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: The prefix 0b or 0B indicates a binary integer literal such as 0b1011 .

or indicates a binary integer literal such as . Octal integer literals: The prefix 0o or 0O indicates an octal integer literal such as 0o660 . The existing octal notation indicated by a leading 0 followed by octal digits remains valid.

or indicates an octal integer literal such as . The existing octal notation indicated by a leading followed by octal digits remains valid. Hexadecimal floating point literals: The prefix 0x or 0X may 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 p or P followed by an exponent in decimal. The exponent scales the mantissa by 2 to the power of the exponent.

or may now be used to express the mantissa of a floating-point number in hexadecimal format such as . A hexadecimal floating-point number must always have an exponent, written as the letter or followed by an exponent in decimal. The exponent scales the mantissa by 2 to the power of the exponent. Imaginary literals: The imaginary suffix i may now be used with any (binary, decimal, hexadecimal) integer or floating-point literal.

may now be used with any (binary, decimal, hexadecimal) integer or floating-point literal. Digit separators: The digits of any number literal may now be separated (grouped) using underscores, such as in 1_000_000 , 0b_1010_0110 , or 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 uint conversions, solely introduced to satisfy this (now removed) restriction of the << and >> operators.

These language changes were implemented by changes to the compiler, and corresponding internal changes to the library packages go/scanner and text/scanner (number literals), and 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 .

Ports

Go 1.13 is the last release that will run on Native Client (NaCl).

For 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

AIX on PPC64 ( aix/ppc64 ) now supports cgo, external linking, and the c-archive and pie build modes.

Android

Go programs are now compatible with Android 10.

Darwin

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.

FreeBSD

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 COMPAT_FREEBSD11 option set (this is the default).

Illumos

Go now supports Illumos with GOOS=illumos . The illumos build tag implies the solaris build tag.

Windows

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.

Modules

Environment variables

The GO111MODULE environment variable continues to default to auto , but the auto setting now activates the module-aware mode of the 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.

The new GOPRIVATE environment variable indicates module paths that are not publicly available. It serves as the default value for the lower-level GONOPROXY and GONOSUMDB variables, which provide finer-grained control over which modules are fetched via proxy and verified using the checksum database.

The GOPROXY environment variable may now be set to a comma-separated list of proxy URLs or the special token direct , and its default value is now 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.

The new GOSUMDB 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 go.sum file. If GOSUMDB does not include an explicit URL, the URL is chosen by probing the 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 to 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 setting GOPROXY to direct , and/or GOSUMDB to off . go env -w can be used to set the default values for these variables independent of platform:

go env -w GOPROXY=direct go env -w GOSUMDB=off

go get

In module-aware mode, go get with the -u flag now updates a smaller set of modules that is more consistent with the set of packages updated by go get -u in GOPATH mode. go get -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 named packages.

Note in particular that go get -u (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 go get -u all .

As a result of the above changes to go get -u , the go get subcommand no longer supports the -m flag, which caused go get to stop before loading packages. The -d flag remains supported, and continues to cause go get to stop after downloading the source code needed to build dependencies of the named packages.

By default, go get -u in module mode upgrades only non-test dependencies, as in GOPATH mode. It now also accepts the -t flag, which (as in GOPATH mode) causes go get to include the packages imported by tests of the packages named on the command line.

In module-aware mode, the go get subcommand now supports the version suffix @patch . The @patch 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 go get 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 behavior. @latest explicitly requests the latest version regardless of the current version.

Version validation

When extracting a module from a version control system, the go command now performs additional validation on the requested version string.

The +incompatible version annotation bypasses the requirement of semantic import versioning for repositories that predate the introduction of modules. The go command now verifies that such a version does not include an explicit go.mod file.

The 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.

, 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 what the go command would generate. (For SHA-1 hashes as used by git , a 12-digit prefix.)

If a 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 as go list -m all or go mod 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 the 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. For example,

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

Go command

The go env 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 go env -w are stored in the go/env file within os.UserConfigDir() .

The go version command now accepts arguments naming executables and directories. When invoked on an executable, go version prints the version of Go used to build the executable. If the -m flag is used, go version prints the executable's embedded module version information, if available. When invoked on a directory, go version prints information about executables contained in the directory and its subdirectories.

The new go build flag -trimpath removes all file system paths from the compiled executable, to improve build reproducibility.

