Go 1.21 Release Candidate

The Go 1.21 first Release Candidate (RC) is available today on the download page! Go 1.21 is packed with new features and improvements. Getting the RC (release candidate) allows you to experiment with it early, try it on your workloads, and report any issues before the final release (scheduled for August). Here are some notable changes and features in Go 1.21; for the full list, refer to the full release notes.

(Please note that the first RC for Go 1.21 is called go1.21rc2 because a bug was found and fixed after tagging go1.21rc1)

Tool improvements

• The Profile Guided Optimization (PGO) feature we announced for preview in 1.20 is now generally available! If a file named default.pgo is present in the main package’s directory, the go command will use it to enable a PGO build. See the PGO documentation for more details. We’ve measured the impact of PGO on a wide set of Go programs and see performance improvements of 2-7%.
• The go tool now supports backward and forward language compatibility.

Language changes

• New built-in functions: min, max and clear.
• Several improvements to type inference for generic functions. The description of type inference in the spec has been expanded and clarified.
• In a future version of Go we’re planning to address one of the most common gotchas of Go programming: loop variable capture. Go 1.21 comes with a preview of this feature that you can enable in your code using an environment variable. See this LoopvarExperiment wiki page for more details.

Standard library additions

• New log/slog package for structured logging.
• New slices package for common operations on slices of any element type. This includes sorting functions that are generally faster and more ergonomic than the sort package.
• New maps package for common operations on maps of any key or element type.
• New cmp package with new utilities for comparing ordered values.

Improved performance

In addition to the performance improvements when enabling PGO:

• The Go compiler itself has been rebuilt with PGO enabled for 1.21, and as a result it builds Go programs 2-4% faster, depending on the host architecture.
• Due to tuning of the garbage collector, some applications may see up to a 40% reduction in tail latency.
• Collecting traces with runtime/trace now incurs a substantially smaller CPU cost on amd64 and arm64.

A new port to WASI

Go 1.21 adds an experimental port for WebAssembly System Interface (WASI), Preview 1 (GOOS=wasip1, GOARCH=wasm).

To facilitate writing more general WebAssembly (WASM) code, the compiler also supports a new directive for importing functions from the WASM host: go:wasmimport.

Please download the Go 1.21 RC and try it! If you notice any problems, please file an issue.