Add TryFrom<Decimal256> impl for Decimal

CHANGELOG.md

@@ -12,9 +12,11 @@ and this project adheres to
 - cosmwasm-std: Add iterators for `Coins` ([#1806]).
 - cosmwasm-std: Make `abs_diff` const for `Uint{256,512}` and
   `Int{64,128,256,512}`. It is now const for all integer types.
+- cosmwasm-std: Implement `TryFrom<Decimal256>` for `Decimal` ([#1832])
 
 [#1799]: https://github.com/CosmWasm/cosmwasm/pull/1799
 [#1806]: https://github.com/CosmWasm/cosmwasm/pull/1806
+[#1832]: https://github.com/CosmWasm/cosmwasm/pull/1832
 
 ### Changed
 

packages/std/src/math/decimal.rs

@@ -11,7 +11,7 @@ use crate::errors::{
     CheckedFromRatioError, CheckedMultiplyRatioError, DivideByZeroError, OverflowError,
     OverflowOperation, RoundUpOverflowError, StdError,
 };
-use crate::forward_ref_partial_eq;
+use crate::{forward_ref_partial_eq, Decimal256};
 
 use super::Fraction;
 use super::Isqrt;
@@ -498,6 +498,18 @@ impl Fraction<Uint128> for Decimal {
     }
 }
 
+impl TryFrom<Decimal256> for Decimal {
+    type Error = DecimalRangeExceeded;
+
+    fn try_from(value: Decimal256) -> Result<Self, Self::Error> {
+        value
+            .atomics()
+            .try_into()
+            .map(Decimal)
+            .map_err(|_| DecimalRangeExceeded)
+    }
+}
+
 impl FromStr for Decimal {
     type Err = StdError;
 
@@ -817,6 +829,22 @@ mod tests {
         assert_eq!(value.0, Decimal::DECIMAL_FRACTIONAL / Uint128::from(80u8));
     }
 
+    #[test]
+    fn decimal_from_decimal256_works() {
+        let too_big = Decimal256::new(Uint256::from(Uint128::MAX) + Uint256::one());
+        assert_eq!(Decimal::try_from(too_big), Err(DecimalRangeExceeded));
+
+        let just_right = Decimal256::new(Uint256::from(Uint128::MAX));
+        assert_eq!(Decimal::try_from(just_right), Ok(Decimal::MAX));
+
+        assert_eq!(Decimal::try_from(Decimal256::zero()), Ok(Decimal::zero()));
+        assert_eq!(Decimal::try_from(Decimal256::one()), Ok(Decimal::one()));
+        assert_eq!(
+            Decimal::try_from(Decimal256::percent(50)),
+            Ok(Decimal::percent(50))
+        );
+    }
+
     #[test]
     fn decimal_from_atomics_works() {
         let one = Decimal::one();