diff --git a/mino/src/piece.rs b/mino/src/piece.rs
index c3d756c..c8c2507 100644
--- a/mino/src/piece.rs
+++ b/mino/src/piece.rs
@@ -124,9 +124,11 @@ impl<'c> Cells<'c> {
 
     /// Translates the cells by the given amount in both directions.
     #[inline]
-    pub fn translate(&mut self, dx: i16, dy: i16) {
-        self.x = self.x.checked_add(dx).expect("overflow/underflow");
-        self.y = self.y.checked_add(dy).expect("overflow/underflow");
+    pub fn translate(&self, dx: i16, dy: i16) -> Self {
+        let mut copy = *self;
+        copy.x = self.x.checked_add(dx).expect("overflow/underflow");
+        copy.y = self.y.checked_add(dy).expect("overflow/underflow");
+        copy
     }
 
     #[cfg(test)]
@@ -157,14 +159,14 @@ pub struct Piece<S> {
 impl<'c, S: Shape<'c>> Piece<S> {
     /// Returns the cells occupied by this piece.
     pub fn cells(&self) -> Cells<'c> {
-        let mut cells = self.shape.cells(self.loc.r);
-        cells.translate(self.loc.x, self.loc.y);
-        cells
+        self.shape
+            .cells(self.loc.r)
+            .translate(self.loc.x, self.loc.y)
     }
 }
 
 #[cfg(test)]
-mod test {
+pub mod test {
     use super::*;
     use crate::mat;