Go 1.15 Release Notes

Introduction to Go 1.15

The latest Go release, version 1.15, arrives six months after Go 1.14. 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.

Go 1.15 includes substantial improvements to the linker, improves allocation for small objects at high core counts, and deprecates X.509 CommonName. GOPROXY now supports skipping proxies that return errors and a new embedded tzdata package has been added.

Changes to the language

There are no changes to the language.



As announced in the Go 1.14 release notes, Go 1.15 requires macOS 10.12 Sierra or later; support for previous versions has been discontinued.

As announced in the Go 1.14 release notes, Go 1.15 drops support for 32-bit binaries on macOS, iOS, iPadOS, watchOS, and tvOS (the darwin/386 and darwin/arm ports). Go continues to support the 64-bit darwin/amd64 and darwin/arm64 ports.


Go now generates Windows ASLR executables when -buildmode=pie cmd/link flag is provided. Go command uses -buildmode=pie by default on Windows.

The -race and -msan flags now always enable -d=checkptr, which checks uses of unsafe.Pointer. This was previously the case on all OSes except Windows.

Go-built DLLs no longer cause the process to exit when it receives a signal (such as Ctrl-C at a terminal).


When linking binaries for Android, Go 1.15 explicitly selects the lld linker available in recent versions of the NDK. The lld linker avoids crashes on some devices, and is planned to become the default NDK linker in a future NDK version.


Go 1.15 adds support for OpenBSD 6.7 on GOARCH=arm and GOARCH=arm64. Previous versions of Go already supported OpenBSD 6.7 on GOARCH=386 and GOARCH=amd64.


There has been progress in improving the stability and performance of the 64-bit RISC-V port on Linux (GOOS=linux, GOARCH=riscv64). It also now supports asynchronous preemption.


Go 1.15 is the last release to support x87-only floating-point hardware (GO386=387). Future releases will require at least SSE2 support on 386, raising Go’s minimum GOARCH=386 requirement to the Intel Pentium 4 (released in 2000) or AMD Opteron/Athlon 64 (released in 2003).


Go command

The GOPROXY environment variable now supports skipping proxies that return errors. Proxy URLs may now be separated with either commas (,) or pipe characters (|). If a proxy URL is followed by a comma, the go command will only try the next proxy in the list after a 404 or 410 HTTP response. If a proxy URL is followed by a pipe character, the go command will try the next proxy in the list after any error. Note that the default value of GOPROXY remains https://proxy.golang.org,direct, which does not fall back to direct in case of errors.

go test

Changing the -timeout flag now invalidates cached test results. A cached result for a test run with a long timeout will no longer count as passing when go test is re-invoked with a short one.

Flag parsing

Various flag parsing issues in go test and go vet have been fixed. Notably, flags specified in GOFLAGS are handled more consistently, and the -outputdir flag now interprets relative paths relative to the working directory of the go command (rather than the working directory of each individual test).

Module cache

The location of the module cache may now be set with the GOMODCACHE environment variable. The default value of GOMODCACHE is GOPATH[0]/pkg/mod, the location of the module cache before this change.

A workaround is now available for Windows “Access is denied” errors in go commands that access the module cache, caused by external programs concurrently scanning the file system (see issue #36568). The workaround is not enabled by default because it is not safe to use when Go versions lower than 1.14.2 and 1.13.10 are running concurrently with the same module cache. It can be enabled by explicitly setting the environment variable GODEBUG=modcacheunzipinplace=1.


New warning for string(x)

The vet tool now warns about conversions of the form string(x) where x has an integer type other than rune or byte. Experience with Go has shown that many conversions of this form erroneously assume that string(x) evaluates to the string representation of the integer x. It actually evaluates to a string containing the UTF-8 encoding of the value of x. For example, string(9786) does not evaluate to the string "9786"; it evaluates to the string "\xe2\x98\xba", or "☺".

Code that is using string(x) correctly can be rewritten to string(rune(x)). Or, in some cases, calling utf8.EncodeRune(buf, x) with a suitable byte slice buf may be the right solution. Other code should most likely use strconv.Itoa or fmt.Sprint.

This new vet check is enabled by default when using go test.

We are considering prohibiting the conversion in a future release of Go. That is, the language would change to only permit string(x) for integer x when the type of x is rune or byte. Such a language change would not be backward compatible. We are using this vet check as a first trial step toward changing the language.

New warning for impossible interface conversions

The vet tool now warns about type assertions from one interface type to another interface type when the type assertion will always fail. This will happen if both interface types implement a method with the same name but with a different type signature.

There is no reason to write a type assertion that always fails, so any code that triggers this vet check should be rewritten.

This new vet check is enabled by default when using go test.

We are considering prohibiting impossible interface type assertions in a future release of Go. Such a language change would not be backward compatible. We are using this vet check as a first trial step toward changing the language.


