WIP adding unique mutable case

compiler/builtins/bitcode/src/list.zig

@@ -852,7 +852,33 @@ pub fn listDropAt(
             dec(element);
         }
 
-        const output = RocList.allocate(alignment, (size - 1), element_width);
+        // NOTE
+        // we need to return an empty list explicitly,
+        // because we rely on the pointer field being null if the list is empty
+        // which also requires duplicating the utils.decref call to spend the RC token
+        if (size < 2) {
+            utils.decref(list.bytes, size * element_width, alignment);
+            return RocList.empty();
+        }
+
+        if (list.isUnique()) {
+            var i = drop_index;
+            while (i < size) : (i += 1) {
+                const copy_target = source_ptr + i * element_width;
+                const copy_source = copy_target + element_width;
+                @memcpy(copy_target, copy_source, element_width);
+            }
+
+            var new_list = list;
+
+            new_list.length -= 1;
+            return new_list;
+        }
+
+        const stdout = std.io.getStdOut().writer();
+        stdout.print("Hit non-unique branch with list, {any}!\n", .{list}) catch unreachable;
+
+        const output = RocList.allocate(alignment, size - 1, element_width);
         const target_ptr = output.bytes orelse unreachable;
 
         const head_size = drop_index * element_width;

compiler/test_gen/src/gen_list.rs

@@ -211,6 +211,25 @@ fn list_drop_at() {
         RocList<i64>
     );
     assert_evals_to!("List.dropAt [] 1", RocList::from_slice(&[]), RocList<i64>);
+    assert_evals_to!("List.dropAt [0] 0", RocList::from_slice(&[]), RocList<i64>);
+
+    assert_evals_to!(
+        indoc!(
+            r#"
+               list : List I64
+               list = [ 1, 2, 3 ]
+
+               { newList: List.dropAt list 0, original: list }
+               "#
+        ),
+        (
+            // new_list
+            RocList::from_slice(&[2, 3]),
+            // original
+            RocList::from_slice(&[1, 2, 3]),
+        ),
+        (RocList<i64>, RocList<i64>,)
+    );
 }
 
 #[test]