https://tip.golang.org/doc/go1.18

Changes to the language
Generics
Go 1.18 includes an implementation of generic features as described by the Type Parameters Proposal. This includes major - but fully backward-compatible - changes to the language.

These new language changes required a large amount of new code that has not had significant testing in production settings. That will only happen as more people write and use generic code. We believe that this feature is well implemented and high quality. However, unlike most aspects of Go, we can’t back up that belief with real world experience. Therefore, while we encourage the use of generics where it makes sense, please use appropriate caution when deploying generic code in production.

While we believe that the new language features are well designed and clearly specified, it is possible that we have made mistakes. We want to stress that the Go 1 compatibility guarantee says “If it becomes necessary to address an inconsistency or incompleteness in the specification, resolving the issue could affect the meaning or legality of existing programs. We reserve the right to address such issues, including updating the implementations.” It also says “If a compiler or library has a bug that violates the specification, a program that depends on the buggy behavior may break if the bug is fixed. We reserve the right to fix such bugs.” In other words, it is possible that there will be code using generics that will work with the 1.18 release but break in later releases. We do not plan or expect to make any such change. However, breaking 1.18 programs in future releases may become necessary for reasons that we cannot today foresee. We will minimize any such breakage as much as possible, but we can’t guarantee that the breakage will be zero.

The following is a list of the most visible changes. For a more comprehensive overview, see the proposal. For details see the language spec.

The syntax for function and type declarations now accepts type parameters.
Parameterized functions and types can be instantiated by following them with a list of type arguments in square brackets.
The new token ~ has been added to the set of operators and punctuation.
The syntax for Interface types now permits the embedding of arbitrary types (not just type names of interfaces) as well as union and ~T type elements. Such interfaces may only be used as type constraints. An interface now defines a set of types as well as a set of methods.
The new predeclared identifier any is an alias for the empty interface. It may be used instead of interface{}.
The new predeclared identifier comparable is an interface that denotes the set of all types which can be compared using == or !=. It may only be used as (or embedded in) a type constraint.
There are three experimental packages using generics that may be useful. These packages are in x/exp repository; their API is not covered by the Go 1 guarantee and may change as we gain more experience with generics.

golang.org/x/exp/constraints
Constraints that are useful for generic code, such as constraints.Ordered.

golang.org/x/exp/slices
A collection of generic functions that operate on slices of any element type.

golang.org/x/exp/maps
A collection of generic functions that operate on maps of any key or element type.

The current generics implementation has the following limitations:

The Go compiler cannot currently handle type declarations inside generic functions or methods. We hope to provide support for this feature in Go 1.19.
The Go compiler currently does not accept arguments of type parameter type with the predeclared functions real, imag, and complex. We hope to remove this restriction in Go 1.19.
The Go compiler currently only supports calling a method m on a value x of type parameter type P if m is explicitly declared by P’s constraint interface. Similarly, method values x.m and method expressions P.m also are only supported if m is explicitly declared by P, even though m might be in the method set of P by virtue of the fact that all types in P implement m. We hope to remove this restriction in Go 1.19.
Embedding a type parameter, or a pointer to a type parameter, as an unnamed field in a struct type is not permitted. Similarly, embedding a type parameter in an interface type is not permitted. Whether these will ever be permitted is unclear at present.
A union element with more than one term may not contain an interface type with a non-empty method set. Whether this will ever be permitted is unclear at present.
Bug fixes
The Go 1.18 compiler now correctly reports declared but not used errors for variables that are set inside a function literal but are never used. Before Go 1.18, the compiler did not report an error in such cases. This fixes long-outstanding compiler issue #8560. As a result of this change, (possibly incorrect) programs may not compile anymore. The necessary fix is straightforward: fix the program if it was in fact incorrect, or use the offending variable, for instance by assigning it to the blank identifier _. Since go vet always pointed out this error, the number of affected programs is likely very small.

The Go 1.18 compiler now reports an overflow when passing a rune constant expression such as ‘1’ « 32 as an argument to the predeclared functions print and println, consistent with the behavior of user-defined functions. Before Go 1.18, the compiler did not report an error in such cases but silently accepted such constant arguments if they fit into an int64. As a result of this change, (possibly incorrect) programs may not compile anymore. The necessary fix is straightforward: fix the program if it was in fact incorrect, or explicitly convert the offending argument to the correct type. Since go vet always pointed out this error, the number of affected programs is likely very small.

