## Introduction to Go 1.11

The latest Go release, version 1.11, arrives six months after Go 1.10. 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

As announced in the Go 1.10 release notes, Go 1.11 now requires OpenBSD 6.2 or later, macOS 10.10 Yosemite or later, or Windows 7 or later; support for previous versions of these operating systems has been removed.

Go 1.11 supports the upcoming OpenBSD 6.4 release. Due to changes in the OpenBSD kernel, older versions of Go will not work on OpenBSD 6.4.

There are known issues with NetBSD on i386 hardware.

The race detector is now supported on `linux/ppc64le` and, to a lesser extent, on `netbsd/amd64`. The NetBSD race detector support has known issues.

The memory sanitizer (`-msan`) is now supported on `linux/arm64`.

The build modes `c-shared` and `c-archive` are now supported on `freebsd/amd64`.

On 64-bit MIPS systems, the new environment variable settings `GOMIPS64=hardfloat` (the default) and `GOMIPS64=softfloat` select whether to use hardware instructions or software emulation for floating-point computations. For 32-bit systems, the environment variable is still `GOMIPS`, as added in Go 1.10.

On soft-float ARM systems (`GOARM=5`), Go now uses a more efficient software floating point interface. This is transparent to Go code, but ARM assembly that uses floating-point instructions not guarded on GOARM will break and must be ported to the new interface.

Go 1.11 on ARMv7 no longer requires a Linux kernel configured with `KUSER_HELPERS`. This setting is enabled in default kernel configurations, but is sometimes disabled in stripped-down configurations.

### WebAssembly

Go 1.11 adds an experimental port to WebAssembly (`js/wasm`).

Go programs currently compile to one WebAssembly module that includes the Go runtime for goroutine scheduling, garbage collection, maps, etc. As a result, the resulting size is at minimum around 2 MB, or 500 KB compressed. Go programs can call into JavaScript using the new experimental `syscall/js` package. Binary size and interop with other languages has not yet been a priority but may be addressed in future releases.

As a result of the addition of the new `GOOS` value "`js`" and `GOARCH` value "`wasm`", Go files named `*_js.go` or `*_wasm.go` will now be ignored by Go tools except when those GOOS/GOARCH values are being used. If you have existing filenames matching those patterns, you will need to rename them.

More information can be found on the WebAssembly wiki page.

### RISC-V GOARCH values reserved

The main Go compiler does not yet support the RISC-V architecture but we've reserved the `GOARCH` values "`riscv`" and "`riscv64`", as used by Gccgo, which does support RISC-V. This means that Go files named `*_riscv.go` will now also be ignored by Go tools except when those GOOS/GOARCH values are being used.

## Tools

### Modules, package versioning, and dependency management

Go 1.11 adds preliminary support for a new concept called “modules,” an alternative to GOPATH with integrated support for versioning and package distribution. Using modules, developers are no longer confined to working inside GOPATH, version dependency information is explicit yet lightweight, and builds are more reliable and reproducible.

Module support is considered experimental. Details are likely to change in response to feedback from Go 1.11 users, and we have more tools planned. Although the details of module support may change, projects that convert to modules using Go 1.11 will continue to work with Go 1.12 and later. If you encounter bugs using modules, please file issues so we can fix them. For more information, see the `go` command documentation.

### Import path restriction

Because Go module support assigns special meaning to the `@` symbol in command line operations, the `go` command now disallows the use of import paths containing `@` symbols. Such import paths were never allowed by `go` `get`, so this restriction can only affect users building custom GOPATH trees by other means.

### Package loading

The new package `golang.org/x/tools/go/packages` provides a simple API for locating and loading packages of Go source code. Although not yet part of the standard library, for many tasks it effectively replaces the `go/build` package, whose API is unable to fully support modules. Because it runs an external query command such as `go list` to obtain information about Go packages, it enables the construction of analysis tools that work equally well with alternative build systems such as Bazel and Buck.

### Build cache requirement

Go 1.11 will be the last release to support setting the environment variable `GOCACHE=off` to disable the build cache, introduced in Go 1.10. Starting in Go 1.12, the build cache will be required, as a step toward eliminating `$GOPATH/pkg`. The module and package loading support described above already require that the build cache be enabled. If you have disabled the build cache to avoid problems you encountered, please file an issue to let us know about them.

### Compiler toolchain

More functions are now eligible for inlining by default, including functions that call `panic`.

The compiler toolchain now supports column information in line directives.

