Fix f64 dec limit edge case (#678)

---
Co-authored-by: Paul Mason <paul@paulmason.me>

src/decimal.rs

@@ -2190,6 +2190,9 @@ fn base2_to_decimal(
             exponent10 -= 1;
             exponent5 += 1;
             ops::array::shl1_internal(bits, 0);
+        } else if exponent10 * 2 > -exponent5 {
+            // Multiplying by >=2 which, due to the previous condition, means an overflow.
+            return None;
         } else {
             // The mantissa would overflow if shifted. Therefore it should be
             // directly divided by 5. This will lose significant digits, unless

tests/decimal_tests.rs

@@ -3136,6 +3136,30 @@ fn it_converts_from_f64_limits() {
     assert!(Decimal::try_from(f64::MIN).is_err(), "try_from(f64::MAX)");
 }
 
+#[test]
+fn it_converts_from_f64_dec_limits() {
+    use num_traits::FromPrimitive;
+
+    // Note Decimal MAX is: 79_228_162_514_264_337_593_543_950_335
+    let over_max = 79_228_162_514_264_355_185_729_994_752_f64;
+    let max_plus_one = 79_228_162_514_264_337_593_543_950_336_f64;
+    let under_max = 79_228_162_514_264_328_797_450_928_128_f64;
+
+    assert!(
+        Decimal::from_f64(over_max).is_none(),
+        "from_f64(79_228_162_514_264_355_185_729_994_752_f64) -> none (too large)"
+    );
+    assert!(
+        Decimal::from_f64(max_plus_one).is_none(),
+        "from_f64(79_228_162_514_264_337_593_543_950_336_f64) -> none (too large)"
+    );
+    assert_eq!(
+        "79228162514264328797450928128",
+        Decimal::from_f64(under_max).unwrap().to_string(),
+        "from_f64(79_228_162_514_264_328_797_450_928_128_f64) -> some (inside limits)"
+    );
+}
+
 #[test]
 fn it_converts_from_f64_retaining_bits() {
     let tests = [