Ports
AMD64
Go 1.18 introduces the new GOAMD64 environment variable, which selects at compile time a minimum target version of the AMD64 architecture. Allowed values are v1, v2, v3, or v4. Each higher level requires, and takes advantage of, additional processor features. A detailed description can be found here.

The GOAMD64 environment variable defaults to v1.

RISC-V
The 64-bit RISC-V architecture on Linux (the linux/riscv64 port) now supports the c-archive and c-shared build modes.

Linux
Go 1.18 requires Linux kernel version 2.6.32 or later.

Windows
The windows/arm and windows/arm64 ports now support non-cooperative preemption, bringing that capability to all four Windows ports, which should hopefully address subtle bugs encountered when calling into Win32 functions that block for extended periods of time.

iOS
On iOS (the ios/arm64 port) and iOS simulator running on AMD64-based macOS (the ios/amd64 port), Go 1.18 now requires iOS 12 or later; support for previous versions has been discontinued.

FreeBSD
Go 1.18 is the last release that is supported on FreeBSD 11.x, which has already reached end-of-life. Go 1.19 will require FreeBSD 12.2+ or FreeBSD 13.0+. FreeBSD 13.0+ will require a kernel with the COMPAT_FREEBSD12 option set (this is the default).

Tools
Fuzzing
Go 1.18 includes an implementation of fuzzing as described by the fuzzing proposal.

See the fuzzing landing page to get started.

Please be aware that fuzzing can consume a lot of memory and may impact your machine’s performance while it runs. Also be aware that the fuzzing engine writes values that expand test coverage to a fuzz cache directory within $GOCACHE/fuzz while it runs. There is currently no limit to the number of files or total bytes that may be written to the fuzz cache, so it may occupy a large amount of storage (possibly several GBs).

Go command
go get
go get no longer builds or installs packages in module-aware mode. go get is now dedicated to adjusting dependencies in go.mod. Effectively, the -d flag is always enabled. To install the latest version of an executable outside the context of the current module, use go install example.com/cmd@latest. Any version query may be used instead of latest. This form of go install was added in Go 1.16, so projects supporting older versions may need to provide install instructions for both go install and go get. go get now reports an error when used outside a module, since there is no go.mod file to update. In GOPATH mode (with GO111MODULE=off), go get still builds and installs packages, as before.

Automatic go.mod and go.sum updates
The go mod graph, go mod vendor, go mod verify, and go mod why subcommands no longer automatically update the go.mod and go.sum files. (Those files can be updated explicitly using go get, go mod tidy, or go mod download.)

go version
The go command now embeds version control information in binaries. It includes the currently checked-out revision, commit time, and a flag indicating whether edited or untracked files are present. Version control information is embedded if the go command is invoked in a directory within a Git, Mercurial, Fossil, or Bazaar repository, and the main package and its containing main module are in the same repository. This information may be omitted using the flag -buildvcs=false.

Additionally, the go command embeds information about the build, including build and tool tags (set with -tags), compiler, assembler, and linker flags (like -gcflags), whether cgo was enabled, and if it was, the values of the cgo environment variables (like CGO_CFLAGS). Both VCS and build information may be read together with module information using go version -m file or runtime/debug.ReadBuildInfo (for the currently running binary) or the new debug/buildinfo package.

The underlying data format of the embedded build information can change with new go releases, so an older version of go may not handle the build information produced with a newer version of go. To read the version information from a binary built with go 1.18, use the go version command and the debug/buildinfo package from go 1.18+.

go mod download
If the main module’s go.mod file specifies go 1.17 or higher, go mod download without arguments now downloads source code for only the modules explicitly required in the main module’s go.mod file. (In a go 1.17 or higher module, that set already includes all dependencies needed to build the packages and tests in the main module.) To also download source code for transitive dependencies, use go mod download all.

go mod vendor
The go mod vendor subcommand now supports a -o flag to set the output directory. (Other go commands still read from the vendor directory at the module root when loading packages with -mod=vendor, so the main use for this flag is for third-party tools that need to collect package source code.)

go mod tidy
The go mod tidy command now retains additional checksums in the go.sum file for modules whose source code is needed to verify that each imported package is provided by only one module in the build list. Because this condition is rare and failure to apply it results in a build error, this change is not conditioned on the go version in the main module’s go.mod file.