A new package export data format has been introduced. This should be transparent to end users, except for speeding up build times for large Go projects. If it does cause problems, it can be turned off again by passing `-gcflags=all=-iexport=false` to the `go` tool when building a binary.

The compiler now rejects unused variables declared in a type switch guard, such as `x` in the following example:

    func f(v interface{}) {
    	switch x := v.(type) {
    	}
    }

This was already rejected by both `gccgo` and go/types.

### Assembler

The assembler for `amd64` now accepts AVX512 instructions.

### Debugging

The compiler now produces significantly more accurate debug information for optimized binaries, including variable location information, line numbers, and breakpoint locations. This should make it possible to debug binaries compiled without `-N` `-l`. There are still limitations to the quality of the debug information, some of which are fundamental, and some of which will continue to improve with future releases.

DWARF sections are now compressed by default because of the expanded and more accurate debug information produced by the compiler. This is transparent to most ELF tools (such as debuggers on Linux and *BSD) and is supported by the Delve debugger on all platforms, but has limited support in the native tools on macOS and Windows. To disable DWARF compression, pass `-ldflags=-compressdwarf=false` to the `go` tool when building a binary.

Go 1.11 adds experimental support for calling Go functions from within a debugger. This is useful, for example, to call `String` methods when paused at a breakpoint. This is currently only supported by Delve (version 1.1.0 and up).

### Test

Since Go 1.10, the `go` `test` command runs `go` `vet` on the package being tested, to identify problems before running the test. Since `vet` typechecks the code with go/types before running, tests that do not typecheck will now fail. In particular, tests that contain an unused variable inside a closure compiled with Go 1.10, because the Go compiler incorrectly accepted them (Issue #3059), but will now fail, since `go/types` correctly reports an "unused variable" error in this case.

The `-memprofile` flag to `go` `test` now defaults to the "allocs" profile, which records the total bytes allocated since the test began (including garbage-collected bytes).

### Vet

The `go` `vet` command now reports a fatal error when the package under analysis does not typecheck. Previously, a type checking error simply caused a warning to be printed, and `vet` to exit with status 1.

Additionally, `go` `vet` has become more robust when format-checking `printf` wrappers. Vet now detects the mistake in this example:

    func wrapper(s string, args ...interface{}) {
    	fmt.Printf(s, args...)
    }

    func main() {
    	wrapper("%s", 42)
    }

### Trace

With the new `runtime/trace` package's user annotation API, users can record application-level information in execution traces and create groups of related goroutines. The `go` `tool` `trace` command visualizes this information in the trace view and the new user task/region analysis page.

### Cgo

Since Go 1.10, cgo has translated some C pointer types to the Go type `uintptr`. These types include the `CFTypeRef` hierarchy in Darwin's CoreFoundation framework and the `jobject` hierarchy in Java's JNI interface. In Go 1.11, several improvements have been made to the code that detects these types. Code that uses these types may need some updating. See the Go 1.10 release notes for details.

### Go command

The environment variable `GOFLAGS` may now be used to set default flags for the `go` command. This is useful in certain situations. Linking can be noticeably slower on underpowered systems due to DWARF, and users may want to set `-ldflags=-w` by default. For modules, some users and CI systems will want vendoring always, so they should set `-mod=vendor` by default. For more information, see the `go` command documentation.

### Godoc

Go 1.11 will be the last release to support `godoc`'s command-line interface. In future releases, `godoc` will only be a web server. Users should use `go` `doc` for command-line help output instead.

The `godoc` web server now shows which version of Go introduced new API features. The initial Go version of types, funcs, and methods are shown right-aligned. For example, see `UserCacheDir`, with "1.11" on the right side. For struct fields, inline comments are added when the struct field was added in a Go version other than when the type itself was introduced. For a struct field example, see `ClientTrace.Got1xxResponse`.

### Gofmt

One minor detail of the default formatting of Go source code has changed. When formatting expression lists with inline comments, the comments were aligned according to a heuristic. However, in some cases the alignment would be split up too easily, or introduce too much whitespace. The heuristic has been changed to behave better for human-written code.

Note that these kinds of minor updates to gofmt are expected from time to time. In general, systems that need consistent formatting of Go source code should use a specific version of the `gofmt` binary. See the go/format package documentation for more information.

## Runtime

The runtime now uses a sparse heap layout so there is no longer a limit to the size of the Go heap (previously, the limit was 512GiB). This also fixes rare "address space conflict" failures in mixed Go/C binaries or binaries compiled with `-race`.

