blog: Go 1.21 RC announcement post

Change-Id: I72b71b2b16d5c9e887563dfeae7cab08eeca360f
Reviewed-on: https://go-review.googlesource.com/c/website/+/502796
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Eli Bendersky <eliben@google.com>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>

_content/blog/go1.21rc.md

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+---
+title: Go 1.21 Release Candidate
+date: 2023-06-21
+by:
+- Eli Bendersky, on behalf of the Go team
+summary: Go 1.21 RC brings language improvements, new standard library packages, PGO GA, backward and forward compatibility in the toolchain and faster builds.
+---
+
+The Go 1.21 first Release Candidate (RC) is available today on the [download
+page](/dl/#go1.21rc2)! 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](https://tip.golang.org/doc/go1.21).
+
+*(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](/blog/pgo-preview) 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](/doc/pgo) 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](/cmd/go) now supports [backward](https://tip.golang.org/doc/godebug)
+  and [forward](/doc/toolchain) language compatibility.
+
+## Language changes
+
+- New built-in functions: [min, max](https://tip.golang.org/ref/spec#Min_and_max)
+  and [clear](https://tip.golang.org/ref/spec#Clear).
+- Several improvements to type inference for generic functions. The description of
+  [type inference in the spec](https://tip.golang.org/ref/spec#Type_inference)
+  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](https://go.dev/wiki/CommonMistakes).
+  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](https://go.dev/wiki/LoopvarExperiment) for more details.
+
+## Standard library additions
+
+- New [log/slog](https://tip.golang.org/pkg/log/slog) package for structured logging.
+- New [slices](https://tip.golang.org/pkg/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](https://tip.golang.org/pkg/sort) package.
+- New [maps](https://tip.golang.org/pkg/maps) package for common operations on maps
+  of any key or element type.
+- New [cmp](https://tip.golang.org/pkg/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](https://pkg.go.dev/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)](https://wasi.dev/),
+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](/dl/#go1.21rc2) and try it! If you notice any
+problems, please [file an issue](/issue/new).