## Introduction to Go 1.12

The latest Go release, version 1.12, arrives six months after Go 1.11. 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.

## Changes to the language

There are no changes to the language specification.

## Ports

The race detector is now supported on `linux/arm64`.

Go 1.12 is the last release that is supported on FreeBSD 10.x, which has already reached end-of-life. Go 1.13 will require FreeBSD 11.2+ or FreeBSD 12.0+. FreeBSD 12.0+ requires a kernel with the COMPAT_FREEBSD11 option set (this is the default).

cgo is now supported on `linux/ppc64`.

`hurd` is now a recognized value for `GOOS`, reserved for the GNU/Hurd system for use with `gccgo`.

### Windows

Go's new `windows/arm` port supports running Go on Windows 10 IoT Core on 32-bit ARM chips such as the Raspberry Pi 3.

### AIX

Go now supports AIX 7.2 and later on POWER8 architectures (`aix/ppc64`). External linking, cgo, pprof and the race detector aren't yet supported.

### Darwin

Go 1.12 is the last release that will run on macOS 10.10 Yosemite. Go 1.13 will require macOS 10.11 El Capitan or later.

`libSystem` is now used when making syscalls on Darwin, ensuring forward-compatibility with future versions of macOS and iOS. The switch to `libSystem` triggered additional App Store checks for private API usage. Since it is considered private, `syscall.Getdirentries` now always fails with `ENOSYS` on iOS.

## Tools

### `go tool vet` no longer supported

The `go vet` command has been rewritten to serve as the base for a range of different source code analysis tools. See the golang.org/x/tools/go/analysis package for details. A side-effect is that `go tool vet` is no longer supported. External tools that use `go tool vet` must be changed to use `go vet`. Using `go vet` instead of `go tool vet` should work with all supported versions of Go.

As part of this change, the experimental `-shadow` option is no longer available with `go vet`. Checking for variable shadowing may now be done using

    go get -u golang.org/x/tools/go/analysis/passes/shadow/cmd/shadow
    go vet -vettool=$(which shadow)

### Tour

The Go tour is no longer included in the main binary distribution. To run the tour locally, instead of running `go` `tool` `tour`, manually install it:

    go get -u golang.org/x/tour
    tour

### Build cache requirement

The build cache is now required as a step toward eliminating `$GOPATH/pkg`. Setting the environment variable `GOCACHE=off` will cause `go` commands that write to the cache to fail.

### Binary-only packages

Go 1.12 is the last release that will support binary-only packages.

### Cgo

Go 1.12 will translate the C type `EGLDisplay` to the Go type `uintptr`. This change is similar to how Go 1.10 and newer treats Darwin's CoreFoundation and Java's JNI types. See the cgo documentation for more information.

Mangled C names are no longer accepted in packages that use Cgo. Use the Cgo names instead. For example, use the documented cgo name `C.char` rather than the mangled name `_Ctype_char` that cgo generates.

### Modules

When `GO111MODULE` is set to `on`, the `go` command now supports module-aware operations outside of a module directory, provided that those operations do not need to resolve import paths relative to the current directory or explicitly edit the `go.mod` file. Commands such as `go` `get`, `go` `list`, and `go` `mod` `download` behave as if in a module with initially-empty requirements. In this mode, `go` `env` `GOMOD` reports the system's null device (`/dev/null` or `NUL`).

`go` commands that download and extract modules are now safe to invoke concurrently. The module cache (`GOPATH/pkg/mod`) must reside in a filesystem that supports file locking.

The `go` directive in a `go.mod` file now indicates the version of the language used by the files within that module. It will be set to the current release (`go` `1.12`) if no existing version is present. If the `go` directive for a module specifies a version newer than the toolchain in use, the `go` command will attempt to build the packages regardless, and will note the mismatch only if that build fails.

