Simplifications followin @bnjbvr review.

Author: AureliaDolo

src/lib.rs

@@ -89,7 +89,7 @@ impl Gtid {
             self.intervals.push(interval);
             return;
         }
-        if let Some(&first) = self.intervals.first() {
+        if let Some(first) = self.intervals.first_mut() {
             // if interval is strictly before intervals
             if interval.1 < first.0 {
                 self.intervals.insert(0, interval);
@@ -98,25 +98,19 @@ impl Gtid {
 
             // if interval merge before interval
             if interval.1 == first.0 {
-                // unwrapping is ok as intervals is not empty
-
-                let first = self.intervals.first_mut().unwrap();
-                *first = (interval.0, first.1);
+                first.0 = interval.0;
                 return;
             }
         }
 
-        if let Some(&last) = self.intervals.last() {
+        if let Some(last) = self.intervals.last_mut() {
             if interval.0 > last.1 {
                 self.intervals.push(interval);
                 return;
             }
 
             if interval.0 == last.1 {
-                // unwrapping is ok as intervals is not empty
-
-                let last = self.intervals.last_mut().unwrap();
-                *last = (last.0, interval.1);
+                last.1 = interval.1;
                 return;
             }
         }
@@ -126,17 +120,12 @@ impl Gtid {
             .binary_search_by(|elem| elem.1.cmp(&interval.0))
         {
             Err(idx) => {
-                // error case so it won't merge with previous
-                // it may merge before the item currently at idx
-                if idx == self.intervals.len() {
-                    // previously treated
-                    unreachable!()
-                }
-                // we can unwrap as the case interval is after the last element is treated before
-                let next = self.intervals.get(idx).unwrap();
+                // there is always a next element, otherwise  we fall
+                // into the previous if let some last case
+                let next = &mut self.intervals[idx];
                 // interval merges with next
                 if next.0 == interval.1 {
-                    *self.intervals.get_mut(idx).unwrap() = (interval.0, next.1);
+                    next.0 = interval.0;
                 } else {
                     // just add interval, nothing to merge
                     self.intervals.insert(idx, interval);
@@ -151,27 +140,30 @@ impl Gtid {
 
                 // interval merges with before and after
                 if interval.0 == before.1 && interval.1 == after.0 {
-                    *self.intervals.get_mut(idx).unwrap() = (before.0, after.1);
+                    self.intervals[idx] = (before.0, after.1);
                     self.intervals.remove(idx + 1);
                 } else if
                 // interval merges with before
                 interval.0 == before.1 {
-                    *self.intervals.get_mut(idx).unwrap() = (before.0, interval.1);
+                    self.intervals[idx] = (before.0, interval.1);
                 } else if
                 // interval merges with after
                 interval.1 == after.0 {
-                    unreachable!("should have been treated in the error branch before");
-                    // *self.intervals.get_mut(idx + 1).unwrap() = (interval.0, after.1);
+                    unreachable!("should have been treated in the error branch before: interval merges with after");
                 } else {
                     // interval does not merge
-                    unreachable!("should have been treated in the error branch before");
-                    //  self.intervals.insert(idx + 1, interval)
+                    unreachable!("should have been treated in the error branch before: interval does not merge");
                 }
             }
         }
     }
 
     /// Add a raw interval into the gtid.
+    ///
+    /// ## Safety
+    ///
+    /// Gtids must not overlap, and be ordered.
+    /// see https://dev.mysql.com/doc/refman/8.0/en/replication-gtids-concepts.html
     fn add_interval(&mut self, interval: &(u64, u64)) -> Result<(), GtidError> {
         if interval.0 == 0 || interval.1 == 0 {
             return Err(GtidError::ZeroInInterval);