go work
The go command now supports a “Workspace” mode. If a go.work file is found in the working directory or a parent directory, or one is specified using the GOWORK environment variable, it will put the go command into workspace mode. In workspace mode, the go.work file will be used to determine the set of main modules used as the roots for module resolution, instead of using the normally-found go.mod file to specify the single main module. For more information see the go work documentation.

go build -asan
The go build command and related commands now support an -asan flag that enables interoperation with C (or C++) code compiled with the address sanitizer (C compiler option -fsanitize=address).

go test
The go command now supports additional command line options for the new fuzzing support described above:

go test supports -fuzz, -fuzztime, and -fuzzminimizetime options. For documentation on these see go help testflag.
go clean supports a -fuzzcache option. For documentation see go help clean.
//go:build lines
Go 1.17 introduced //go:build lines as a more readable way to write build constraints, instead of // +build lines. As of Go 1.17, gofmt adds //go:build lines to match existing +build lines and keeps them in sync, while go vet diagnoses when they are out of sync.

Since the release of Go 1.18 marks the end of support for Go 1.16, all supported versions of Go now understand //go:build lines. In Go 1.18, go fix now removes the now-obsolete // +build lines in modules declaring go 1.17 or later in their go.mod files.

For more information, see https://go.dev/design/draft-gobuild.

Gofmt
gofmt now reads and formats input files concurrently, with a memory limit proportional to GOMAXPROCS. On a machine with multiple CPUs, gofmt should now be significantly faster.

Vet
Updates for Generics
The vet tool is updated to support generic code. In most cases, it reports an error in generic code whenever it would report an error in the equivalent non-generic code after substituting for type parameters with a type from their type set. For example, vet reports a format error in

func PrintT ~int|~string {
fmt.Printf(“%d”, t)
}
because it would report a format error in the non-generic equivalent of Print[string]:
func PrintString(x string) {
fmt.Printf(“%d”, x)
}
Precision improvements for existing checkers
The cmd/vet checkers copylock, printf, sortslice, testinggoroutine, and tests have all had moderate precision improvements to handle additional code patterns. This may lead to newly reported errors in existing packages. For example, the printf checker now tracks formatting strings created by concatenating string constants. So vet will report an error in:

// fmt.Printf formatting directive %d is being passed to Println.
fmt.Println(“%d”+ ≡ x (mod 2)+”\n”, x%2)
Runtime
The garbage collector now includes non-heap sources of garbage collector work (e.g., stack scanning) when determining how frequently to run. As a result, garbage collector overhead is more predictable when these sources are significant. For most applications these changes will be negligible; however, some Go applications may now use less memory and spend more time on garbage collection, or vice versa, than before. The intended workaround is to tweak GOGC where necessary.

The runtime now returns memory to the operating system more efficiently and has been tuned to work more aggressively as a result.

Go 1.17 generally improved the formatting of arguments in stack traces, but could print inaccurate values for arguments passed in registers. This is improved in Go 1.18 by printing a question mark (?) after each value that may be inaccurate.

The built-in function append now uses a slightly different formula when deciding how much to grow a slice when it must allocate a new underlying array. The new formula is less prone to sudden transitions in allocation behavior.

Compiler
Go 1.17 implemented a new way of passing function arguments and results using registers instead of the stack on 64-bit x86 architecture on selected operating systems. Go 1.18 expands the supported platforms to include 64-bit ARM (GOARCH=arm64), big- and little-endian 64-bit PowerPC (GOARCH=ppc64, ppc64le), as well as 64-bit x86 architecture (GOARCH=amd64) on all operating systems. On 64-bit ARM and 64-bit PowerPC systems, benchmarking shows typical performance improvements of 10% or more.

As mentioned in the Go 1.17 release notes, this change does not affect the functionality of any safe Go code and is designed to have no impact on most assembly code. See the Go 1.17 release notes for more details.

The compiler now can inline functions that contain range loops or labeled for loops.

The new -asan compiler option supports the new go command -asan option.

Because the compiler’s type checker was replaced in its entirety to support generics, some error messages now may use different wording than before. In some cases, pre-Go 1.18 error messages provided more detail or were phrased in a more helpful way. We intend to address these cases in Go 1.19.

Because of changes in the compiler related to supporting generics, the Go 1.18 compile speed can be roughly 15% slower than the Go 1.17 compile speed. The execution time of the compiled code is not affected. We intend to improve the speed of the compiler in Go 1.19.

Linker
The linker emits far fewer relocations. As a result, most codebases will link faster, require less memory to link, and generate smaller binaries. Tools that process Go binaries should use Go 1.18’s debug/gosym package to transparently handle both old and new binaries.

