comment on significance of 53 bit significand

decimal.go

@@ -1231,6 +1231,7 @@ func (d Decimal) NumDigits() int {
 
 	if d.value.IsInt64() {
 		i64 := d.value.Int64()
+		// restrict fast path to integers with exact conversion to float64
 		if i64 <= (1<<53) && i64 >= -(1<<53) {
 			if i64 == 0 {
 				return 1

decimal_bench_test.go

@@ -125,7 +125,7 @@ func numDigits(b *testing.B, want int, val Decimal) {
 	b.Helper()
 	for i := 0; i < b.N; i++ {
 		if have := val.NumDigits(); have != want {
-			b.Fatalf("\nHave: %q\nWant: %q", have, want)
+			b.Fatalf("\nHave: %d\nWant: %d", have, want)
 		}
 	}
 }