cmd/go/internal/vet: only set work.VetExplicit if the list of explicit flags is non-empty

Updates #35837
Fixes #37030

Change-Id: Ifd3435803622a8624bab55a0f3fbc8855025282f
Reviewed-on: https://go-review.googlesource.com/c/go/+/217897
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/internal/vet/vet.go

@@ -51,7 +51,9 @@ func runVet(cmd *base.Command, args []string) {
 
 	work.BuildInit()
 	work.VetFlags = vetFlags
-	work.VetExplicit = true
+	if len(vetFlags) > 0 {
+		work.VetExplicit = true
+	}
 	if vetTool != "" {
 		var err error
 		work.VetTool, err = filepath.Abs(vetTool)

src/cmd/go/testdata/script/vet_flags.txt

@@ -6,6 +6,17 @@ go vet -n -unreachable=false encoding/binary
 stderr '-unreachable=false'
 ! stderr '-unsafeptr=false'
 
+# Issue 37030: "go vet <std package>" without other flags should disable the
+# unsafeptr check by default.
+go vet -n encoding/binary
+stderr '-unsafeptr=false'
+! stderr '-unreachable=false'
+
+# However, it should be enabled if requested explicitly.
+go vet -n -unsafeptr encoding/binary
+stderr '-unsafeptr'
+! stderr '-unsafeptr=false'
+
 [short] stop
 env GOCACHE=$WORK/gocache
 env GOTMPDIR=$WORK/tmp