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Go 1.17 Release Notes

DRAFT RELEASE NOTES — Introduction to Go 1.17

Go 1.17 is not yet released. These are work-in-progress release notes. Go 1.17 is expected to be released in August 2021.

Changes to the language

Go 1.17 includes three small enhancements to the language.

The package unsafe enhancements were added to simplify writing code that conforms to unsafe.Pointer's safety rules, but the rules remain unchanged. In particular, existing programs that correctly use unsafe.Pointer remain valid, and new programs must still follow the rules when using unsafe.Add or unsafe.Slice.

Note that the new conversion from slice to array pointer is the first case in which a type conversion can panic at run time. Analysis tools that assume type conversions can never panic should be updated to consider this possibility.



As announced in the Go 1.16 release notes, Go 1.17 requires macOS 10.13 High Sierra or later; support for previous versions has been discontinued.


Go 1.17 adds support of 64-bit ARM architecture on Windows (the windows/arm64 port). This port supports cgo.


The 64-bit MIPS architecture on OpenBSD (the openbsd/mips64 port) now supports cgo.

In Go 1.16, on the 64-bit x86 and 64-bit ARM architectures on OpenBSD (the openbsd/amd64 and openbsd/arm64 ports) system calls are made through libc, instead of directly using machine instructions. In Go 1.17, this is also done on the 32-bit x86 and 32-bit ARM architectures on OpenBSD (the openbsd/386 and openbsd/arm ports). This ensures compatibility with OpenBSD 6.9 onwards, which require system calls to be made through libc for non-static Go binaries.


Go programs now maintain stack frame pointers on the 64-bit ARM architecture on all operating systems. Previously it maintained stack frame pointers only on Linux, macOS, and iOS.


Go command

Lazy module loading

If a module specifies go 1.17 or higher in its go.mod file, its transitive requirements are now loaded lazily, avoiding the need to download or read go.mod files for otherwise-irrelevant dependencies. To support lazy loading, in Go 1.17 modules the go command maintains explicit requirements in the go.mod file for every dependency that provides any package transitively imported by any package or test within the module. See the design document for more detail.

Because the number of additional explicit requirements in the go.mod file may be substantial, in a Go 1.17 module the newly-added requirements on indirect dependencies are maintained in a separate require block from the block containing direct dependencies.

To facilitate the upgrade to lazy loading, the go mod tidy subcommand now supports a -go flag to set or change the go version in the go.mod file. To enable lazy loading for an existing module without changing the selected versions of its dependencies, run:

  go mod tidy -go=1.17

By default, go mod tidy verifies that the selected versions of dependencies relevant to the main module are the same versions that would be used by the prior Go release (Go 1.16 for a module that specifies go 1.17), and preserves the go.sum entries needed by that release even for dependencies that are not normally needed by other commands.

The -compat flag allows that version to be overridden to support older (or only newer) versions, up to the version specified by the go directive in the go.mod file. To tidy a go 1.17 module for Go 1.17 only, without saving checksums for (or checking for consistency with) Go 1.16:

  go mod tidy -compat=1.17

Note that even if the main module is tidied with -compat=1.17, users who require the module from a go 1.16 or earlier module will still be able to use it, provided that the packages use only compatible language and library features.

Module deprecation comments

Module authors may deprecate a module by adding a // Deprecated: comment to go.mod, then tagging a new version. go get now prints a warning if a module needed to build packages named on the command line is deprecated. go list -m -u prints deprecations for all dependencies (use -f or -json to show the full message). The go command considers different major versions to be distinct modules, so this mechanism may be used, for example, to provide users with migration instructions for a new major version.

go get

The go get -insecure flag is deprecated and has been removed. To permit the use of insecure schemes when fetching dependencies, please use the GOINSECURE environment variable. The -insecure flag also bypassed module sum validation, use GOPRIVATE or GONOSUMDB if you need that functionality. See go help environment for details.

go.mod files missing go directives

If the main module's go.mod file does not contain a go directive and the go command cannot update the go.mod file, the go command now assumes go 1.11 instead of the current release. (go mod init has added go directives automatically since Go 1.12.)

If a module dependency lacks an explicit go.mod file, or its go.mod file does not contain a go directive, the go command now assumes go 1.16 for that dependency instead of the current release. (Dependencies developed in GOPATH mode may lack a go.mod file, and the vendor/modules.txt has to date never recorded the go versions indicated by dependencies' go.mod files.)

vendor contents

If the main module specifies go 1.17 or higher, go mod vendor now annotates vendor/modules.txt with the go version indicated by each vendored module in its own go.mod file. The annotated version is used when building the module's packages from vendored source code.