If the -o flag passed to go build refers to an existing directory, go build will now write executable files within that directory for main packages matching its package arguments.

go 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.

Compiler toolchain

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 violates the unsafe.Pointer safety rules). If you notice any regressions that appear related, the old escape analysis pass can be re-enabled with go build -gcflags=all=-newescape=false . The option to use the old escape analysis will be removed in a future release.

The compiler no longer emits floating point or complex constants to go_asm.h files. These have always been emitted in a form that could not be used as numeric constant in assembly code.

Assembler

The assembler now supports many of the atomic instructions introduced in ARM v8.1.

gofmt

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 becomes 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 starting with 0 is considered a decimal, not an octal number. Removing superfluous leading zeroes avoids potential confusion.) For instance, 0B1010 , 0XabcDEF , 0O660 , 1.2E3 , and 01i become 0b1010 , 0xabcDEF , 0o660 , 1.2e3 , and 1i after applying gofmt .

godoc and go doc

The 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

The go doc 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.

Runtime

Out of range panic messages now include the index that was out of bounds and the length (or capacity) of the slice. For example, s[3] on a slice of length 1 will panic with "runtime error: index out of range [3] with length 1".

This release improves performance of most uses of defer by 30%.

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.

Core library

TLS 1.3

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 value tls13=0 to the GODEBUG environment variable. The opt-out will be removed in Go 1.14.

See the Go 1.12 release notes for important compatibility information.

The new crypto/ed25519 package implements the Ed25519 signature scheme. This functionality was previously provided by the golang.org/x/crypto/ed25519 package, which becomes a wrapper for crypto/ed25519 when used with Go 1.13+.

Error wrapping

Go 1.13 contains support for error wrapping, as first proposed in the Error Values proposal and discussed on the associated issue.

An error e can wrap another error w by providing an Unwrap method that returns w . Both e and 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 w .

To support wrapping, fmt.Errorf now has a %w verb for creating wrapped errors, and three new functions in the errors package ( errors.Unwrap , errors.Is and errors.As ) simplify unwrapping and inspecting wrapped errors.

For more information, read the errors package documentation, or see the Error Value FAQ. There will soon be a blog post as well.

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.

bytes The new ToValidUTF8 function returns a copy of a given byte slice with each run of invalid UTF-8 byte sequences replaced by a given slice.

context The formatting of contexts returned by WithValue no longer depends on fmt and will not stringify in the same way. Code that depends on the exact previous stringification might be affected.

crypto/tls Support for SSL version 3.0 (SSLv3) is now deprecated and will be removed in Go 1.14. Note that SSLv3 is the cryptographically broken protocol predating TLS. 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.

crypto/x509 Ed25519 keys are now supported in certificates and certificate requests according to RFC 8410, as well as by the ParsePKCS8PrivateKey , MarshalPKCS8PrivateKey , and ParsePKIXPublicKey functions. The paths searched for system roots now include /etc/ssl/cert.pem to support the default location in Alpine Linux 3.7+.

database/sql The new NullTime type represents a time.Time that may be null. The new NullInt32 type represents an int32 that may be null.

debug/dwarf The Data.Type method no longer panics if it encounters an unknown DWARF tag in the type graph. Instead, it represents that component of the type with an UnsupportedType object.

errors The new function As finds 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 Is reports whether a given error value matches an error in another’s chain. The new function Unwrap returns the result of calling Unwrap on a given error, if one exists.

fmt The printing verbs %x and %X now format floating-point and complex numbers in hexadecimal notation, in lower-case and upper-case respectively. The new printing verb %O formats integers in base 8, emitting the 0o prefix. The scanner now accepts hexadecimal floating-point values, digit-separating underscores and leading 0b and 0o prefixes. See the Changes to the language for details. The Errorf function has a new verb, %w , whose operand must be an error. The error returned from Errorf will have an Unwrap method which returns the operand of %w .

go/scanner The scanner has been updated to recognize the new Go number literals, specifically binary literals with 0b / 0B prefix, octal literals with 0o / 0O prefix, and floating-point numbers with hexadecimal mantissa. The imaginary suffix i may now be used with any number literal, and underscores may used as digit separators for grouping. See the Changes to the language for details.

go/types The type-checker has been updated to follow the new rules for integer shifts. See the Changes to the language for details.

