feat: hex formatting helper (starknet-io#26)

crates/starknet-types-core/src/felt.rs

@@ -210,6 +210,13 @@ impl Felt {
         BitArray::new(limbs)
     }
 
+    /// Helper to produce a hexadecimal formatted string.
+    /// Equivalent to calling `format!("{self:#x}")`.
+    #[cfg(feature = "alloc")]
+    pub fn to_hex_string(&self) -> alloc::string::String {
+        alloc::format!("{self:#x}")
+    }
+
     /// Converts to little-endian bit representation.
     /// This is as performant as [to_bits_be](Felt::to_bits_be)
     pub fn to_bits_le(&self) -> BitArray<BitArrayStore> {
@@ -1060,6 +1067,12 @@ mod test {
             prop_assert_eq!(x, y);
         }
 
+        #[test]
+        #[cfg(feature = "alloc")]
+        fn to_hex_string_is_same_as_format(ref x in any::<Felt>()) {
+            prop_assert_eq!(alloc::format!("{x:#x}"), x.to_hex_string());
+        }
+
         #[test]
         fn from_bytes_le_slice_works_for_all_lengths(x in 0..1000usize) {
             let bytes: [u8; 1000] = core::array::from_fn(|i| i as u8);