Use unsafe Array initialization to avoid manually counting number of …

…None's

The number of standard gates is specified by STANDARD_GATE_SIZE. To add a gate, the dev
has to read the value from one source file, then add (possible count) `None`s in two
places in another file to initialize arrays. After learning it all once it's not so bad,
but it still could be better.

After this commit the dev is required to change only the value of STANDARD_GATE_SIZE.

crates/circuit/src/imports.rs

@@ -100,19 +100,28 @@ static STDGATE_IMPORT_PATHS: [[&str; 2]; STANDARD_GATE_SIZE] = [
 static mut STDGATE_PYTHON_GATES: GILOnceCell<[Option<PyObject>; STANDARD_GATE_SIZE]> =
     GILOnceCell::new();
 
+// A fixed size array is initialized like this because using the `[T; 5]` syntax
+// requires T to be `Copy`. But `PyObject` isn't Copy so therefore Option<PyObject>
+// as T isn't Copy.
+fn _array_of_option_pyobject() -> [Option<PyObject>; STANDARD_GATE_SIZE] {
+    use std::mem::{self, MaybeUninit};
+    let array = unsafe {
+        let mut data: [MaybeUninit<Option<PyObject>>; STANDARD_GATE_SIZE] = MaybeUninit::uninit().assume_init();
+        for elem in &mut data[..] {
+            *elem = MaybeUninit::new(None);
+        }
+        mem::transmute::<_, [Option<PyObject>; STANDARD_GATE_SIZE]>(data)
+    };
+    array
+}
+
 #[inline]
 pub fn populate_std_gate_map(py: Python, rs_gate: StandardGate, py_gate: PyObject) {
     let gate_map = unsafe {
         match STDGATE_PYTHON_GATES.get_mut() {
             Some(gate_map) => gate_map,
             None => {
-                // A fixed size array is initialized like this because using the `[T; 5]` syntax
-                // requires T to be `Copy`. But `PyObject` isn't Copy so therefore Option<PyObject>
-                // as T isn't Copy. To avoid that we just list out None STANDARD_GATE_SIZE times
-                let array: [Option<PyObject>; STANDARD_GATE_SIZE] = [
-                    None, None, None, None, None, None, None, None, None, None, None, None, None,
-                    None, None, None, None, None, None,
-                ];
+                let array = _array_of_option_pyobject();
                 STDGATE_PYTHON_GATES.set(py, array).unwrap();
                 STDGATE_PYTHON_GATES.get_mut().unwrap()
             }
@@ -127,16 +136,7 @@ pub fn populate_std_gate_map(py: Python, rs_gate: StandardGate, py_gate: PyObjec
 #[inline]
 pub fn get_std_gate_class(py: Python, rs_gate: StandardGate) -> PyResult<PyObject> {
     let gate_map = unsafe {
-        STDGATE_PYTHON_GATES.get_or_init(py, || {
-            // A fixed size array is initialized like this because using the `[T; 5]` syntax
-            // requires T to be `Copy`. But `PyObject` isn't Copy so therefore Option<PyObject>
-            // as T isn't Copy. To avoid that we just list out None STANDARD_GATE_SIZE times
-            let array: [Option<PyObject>; STANDARD_GATE_SIZE] = [
-                None, None, None, None, None, None, None, None, None, None, None, None, None, None,
-                None, None, None, None, None,
-            ];
-            array
-        })
+        STDGATE_PYTHON_GATES.get_or_init(py, _array_of_option_pyobject)
     };
     let gate = &gate_map[rs_gate as usize];
     let populate = gate.is_none();