The new -asan linker option supports the new go command -asan option.

Bootstrap
When building a Go release from source and GOROOT_BOOTSTRAP is not set, previous versions of Go looked for a Go 1.4 or later bootstrap toolchain in the directory $HOME/go1.4 (%HOMEDRIVE%%HOMEPATH%\go1.4 on Windows). Go now looks first for $HOME/go1.17 or $HOME/sdk/go1.17 before falling back to $HOME/go1.4. We intend for Go 1.19 to require Go 1.17 or later for bootstrap, and this change should make the transition smoother. For more details, see go.dev/issue/44505.

Core library
New debug/buildinfo package
The new debug/buildinfo package provides access to module versions, version control information, and build flags embedded in executable files built by the go command. The same information is also available via runtime/debug.ReadBuildInfo for the currently running binary and via go version -m on the command line.

New net/netip package
The new net/netip package defines a new IP address type, Addr. Compared to the existing net.IP type, the netip.Addr type takes less memory, is immutable, and is comparable so it supports == and can be used as a map key.

In addition to Addr, the package defines AddrPort, representing an IP and port, and Prefix, representing a network CIDR prefix.

The package also defines several functions to create and examine these new types: AddrFrom4, AddrFrom16, AddrFromSlice, AddrPortFrom, IPv4Unspecified, IPv6LinkLocalAllNodes, IPv6Unspecified, MustParseAddr, MustParseAddrPort, MustParsePrefix, ParseAddr, ParseAddrPort, ParsePrefix, PrefixFrom.

The net package includes new methods that parallel existing methods, but return netip.AddrPort instead of the heavier-weight net.IP or *net.UDPAddr types: Resolver.LookupNetIP, UDPConn.ReadFromUDPAddrPort, UDPConn.ReadMsgUDPAddrPort, UDPConn.WriteToUDPAddrPort, UDPConn.WriteMsgUDPAddrPort. The new UDPConn methods support allocation-free I/O.

The net package also now includes functions and methods to convert between the existing TCPAddr/UDPAddr types and netip.AddrPort: TCPAddrFromAddrPort, UDPAddrFromAddrPort, TCPAddr.AddrPort, UDPAddr.AddrPort.

TLS 1.0 and 1.1 disabled by default client-side
If Config.MinVersion is not set, it now defaults to TLS 1.2 for client connections. Any safely up-to-date server is expected to support TLS 1.2, and browsers have required it since 2020. TLS 1.0 and 1.1 are still supported by setting Config.MinVersion to VersionTLS10. The server-side default is unchanged at TLS 1.0.

The default can be temporarily reverted to TLS 1.0 by setting the GODEBUG=tls10default=1 environment variable. This option will be removed in Go 1.19.

Rejecting SHA-1 certificates
crypto/x509 will now reject certificates signed with the SHA-1 hash function. This doesn’t apply to self-signed root certificates. Practical attacks against SHA-1 have been demonstrated since 2017 and publicly trusted Certificate Authorities have not issued SHA-1 certificates since 2015.

This can be temporarily reverted by setting the GODEBUG=x509sha1=1 environment variable. This option will be removed in Go 1.19.

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
The new Writer.AvailableBuffer method returns an empty buffer with a possibly non-empty capacity for use with append-like APIs. After appending, the buffer can be provided to a succeeding Write call and possibly avoid any copying.

The Reader.Reset and Writer.Reset methods now use the default buffer size when called on objects with a nil buffer.

bytes
The new Cut function slices a []byte around a separator. It can replace and simplify many common uses of Index, IndexByte, IndexRune, and SplitN.

Trim, TrimLeft, and TrimRight are now allocation free and, especially for small ASCII cutsets, up to 10 times faster.

The Title function is now deprecated. It doesn’t handle Unicode punctuation and language-specific capitalization rules, and is superseded by the golang.org/x/text/cases package.

crypto/elliptic
The P224, P384, and P521 curve implementations are now all backed by code generated by the addchain and fiat-crypto projects, the latter of which is based on a formally-verified model of the arithmetic operations. They now use safer complete formulas and internal APIs. P-224 and P-384 are now approximately four times faster. All specific curve implementations are now constant-time.

