## 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.

- Conversions from slice to array pointer: An expression `s` of type `[]T` may now be converted to array pointer type `*[N]T`. If `a` is the result of such a conversion, then corresponding indices that are in range refer to the same underlying elements: `&a[i] == &s[i]` for `0 <= i < N`. The conversion panics if `len(s)` is less than `N`.
- `unsafe.Add`: `unsafe.Add(ptr, len)` adds `len` to `ptr` and returns the updated pointer `unsafe.Pointer(uintptr(ptr) + uintptr(len))`.
- `unsafe.Slice`: For expression `ptr` of type `*T`, `unsafe.Slice(ptr, len)` returns a slice of type `[]T` whose underlying array starts at `ptr` and whose length and capacity are `len`.

These 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.

## Ports

### Darwin

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.

### Windows

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

### OpenBSD

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 forward-compatibility with future versions of OpenBSD, in particular, with OpenBSD 6.9 onwards, which requires system calls to be made through `libc` for non-static Go binaries.

### ARM64

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.

## Tools

### 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 spsecifies `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 https://golang.org/design/draft-gobuild.

### `gofmt`

`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 https://golang.org/design/draft-gobuild.

### Vet

#### 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 https://golang.org/design/draft-gobuild.

#### 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.
    }

### Cover

The `cover` tool now uses an optimized parser from `golang.org/x/tools/cover`, which may be noticeably faster when parsing large coverage profiles.

## Compiler

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. Results (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

### Cgo

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.

archive/zip

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

bufio

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

bytes

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

compress/lzw

  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`.

crypto/ed25519

  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.

crypto/elliptic

  `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.

crypto/rand

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

crypto/tls

  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.

crypto/x509

  `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+.

database/sql

  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.

debug/elf

  The `SHT_MIPS_ABIFLAGS` constant has been added.

encoding/binary

  `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.

encoding/csv

  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

  Flag declarations now panic if an invalid name is specified.

go/build

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

go/format

  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 https://golang.org/design/draft-gobuild.

io/fs

  The new `FileInfoToDirEntry` function converts a `FileInfo` to a `DirEntry`.

math

  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.

mime

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

net

  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-encoded values after validation, which avoids this class of parser misalignment issues.

net/http

  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.

net/http/httptest

  `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.

net/url

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

os

  The `File.WriteString` method has been optimized to no longer make a copy of the input string.

reflect

  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.

runtime/metrics

  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.

runtime/pprof

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

strconv

  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.

strings

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

sync/atomic

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

syscall

  The `GetQueuedCompletionStatus` and `PostQueuedCompletionStatus` functions are now deprecated. These functions have incorrect signatures and are superseded by equivalents in the `golang.org/x/sys/windows` 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).

testing

  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.

text/template/parse

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

time

  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.

unicode

  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.