Support underscores in Dec numbers

Closes #5303

crates/roc_std/src/lib.rs

@@ -298,12 +298,36 @@ impl RocDec {
         };
 
         // Calculate the high digits - the ones before the decimal point.
-        match before_point.parse::<i128>() {
+        let (is_pos, digits) = match before_point.chars().next() {
-            Ok(answer) => match answer.checked_mul(Self::ONE_POINT_ZERO) {
+            Some('+') => (true, &before_point[1..]),
-                Some(hi) => hi.checked_add(lo).map(|num| Self(num.to_ne_bytes())),
+            Some('-') => (false, &before_point[1..]),
-                None => None,
+            _ => (true, before_point),
-            },
+        };
-            Err(_) => None,
+
+        let mut hi: i128 = 0;
+        macro_rules! adjust_hi {
+            ($op:ident) => {{
+                for digit in digits.chars() {
+                    if digit == '_' {
+                        continue;
+                    }
+
+                    let digit = digit.to_digit(10)?;
+                    hi = hi.checked_mul(10)?;
+                    hi = hi.$op(digit as _)?;
+                }
+            }};
+        }
+
+        if is_pos {
+            adjust_hi!(checked_add);
+        } else {
+            adjust_hi!(checked_sub);
+        }
+
+        match hi.checked_mul(Self::ONE_POINT_ZERO) {
+            Some(hi) => hi.checked_add(lo).map(|num| Self(num.to_ne_bytes())),
+            None => None,
         }
     }
 

crates/roc_std/tests/test_roc_std.rs

@@ -295,6 +295,9 @@ mod test_roc_std {
 
         let example = RocDec::from_str("1234.5678").unwrap();
         assert_eq!(format!("{}", example), "1234.5678");
+
+        let example = RocDec::from_str("1_000.5678").unwrap();
+        assert_eq!(format!("{}", example), "1000.5678");
     }
 
     #[test]