perf(vm): preallocate map literal HashMap with known pair count 🗺️

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

Author: timfennis

benches/programs/map_literal_build.ndc

@@ -0,0 +1,11 @@
+let total = 0;
+for _ in 0..50_000 {
+    let m = %{
+        1: 1,  2: 2,  3: 3,  4: 4,  5: 5,  6: 6,  7: 7,  8: 8,
+        9: 9, 10: 10, 11: 11, 12: 12, 13: 13, 14: 14, 15: 15, 16: 16,
+        17: 17, 18: 18, 19: 19, 20: 20, 21: 21, 22: 22, 23: 23, 24: 24,
+        25: 25, 26: 26, 27: 27, 28: 28, 29: 29, 30: 30, 31: 31, 32: 32
+    };
+    total += m[1];
+}
+print(total);

ndc_vm/src/vm.rs

@@ -351,7 +351,7 @@ impl Vm {
                         None
                     };
                     let flat = self.stack.split_off(self.stack.len() - pairs * 2);
-                    let mut map = HashMap::new();
+                    let mut map = HashMap::with_capacity(pairs);
                     let mut flat_iter = flat.into_iter();
                     for _ in 0..pairs {
                         let key = flat_iter.next().expect("expected key");