Operating on invalid curve points (those for which the IsOnCurve method returns false, and which are never returned by Unmarshal or a Curve method operating on a valid point) has always been undefined behavior, can lead to key recovery attacks, and is now unsupported by the new backend. If an invalid point is supplied to a P224, P384, or P521 method, that method will now return a random point. The behavior might change to an explicit panic in a future release.

crypto/tls
The new Conn.NetConn method allows access to the underlying net.Conn.

crypto/x509
Certificate.Verify now uses platform APIs to verify certificate validity on macOS and iOS when it is called with a nil VerifyOpts.Roots or when using the root pool returned from SystemCertPool.

SystemCertPool is now available on Windows.

On Windows, macOS, and iOS, when a CertPool returned by SystemCertPool has additional certificates added to it, Certificate.Verify will do two verifications: one using the platform verifier APIs and the system roots, and one using the Go verifier and the additional roots. Chains returned by the platform verifier APIs will be prioritized.

CertPool.Subjects is deprecated. On Windows, macOS, and iOS the CertPool returned by SystemCertPool will return a pool which does not include system roots in the slice returned by Subjects, as a static list can’t appropriately represent the platform policies and might not be available at all from the platform APIs.

debug/dwarf
The StructField and BasicType structs both now have a DataBitOffset field, which holds the value of the DW_AT_data_bit_offset attribute if present.

debug/elf
The R_PPC64_RELATIVE constant has been added.

debug/plan9obj
The File.Symbols method now returns the new exported error value ErrNoSymbols if the file has no symbol section.

go/ast
Per the proposal Additions to go/ast and go/token to support parameterized functions and types the following additions are made to the go/ast package:

the FuncType and TypeSpec nodes have a new field TypeParams to hold type parameters, if any.
The new expression node IndexListExpr represents index expressions with multiple indices, used for function and type instantiations with more than one explicit type argument.
go/constant
The new Kind.String method returns a human-readable name for the receiver kind.

go/token
The new constant TILDE represents the ~ token per the proposal Additions to go/ast and go/token to support parameterized functions and types .

go/types
The new Config.GoVersion field sets the accepted Go language version.

Per the proposal Additions to go/types to support type parameters the following additions are made to the go/types package:

The new type TypeParam, factory function NewTypeParam, and associated methods are added to represent a type parameter.
The new type TypeParamList holds a list of type parameters.
The new type TypeList holds a list of types.
The new factory function NewSignatureType allocates a Signature with (receiver or function) type parameters. To access those type parameters, the Signature type has two new methods Signature.RecvTypeParams and Signature.TypeParams.
Named types have four new methods: Named.Origin to get the original parameterized types of instantiated types, Named.TypeArgs and Named.TypeParams to get the type arguments or type parameters of an instantiated or parameterized type, and Named.SetTypeParams to set the type parameters (for instance, when importing a named type where allocation of the named type and setting of type parameters cannot be done simultaneously due to possible cycles).
The Interface type has four new methods: Interface.IsComparable and Interface.IsMethodSet to query properties of the type set defined by the interface, and Interface.MarkImplicit and Interface.IsImplicit to set and test whether the interface is an implicit interface around a type constraint literal.
The new types Union and Term, factory functions NewUnion and NewTerm, and associated methods are added to represent type sets in interfaces.
The new function Instantiate instantiates a parameterized type.
The new Info.Instances map records function and type instantiations through the new Instance type.
The new type ArgumentError and associated methods are added to represent an error related to a type argument.
The new type Context and factory function NewContext are added to facilitate sharing of identical type instances across type-checked packages, via the new Config.Context field.
The predicates AssignableTo, ConvertibleTo, Implements, Identical, IdenticalIgnoreTags, and AssertableTo now also work with arguments that are or contain generalized interfaces, i.e. interfaces that may only be used as type constraints in Go code. Note that the behavior of AssertableTo is undefined if the first argument is a generalized interface.

html/template
Within a range pipeline the new command will end the loop early and the new command will immediately start the next loop iteration.

The and function no longer always evaluates all arguments; it stops evaluating arguments after the first argument that evaluates to false. Similarly, the or function now stops evaluating arguments after the first argument that evaluates to true. This makes a difference if any of the arguments is a function call.

image/draw
The Draw and DrawMask fallback implementations (used when the arguments are not the most common image types) are now faster when those arguments implement the optional draw.RGBA64Image and image.RGBA64Image interfaces that were added in Go 1.17.

net
net.Error.Temporary has been deprecated.

net/http
On WebAssembly targets, the Dial, DialContext, DialTLS and DialTLSContext method fields in Transport will now be correctly used, if specified, for making HTTP requests.