html/template When using a <script> tag with "module" set as the type attribute, code will now be interpreted as JavaScript module script.

log The new Writer function returns the output destination for the standard logger.

math/bits The execution time of Add , Sub , Mul , RotateLeft , and ReverseBytes is now guaranteed to be independent of the inputs.

net On Unix systems where use-vc is set in resolv.conf , TCP is used for DNS resolution. The new field ListenConfig.KeepAlive specifies 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 Timeout method that returns true if called. This can make a keep-alive error difficult to distinguish from an error returned due to a missed deadline as set by the SetDeadline method and similar methods. Code that uses deadlines and checks for them with the Timeout method or with os.IsTimeout may 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.

net/http The new fields Transport.WriteBufferSize and Transport.ReadBufferSize allow 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.ForceAttemptHTTP2 controls whether HTTP/2 is enabled when a non-zero Dial , DialTLS , or DialContext func or TLSClientConfig is provided. Transport.MaxConnsPerHost now works properly with HTTP/2. TimeoutHandler 's ResponseWriter now implements the Pusher interface. The StatusCode 103 "Early Hints" has been added. Transport now uses the Request.Body 's io.ReaderFrom implementation if available, to optimize writing the body. On encountering unsupported transfer-encodings, http.Server now returns a "501 Unimplemented" status as mandated by the HTTP specification RFC 7230 Section 3.3.1. The new Server fields BaseContext and ConnContext allow finer control over the Context values provided to requests and connections. http.DetectContentType now correctly detects RAR signatures, and can now also detect RAR v5 signatures. The new Header method Clone returns a copy of the receiver. A new function NewRequestWithContext has been added and it accepts a Context that controls the entire lifetime of the created outgoing Request , suitable for use with Client.Do and Transport.RoundTrip . The Transport no longer logs errors when servers gracefully shut down idle connections using a "408 Request Timeout" response.

os The new UserConfigDir function returns the default directory to use for user-specific configuration data. If a File is opened using the O_APPEND flag, its WriteAt method will always return an error.

os/exec On Windows, the environment for a Cmd always inherits the %SYSTEMROOT% value of the parent process unless the Cmd.Env field includes an explicit value for it.

reflect The new Value.IsZero method reports whether a Value is the zero value for its type. The MakeFunc function 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.

runtime Tracebacks, runtime.Caller , and runtime.Callers now refer to the function that initializes the global variables of PKG as PKG.init instead of PKG.init.ializers .

strings The new ToValidUTF8 function returns a copy of a given string with each run of invalid UTF-8 byte sequences replaced by a given string.

sync The fast paths of Mutex.Lock , Mutex.Unlock , RWMutex.Lock , RWMutex.RUnlock , and Once.Do are now inlined in their callers. For the uncontended cases on amd64, these changes make Once.Do twice as fast, and the Mutex / RWMutex methods up to 10% faster. Large Pool no longer increase stop-the-world pause times. Pool no 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 Pool .

syscall Uses of _getdirentries64 have been removed from Darwin builds, to allow Go binaries to be uploaded to the macOS App Store. The new ProcessAttributes and ThreadAttributes fields in SysProcAttr have been introduced for Windows, exposing security settings when creating new processes. EINVAL is no longer returned in zero Chmod mode on Windows. Values of type Errno can be tested against error values in the os package, like ErrExist , using errors.Is .

syscall/js TypedArrayOf has been replaced by CopyBytesToGo and CopyBytesToJS for copying bytes between a byte slice and a Uint8Array .

testing When running benchmarks, B.N is no longer rounded. The new method B.ReportMetric lets users report custom benchmark metrics and override built-in metrics. Testing flags are now registered in the new Init function, which is invoked by the generated main function for the test. As a result, testing flags are now only registered when running a test binary, and packages that call flag.Parse during package initialization may cause tests to fail.

text/scanner The scanner has been updated to recognize the new Go number literals, specifically binary literals with 0b / 0B prefix, octal literals with 0o / 0O prefix, and floating-point numbers with hexadecimal mantissa. Also, the new AllowDigitSeparators mode allows number literals to contain underscores as digit separators (off by default for backwards-compatibility). See the Changes to the language for details.

text/template The new slice function returns the result of slicing its first argument by the following arguments.

time Day-of-year is now supported by Format and Parse . The new Duration methods Microseconds and Milliseconds return the duration as an integer count of their respectively named units.