This changed use of the `go` directive means that if you use Go 1.12 to build a module, thus recording `go 1.12` in the `go.mod` file, you will get an error when attempting to build the same module with Go 1.11 through Go 1.11.3. Go 1.11.4 or later will work fine, as will releases older than Go 1.11. If you must use Go 1.11 through 1.11.3, you can avoid the problem by setting the language version to 1.11, using the Go 1.12 go tool, via `go mod edit -go=1.11`.

When an import cannot be resolved using the active modules, the `go` command will now try to use the modules mentioned in the main module's `replace` directives before consulting the module cache and the usual network sources. If a matching replacement is found but the `replace` directive does not specify a version, the `go` command uses a pseudo-version derived from the zero `time.Time` (such as `v0.0.0-00010101000000-000000000000`).

### Compiler toolchain

The compiler's live variable analysis has improved. This may mean that finalizers will be executed sooner in this release than in previous releases. If that is a problem, consider the appropriate addition of a `runtime.KeepAlive` call.

More functions are now eligible for inlining by default, including functions that do nothing but call another function. This extra inlining makes it additionally important to use `runtime.CallersFrames` instead of iterating over the result of `runtime.Callers` directly.

    // Old code which no longer works correctly (it will miss inlined call frames).
    var pcs [10]uintptr
    n := runtime.Callers(1, pcs[:])
    for _, pc := range pcs[:n] {
    	f := runtime.FuncForPC(pc)
    	if f != nil {
    		fmt.Println(f.Name())
    	}
    }

    // New code which will work correctly.
    var pcs [10]uintptr
    n := runtime.Callers(1, pcs[:])
    frames := runtime.CallersFrames(pcs[:n])
    for {
    	frame, more := frames.Next()
    	fmt.Println(frame.Function)
    	if !more {
    		break
    	}
    }

Wrappers generated by the compiler to implement method expressions are no longer reported by `runtime.CallersFrames` and `runtime.Stack`. They are also not printed in panic stack traces. This change aligns the `gc` toolchain to match the `gccgo` toolchain, which already elided such wrappers from stack traces. Clients of these APIs might need to adjust for the missing frames. For code that must interoperate between 1.11 and 1.12 releases, you can replace the method expression `x.M` with the function literal `func (...) { x.M(...) }` .

The compiler now accepts a `-lang` flag to set the Go language version to use. For example, `-lang=go1.8` causes the compiler to emit an error if the program uses type aliases, which were added in Go 1.9. Language changes made before Go 1.12 are not consistently enforced.

The compiler toolchain now uses different conventions to call Go functions and assembly functions. This should be invisible to users, except for calls that simultaneously cross between Go and assembly and cross a package boundary. If linking results in an error like "relocation target not defined for ABIInternal (but is defined for ABI0)", please refer to the compatibility section of the ABI design document.

There have been many improvements to the DWARF debug information produced by the compiler, including improvements to argument printing and variable location information.

Go programs now also maintain stack frame pointers on `linux/arm64` for the benefit of profiling tools like `perf`. The frame pointer maintenance has a small run-time overhead that varies but averages around 3%. To build a toolchain that does not use frame pointers, set `GOEXPERIMENT=noframepointer` when running `make.bash`.

The obsolete "safe" compiler mode (enabled by the `-u` gcflag) has been removed.

### `godoc` and `go` `doc`

In Go 1.12, `godoc` no longer has a command-line interface and is only a web server. Users should use `go` `doc` for command-line help output instead. Go 1.12 is the last release that will include the `godoc` webserver; in Go 1.13 it will be available via `go` `get`.

`go` `doc` now supports the `-all` flag, which will cause it to print all exported APIs and their documentation, as the `godoc` command line used to do.

`go` `doc` also now includes the `-src` flag, which will show the target's source code.

### Trace

The trace tool now supports plotting mutator utilization curves, including cross-references to the execution trace. These are useful for analyzing the impact of the garbage collector on application latency and throughput.

### Assembler

On `arm64`, the platform register was renamed from `R18` to `R18_PLATFORM` to prevent accidental use, as the OS could choose to reserve this register.