If the main module specifies go 1.17 or higher, go mod vendor now omits go.mod and go.sum files for vendored dependencies, which can otherwise interfere with the ability of the go command to identify the correct module root when invoked within the vendor tree.

Password prompts

The go command by default now suppresses SSH password prompts and Git Credential Manager prompts when fetching Git repositories using SSH, as it already did previously for other Git password prompts. Users authenticating to private Git repos with password-protected SSH may configure an ssh-agent to enable the go command to use password-protected SSH keys.

go mod download

When go mod download is invoked without arguments, it will no longer save sums for downloaded module content to go.sum. It may still make changes to go.mod and go.sum needed to load the build list. This is the same as the behavior in Go 1.15. To save sums for all modules, use go mod download all.

//go:build lines

The go command now understands //go:build lines and prefers them over // +build lines. The new syntax uses boolean expressions, just like Go, and should be less error-prone. As of this release, the new syntax is fully supported, and all Go files should be updated to have both forms with the same meaning. To aid in migration, gofmt now automatically synchronizes the two forms. For more details on the syntax and migration plan, see

go run

go run now accepts arguments with version suffixes (for example, go run This causes go run to build and run 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 running executables without installing them or without changing dependencies of the current module.


gofmt (and go fmt) now synchronizes //go:build lines with // +build lines. If a file only has // +build lines, they will be moved to the appropriate location in the file, and matching //go:build lines will be added. Otherwise, // +build lines will be overwritten based on any existing //go:build lines. For more information, see


New warning for mismatched //go:build and // +build lines

The vet tool now verifies that //go:build and // +build lines are in the correct part of the file and synchronized with each other. If they aren't, gofmt can be used to fix them. For more information, see

New warning for calling signal.Notify on unbuffered channels

The vet tool now warns about calls to signal.Notify with incoming signals being sent to an unbuffered channel. Using an unbuffered channel risks missing signals sent on them as signal.Notify does not block when sending to a channel. For example:

c := make(chan os.Signal)
// signals are sent on c before the channel is read from.
// This signal may be dropped as c is unbuffered.
signal.Notify(c, os.Interrupt)

Users of signal.Notify should use channels with sufficient buffer space to keep up with the expected signal rate.

New warnings for Is, As and Unwrap methods

The vet tool now warns about methods named As, Is or Unwrap on types implementing the error interface that have a different signature than the one expected by the errors package. The errors.{As,Is,Unwrap} functions expect such methods to implement either Is(error) bool, As(interface{}) bool, or Unwrap() error respectively. The functions errors.{As,Is,Unwrap} will ignore methods with the same names but a different signature. For example:

type MyError struct { hint string }
func (m MyError) Error() string { ... } // MyError implements error.
func (MyError) Is(target interface{}) bool { ... } // target is interface{} instead of error.
func Foo() bool {
	x, y := MyError{"A"}, MyError{"B"}
	return errors.Is(x, y) // returns false as x != y and MyError does not have an `Is(error) bool` function.


The cover tool now uses an optimized parser from, which may be noticeably faster when parsing large coverage profiles.


Go 1.17 implements a new way of passing function arguments and results using registers instead of the stack. This work is enabled for Linux, MacOS, and Windows on the 64-bit x86 architecture (the linux/amd64, darwin/amd64, windows/amd64 ports). For a representative set of Go packages and programs, benchmarking has shown performance improvements of about 5%, and a typical reduction in binary size of about 2%.

This change does not affect the functionality of any safe Go code. It can affect code outside the compatibility guidelines with minimal impact. To maintain compatibility with existing assembly functions, adapter functions converting between the new register-based calling convention and the previous stack-based calling convention (also known as ABI wrappers) are sometimes used. This is mostly invisible to users, except for assembly functions that have their addresses taken in Go. Using reflect.ValueOf(fn).Pointer() (or similar approaches such as via unsafe.Pointer) to get the address of an assembly function will now return the address of the ABI wrapper. This is mostly harmless, except for special-purpose assembly code (such as accessing thread-local storage or requiring a special stack alignment). Assembly functions called indirectly from Go via func values will now be made through ABI wrappers, which may cause a very small performance overhead. Also, calling Go functions from assembly may now go through ABI wrappers, with a very small performance overhead.

The format of stack traces from the runtime (printed when an uncaught panic occurs, or when runtime.Stack is called) is improved. Previously, the function arguments were printed as hexadecimal words based on the memory layout. Now each argument in the source code is printed separately, separated by commas. Aggregate-typed (struct, array, string, slice, interface, and complex) arguments are delimited by curly braces. A caveat is that the value of an argument that only lives in a register and is not stored to memory may be inaccurate. Function return values (which were usually inaccurate) are no longer printed.

Functions containing closures can now be inlined. One effect of this change is that a function with a closure may actually produce a distinct closure function for each place that the function is inlined. Hence, this change could reveal bugs where Go functions are compared (incorrectly) by pointer value. Go functions are by definition not comparable.

