Go 1.13 Release Notes
Introduction to Go 1.13
The latest Go release, version 1.13, arrives six months after Go 1.12. Most of its changes are in the implementation of the toolchain, runtime, and libraries. As always, the release maintains the Go 1 promise of compatibility. We expect almost all Go programs to continue to compile and run as before.
As of Go 1.13, the go command by default downloads and authenticates modules using the Go module mirror and Go checksum database run by Google. See https://proxy.golang.org/privacy for privacy information about these services and the go command documentation for configuration details including how to disable the use of these servers or use different ones. If you depend on non-public modules, see the documentation for configuring your environment.
Changes to the language
Per the number literal proposal, Go 1.13 supports a more uniform and modernized set of number literal prefixes.
Binary integer literals:
0Bindicates a binary integer literal such as
Octal integer literals:
0Oindicates an octal integer literal such as
0o660. The existing octal notation indicated by a leading
0followed by octal digits remains valid.
Hexadecimal floating point literals:
0Xmay now be used to express the mantissa of a floating-point number in hexadecimal format such as
0x1.0p-1021. A hexadecimal floating-point number must always have an exponent, written as the letter
Pfollowed by an exponent in decimal. The exponent scales the mantissa by 2 to the power of the exponent.
The imaginary suffix
imay now be used with any (binary, decimal, hexadecimal) integer or floating-point literal.
The digits of any number literal may now be separated (grouped) using underscores, such as
3.1415_9265. An underscore may appear between any two digits or the literal prefix and the first digit.
Per the signed shift counts proposal
Go 1.13 removes the restriction that a shift count
must be unsigned. This change eliminates the need for many artificial
solely introduced to satisfy this (now removed) restriction of the
These language changes were implemented by changes to the compiler, and corresponding internal changes to the library
text/scanner (number literals),
go/types (signed shift counts).
If your code uses modules and your
go.mod files specifies a language version, be sure
it is set to at least
1.13 to get access to these language changes.
You can do this by editing the
go.mod file directly, or you can run
go mod edit -go=1.13.
Go 1.13 is the last release that will run on Native Client (NaCl).
GOARCH=wasm, the new environment variable
GOWASM takes a comma-separated list of experimental features that the binary gets compiled with.
The valid values are documented here.
AIX on PPC64 (
aix/ppc64) now supports cgo, external
linking, and the
Go programs are now compatible with Android 10.
As announced in the Go 1.12 release notes, Go 1.13 now requires macOS 10.11 El Capitan or later; support for previous versions has been discontinued.
As announced in the Go 1.12 release notes,
Go 1.13 now requires FreeBSD 11.2 or later;
support for previous versions has been discontinued.
FreeBSD 12.0 or later requires a kernel with the
option set (this is the default).
Go now supports Illumos with
illumos build tag implies the
The Windows version specified by internally-linked Windows binaries is now Windows 7 rather than NT 4.0. This was already the minimum required version for Go, but can affect the behavior of system calls that have a backwards-compatibility mode. These will now behave as documented. Externally-linked binaries (any program using cgo) have always specified a more recent Windows version.
environment variable continues to default to
auto setting now activates the module-aware mode of
go command whenever the current working directory contains,
or is below a directory containing, a
go.mod file — even if the
current directory is within
GOPATH/src. This change simplifies
the migration of existing code within
GOPATH/src and the ongoing
maintenance of module-aware packages alongside non-module-aware importers.
environment variable indicates module paths that are not publicly available.
It serves as the default value for the lower-level
GONOSUMDB variables, which provide finer-grained control over
which modules are fetched via proxy and verified using the checksum database.
environment variable may now be set to a comma-separated list of proxy
URLs or the special token
its default value is
https://proxy.golang.org,direct. When resolving a package
path to its containing module, the
go command will try all
candidate module paths on each proxy in the list in succession. An unreachable
proxy or HTTP status code other than 404 or 410 terminates the search without
consulting the remaining proxies.
environment variable identifies the name, and optionally the public key and
server URL, of the database to consult for checksums of modules that are not
yet listed in the main module's
GOSUMDB does not include an explicit URL, the URL is chosen by
GOPROXY URLs for an endpoint indicating support for
the checksum database, falling back to a direct connection to the named
database if it is not supported by any proxy. If
GOSUMDB is set
off, the checksum database is not consulted and only the
existing checksums in the
go.sum file are verified.
