type Cell = u8;

pub struct VM {
    cells: [Cell; 30_000],
    data_pointer: usize,
}


impl VM {
    pub fn new() -> VM {
        VM { cells: [0; 30_000], data_pointer: 0 }
    }


    pub fn output(&self) -> Cell {
        self.cells[self.data_pointer]
    }


    pub fn increment(&mut self) {
        self.cells[self.data_pointer] = self.cells[self.data_pointer] + 1;
    }


    pub fn decrement(&mut self) {
        self.cells[self.data_pointer] = self.cells[self.data_pointer] - 1;
    }


    pub fn right_shift(&mut self) {
        self.data_pointer = self.data_pointer + 1;
    }


    pub fn left_shift(&mut self) {
        self.data_pointer = self.data_pointer - 1;
    }
}

type Cell = u8;

pub struct VM {
    cells: [Cell; 30_000],
    data_pointer: usize,
}


impl VM {
    pub fn new() -> VM {
        VM { cells: [0; 30_000], data_pointer: 0 }
    }


    pub fn output(&self) -> Cell {
        self.cells[self.data_pointer]
    }


    pub fn increment(&mut self) {
        let value = self.cells[self.data_pointer];
        self.cells[self.data_pointer] = value + 1;
    }


    pub fn decrement(&mut self) {
        self.cells[self.data_pointer] = self.cells[self.data_pointer] - 1;
    }


    pub fn right_shift(&mut self) {
        self.data_pointer = self.data_pointer + 1;
    }


    pub fn left_shift(&mut self) {
        self.data_pointer = self.data_pointer - 1;
    }
}

--- 
+++ 
@@ -18,8 +18,7 @@
 
 
     pub fn increment(&mut self) {
-        let value = self.cells[self.data_pointer];
-        self.cells[self.data_pointer] = value + 1;
+        self.cells[self.data_pointer] = self.cells[self.data_pointer] + 1;
     }
 
 
Simplify definition of increment function