If panic is invoked with a value whose type is derived from any of: bool, complex64, complex128, float32, float64, int, int8, int16, int32, int64, string, uint, uint8, uint16, uint32, uint64, uintptr, then the value will be printed, instead of just its address. Previously, this was only true for values of exactly these types.

On a Unix system, if the kill command or kill system call is used to send a SIGSEGV, SIGBUS, or SIGFPE signal to a Go program, and if the signal is not being handled via os/signal.Notify, the Go program will now reliably crash with a stack trace. In earlier releases the behavior was unpredictable.

Allocation of small objects now performs much better at high core counts, and has lower worst-case latency.

Converting a small integer value into an interface value no longer causes allocation.

Non-blocking receives on closed channels now perform as well as non-blocking receives on open channels.


Package unsafe’s safety rules allow converting an unsafe.Pointer into uintptr when calling certain functions. Previously, in some cases, the compiler allowed multiple chained conversions (for example, syscall.Syscall(…, uintptr(uintptr(ptr)), …)). The compiler now requires exactly one conversion. Code that used multiple conversions should be updated to satisfy the safety rules.

Go 1.15 reduces typical binary sizes by around 5% compared to Go 1.14 by eliminating certain types of GC metadata and more aggressively eliminating unused type metadata.

The toolchain now mitigates Intel CPU erratum SKX102 on GOARCH=amd64 by aligning functions to 32 byte boundaries and padding jump instructions. While this padding increases binary sizes, this is more than made up for by the binary size improvements mentioned above.

Go 1.15 adds a -spectre flag to both the compiler and the assembler, to allow enabling Spectre mitigations. These should almost never be needed and are provided mainly as a “defense in depth” mechanism. See the Spectre wiki page for details.

The compiler now rejects //go: compiler directives that have no meaning for the declaration they are applied to with a “misplaced compiler directive” error. Such misapplied directives were broken before, but were silently ignored by the compiler.

The compiler’s -json optimization logging now reports large (>= 128 byte) copies and includes explanations of escape analysis decisions.


This release includes substantial improvements to the Go linker, which reduce linker resource usage (both time and memory) and improve code robustness/maintainability.

For a representative set of large Go programs, linking is 20% faster and requires 30% less memory on average, for ELF-based OSes (Linux, FreeBSD, NetBSD, OpenBSD, Dragonfly, and Solaris) running on amd64 architectures, with more modest improvements for other architecture/OS combinations.

The key contributors to better linker performance are a newly redesigned object file format, and a revamping of internal phases to increase concurrency (for example, applying relocations to symbols in parallel). Object files in Go 1.15 are slightly larger than their 1.14 equivalents.

These changes are part of a multi-release project to modernize the Go linker, meaning that there will be additional linker improvements expected in future releases.

The linker now defaults to internal linking mode for -buildmode=pie on linux/amd64 and linux/arm64, so these configurations no longer require a C linker. External linking mode (which was the default in Go 1.14 for -buildmode=pie) can still be requested with -ldflags=-linkmode=external flag.


The objdump tool now supports disassembling in GNU assembler syntax with the -gnu flag.

Standard library

New embedded tzdata package

Go 1.15 includes a new package, time/tzdata, that permits embedding the timezone database into a program. Importing this package (as import _ "time/tzdata") permits the program to find timezone information even if the timezone database is not available on the local system. You can also embed the timezone database by building with -tags timetzdata. Either approach increases the size of the program by about 800 KB.


Go 1.15 will translate the C type EGLConfig to the Go type uintptr. This change is similar to how Go 1.12 and newer treats EGLDisplay, Darwin’s CoreFoundation and Java’s JNI types. See the cgo documentation for more information.

In Go 1.15.3 and later, cgo will not permit Go code to allocate an undefined struct type (a C struct defined as just struct S; or similar) on the stack or heap. Go code will only be permitted to use pointers to those types. Allocating an instance of such a struct and passing a pointer, or a full struct value, to C code was always unsafe and unlikely to work correctly; it is now forbidden. The fix is to either rewrite the Go code to use only pointers, or to ensure that the Go code sees the full definition of the struct by including the appropriate C header file.

X.509 CommonName deprecation

The deprecated, legacy behavior of treating the CommonName field on X.509 certificates as a host name when no Subject Alternative Names are present is now disabled by default. It can be temporarily re-enabled by adding the value x509ignoreCN=0 to the GODEBUG environment variable.

Note that if the CommonName is an invalid host name, it’s always ignored, regardless of GODEBUG settings. Invalid names include those with any characters other than letters, digits, hyphens and underscores, and those with empty labels or trailing dots.

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.


When a Scanner is used with an invalid io.Reader that incorrectly returns a negative number from Read, the Scanner will no longer panic, but will instead return the new error ErrBadReadCount.


