art: avoid constructing boxed arrays on the stack

I was hoping for copy elision here, but it seems like this might
overflow on Windows platforms. Until rust-lang/rust#53827 gets a proper
fix, this may be the best we can do, even if it's uglier.

(We still construct the inner dimension on the stack, which seems fine.)

Author: wchargin

src/art.rs

@@ -552,10 +552,14 @@ impl FlowField {
         ff
     }
 
+    fn constant_flow_field(theta: f64) -> Box<[[f64; FLOW_FIELD_ROWS]; FLOW_FIELD_COLS]> {
+        let vec_of_arrays = vec![[theta; FLOW_FIELD_ROWS]; FLOW_FIELD_COLS];
+        let slice_of_arrays = vec_of_arrays.into_boxed_slice();
+        slice_of_arrays.try_into().unwrap()
+    }
+
     fn raw_linear(default_theta: f64) -> Self {
-        FlowField(Box::new(
-            [[default_theta; FLOW_FIELD_ROWS]; FLOW_FIELD_COLS],
-        ))
+        FlowField(Self::constant_flow_field(default_theta))
     }
 
     fn raw_circular(
@@ -574,7 +578,7 @@ impl FlowField {
         }
         rot = pi(rot);
 
-        let mut flow_points = Box::new([[0.0; FLOW_FIELD_ROWS]; FLOW_FIELD_COLS]);
+        let mut flow_points = Self::constant_flow_field(0.0);
 
         let cx = {
             let fst = rng.uniform(w(0.0), w(1.0));

src/sectors.rs

@@ -41,7 +41,12 @@ impl Sectors {
     pub fn new(config: &Config, left: f64, right: f64, top: f64, bottom: f64) -> Self {
         let ix = Indexer::new(f64::min(left, right), f64::max(left, right));
         let iy = Indexer::new(f64::min(top, bottom), f64::max(top, bottom));
-        let sectors = Box::new(std::array::from_fn(|_| std::array::from_fn(|_| Vec::new())));
+        let sectors = {
+            const EMPTY_SECTOR: Vec<Collider> = Vec::new();
+            let vec_of_arrays = vec![[EMPTY_SECTOR; NUM_SECTORS]; NUM_SECTORS];
+            let slice_of_arrays = vec_of_arrays.into_boxed_slice();
+            slice_of_arrays.try_into().unwrap()
+        };
         Sectors {
             fast_collisions: config.fast_collisions,
             ix,