Users who cannot reach the default proxy and checksum database (for example,
due to a firewalled or sandboxed configuration) may disable their use by
can be used to set the default values for these variables independent of
go env -w GOPROXY=direct go env -w GOSUMDB=off
In module-aware mode,
-u flag now updates a smaller set of modules that is
more consistent with the set of packages updated by
-u in GOPATH mode.
-u continues to update the
modules and packages named on the command line, but additionally updates only
the modules containing the packages imported by the named packages,
rather than the transitive module requirements of the modules containing the
Note in particular that
(without additional arguments) now updates only the transitive imports of the
package in the current directory. To instead update all of the packages
transitively imported by the main module (including test dependencies), use
As a result of the above changes to
get subcommand no longer supports
-m flag, which caused
stop before loading packages. The
-d flag remains supported, and
continues to cause
get to stop after downloading
the source code needed to build dependencies of the named packages.
-u in module mode
upgrades only non-test dependencies, as in GOPATH mode. It now also accepts
-t flag, which (as in GOPATH mode)
get to include the packages imported
by tests of the packages named on the command line.
In module-aware mode, the
get subcommand now
supports the version suffix
suffix indicates that the named module, or module containing the named
package, should be updated to the highest patch release with the same
major and minor versions as the version found in the build list.
If a module passed as an argument to
without a version suffix is already required at a newer version than the
latest released version, it will remain at the newer version. This is
consistent with the behavior of the
-u flag for module
dependencies. This prevents unexpected downgrades from pre-release versions.
The new version suffix
@upgrade explicitly requests this
@latest explicitly requests the latest version
regardless of the current version.
When extracting a module from a version control system, the
command now performs additional validation on the requested version string.
+incompatible version annotation bypasses the requirement
import versioning for repositories that predate the introduction of
go command now verifies that such a version does not
include an explicit
go command now verifies the mapping
between pseudo-versions and
version-control metadata. Specifically:
- The version prefix must be of the form
vX.0.0, or derived from a tag on an ancestor of the named revision, or derived from a tag that includes build metadata on the named revision itself.
- The date string must match the UTC timestamp of the revision.
- The short name of the revision must use the same number of characters as
gocommand would generate. (For SHA-1 hashes as used by
git, a 12-digit prefix.)
require directive in the
main module uses
an invalid pseudo-version, it can usually be corrected by redacting the
version to just the commit hash and re-running a
go command, such
tidy. For example,
require github.com/docker/docker v1.14.0-0.20190319215453-e7b5f7dbe98c
can be redacted to
require github.com/docker/docker e7b5f7dbe98c
which currently resolves to
require github.com/docker/docker v0.7.3-0.20190319215453-e7b5f7dbe98c
If one of the transitive dependencies of the main module requires an invalid
version or pseudo-version, the invalid version can be replaced with a valid
one using a
replace directive in
go.mod file of the main module. If the replacement is a
commit hash, it will be resolved to the appropriate pseudo-version as above.
replace github.com/docker/docker v1.14.0-0.20190319215453-e7b5f7dbe98c => github.com/docker/docker e7b5f7dbe98c
currently resolves to
replace github.com/docker/docker v1.14.0-0.20190319215453-e7b5f7dbe98c => github.com/docker/docker v0.7.3-0.20190319215453-e7b5f7dbe98c
command now accepts a
-w flag to set the per-user default value
of an environment variable recognized by the
go command, and a corresponding
-u flag to unset a
previously-set default. Defaults set via
-w are stored in the
go/env file within
version command now accepts arguments naming
executables and directories. When invoked on an executable,
version prints the version of Go used to build
the executable. If the
-m flag is used,
version prints the executable's embedded module
version information, if available. When invoked on a directory,
version prints information about executables
contained in the directory and its subdirectories.
-trimpath removes all file system paths
from the compiled executable, to improve build reproducibility.
-o flag passed to
refers to an existing directory,
build will now
write executable files within that directory for
matching its package arguments.
generate now sets the
generate build tag so that
files may be searched for directives but ignored during build.