The new Cookie.Valid method reports whether the cookie is valid.

The new MaxBytesHandler function creates a Handler that wraps its ResponseWriter and Request.Body with a MaxBytesReader.

When looking up a domain name containing non-ASCII characters, the Unicode-to-ASCII conversion is now done in accordance with Nontransitional Processing as defined in the Unicode IDNA Compatibility Processing standard (UTS #46). The interpretation of four distinct runes are changed: ß, ς, zero-width joiner U+200D, and zero-width non-joiner U+200C. Nontransitional Processing is consistent with most applications and web browsers.

os/user
User.GroupIds now uses a Go native implementation when cgo is not available.

reflect
The new Value.SetIterKey and Value.SetIterValue methods set a Value using a map iterator as the source. They are equivalent to Value.Set(iter.Key()) and Value.Set(iter.Value()), but do fewer allocations.

The new Value.UnsafePointer method returns the Value’s value as an unsafe.Pointer. This allows callers to migrate from Value.UnsafeAddr and Value.Pointer to eliminate the need to perform uintptr to unsafe.Pointer conversions at the callsite (as unsafe.Pointer rules require).

The new MapIter.Reset method changes its receiver to iterate over a different map. The use of MapIter.Reset allows allocation-free iteration over many maps.

A number of methods ( Value.CanInt, Value.CanUint, Value.CanFloat, Value.CanComplex ) have been added to Value to test if a conversion is safe.

Value.FieldByIndexErr has been added to avoid the panic that occurs in Value.FieldByIndex when stepping through a nil pointer to an embedded struct.

reflect.Ptr and reflect.PtrTo have been renamed to reflect.Pointer and reflect.PointerTo, respectively, for consistency with the rest of the reflect package. The old names will continue to work, but will be deprecated in a future Go release.

regexp
regexp now treats each invalid byte of a UTF-8 string as U+FFFD.

runtime/debug
The BuildInfo struct has two new fields, containing additional information about how the binary was built:

GoVersion holds the version of Go used to build the binary.
Settings is a slice of BuildSettings structs holding key/value pairs describing the build.
runtime/pprof
The CPU profiler now uses per-thread timers on Linux. This increases the maximum CPU usage that a profile can observe, and reduces some forms of bias.

strconv
strconv.Unquote now rejects Unicode surrogate halves.

strings
The new Cut function slices a string around a separator. It can replace and simplify many common uses of Index, IndexByte, IndexRune, and SplitN.

The new Clone function copies the input string without the returned cloned string referencing the input string’s memory.

Trim, TrimLeft, and TrimRight are now allocation free and, especially for small ASCII cutsets, up to 10 times faster.

The Title function is now deprecated. It doesn’t handle Unicode punctuation and language-specific capitalization rules, and is superseded by the golang.org/x/text/cases package.

sync
The new methods Mutex.TryLock, RWMutex.TryLock, and RWMutex.TryRLock, will acquire the lock if it is not currently held.

syscall
The new function SyscallN has been introduced for Windows, allowing for calls with arbitrary number of arguments. As a result, Syscall, Syscall6, Syscall9, Syscall12, Syscall15, and Syscall18 are deprecated in favor of SyscallN.

SysProcAttr.Pdeathsig is now supported in FreeBSD.

syscall/js
The Wrapper interface has been removed.

testing
The precedence of / in the argument for -run and -bench has been increased. A/B|C/D used to be treated as A/(B|C)/D and is now treated as (A/B)|(C/D).

If the -run option does not select any tests, the -count option is ignored. This could change the behavior of existing tests in the unlikely case that a test changes the set of subtests that are run each time the test function itself is run.

The new testing.F type is used by the new fuzzing support described above. Tests also now support the command line options -test.fuzz, -test.fuzztime, and -test.fuzzminimizetime.

text/template
Within a range pipeline the new command will end the loop early and the new command will immediately start the next loop iteration.

The and function no longer always evaluates all arguments; it stops evaluating arguments after the first argument that evaluates to false. Similarly, the or function now stops evaluating arguments after the first argument that evaluates to true. This makes a difference if any of the arguments is a function call.

text/template/parse
The package supports the new text/template and html/template command via the new constant NodeBreak and the new type BreakNode, and similarly supports the new command via the new constant NodeContinue and the new type ContinueNode.

unicode/utf8
The new AppendRune function appends the UTF-8 encoding of a rune to a []byte.