go/build: recognize "unix" build tag

The new "unix" build tag matches any Unix or Unix-like system.
This is only recognized on go:build lines, not in file names.

For #20322
Fixes #51572

Change-Id: I3a991f9e69353b25e259bc6462709cdcd83640fb
Reviewed-on: https://go-review.googlesource.com/c/go/+/389934
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

1 files changed, 33 insertions, 4 deletions

diff --git a/src/cmd/dist/build.go b/src/cmd/dist/build.go
index ba09ce9a7b..4dfaf83ef7 100644
--- a/src/cmd/dist/build.go
+++ b/src/cmd/dist/build.go
@@ -976,12 +976,41 @@ func packagefile(pkg string) string {
 	return pathf("%s/pkg/%s_%s/%s.a", goroot, goos, goarch, pkg)
 }
 
+// unixOS is the set of GOOS values matched by the "unix" build tag.
+// This is the same list as in go/build/syslist.go.
+var unixOS = map[string]bool{
+	"aix":       true,
+	"android":   true,
+	"darwin":    true,
+	"dragonfly": true,
+	"freebsd":   true,
+	"hurd":      true,
+	"illumos":   true,
+	"ios":       true,
+	"linux":     true,
+	"netbsd":    true,
+	"openbsd":   true,
+	"solaris":   true,
+}
+
 // matchtag reports whether the tag matches this build.
 func matchtag(tag string) bool {
-	return tag == "gc" || tag == goos || tag == goarch || tag == "cmd_go_bootstrap" || tag == "go1.1" ||
-		(goos == "android" && tag == "linux") ||
-		(goos == "illumos" && tag == "solaris") ||
-		(goos == "ios" && tag == "darwin")
+	switch tag {
+	case "gc", "cmd_go_bootstrap", "go1.1":
+		return true
+	case "linux":
+		return goos == "linux" || goos == "android"
+	case "solaris":
+		return goos == "solaris" || goos == "illumos"
+	case "darwin":
+		return goos == "darwin" || goos == "ios"
+	case goos, goarch:
+		return true
+	case "unix":
+		return unixOS[goos]
+	default:
+		return false
+	}
 }
 
 // shouldbuild reports whether we should build this file.