feat: track_caller so panic reports call site, not lib

src/lib.rs

@@ -111,6 +111,7 @@ macro_rules! implement_common {
             /// # Panic
             ///
             /// This function will panic if `value` is not representable by this type
+            #[track_caller]
             pub const fn new(value: $type) -> $name {
                 assert!(value <= $name::MAX.0 && value >= $name::MIN.0);
                 $name(value)
@@ -361,6 +362,7 @@ macro_rules! implement_common {
         impl lib::core::ops::Add<$name> for $name {
             type Output = $name;
             #[allow(unused_comparisons)]
+            #[track_caller]
             fn add(self, other: $name) -> $name {
                 if self.0 > 0 && other.0 > 0 {
                     debug_assert!(Self::MAX.0 - other.0 >= self.0);
@@ -374,6 +376,7 @@ macro_rules! implement_common {
         impl lib::core::ops::Sub<$name> for $name {
             type Output = $name;
             #[allow(unused_comparisons)]
+            #[track_caller]
             fn sub(self, other: $name) -> $name {
                 if self > other {
                     debug_assert!(Self::MAX.0 + other.0 >= self.0);
@@ -966,4 +969,44 @@ mod tests {
             _ => (),
         }
     }
+
+    /// For manual confirmation:
+    /// 1. Comment-out the line containing `should_panic`
+    /// 2. cargo test --features=std test_panic_track_caller
+    /// 3. Verify line numbers match this call site, NOT `const fn new(...)`
+    #[cfg(feature = "std")]
+    #[test]
+    #[should_panic(expected = "assertion failed: value <= u20::MAX.0 && value >= u20::MIN.0")]
+    fn test_panic_track_caller_1() {
+        std::eprintln!(
+            "Expected:\nthread 'tests::test_panic_track_caller_1' panicked at {}:{}:9",
+            file!(),
+            line!() + 2
+        );
+        u20::new(u32::MAX);
+    }
+
+    #[cfg(feature = "std")]
+    #[test]
+    #[should_panic(expected = "assertion failed: Self::MAX.0 - other.0 >= self.0")]
+    fn test_panic_track_caller_2() {
+        std::eprintln!(
+            "Expected:\nthread 'tests::test_panic_track_caller_2' panicked at {}:{}:26",
+            file!(),
+            line!() + 2
+        );
+        let _overflows = u20::new(u20::MAX.0) + u20::new(1);
+    }
+
+    #[cfg(feature = "std")]
+    #[test]
+    #[should_panic(expected = "assertion failed: Self::MIN.0 + other.0 <= self.0")]
+    fn test_panic_track_caller_3() {
+        std::eprintln!(
+            "Expected:\nthread 'tests::test_panic_track_caller_3' panicked at {}:{}:27",
+            file!(),
+            line!() + 2
+        );
+        let _underflows = u20::new(1) - u20::new(u20::MAX.0);
+    }
 }