cynic-parser: add overlaps() method to Span (#1065)

cynic-parser/src/span.rs

@@ -8,4 +8,59 @@ impl Span {
     pub fn new(start: usize, end: usize) -> Self {
         Self { start, end }
     }
+
+    /// Returns `true` if this [Span] overlaps with the other [Span].
+    ///
+    /// Will always return `false` when one of the spans is empty or invalid (`end < start`).
+    pub fn overlaps(&self, other: Span) -> bool {
+        self.start < other.end && other.start < self.end
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[track_caller]
+    fn assert_overlap(a: Span, b: Span) {
+        let forward = a.overlaps(b);
+        let back = b.overlaps(a);
+        assert_eq!(forward, back);
+
+        assert!(
+            forward,
+            "Expected {a:?} and {b:?} to overlap but they don't."
+        );
+    }
+
+    #[track_caller]
+    fn assert_no_overlap(a: Span, b: Span) {
+        let forward = a.overlaps(b);
+        let back = b.overlaps(a);
+        assert_eq!(forward, back);
+
+        assert!(
+            !forward,
+            "Expected {a:?} and {b:?} not to overlap but they do."
+        );
+    }
+
+    #[test]
+    fn empty_spans_never_overlap() {
+        assert_no_overlap(Span { start: 0, end: 0 }, Span { start: 0, end: 0 });
+        assert_no_overlap(Span { start: 0, end: 0 }, Span { start: 10, end: 10 });
+        assert_no_overlap(Span { start: 100, end: 0 }, Span { start: 10, end: 10 });
+    }
+
+    #[test]
+    fn overlap_tests() {
+        assert_overlap(Span { start: 0, end: 10 }, Span { start: 5, end: 15 });
+        assert_overlap(Span { start: 5, end: 15 }, Span { start: 0, end: 10 });
+        assert_overlap(Span { start: 0, end: 10 }, Span { start: 0, end: 10 });
+        assert_overlap(Span { start: 0, end: 10 }, Span { start: 0, end: 5 });
+        assert_overlap(Span { start: 0, end: 10 }, Span { start: 5, end: 10 });
+
+        assert_no_overlap(Span { start: 10, end: 20 }, Span { start: 20, end: 30 });
+        assert_no_overlap(Span { start: 10, end: 20 }, Span { start: 30, end: 40 });
+    }
 }