Core library


The runtime/cgo package now provides a new facility that allows to turn any Go values to a safe representation that can be used to pass values between C and Go safely. See runtime/cgo.Handle for more information.

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.


The new methods File.OpenRaw, Writer.CreateRaw, Writer.Copy provide support for cases where performance is a primary concern.


The Writer.WriteRune method now writes the replacement character U+FFFD for negative rune values, as it does for other invalid runes.


The Buffer.WriteRune method now writes the replacement character U+FFFD for negative rune values, as it does for other invalid runes.


The NewReader function is guaranteed to return a value of the new type Reader, and similarly NewWriter is guaranteed to return a value of the new type Writer. These new types both implement a Reset method (Reader.Reset, Writer.Reset) that allows reuse of the Reader or Writer.


The crypto/ed25519 package has been rewritten, and all operations are now approximately twice as fast on amd64 and arm64. The observable behavior has not otherwise changed.


CurveParams methods now automatically invoke faster and safer dedicated implementations for known curves (P-224, P-256, and P-521) when available. Note that this is a best-effort approach and applications should avoid using the generic, not constant-time CurveParams methods and instead use dedicated Curve implementations such as P256.

The P521 curve implementation has been rewritten using code generated by the fiat-crypto project, which is based on a formally-verified model of the arithmetic operations. It is now constant-time and three times faster on amd64 and arm64. The observable behavior has not otherwise changed.


The crypto/rand package now uses the getentropy syscall on macOS and the getrandom syscall on Solaris, Illumos, and DragonFlyBSD.


The new Conn.HandshakeContext method allows the user to control cancellation of an in-progress TLS handshake. The provided context is accessible from various callbacks through the new ClientHelloInfo.Context and CertificateRequestInfo.Context methods. Canceling the context after the handshake has finished has no effect.

When Config.NextProtos is set, servers now enforce that there is an overlap between the configured protocols and the protocols advertised by the client, if any. If there is no overlap the connection is closed with the no_application_protocol alert, as required by RFC 7301.

Cipher suite ordering is now handled entirely by the crypto/tls package. Currently, cipher suites are sorted based on their security, performance, and hardware support taking into account both the local and peer's hardware. The order of the Config.CipherSuites field is now ignored, as well as the Config.PreferServerCipherSuites field. Note that Config.CipherSuites still allows applications to choose what TLS 1.0–1.2 cipher suites to enable.

The 3DES cipher suites have been moved to InsecureCipherSuites due to fundamental block size-related weakness. They are still enabled by default but only as a last resort, thanks to the cipher suite ordering change above.


CreateCertificate now returns an error if the provided private key doesn't match the parent's public key, if any. The resulting certificate would have failed to verify.

The temporary GODEBUG=x509ignoreCN=0 flag has been removed.

ParseCertificate has been rewritten, and now consumes ~70% fewer resources. The observable behavior has not otherwise changed, except for error messages.

On BSD systems, /etc/ssl/certs is now searched for trusted roots. This adds support for the new system trusted certificate store in FreeBSD 12.2+.


The DB.Close method now closes the connector field if the type in this field implements the io.Closer interface.

The new NullInt16 and NullByte structs represent the int16 and byte values that may be null. These can be used as destinations of the Scan method, similar to NullString.


The SHT_MIPS_ABIFLAGS constant has been added.


binary.Uvarint will stop reading after 10 bytes to avoid wasted computations. If more than 10 bytes are needed, the byte count returned is -11.
Previous Go versions could return larger negative counts when reading incorrectly encoded varints.


The new Reader.FieldPos method returns the line and column corresponding to the start of a given field in the record most recently returned by Read.


Flag declarations now panic if an invalid name is specified.


The new Context.ToolTags field holds the build tags appropriate to the current Go toolchain configuration.


The Source and Node functions now synchronize //go:build lines with // +build lines. If a file only has // +build lines, they will be moved to the appropriate location in the file, and matching //go:build lines will be added. Otherwise, // +build lines will be overwritten based on any existing //go:build lines. For more information, see


The concrete image types (RGBA, Gray16 and so on) now implement a new RGBA64Image interface. Those concrete types, other than the chroma-subsampling related YCbCr and NYCbCrA, also now implement draw.RGBA64Image, a new interface in the image/draw package.


The new FileInfoToDirEntry function converts a FileInfo to a DirEntry.


The math package now defines three more constants: MaxUint, MaxInt and MinInt. For 32-bit systems their values are 2^32 - 1, 2^31 - 1 and -2^31, respectively. For 64-bit systems their values are 2^64 - 1, 2^63 - 1 and -2^63, respectively.


