[dev.boringcrypto] crypto/boring: expose boring.Enabled()

This will let applications check whether they are using BoringCrypto
without making assumptions.

Change-Id: I704304aa4f2095029d536a91708e8b9efddec01c
Reviewed-on: https://go-review.googlesource.com/c/go/+/250499
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>

src/crypto/boring/boring.go

@@ -0,0 +1,16 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package boring exposes functions that are only available when building with
+// Go+BoringCrypto. This package is available on all targets as long as the
+// Go+BoringCrypto toolchain is used. Use the Enabled function to determine
+// whether the BoringCrypto core is actually in use.
+package boring
+
+import "crypto/internal/boring"
+
+// Enabled reports whether BoringCrypto handles supported crypto operations.
+func Enabled() bool {
+	return boring.Enabled
+}

src/crypto/boring/boring_test.go

@@ -0,0 +1,20 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package boring_test
+
+import (
+	"crypto/boring"
+	"runtime"
+	"testing"
+)
+
+func TestEnabled(t *testing.T) {
+	supportedPlatform := runtime.GOOS == "linux" && runtime.GOARCH == "amd64"
+	if supportedPlatform && !boring.Enabled() {
+		t.Error("Enabled returned false on a supported platform")
+	} else if !supportedPlatform && boring.Enabled() {
+		t.Error("Enabled returned true on an unsupported platform")
+	}
+}

src/go/build/deps_test.go

@@ -393,6 +393,9 @@ var depsRules = `
 	crypto/internal/boring/sig, crypto/internal/boring/fipstls
 	< crypto/tls/fipsonly;
 
+	crypto/internal/boring
+	< crypto/boring;
+
 	# crypto-aware packages
 
 	NET, crypto/rand, mime/quotedprintable