util/slicesx: add AppendNonzero

By request of @agottardo.

Updates #cleanup

Change-Id: I2f02314eb9533b1581e47b66b45b6fb8ac257bb7
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

util/slicesx/slicesx.go

@@ -95,6 +95,17 @@ func Filter[S ~[]T, T any](dst, src S, fn func(T) bool) S {
 	return dst
 }
 
+// AppendNonzero appends all non-zero elements of src to dst.
+func AppendNonzero[S ~[]T, T comparable](dst, src S) S {
+	var zero T
+	for _, v := range src {
+		if v != zero {
+			dst = append(dst, v)
+		}
+	}
+	return dst
+}
+
 // AppendMatching appends elements in ps to dst if f(x) is true.
 func AppendMatching[T any](dst, ps []T, f func(T) bool) []T {
 	for _, p := range ps {

util/slicesx/slicesx_test.go

@@ -137,6 +137,19 @@ func TestFilterNoAllocations(t *testing.T) {
 	}
 }
 
+func TestAppendNonzero(t *testing.T) {
+	v := []string{"one", "two", "", "four"}
+	got := AppendNonzero(nil, v)
+	want := []string{"one", "two", "four"}
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("got %v; want %v", got, want)
+	}
+	got = AppendNonzero(v[:0], v)
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("got %v; want %v", got, want)
+	}
+}
+
 func TestAppendMatching(t *testing.T) {
 	v := []string{"one", "two", "three", "four"}
 	got := AppendMatching(v[:0], v, func(s string) bool { return len(s) > 3 })