## Runtime

Go 1.12 significantly improves the performance of sweeping when a large fraction of the heap remains live. This reduces allocation latency immediately following a garbage collection.

The Go runtime now releases memory back to the operating system more aggressively, particularly in response to large allocations that can't reuse existing heap space.

The Go runtime's timer and deadline code is faster and scales better with higher numbers of CPUs. In particular, this improves the performance of manipulating network connection deadlines.

On Linux, the runtime now uses `MADV_FREE` to release unused memory. This is more efficient but may result in higher reported RSS. The kernel will reclaim the unused data when it is needed. To revert to the Go 1.11 behavior (`MADV_DONTNEED`), set the environment variable `GODEBUG=madvdontneed=1`.

Adding cpu.extension=off to the GODEBUG environment variable now disables the use of optional CPU instruction set extensions in the standard library and runtime. This is not yet supported on Windows.

Go 1.12 improves the accuracy of memory profiles by fixing overcounting of large heap allocations.

Tracebacks, `runtime.Caller`, and `runtime.Callers` no longer include compiler-generated initialization functions. Doing a traceback during the initialization of a global variable will now show a function named `PKG.init.ializers`.

## Core library

### TLS 1.3

Go 1.12 adds opt-in support for TLS 1.3 in the `crypto/tls` package as specified by RFC 8446. It can be enabled by adding the value `tls13=1` to the `GODEBUG` environment variable. It will be enabled by default in Go 1.13.

To negotiate TLS 1.3, make sure you do not set an explicit `MaxVersion` in `Config` and run your program with the environment variable `GODEBUG=tls13=1` set.

All TLS 1.2 features except `TLSUnique` in `ConnectionState` and renegotiation are available in TLS 1.3 and provide equivalent or better security and performance. Note that even though TLS 1.3 is backwards compatible with previous versions, certain legacy systems might not work correctly when attempting to negotiate it. RSA certificate keys too small to be secure (including 512-bit keys) will not work with TLS 1.3.

TLS 1.3 cipher suites are not configurable. All supported cipher suites are safe, and if `PreferServerCipherSuites` is set in `Config` the preference order is based on the available hardware.

Early data (also called "0-RTT mode") is not currently supported as a client or server. Additionally, a Go 1.12 server does not support skipping unexpected early data if a client sends it. Since TLS 1.3 0-RTT mode involves clients keeping state regarding which servers support 0-RTT, a Go 1.12 server cannot be part of a load-balancing pool where some other servers do support 0-RTT. If switching a domain from a server that supported 0-RTT to a Go 1.12 server, 0-RTT would have to be disabled for at least the lifetime of the issued session tickets before the switch to ensure uninterrupted operation.

In TLS 1.3 the client is the last one to speak in the handshake, so if it causes an error to occur on the server, it will be returned on the client by the first `Read`, not by `Handshake`. For example, that will be the case if the server rejects the client certificate. Similarly, session tickets are now post-handshake messages, so are only received by the client upon its first `Read`.

### 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.

bufio

  `Reader`'s `UnreadRune` and `UnreadByte` methods will now return an error if they are called after `Peek`.

bytes

  The new function `ReplaceAll` returns a copy of a byte slice with all non-overlapping instances of a value replaced by another.

  A pointer to a zero-value `Reader` is now functionally equivalent to `NewReader``(nil)`. Prior to Go 1.12, the former could not be used as a substitute for the latter in all cases.

crypto/rand

  A warning will now be printed to standard error the first time `Reader.Read` is blocked for more than 60 seconds waiting to read entropy from the kernel.

