Implement Selection::replace to replace a single range

Fixes #985

Co-authored-by: Daniel S Poulin <crimsonmage+github@gmail.com>

helix-core/src/selection.rs

@@ -362,6 +362,11 @@ impl Selection {
 
     /// Adds a new range to the selection and makes it the primary range.
     pub fn remove(mut self, index: usize) -> Self {
+        assert!(
+            self.ranges.len() > 1,
+            "can't remove the last range from a selection!"
+        );
+
         self.ranges.remove(index);
         if index < self.primary_index || self.primary_index == self.ranges.len() {
             self.primary_index -= 1;
@@ -369,6 +374,12 @@ impl Selection {
         self
     }
 
+    /// Replace a range in the selection with a new range.
+    pub fn replace(mut self, index: usize, range: Range) -> Self {
+        self.ranges[index] = range;
+        self.normalize()
+    }
+
     /// Map selections over a set of changes. Useful for adjusting the selection position after
     /// applying changes to a document.
     pub fn map(self, changes: &ChangeSet) -> Self {

helix-term/src/commands.rs

@@ -1227,8 +1227,7 @@ fn search_impl(
             Movement::Extend => selection.clone().push(Range::new(start, end)),
             Movement::Move => selection
                 .clone()
-                .remove(selection.primary_index())
-                .push(Range::new(start, end)),
+                .replace(selection.primary_index(), Range::new(start, end)),
         };
 
         doc.set_selection(view.id, selection);