remove allocations from _next_composite

src/func/instances/stackless.rs

@@ -47,15 +47,30 @@ enum EvalTree<'a> {
 
 type Oet<'a> = Option<EvalTree<'a>>;
 
+struct Temp;
+
+impl Atom for Temp {
+    fn next<'t>(self: Box<Self>) -> Oet<'t>
+    where
+        Self: 't,
+    {
+        unreachable!()
+    }
+}
+
 impl<'a> EvalTree<'a> {
-    fn _next_composite(self, other: Box<Self>) -> Self {
-        match self {
+    fn _next_composite(mut self: Box<Self>, other: Box<Self>) -> Self {
+        match *self {
             Self::Atom(f) => match f.next() {
-                Some(newleft) => Self::Composite(Box::new(newleft), other),
+                Some(newleft) => {
+                    *self = newleft;
+                    Self::Composite(self, other)
+                }
                 None => *other,
             },
             Self::Composite(etll, etlr) => {
-                Self::Composite(etll, Box::new(Self::Composite(etlr, other)))
+                *self = Self::Composite(etlr, other);
+                Self::Composite(etll, self)
             }
         }
     }