  On FreeBSD, `Reader` now uses the `getrandom` system call if available, `/dev/urandom` otherwise.

crypto/rc4

  This release removes the assembly implementations, leaving only the pure Go version. The Go compiler generates code that is either slightly better or slightly worse, depending on the exact CPU. RC4 is insecure and should only be used for compatibility with legacy systems.

crypto/tls

  If a client sends an initial message that does not look like TLS, the server will no longer reply with an alert, and it will expose the underlying `net.Conn` in the new field `Conn` of `RecordHeaderError`.

database/sql

  A query cursor can now be obtained by passing a `*Rows` value to the `Row.Scan` method.

expvar

  The new `Delete` method allows for deletion of key/value pairs from a `Map`.

fmt

  Maps are now printed in key-sorted order to ease testing. The ordering rules are:

  - When applicable, nil compares low
  - ints, floats, and strings order by <
  - NaN compares less than non-NaN floats
  - bool compares false before true
  - Complex compares real, then imaginary
  - Pointers compare by machine address
  - Channel values compare by machine address
  - Structs compare each field in turn
  - Arrays compare each element in turn
  - Interface values compare first by `reflect.Type` describing the concrete type and then by concrete value as described in the previous rules.

  When printing maps, non-reflexive key values like `NaN` were previously displayed as `<nil>`. As of this release, the correct values are printed.

go/doc

  To address some outstanding issues in `cmd/doc`, this package has a new `Mode` bit, `PreserveAST`, which controls whether AST data is cleared.

go/token

  The `File` type has a new `LineStart` field, which returns the position of the start of a given line. This is especially useful in programs that occasionally handle non-Go files, such as assembly, but wish to use the `token.Pos` mechanism to identify file positions.

image

  The `RegisterFormat` function is now safe for concurrent use.

image/png

  Paletted images with fewer than 16 colors now encode to smaller outputs.

io

  The new `StringWriter` interface wraps the `WriteString` function.

math

  The functions `Sin`, `Cos`, `Tan`, and `Sincos` now apply Payne-Hanek range reduction to huge arguments. This produces more accurate answers, but they will not be bit-for-bit identical with the results in earlier releases.

math/bits

  New extended precision operations `Add`, `Sub`, `Mul`, and `Div` are available in `uint`, `uint32`, and `uint64` versions.

net

  The `Dialer.DualStack` setting is now ignored and deprecated; RFC 6555 Fast Fallback ("Happy Eyeballs") is now enabled by default. To disable, set `Dialer.FallbackDelay` to a negative value.

  Similarly, TCP keep-alives are now enabled by default if `Dialer.KeepAlive` is zero. To disable, set it to a negative value.

  On Linux, the `splice` system call is now used when copying from a `UnixConn` to a `TCPConn`.

net/http

  The HTTP server now rejects misdirected HTTP requests to HTTPS servers with a plaintext "400 Bad Request" response.

  The new `Client.CloseIdleConnections` method calls the `Client`'s underlying `Transport`'s `CloseIdleConnections` if it has one.

  The `Transport` no longer rejects HTTP responses which declare HTTP Trailers but don't use chunked encoding. Instead, the declared trailers are now just ignored.

  The `Transport` no longer handles `MAX_CONCURRENT_STREAMS` values advertised from HTTP/2 servers as strictly as it did during Go 1.10 and Go 1.11. The default behavior is now back to how it was in Go 1.9: each connection to a server can have up to `MAX_CONCURRENT_STREAMS` requests active and then new TCP connections are created as needed. In Go 1.10 and Go 1.11 the `http2` package would block and wait for requests to finish instead of creating new connections. To get the stricter behavior back, import the `golang.org/x/net/http2` package directly and set `Transport.StrictMaxConcurrentStreams` to `true`.

net/url

  `Parse`, `ParseRequestURI`, and `URL.Parse` now return an error for URLs containing ASCII control characters, which includes NULL, tab, and newlines.

net/http/httputil

  The `ReverseProxy` now automatically proxies WebSocket requests.

os

  The new `ProcessState.ExitCode` method returns the process's exit code.

  `ModeCharDevice` has been added to the `ModeType` bitmask, allowing for `ModeDevice | ModeCharDevice` to be recovered when masking a `FileMode` with `ModeType`.

