Go 1 21

Go 1.21

This overview applies when the project's target Go version is at least 1.21.

Language

• The built-in min and max functions select an extremum from a fixed set of ordered values without hand-written comparisons.
• clear can empty a map or zero every element of a slice without deletion or zeroing loops.
• Type inference covers more generic function arguments, generic function assignments, and interface method matching, reducing the need for explicit type arguments.
• panic(nil) now produces *runtime.PanicNilError, ensuring that a directly deferred recover returns a non-nil value. GODEBUG=panicnil=1 provides temporary compatibility with the older behaviour.

Toolchains and builds

• The go directive in go.mod or go.work is a strict minimum version, while the toolchain directive can suggest a newer toolchain without raising the language and module-semantics minimum.
• Automatic toolchain selection can find or download a newer Go release when a module requires it.
• Version-sensitive GODEBUG defaults follow the go directive, allowing a newer toolchain to preserve older language-version behaviour.
• A representative production CPU profile named default.pgo in a main package enables profile-guided optimisation automatically; -pgo controls an explicit profile location or opt-out.
• go test -fullpath emits unambiguous source paths, and a directory argument to go test -o supports several test binaries.

Generic collection and logging APIs

• The generic slices and maps packages provide common collection operations that previously required local implementations.
• cmp.Compare, cmp.Less, and cmp.Ordered support ordered generic algorithms and comparators for slices helpers.
• log/slog provides structured, levelled logging with keys and values or slog.Attr values.
• testing/slogtest validates custom slog.Handler implementations.

Standard library

• context.WithoutCancel allows work to outlive parent cancellation while retaining values. WithDeadlineCause, WithTimeoutCause, and Cause preserve deadline reasons, while AfterFunc registers cancellation-triggered work.
• errors.ErrUnsupported gives unsupported operations a portable identity through errors.Is.
• sync.OnceFunc, sync.OnceValue, and sync.OnceValues cover common function and result-caching forms of sync.Once.
• reflect.SliceHeader and reflect.StringHeader are deprecated; unsafe.Slice, unsafe.SliceData, unsafe.String, and unsafe.StringData are their safer replacements.
• Low-level crypto/elliptic curve operations are deprecated. crypto/ecdh covers ECDH, while lower-level curve arithmetic requires a maintained implementation.
• runtime.Pinner allows non-Go code to retain references to explicitly pinned Go memory until it is unpinned.
• testing.Testing reports whether the running program is a go test binary.

Experimental and compatibility features

• The per-iteration loop-variable behaviour is a preview in Go 1.21; the language still uses one variable per loop by default.
• GOEXPERIMENT=cgocheck2 provides the thorough cgo pointer checker, replacing the removed runtime setting GODEBUG=cgocheck=2.
• Supported systems start at macOS 10.15, Windows 10, and Windows Server 2016.