On macOS and iOS, the runtime now uses `libSystem.dylib` instead of calling the kernel directly. This should make Go binaries more compatible with future versions of macOS and iOS. The syscall package still makes direct system calls; fixing this is planned for a future release.

## Performance

As always, the changes are so general and varied that precise statements about performance are difficult to make. Most programs should run a bit faster, due to better generated code and optimizations in the core library.

There were multiple performance changes to the `math/big` package as well as many changes across the tree specific to `GOARCH=arm64`.

### Compiler toolchain

The compiler now optimizes map clearing operations of the form:

    for k := range m {
    	delete(m, k)
    }

The compiler now optimizes slice extension of the form `append(s,` `make([]T,` `n)...)`.

The compiler now performs significantly more aggressive bounds-check and branch elimination. Notably, it now recognizes transitive relations, so if `i<j` and `j<len(s)`, it can use these facts to eliminate the bounds check for `s[i]`. It also understands simple arithmetic such as `s[i-10]` and can recognize more inductive cases in loops. Furthermore, the compiler now uses bounds information to more aggressively optimize shift operations.

## Core library

All of the changes to the standard library are minor.

### 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.

crypto

  Certain crypto operations, including `ecdsa.Sign`, `rsa.EncryptPKCS1v15` and `rsa.GenerateKey`, now randomly read an extra byte of randomness to ensure tests don't rely on internal behavior.

crypto/cipher

  The new function `NewGCMWithTagSize` implements Galois Counter Mode with non-standard tag lengths for compatibility with existing cryptosystems.

crypto/rsa

  `PublicKey` now implements a `Size` method that returns the modulus size in bytes.

crypto/tls

  `ConnectionState`'s new `ExportKeyingMaterial` method allows exporting keying material bound to the connection according to RFC 5705.

crypto/x509

  The deprecated, legacy behavior of treating the `CommonName` field as a hostname when no Subject Alternative Names are present is now disabled when the CN is not a valid hostname. The `CommonName` can be completely ignored by adding the experimental value `x509ignoreCN=1` to the `GODEBUG` environment variable. When the CN is ignored, certificates without SANs validate under chains with name constraints instead of returning `NameConstraintsWithoutSANs`.

  Extended key usage restrictions are again checked only if they appear in the `KeyUsages` field of `VerifyOptions`, instead of always being checked. This matches the behavior of Go 1.9 and earlier.

  The value returned by `SystemCertPool` is now cached and might not reflect system changes between invocations.

debug/elf

  More `ELFOSABI` and `EM` constants have been added.

encoding/asn1

  `Marshal` and `Unmarshal` now support "private" class annotations for fields.

encoding/base32

  The decoder now consistently returns `io.ErrUnexpectedEOF` for an incomplete chunk. Previously it would return `io.EOF` in some cases.

encoding/csv

  The `Reader` now rejects attempts to set the `Comma` field to a double-quote character, as double-quote characters already have a special meaning in CSV.

html/template

  The package has changed its behavior when a typed interface value is passed to an implicit escaper function. Previously such a value was written out as (an escaped form) of `<nil>`. Now such values are ignored, just as an untyped `nil` value is (and always has been) ignored.

image/gif

  Non-looping animated GIFs are now supported. They are denoted by having a `LoopCount` of -1.

io/ioutil

  The `TempFile` function now supports specifying where the random characters in the filename are placed. If the `prefix` argument includes a "`*`", the random string replaces the "`*`". For example, a `prefix` argument of "`myname.*.bat`" will result in a random filename such as "`myname.123456.bat`". If no "`*`" is included the old behavior is retained, and the random digits are appended to the end.

math/big

  `ModInverse` now returns nil when g and n are not relatively prime. The result was previously undefined.

mime/multipart

  The handling of form-data with missing/empty file names has been restored to the behavior in Go 1.9: in the `Form` for the form-data part the value is available in the `Value` field rather than the `File` field. In Go releases 1.10 through 1.10.3 a form-data part with a missing/empty file name and a non-empty "Content-Type" field was stored in the `File` field. This change was a mistake in 1.10 and has been reverted to the 1.9 behavior.

mime/quotedprintable

  To support invalid input found in the wild, the package now permits non-ASCII bytes but does not validate their encoding.

net

  The new `ListenConfig` type and the new `Dialer.Control` field permit setting socket options before accepting and creating connections, respectively.

  The `syscall.RawConn` `Read` and `Write` methods now work correctly on Windows.

  The `net` package now automatically uses the `splice` system call on Linux when copying data between TCP connections in `TCPConn.ReadFrom`, as called by `io.Copy`. The result is faster, more efficient TCP proxying.

