Fix use after free when dropping zerowidth data

Commit ec42b42ce6 added an optional
pointer for each cell, thus some old code that was optimizing copying
with 'ptr::copy' was duplicating "unique" pointers ('Box'), which was
resulting in use after free, after attempting to free both of these
pointers.

By replacing these unsafe blocks with safe Rust, the issue itself is
fixed and the potential for future memory problems is eliminated from
this area of the code.

alacritty_terminal/src/term/mod.rs

@@ -1486,12 +1486,10 @@ impl<T: EventListener> Handler for Term<T> {
         if self.mode.contains(TermMode::INSERT) && self.grid.cursor.point.col + width < num_cols {
             let line = self.grid.cursor.point.line;
             let col = self.grid.cursor.point.col;
-            let line = &mut self.grid[line];
+            let row = &mut self.grid[line][..];
 
-            let src = line[col..].as_ptr();
-            let dst = line[(col + width)..].as_mut_ptr();
-            unsafe {
-                ptr::copy(src, dst, (num_cols - col - width).0);
+            for col in (col.0..(num_cols - width).0).rev() {
+                row.swap(col + width, col);
             }
         }
 
@@ -1574,18 +1572,15 @@ impl<T: EventListener> Handler for Term<T> {
         let num_cells = (self.cols() - destination).0;
 
         let line = cursor.point.line;
-        let row = &mut self.grid[line];
+        let row = &mut self.grid[line][..];
 
-        unsafe {
-            let src = row[source..].as_ptr();
-            let dst = row[destination..].as_mut_ptr();
-
-            ptr::copy(src, dst, num_cells);
+        for offset in (0..num_cells).rev() {
+            row.swap(destination.0 + offset, source.0 + offset);
         }
 
         // Cells were just moved out toward the end of the line;
         // fill in between source and dest with blanks.
-        for cell in &mut row[source..destination] {
+        for cell in &mut row[source.0..destination.0] {
             *cell = bg.into();
         }
     }
@@ -1841,21 +1836,18 @@ impl<T: EventListener> Handler for Term<T> {
 
         let start = cursor.point.col;
         let end = min(start + count, cols - 1);
-        let n = (cols - end).0;
+        let num_cells = (cols - end).0;
 
         let line = cursor.point.line;
-        let row = &mut self.grid[line];
+        let row = &mut self.grid[line][..];
 
-        unsafe {
-            let src = row[end..].as_ptr();
-            let dst = row[start..].as_mut_ptr();
-
-            ptr::copy(src, dst, n);
+        for offset in 0..num_cells {
+            row.swap(start.0 + offset, end.0 + offset);
         }
 
         // Clear last `count` cells in the row. If deleting 1 char, need to delete
         // 1 cell.
-        let end = cols - count;
+        let end = (cols - count).0;
         for cell in &mut row[end..] {
             *cell = bg.into();
         }