  The new function `UserHomeDir` returns the current user's home directory.

  `RemoveAll` now supports paths longer than 4096 characters on most Unix systems.

  `File.Sync` now uses `F_FULLFSYNC` on macOS to correctly flush the file contents to permanent storage. This may cause the method to run more slowly than in previous releases.

  `File` now supports a `SyscallConn` method returning a `syscall.RawConn` interface value. This may be used to invoke system-specific operations on the underlying file descriptor.

path/filepath

  The `IsAbs` function now returns true when passed a reserved filename on Windows such as `NUL`. List of reserved names.

reflect

  A new `MapIter` type is an iterator for ranging over a map. This type is exposed through the `Value` type's new `MapRange` method. This follows the same iteration semantics as a range statement, with `Next` to advance the iterator, and `Key`/`Value` to access each entry.

regexp

  `Copy` is no longer necessary to avoid lock contention, so it has been given a partial deprecation comment. `Copy` may still be appropriate if the reason for its use is to make two copies with different `Longest` settings.

runtime/debug

  A new `BuildInfo` type exposes the build information read from the running binary, available only in binaries built with module support. This includes the main package path, main module information, and the module dependencies. This type is given though the `ReadBuildInfo` function on `BuildInfo`.

strings

  The new function `ReplaceAll` returns a copy of a string with all non-overlapping instances of a value replaced by another.

  A pointer to a zero-value `Reader` is now functionally equivalent to `NewReader``(nil)`. Prior to Go 1.12, the former could not be used as a substitute for the latter in all cases.

  The new `Builder.Cap` method returns the capacity of the builder's underlying byte slice.

  The character mapping functions `Map`, `Title`, `ToLower`, `ToLowerSpecial`, `ToTitle`, `ToTitleSpecial`, `ToUpper`, and `ToUpperSpecial` now always guarantee to return valid UTF-8. In earlier releases, if the input was invalid UTF-8 but no character replacements needed to be applied, these routines incorrectly returned the invalid UTF-8 unmodified.

syscall

  64-bit inodes are now supported on FreeBSD 12. Some types have been adjusted accordingly.

  The Unix socket (`AF_UNIX`) address family is now supported for compatible versions of Windows.

  The new function `Syscall18` has been introduced for Windows, allowing for calls with up to 18 arguments.

syscall/js

  The `Callback` type and `NewCallback` function have been renamed; they are now called `Func` and `FuncOf`, respectively. This is a breaking change, but WebAssembly support is still experimental and not yet subject to the Go 1 compatibility promise. Any code using the old names will need to be updated.

  If a type implements the new `Wrapper` interface, `ValueOf` will use it to return the JavaScript value for that type.

  The meaning of the zero `Value` has changed. It now represents the JavaScript `undefined` value instead of the number zero. This is a breaking change, but WebAssembly support is still experimental and not yet subject to the Go 1 compatibility promise. Any code relying on the zero `Value` to mean the number zero will need to be updated.

  The new `Value.Truthy` method reports the JavaScript "truthiness" of a given value.

testing

  The `-benchtime` flag now supports setting an explicit iteration count instead of a time when the value ends with an "`x`". For example, `-benchtime=100x` runs the benchmark 100 times.

text/template

  When executing a template, long context values are no longer truncated in errors.

  `executing "tmpl" at <.very.deep.context.v...>: map has no entry for key "notpresent"`

  is now

  `executing "tmpl" at <.very.deep.context.value.notpresent>: map has no entry for key "notpresent"`

  If a user-defined function called by a template panics, the panic is now caught and returned as an error by the `Execute` or `ExecuteTemplate` method.

time

  The time zone database in `$GOROOT/lib/time/zoneinfo.zip` has been updated to version 2018i. Note that this ZIP file is only used if a time zone database is not provided by the operating system.

unsafe

  It is invalid to convert a nil `unsafe.Pointer` to `uintptr` and back with arithmetic. (This was already invalid, but will now cause the compiler to misbehave.)