  The `TCPConn.File`, `UDPConn.File`, `UnixConn.File`, and `IPConn.File` methods no longer put the returned `*os.File` into blocking mode.

net/http

  The `Transport` type has a new `MaxConnsPerHost` option that permits limiting the maximum number of connections per host.

  The `Cookie` type has a new `SameSite` field (of new type also named `SameSite`) to represent the new cookie attribute recently supported by most browsers. The `net/http`'s `Transport` does not use the `SameSite` attribute itself, but the package supports parsing and serializing the attribute for browsers to use.

  It is no longer allowed to reuse a `Server` after a call to `Shutdown` or `Close`. It was never officially supported in the past and had often surprising behavior. Now, all future calls to the server's `Serve` methods will return errors after a shutdown or close.

  The constant `StatusMisdirectedRequest` is now defined for HTTP status code 421.

  The HTTP server will no longer cancel contexts or send on `CloseNotifier` channels upon receiving pipelined HTTP/1.1 requests. Browsers do not use HTTP pipelining, but some clients (such as Debian's `apt`) may be configured to do so.

  `ProxyFromEnvironment`, which is used by the `DefaultTransport`, now supports CIDR notation and ports in the `NO_PROXY` environment variable.

net/http/httputil

  The `ReverseProxy` has a new `ErrorHandler` option to permit changing how errors are handled.

  The `ReverseProxy` now also passes "`TE:` `trailers`" request headers through to the backend, as required by the gRPC protocol.

os

  The new `UserCacheDir` function returns the default root directory to use for user-specific cached data.

  The new `ModeIrregular` is a `FileMode` bit to represent that a file is not a regular file, but nothing else is known about it, or that it's not a socket, device, named pipe, symlink, or other file type for which Go has a defined mode bit.

  `Symlink` now works for unprivileged users on Windows 10 on machines with Developer Mode enabled.

  When a non-blocking descriptor is passed to `NewFile`, the resulting `*File` will be kept in non-blocking mode. This means that I/O for that `*File` will use the runtime poller rather than a separate thread, and that the `SetDeadline` methods will work.

os/signal

  The new `Ignored` function reports whether a signal is currently ignored.

os/user

  The `os/user` package can now be built in pure Go mode using the build tag "`osusergo`", independent of the use of the environment variable `CGO_ENABLED=0`. Previously the only way to use the package's pure Go implementation was to disable `cgo` support across the entire program.

runtime

  Setting the `GODEBUG=tracebackancestors=N` environment variable now extends tracebacks with the stacks at which goroutines were created, where N limits the number of ancestor goroutines to report.

runtime/pprof

  This release adds a new "allocs" profile type that profiles total number of bytes allocated since the program began (including garbage-collected bytes). This is identical to the existing "heap" profile viewed in `-alloc_space` mode. Now `go test -memprofile=...` reports an "allocs" profile instead of "heap" profile.

sync

  The mutex profile now includes reader/writer contention for `RWMutex`. Writer/writer contention was already included in the mutex profile.

syscall

  On Windows, several fields were changed from `uintptr` to a new `Pointer` type to avoid problems with Go's garbage collector. The same change was made to the `golang.org/x/sys/windows` package. For any code affected, users should first migrate away from the `syscall` package to the `golang.org/x/sys/windows` package, and then change to using the `Pointer`, while obeying the `unsafe.Pointer` conversion rules.

  On Linux, the `flags` parameter to `Faccessat` is now implemented just as in glibc. In earlier Go releases the flags parameter was ignored.

  On Linux, the `flags` parameter to `Fchmodat` is now validated. Linux's `fchmodat` doesn't support the `flags` parameter so we now mimic glibc's behavior and return an error if it's non-zero.

text/scanner

  The `Scanner.Scan` method now returns the `RawString` token instead of `String` for raw string literals.

text/template

  Modifying template variables via assignments is now permitted via the `=` token:

        {{"{{"}} $v := "init" {{"}}"}}
        {{"{{"}} if true {{"}}"}}
          {{"{{"}} $v = "changed" {{"}}"}}
        {{"{{"}} end {{"}}"}}
        v: {{"{{"}} $v {{"}}"}} {{"{{"}}/* "changed" */{{"}}"}}

  In previous versions untyped `nil` values passed to template functions were ignored. They are now passed as normal arguments.

time

  Parsing of timezones denoted by sign and offset is now supported. In previous versions, numeric timezone names (such as `+03`) were not considered valid, and only three-letter abbreviations (such as `MST`) were accepted when expecting a timezone name.