cmd/go: don't generate output for "go build -o /dev/null x.go"

We look for "-o /dev/null", and, if found, pretend that there was no
"-o" option and don't generate an action to create the final executable.

We look for "go build x.go", and, if found, and if -o was not used,
pretend that the user specified "-o x".

Unfortunately, we were doing those in the wrong order, so that "go
build -o /dev/null x.go" would first clear the "-o" option and then
set it to "-o x".

This CL flips the order so that the right thing happens.

Fixes #25579

Change-Id: Ic9556ac0a57f7b45b685951bc96ba5ea4633b860
Reviewed-on: https://go-review.googlesource.com/114715
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/go_test.go

@@ -6325,3 +6325,14 @@ func TestCDAndGOPATHAreDifferent(t *testing.T) {
 		testCDAndGOPATHAreDifferent(tg, cd, strings.ToLower(gopath))
 	}
 }
+
+// Issue 25579.
+func TestGoBuildDashODevNull(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.setenv("GOPATH", filepath.Join(tg.pwd(), "testdata"))
+	tg.run("build", "-o", os.DevNull, filepath.Join(tg.pwd(), "testdata", "src", "hello", "hello.go"))
+	tg.mustNotExist("hello")
+	tg.mustNotExist("hello.exe")
+}

src/cmd/go/internal/work/build.go

@@ -284,11 +284,6 @@ func runBuild(cmd *base.Command, args []string) {
 		cfg.BuildO += cfg.ExeSuffix
 	}
 
-	// Special case -o /dev/null by not writing at all.
-	if cfg.BuildO == os.DevNull {
-		cfg.BuildO = ""
-	}
-
 	// sanity check some often mis-used options
 	switch cfg.BuildContext.Compiler {
 	case "gccgo":
@@ -311,6 +306,11 @@ func runBuild(cmd *base.Command, args []string) {
 
 	pkgs = pkgsFilter(load.Packages(args))
 
+	// Special case -o /dev/null by not writing at all.
+	if cfg.BuildO == os.DevNull {
+		cfg.BuildO = ""
+	}
+
 	if cfg.BuildO != "" {
 		if len(pkgs) > 1 {
 			base.Fatalf("go build: cannot use -o with multiple packages")