When iterating fields, check tag for `spew:"-"` and don't print if present

Specifically checks to see if any of the remaining fields will be printed so it can accurately add a comma when more than one field are printed out.
Also there's a test for it.

spew/dump.go

@@ -415,11 +415,24 @@ func (d *dumpState) dump(v reflect.Value) {
 			for i := 0; i < numFields; i++ {
 				d.indent()
 				vtf := vt.Field(i)
+				tag := vtf.Tag.Get("spew")
+				if tag == "-" {
+					continue
+				}
 				d.w.Write([]byte(vtf.Name))
 				d.w.Write(colonSpaceBytes)
 				d.ignoreNextIndent = true
 				d.dump(d.unpackValue(v.Field(i)))
-				if i < (numFields - 1) {
+
+				hasAnotherFieldToWrite := false
+				for j := i + 1; j < numFields; j++ {
+					vtf = vt.Field(j)
+					tag = vtf.Tag.Get("spew")
+					if tag != "-" {
+						hasAnotherFieldToWrite = true
+					}
+				}
+				if i < (numFields-1) && hasAnotherFieldToWrite {
 					d.w.Write(commaNewlineBytes)
 				} else {
 					d.w.Write(newlineBytes)

spew/spew_test.go

@@ -154,6 +154,13 @@ func initSpewTests() {
 	dt := depthTester{indirCir1{nil}, [1]string{"arr"}, []string{"slice"},
 		map[string]int{"one": 1}}
 
+	type ignoreTester struct {
+		visible   bool
+		invisible bool `spew:"-"`
+	}
+
+	it := ignoreTester{true, false}
+
 	// Variable for tests on types which implement error interface.
 	te := customError(10)
 
@@ -179,6 +186,8 @@ func initSpewTests() {
 		{scsDefault, fSprint, "", complex(-1, -2), "(-1-2i)"},
 		{scsDefault, fSprintf, "%v", complex(float32(-3), -4), "(-3-4i)"},
 		{scsDefault, fSprintln, "", complex(float64(-5), -6), "(-5-6i)\n"},
+		{scsDefault, fCSFdump, "", it, "(spew_test.ignoreTester) {\n" +
+			" visible: (bool) true\n }\n"},
 		{scsNoMethods, fCSFprint, "", ts, "test"},
 		{scsNoMethods, fCSFprint, "", &ts, "<*>test"},
 		{scsNoMethods, fCSFprint, "", tps, "test"},