Creating a derived Context using a nil parent is now explicitly disallowed. Any attempt to do so with the WithValue, WithDeadline, or WithCancel functions will cause a panic.


The PrivateKey and PublicKey types in the crypto/rsa, crypto/ecdsa, and crypto/ed25519 packages now have an Equal method to compare keys for equivalence or to make type-safe interfaces for public keys. The method signature is compatible with go-cmp’s definition of equality.

Hash now implements fmt.Stringer.


The new SignASN1 and VerifyASN1 functions allow generating and verifying ECDSA signatures in the standard ASN.1 DER encoding.


The new MarshalCompressed and UnmarshalCompressed functions allow encoding and decoding NIST elliptic curve points in compressed format.


VerifyPKCS1v15 now rejects invalid short signatures with missing leading zeroes, according to RFC 8017.


The new Dialer type and its DialContext method permit using a context to both connect and handshake with a TLS server.

The new VerifyConnection callback on the Config type allows custom verification logic for every connection. It has access to the ConnectionState which includes peer certificates, SCTs, and stapled OCSP responses.

Auto-generated session ticket keys are now automatically rotated every 24 hours, with a lifetime of 7 days, to limit their impact on forward secrecy.

Session ticket lifetimes in TLS 1.2 and earlier, where the session keys are reused for resumed connections, are now limited to 7 days, also to limit their impact on forward secrecy.

The client-side downgrade protection checks specified in RFC 8446 are now enforced. This has the potential to cause connection errors for clients encountering middleboxes that behave like unauthorized downgrade attacks.

SignatureScheme, CurveID, and ClientAuthType now implement fmt.Stringer.

The ConnectionState fields OCSPResponse and SignedCertificateTimestamps are now repopulated on client-side resumed connections.

tls.Conn now returns an opaque error on permanently broken connections, wrapping the temporary net.Error. To access the original net.Error, use errors.As (or errors.Unwrap) instead of a type assertion.


If either the name on the certificate or the name being verified (with VerifyOptions.DNSName or VerifyHostname) are invalid, they will now be compared case-insensitively without further processing (without honoring wildcards or stripping trailing dots). Invalid names include those with any characters other than letters, digits, hyphens and underscores, those with empty labels, and names on certificates with trailing dots.

The new CreateRevocationList function and RevocationList type allow creating RFC 5280-compliant X.509 v2 Certificate Revocation Lists.

CreateCertificate now automatically generates the SubjectKeyId if the template is a CA and doesn’t explicitly specify one.

CreateCertificate now returns an error if the template specifies MaxPathLen but is not a CA.

On Unix systems other than macOS, the SSL_CERT_DIR environment variable can now be a colon-separated list.

On macOS, binaries are now always linked against Security.framework to extract the system trust roots, regardless of whether cgo is available. The resulting behavior should be more consistent with the OS verifier.


Name.String now prints non-standard attributes from Names if ExtraNames is nil.


The new DB.SetConnMaxIdleTime method allows removing a connection from the connection pool after it has been idle for a period of time, without regard to the total lifespan of the connection. The DBStats.MaxIdleTimeClosed field shows the total number of connections closed due to DB.SetConnMaxIdleTime.

The new Row.Err getter allows checking for query errors without calling Row.Scan.


The new Validator interface may be implemented by Conn to allow drivers to signal if a connection is valid or if it should be discarded.


The package now defines the IMAGE_FILE, IMAGE_SUBSYSTEM, and IMAGE_DLLCHARACTERISTICS constants used by the PE file format.


Marshal now sorts the components of SET OF according to X.690 DER.

Unmarshal now rejects tags and Object Identifiers which are not minimally encoded according to X.690 DER.


The package now has an internal limit to the maximum depth of nesting when decoding. This reduces the possibility that a deeply nested input could use large quantities of stack memory, or even cause a “goroutine stack exceeds limit” panic.


When the flag package sees -h or -help, and those flags are not defined, it now prints a usage message. If the FlagSet was created with ExitOnError, FlagSet.Parse would then exit with a status of 2. In this release, the exit status for -h or -help has been changed to 0. In particular, this applies to the default handling of command line flags.


The printing verbs %#g and %#G now preserve trailing zeros for floating-point values.


The Source and Node functions now canonicalize number literal prefixes and exponents as part of formatting Go source code. This matches the behavior of the gofmt command as it was implemented since Go 1.13.


The package now uses Unicode escapes (\uNNNN) in all JavaScript and JSON contexts. This fixes escaping errors in application/ld+json and application/json contexts.


TempDir and TempFile now reject patterns that contain path separators. That is, calls such as ioutil.TempFile("/tmp", "../base*") will no longer succeed. This prevents unintended directory traversal.


