[dev.boringcrypto] go/build: satisfy the boringcrypto build tag

This will let applications target Go+BoringCrypto specific APIs cleanly.

Change-Id: I49cbe3a7f044be043f1b98c53112e5147914eaed
Reviewed-on: https://go-review.googlesource.com/c/go/+/250500
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

@@ -6,6 +6,9 @@
 // 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.
+//
+// Any time the Go+BoringCrypto toolchain is used, the "boringcrypto" build tag
+// is satisfied, so that applications can tag files that use this package.
 package boring
 
 import "crypto/internal/boring"

src/crypto/boring/notboring_test.go

@@ -0,0 +1,13 @@
+// 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.
+
+// +build !boringcrypto
+
+package boring_test
+
+import "testing"
+
+func TestNotBoring(t *testing.T) {
+	t.Error("a file tagged !boringcrypto should not build under Go+BoringCrypto")
+}

src/go/build/build.go

@@ -1697,6 +1697,7 @@ func splitQuoted(s string) (r []string, err error) {
 //	$GOARCH
 //	cgo (if cgo is enabled)
 //	!cgo (if cgo is disabled)
+//	boringcrypto
 //	ctxt.Compiler
 //	!ctxt.Compiler
 //	tag (if tag is listed in ctxt.BuildTags or ctxt.ReleaseTags)
@@ -1748,6 +1749,10 @@ func (ctxt *Context) match(name string, allTags map[string]bool) bool {
 	if ctxt.GOOS == "illumos" && name == "solaris" {
 		return true
 	}
+	// Let applications know that the Go+BoringCrypto toolchain is in use.
+	if name == "boringcrypto" {
+		return true
+	}
 
 	// other tags
 	for _, tag := range ctxt.BuildTags {