As announced in the Go 1.12 release
notes, binary-only packages are no longer supported. Building a binary-only
package (marked with a
//go:binary-only-package comment) now
results in an error.
The compiler has a new implementation of escape analysis that is
more precise. For most Go code should be an improvement (in other
words, more Go variables and expressions allocated on the stack
instead of heap). However, this increased precision may also break
invalid code that happened to work before (for example, code that
safety rules). If you notice any regressions that appear
related, the old escape analysis pass can be re-enabled
The option to use the old escape analysis will be removed in a
The compiler no longer emits floating point or complex constants
go_asm.h files. These have always been emitted in a
form that could not be used as numeric constant in assembly code.
The assembler now supports many of the atomic instructions introduced in ARM v8.1.
gofmt (and with that
go fmt) now canonicalizes
number literal prefixes and exponents to use lower-case letters, but
leaves hexadecimal digits alone. This improves readability when using the new octal prefix
0o), and the rewrite is applied consistently.
gofmt now also removes unnecessary leading zeroes from a decimal integer
imaginary literal. (For backwards-compatibility, an integer imaginary literal
0 is considered a decimal, not an octal number.
Removing superfluous leading zeroes avoids potential confusion.)
1i after applying
godoc webserver is no longer included in the main binary distribution.
To run the
godoc webserver locally, manually install it first:
go get golang.org/x/tools/cmd/godoc godoc
command now always includes the package clause in its output, except for
commands. This replaces the previous behavior where a heuristic was used,
causing the package clause to be omitted under certain conditions.
Out of range panic messages now include the index that was out of
bounds and the length (or capacity) of the slice. For
s on a slice of length 1 will panic with
"runtime error: index out of range  with length 1".
This release improves performance of most uses of
The runtime is now more aggressive at returning memory to the operating system to make it available to co-tenant applications. Previously, the runtime could retain memory for five or more minutes following a spike in the heap size. It will now begin returning it promptly after the heap shrinks. However, on many OSes, including Linux, the OS itself reclaims memory lazily, so process RSS will not decrease until the system is under memory pressure.
As announced in Go 1.12, Go 1.13 enables support for TLS 1.3 in the
crypto/tls package by default. It can be disabled by adding the
tls13=0 to the
environment variable. The opt-out will be removed in Go 1.14.
See the Go 1.12 release notes for important compatibility information.
package implements the Ed25519 signature
scheme. This functionality was previously provided by the
package, which becomes a wrapper for
crypto/ed25519 when used with Go 1.13+.
e can wrap another error
w by providing
Unwrap method that returns
w are available to programs, allowing
e to provide
additional context to
w or to reinterpret it while still allowing
programs to make decisions based on
To support wrapping,
fmt.Errorf now has a
verb for creating wrapped errors, and three new functions in
errors package (
errors.As) simplify unwrapping
and inspecting wrapped errors.
Minor changes to the library
As always, there are various minor changes and updates to the library, made with the Go 1 promise of compatibility in mind.
ToValidUTF8function returns a copy of a given byte slice with each run of invalid UTF-8 byte sequences replaced by a given slice.
The formatting of contexts returned by
WithValueno longer depends on
fmtand will not stringify in the same way. Code that depends on the exact previous stringification might be affected.
SSLv3 was always disabled by default, other than in Go 1.12, when it was mistakenly enabled by default server-side. It is now again disabled by default. (SSLv3 was never supported client-side.)
Ed25519 certificates are now supported in TLS versions 1.2 and 1.3.
The paths searched for system roots now include
/etc/ssl/cert.pemto support the default location in Alpine Linux 3.7+.
NullTimetype represents a
time.Timethat may be null.
NullInt32type represents an
int32that may be null.
The new function
Asfinds the first error in a given error’s chain (sequence of wrapped errors) that matches a given target’s type, and if so, sets the target to that error value.
The new function
Isreports whether a given error value matches an error in another’s chain.
The new function
Unwrapreturns the result of calling
Unwrapon a given error, if one exists.
The printing verbs
%Xnow format floating-point and complex numbers in hexadecimal notation, in lower-case and upper-case respectively.
