[cli] Improve span merging in the coverage tool

- Improve merging spans in the coverage tool:
  - The old solution failed to merge neighbouring spans
    e.g. `[3, 5]` and `[5, 7]` into `[3, 7]`
- Also, make the `merge_spans()` function public so it could be used by the
  `move-mutation-test` tool which also parses the coverage output file.

third_party/move/tools/move-coverage/src/source_coverage.rs

@@ -398,34 +398,107 @@ impl SourceCoverage {
     }
 }
 
-fn merge_spans(file_hash: FileHash, cov: FunctionSourceCoverage) -> Vec<Span> {
+/// Merge overlapping spans.
+pub fn merge_spans(file_hash: FileHash, cov: FunctionSourceCoverage) -> Vec<Span> {
     if cov.uncovered_locations.is_empty() {
         return vec![];
     }
 
-    let mut covs: Vec<_> = cov
+    let mut spans = cov
         .uncovered_locations
         .iter()
         .filter(|loc| loc.file_hash() == file_hash)
         .map(|loc| Span::new(loc.start(), loc.end()))
-        .collect();
-    if covs.is_empty() {
+        .collect::<Vec<_>>();
+    if spans.is_empty() {
         return vec![];
     }
-    covs.sort();
+    spans.sort();
 
-    let mut unioned = Vec::new();
-    let mut curr = covs.remove(0);
+    let mut unioned = Vec::with_capacity(spans.len());
+    let mut curr = spans.remove(0);
 
-    for interval in covs {
-        if curr.disjoint(interval) {
-            unioned.push(curr);
-            curr = interval;
+    for span in spans {
+        if curr.end() >= span.start() {
+            curr = curr.merge(span);
         } else {
-            curr = curr.merge(interval);
+            unioned.push(curr);
+            curr = span;
         }
     }
 
     unioned.push(curr);
     unioned
 }
+
+#[cfg(test)]
+mod tests {
+    use super::FunctionSourceCoverage;
+    use crate::source_coverage::merge_spans;
+    use codespan::Span;
+    use move_command_line_common::files::FileHash;
+    use move_ir_types::location::Loc;
+
+    /// Ensure merging spans works as expected.
+    #[test]
+    fn test_merge_spans_works() {
+        let test_hash = FileHash([0; 32]);
+        let other_hash = FileHash([1; 32]);
+
+        // Let's use unsorted array on purpose.
+        let uncovered_locations = vec![
+            // Should be merged:
+            Loc::new(test_hash, 50, 51),
+            Loc::new(test_hash, 51, 52),
+            // Should be merged:
+            Loc::new(test_hash, 3, 5),
+            Loc::new(test_hash, 2, 10),
+            Loc::new(test_hash, 5, 11),
+            // Should stay the same:
+            Loc::new(test_hash, 15, 16),
+            // Should be merged:
+            Loc::new(test_hash, 20, 25),
+            Loc::new(test_hash, 24, 25),
+            Loc::new(test_hash, 21, 25),
+            // Should stay the same:
+            Loc::new(test_hash, 26, 29),
+            // Shouldn't intefere with other hashes.
+            Loc::new(other_hash, 2, 20),
+        ];
+        let expected_spans = vec![
+            Span::new(2, 11),
+            Span::new(15, 16),
+            Span::new(20, 25),
+            Span::new(26, 29),
+            Span::new(50, 52),
+        ];
+
+        let cov = FunctionSourceCoverage {
+            fn_is_native: false,
+            uncovered_locations,
+        };
+        assert_eq!(expected_spans, merge_spans(test_hash, cov));
+    }
+
+    /// Ensure merging spans works fine when there is no spans to be merged.
+    #[test]
+    fn test_merge_spans_when_spans_are_empty() {
+        let test_hash = FileHash([0; 32]);
+        let other_hash = FileHash([1; 32]);
+
+        // Check that it works when are files have full coverage.
+        let cov = FunctionSourceCoverage {
+            fn_is_native: false,
+            uncovered_locations: vec![],
+        };
+        assert!(merge_spans(test_hash, cov).is_empty());
+
+        // Check that it works when the hash under test has the full coverage.
+        let uncovered_locations = vec![Loc::new(other_hash, 2, 20)];
+        let cov = FunctionSourceCoverage {
+            fn_is_native: false,
+            uncovered_locations,
+        };
+        assert!(merge_spans(test_hash, cov).is_empty());
+    }
+}