On Unix systems, the table of MIME types is now read from the local system's Shared MIME-info Database when available.


The new method IP.IsPrivate reports whether an address is a private IPv4 address according to RFC 1918 or a local IPv6 address according RFC 4193.

The Go DNS resolver now only sends one DNS query when resolving an address for an IPv4-only or IPv6-only network, rather than querying for both address families.

The ErrClosed sentinel error and ParseError error type now implement the net.Error interface.

The ParseIP and ParseCIDR functions now reject IPv4 addresses which contain decimal components with leading zeros. These components were always interpreted as decimal, but some operating systems treat them as octal. This mismatch could hypothetically lead to security issues if a Go application was used to validate IP addresses which were then used in their original form with non-Go applications which interpreted components as octal. Generally, it is advisable to always re-encode values after validation, which avoids this class of parser misalignment issues.


The net/http package now uses the new (*tls.Conn).HandshakeContext with the Request context when performing TLS handshakes in the client or server.

Setting the Server ReadTimeout or WriteTimeout fields to a negative value now indicates no timeout rather than an immediate timeout.

The ReadRequest function now returns an error when the request has multiple Host headers.


ResponseRecorder.WriteHeader now panics when the provided code is not a valid three-digit HTTP status code. This matches the behavior of ResponseWriter implementations in the net/http package.


The new method Values.Has reports whether a query parameter is set.


The File.WriteString method has been optimized to not make a copy of the input string.


The new StructField.IsExported and Method.IsExported methods report whether a struct field or type method is exported. They provide a more readable alternative to checking whether PkgPath is empty.

The new VisibleFields function returns all the visible fields in a struct type, including fields inside anonymous struct members.

The ArrayOf function now panics when called with a negative length.

Checking the Type.ConvertibleTo method is no longer sufficient to guarantee that a call to Value.Convert will not panic. It may panic when converting `[]T` to `*[N]T` if the slice's length is less than N. See the language changes section above.


New metrics were added that track total bytes and objects allocated and freed. A new metric tracking the distribution of goroutine scheduling latencies was also added.


Block profiles are no longer biased to favor infrequent long events over frequent short events.


The strconv package now uses Ulf Adams's Ryū algorithm for formatting floating-point numbers. This algorithm improves performance on most inputs and is more than 99% faster on worst-case inputs.

The new QuotedPrefix function returns the quoted string (as understood by Unquote) at the start of input.


The Builder.WriteRune method now writes the replacement character U+FFFD for negative rune values, as it does for other invalid runes.


atomic.Value now has Swap and CompareAndSwap methods that provide additional atomic operations.


The GetQueuedCompletionStatus and PostQueuedCompletionStatus functions are now deprecated. These functions have incorrect signatures and are superseded by equivalents in the package.

On Unix-like systems, the process group of a child process is now set with signals blocked. This avoids sending a SIGTTOU to the child when the parent is in a background process group.

The Windows version of SysProcAttr has two new fields. AdditionalInheritedHandles is a list of additional handles to be inherited by the new child process. ParentProcess permits specifying the parent process of the new process.

The constant MSG_CMSG_CLOEXEC is now defined on DragonFly and all OpenBSD systems (it was already defined on some OpenBSD systems and all FreeBSD, NetBSD, and Linux systems).

The constants SYS_WAIT6 and WEXITED are now defined on NetBSD systems (SYS_WAIT6 was already defined on DragonFly and FreeBSD systems; WEXITED was already defined on Darwin, DragonFly, FreeBSD, Linux, and Solaris systems).


Added a new testing flag -shuffle which controls the execution order of tests and benchmarks.

The new T.Setenv and B.Setenv methods support setting an environment variable for the duration of the test or benchmark.


The new SkipFuncCheck Mode value changes the template parser to not verify that functions are defined.


The Time type now has a GoString method that will return a more useful value for times when printed with the %#v format specifier in the fmt package.

The new Time.IsDST method can be used to check whether the time is in Daylight Savings Time in its configured location.

The new Time.UnixMilli and Time.UnixMicro methods return the number of milliseconds and microseconds elapsed since January 1, 1970 UTC respectively.
The new UnixMilli and UnixMicro functions return local Time corresponding to given Unix time.

The package now accepts comma "," as a separator for fractional seconds when parsing and formatting time. The following time formats are now accepted:

  • 2006-01-02 14:06:03,999999999 -0700 MST
  • Mon Jan _2 14:06:03,120007 2006
  • Mon Jan 2 14:06:03,120007 2006

The new constant Layout defines the reference time.


The Is, IsGraphic, IsLetter, IsLower, IsMark, IsNumber, IsPrint, IsPunct, IsSpace, IsSymbol, and IsUpper functions now return false on negative rune values, as they do for other invalid runes.