The new printing verb
%Oformats integers in base 8, emitting the
The scanner now accepts hexadecimal floating-point values, digit-separating underscores and leading
0oprefixes. See the Changes to the language for details.
Errorffunction has a new verb,
%w, whose operand must be an error. The error returned from
Errorfwill have an
Unwrapmethod which returns the operand of
The scanner has been updated to recognize the new Go number literals, specifically binary literals with
0Bprefix, octal literals with
0Oprefix, and floating-point numbers with hexadecimal mantissa. The imaginary suffix
imay now be used with any number literal, and underscores may used as digit separators for grouping. See the Changes to the language for details.
The type-checker has been updated to follow the new rules for integer shifts. See the Changes to the language for details.
When using a
Rat.SetUint64method sets the
Rat.SetStringnow accepts non-decimal floating point representations.
On Unix systems where
use-vcis set in
resolv.conf, TCP is used for DNS resolution.
The new field
ListenConfig.KeepAlivespecifies the keep-alive period for network connections accepted by the listener. If this field is 0 (the default) TCP keep-alives will be enabled. To disable them, set it to a negative value.
Note that the error returned from I/O on a connection that was closed by a keep-alive timeout will have a
Timeoutmethod that returns
trueif called. This can make a keep-alive error difficult to distinguish from an error returned due to a missed deadline as set by the
SetDeadlinemethod and similar methods. Code that uses deadlines and checks for them with the
Timeoutmethod or with
os.IsTimeoutmay want to disable keep-alives, or use
errors.Is(syscall.ETIMEDOUT)(on Unix systems) which will return true for a keep-alive timeout and false for a deadline timeout.
The new fields
Transport.ReadBufferSizeallow one to specify the sizes of the write and read buffers for a
Transport. If either field is zero, a default size of 4KB is used.
The new field
Transport.ForceAttemptHTTP2controls whether HTTP/2 is enabled when a non-zero
Transport.MaxConnsPerHostnow works properly with HTTP/2.
"Early Hints"has been added.
http.DetectContentTypenow correctly detects RAR signatures, and can now also detect RAR v5 signatures.
A new function
NewRequestWithContexthas been added and it accepts a
Contextthat controls the entire lifetime of the created outgoing
Request, suitable for use with
Transportno longer logs errors when servers gracefully shut down idle connections using a
"408 Request Timeout"response.
UserConfigDirfunction returns the default directory to use for user-specific configuration data.
Value.IsZeromethod reports whether a
Valueis the zero value for its type.
MakeFuncfunction now allows assignment conversions on returned values, instead of requiring exact type match. This is particularly useful when the type being returned is an interface type, but the value actually returned is a concrete value implementing that type.
ToValidUTF8function returns a copy of a given string with each run of invalid UTF-8 byte sequences replaced by a given string.
The fast paths of
Once.Doare now inlined in their callers. For the uncontended cases on amd64, these changes make
Once.Dotwice as fast, and the
RWMutexmethods up to 10% faster.
Poolno longer increase stop-the-world pause times.
Poolno longer needs to be completely repopulated after every GC. It now retains some objects across GCs, as opposed to releasing all objects, reducing load spikes for heavy users of
_getdirentries64have been removed from Darwin builds, to allow Go binaries to be uploaded to the macOS App Store.
SysProcAttrhave been introduced for Windows, exposing security settings when creating new processes.
EINVALis no longer returned in zero
Chmodmode on Windows.
When running benchmarks,
B.Nis no longer rounded.
The new method
B.ReportMetriclets users report custom benchmark metrics and override built-in metrics.
Testing flags are now registered in the new
Initfunction, which is invoked by the generated
mainfunction for the test. As a result, testing flags are now only registered when running a test binary, and packages that call
flag.Parseduring package initialization may cause tests to fail.
The scanner has been updated to recognize the new Go number literals, specifically binary literals with
0Bprefix, octal literals with
0Oprefix, and floating-point numbers with hexadecimal mantissa. Also, the new
AllowDigitSeparatorsmode allows number literals to contain underscores as digit separators (off by default for backwards-compatibility). See the Changes to the language for details.
The new slice function returns the result of slicing its first argument by the following arguments.