The new Int.FillBytes method allows serializing to fixed-size pre-allocated byte slices.


The functions in this package were updated to conform to the C99 standard (Annex G IEC 60559-compatible complex arithmetic) with respect to handling of special arguments such as infinity, NaN and signed zero.


If an I/O operation exceeds a deadline set by the Conn.SetDeadline, Conn.SetReadDeadline, or Conn.SetWriteDeadline methods, it will now return an error that is or wraps os.ErrDeadlineExceeded. This may be used to reliably detect whether an error is due to an exceeded deadline. Earlier releases recommended calling the Timeout method on the error, but I/O operations can return errors for which Timeout returns true although a deadline has not been exceeded.

The new Resolver.LookupIP method supports IP lookups that are both network-specific and accept a context.


Parsing is now stricter as a hardening measure against request smuggling attacks: non-ASCII white space is no longer trimmed like SP and HTAB, and support for the “identityTransfer-Encoding was dropped.


ReverseProxy now supports not modifying the X-Forwarded-For header when the incoming Request.Header map entry for that field is nil.

When a Switching Protocol (like WebSocket) request handled by ReverseProxy is canceled, the backend connection is now correctly closed.


All profile endpoints now support a “seconds” parameter. When present, the endpoint profiles for the specified number of seconds and reports the difference. The meaning of the “seconds” parameter in the cpu profile and the trace endpoints is unchanged.


The new URL field RawFragment and method EscapedFragment provide detail about and control over the exact encoding of a particular fragment. These are analogous to RawPath and EscapedPath.

The new URL method Redacted returns the URL in string form with any password replaced with xxxxx.


If an I/O operation exceeds a deadline set by the File.SetDeadline, File.SetReadDeadline, or File.SetWriteDeadline methods, it will now return an error that is or wraps os.ErrDeadlineExceeded. This may be used to reliably detect whether an error is due to an exceeded deadline. Earlier releases recommended calling the Timeout method on the error, but I/O operations can return errors for which Timeout returns true although a deadline has not been exceeded.

Packages os and net now automatically retry system calls that fail with EINTR. Previously this led to spurious failures, which became more common in Go 1.14 with the addition of asynchronous preemption. Now this is handled transparently.

The os.File type now supports a ReadFrom method. This permits the use of the copy_file_range system call on some systems when using io.Copy to copy data from one os.File to another. A consequence is that io.CopyBuffer will not always use the provided buffer when copying to a os.File. If a program wants to force the use of the provided buffer, it can be done by writing io.CopyBuffer(struct{ io.Writer }{dst}, src, buf).


DWARF generation is now supported (and enabled by default) for -buildmode=plugin on macOS.

Building with -buildmode=plugin is now supported on freebsd/amd64.


Package reflect now disallows accessing methods of all non-exported fields, whereas previously it allowed accessing those of non-exported, embedded fields. Code that relies on the previous behavior should be updated to instead access the corresponding promoted method of the enclosing variable.


The new Regexp.SubexpIndex method returns the index of the first subexpression with the given name within the regular expression.


Several functions, including ReadMemStats and GoroutineProfile, no longer block if a garbage collection is in progress.


The goroutine profile now includes the profile labels associated with each goroutine at the time of profiling. This feature is not yet implemented for the profile reported with debug=2.


FormatComplex and ParseComplex are added for working with complex numbers.

FormatComplex converts a complex number into a string of the form (a+bi), where a and b are the real and imaginary parts.

ParseComplex converts a string into a complex number of a specified precision. ParseComplex accepts complex numbers in the format N+Ni.


The new method Map.LoadAndDelete atomically deletes a key and returns the previous value if present.

The method Map.Delete is more efficient.


On Unix systems, functions that use SysProcAttr will now reject attempts to set both the Setctty and Foreground fields, as they both use the Ctty field but do so in incompatible ways. We expect that few existing programs set both fields.

Setting the Setctty field now requires that the Ctty field be set to a file descriptor number in the child process, as determined by the ProcAttr.Files field. Using a child descriptor always worked, but there were certain cases where using a parent file descriptor also happened to work. Some programs that set Setctty will need to change the value of Ctty to use a child descriptor number.

It is now possible to call system calls that return floating point values on windows/amd64.


The testing.T type now has a Deadline method that reports the time at which the test binary will have exceeded its timeout.

A TestMain function is no longer required to call os.Exit. If a TestMain function returns, the test binary will call os.Exit with the value returned by m.Run.

The new methods T.TempDir and B.TempDir return temporary directories that are automatically cleaned up at the end of the test.

go test -v now groups output by test name, rather than printing the test name on each line.


JSEscape now consistently uses Unicode escapes (\u00XX), which are compatible with JSON.


The new method Ticker.Reset supports changing the duration of a ticker.

When returning an error, ParseDuration now quotes the original value.