing; this is common on musl-based systems and makes
Go programs match the behavior of C programs on those systems.
In the net/http package, the
behavior of StripPrefix
has been changed to strip the prefix from the request URL's
RawPath field in addition to its Path field.
In past releases, only the Path field was trimmed, and so if the
request URL contained any escaped characters the URL would be modified to
have mismatched Path and RawPath fields.
In Go 1.16, StripPrefix 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 Path/RawPath pair.
The net/http package now rejects HTTP range requests
of the form "Range": "bytes=--N" where "-N" is a negative suffix length, for
example "Range": "bytes=--2". It now replies with a 416 "Range Not Satisfiable" response.
Cookies set with SameSiteDefaultMode
now behave according to the current spec (no attribute is set) instead of
generating a SameSite key without a value.
The Client now sends
an explicit Content-Length: 0
header in PATCH requests with empty bodies,
matching the existing behavior of POST and PUT.
The ProxyFromEnvironment
function no longer returns the setting of the HTTP_PROXY
environment variable for https:// URLs when
HTTPS_PROXY is unset.
The Transport
type has a new field
GetProxyConnectHeader
which may be set to a function that returns headers to send to a
proxy during a CONNECT request.
In effect GetProxyConnectHeader is a dynamic
version of the existing field
ProxyConnectHeader;
if GetProxyConnectHeader is not nil,
then ProxyConnectHeader is ignored.
The new http.FS
function converts an fs.FS
to an http.Handler.
ReverseProxy
now flushes buffered data more aggressively when proxying
streamed responses with unknown body lengths.
The Client's
Mail
method now sends the SMTPUTF8 directive to
servers that support it, signaling that addresses are encoded in UTF-8.
Process.Signal now
returns ErrProcessDone
instead of the unexported errFinished when the process has
already finished.
The package defines a new type
DirEntry
as an alias for fs.DirEntry.
The new ReadDir
function and the new
File.ReadDir
method can be used to read the contents of a directory into a
slice of DirEntry.
The File.Readdir
method (note the lower case d in dir)
still exists, returning a slice of
FileInfo, but for
most programs it will be more efficient to switch to
File.ReadDir.
The package now defines
CreateTemp,
MkdirTemp,
ReadFile, and
WriteFile,
to be used instead of functions defined in the
io/ioutil package.
The types FileInfo,
FileMode, and
PathError
are now aliases for types of the same name in the
io/fs package.
Function signatures in the os
package have been updated to refer to the names in the
io/fs package.
This should not affect any existing code.
The new DirFS function
provides an implementation of
fs.FS backed by a tree
of operating system files.
The new
NotifyContext
function allows creating contexts that are canceled upon arrival of
specific signals.
The Match function now
returns an error if the unmatched part of the pattern has a
syntax error. Previously, the function returned early on a failed
match, and thus did not report any later syntax error in the
pattern.
The new function
WalkDir
is similar to
Walk,
but is typically more efficient.
The function passed to WalkDir receives a
fs.DirEntry
instead of a
fs.FileInfo.
(To clarify for those who recall the Walk function
as taking an os.FileInfo,
os.FileInfo is now an alias for fs.FileInfo.)
The Match and
Glob 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.
The runtime.Error values
used when SetPanicOnFault is enabled may now have an
Addr method. If that method exists, it returns the memory
address that triggered the fault.
ParseFloat now uses
the Eisel-Lemire
algorithm, improving performance by up to a factor of 2. This can
also speed up decoding textual formats like encoding/json.
NewCallback
and
NewCallbackCDecl
now correctly support callback functions with multiple
sub-uintptr-sized arguments in a row. This may
require changing uses of these functions to eliminate manual
padding between small arguments.
SysProcAttr on Windows has a new NoInheritHandles field that disables inheriting handles when creating a new process.
DLLError on Windows now has an Unwrap method for unwrapping its underlying error.
On Linux,
Setgid,
Setuid,
and related calls are now implemented.
Previously, they returned an syscall.EOPNOTSUPP error.
On Linux, the new functions
AllThreadsSyscall
and AllThreadsSyscall6
may be used to make a system call on all Go threads in the process.
These functions may only be used by programs that do not use cgo;
if a program uses cgo, they will always return
syscall.ENOTSUP.
The new
ErrReader
function returns an
io.Reader that always
returns an error.
The new
TestReader
function tests that an io.Reader
behaves correctly.
Newlines characters are now allowed inside action delimiters, permitting actions to span multiple lines.
The new template.ParseFS
function and template.Template.ParseFS
method are like template.ParseGlob
and template.Template.ParseGlob,
but read the templates from an fs.FS.
A new CommentNode
was added to the parse tree. The Mode
field in the parse.Tree enables access to it.
The slim timezone data format is now used for the timezone database in
$GOROOT/lib/time/zoneinfo.zip and the embedded copy in this
package. This reduces the size of the timezone database by about 350 KB.
The unicode package and associated
support throughout the system has been upgraded from Unicode 12.0.0 to
Unicode 13.0.0,
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.