Fix most of deref logic

Still doesn't handle empty lists properly.

compiler/build/src/link.rs

@@ -466,8 +466,8 @@ fn link_macos(
                 "-lc++",
                 // "-lc++abi",
                 // "-lunwind", // TODO will eventually need this, see https://github.com/rtfeldman/roc/pull/554#discussion_r496370840
-                "-framework",
+                //"-framework",
-                "Security", // This "-framework Security" arg is needed for the `rand` crate in examples/cli
+                //"Security", // This "-framework Security" arg is needed for the `rand` crate in examples/cli
                 // Output
                 "-o",
                 output_path.to_str().unwrap(), // app

compiler/builtins/bitcode/src/list.zig

@@ -665,26 +665,25 @@ pub fn listAppend(list: RocList, alignment: usize, element: Opaque, element_widt
     return output;
 }
 
-pub fn listDrop(list: RocList, alignment: usize, element_width: usize, count: usize) callconv(.C) RocList {
+pub fn listDrop(list: RocList, alignment: usize, element_width: usize, drop_count: usize, dec: Dec) callconv(.C) RocList {
     const size = list.len();
-    if (size <= count) {
+    const keep_count = size - drop_count;
+    if (size <= drop_count) {
         return RocList.empty();
     }
     if (list.bytes) |source_ptr| {
-        var i: usize = 0;
+        const output = RocList.allocate(std.heap.c_allocator, alignment, keep_count, element_width);
-
-        const output = RocList.allocate(std.heap.c_allocator, alignment, size - count, element_width);
         const target_ptr = output.bytes orelse unreachable;
 
-        while (i < size - count) : (i += 1) {
+        @memcpy(target_ptr, source_ptr + drop_count * element_width, keep_count * element_width);
-            @memcpy(target_ptr + (i * element_width), source_ptr + ((i + count) * element_width), element_width);
+
+        var i: usize = 0;
+        while (i < drop_count) : (i += 1) {
+            const element = source_ptr + i * element_width;
+            dec(element);
         }
 
-        // if (list.isUnique()) {
+        utils.decref(std.heap.c_allocator, alignment, list.bytes, size * element_width);
-        //     std.heap.c_allocator.free(source_ptr);
-        // } else {
-        //     utils.decref(std.heap.c_allocator, alignment, list.bytes, size * element_width);
-        // }
 
         return output;
     } else {

compiler/gen/src/llvm/build.rs

@@ -3891,15 +3891,18 @@ fn run_low_level<'a, 'ctx, 'env>(
             let original_wrapper = list.into_struct_value();
 
             let count = load_symbol(scope, &args[1]);
-            let inplace = get_inplace_from_layout(layout);
 
-            list_drop(
+            match list_layout {
-                env,
+                Layout::Builtin(Builtin::EmptyList) => empty_list(env),
-                inplace,
+                Layout::Builtin(Builtin::List(_, element_layout)) => list_drop(
-                original_wrapper,
+                    env,
-                count.into_int_value(),
+                    layout_ids,
-                list_layout,
+                    original_wrapper,
-            )
+                    count.into_int_value(),
+                    element_layout,
+                ),
+                _ => unreachable!("Invalid layout {:?} in List.drop", list_layout),
+            }
         }
         ListPrepend => {
             // List.prepend : List elem, elem -> List elem

compiler/gen/src/llvm/build_list.rs

@@ -325,27 +325,12 @@ pub fn list_append<'a, 'ctx, 'env>(
 /// List.drop : List elem, Nat -> List elem
 pub fn list_drop<'a, 'ctx, 'env>(
     env: &Env<'a, 'ctx, 'env>,
-    _inplace: InPlace,
+    layout_ids: &mut LayoutIds<'a>,
     original_wrapper: StructValue<'ctx>,
     count: IntValue<'ctx>,
-    list_layout: &Layout<'a>,
+    element_layout: &Layout<'a>,
 ) -> BasicValueEnum<'ctx> {
-    let (_, element_layout) = match *list_layout {
+    let dec_element_fn = build_dec_wrapper(env, layout_ids, &element_layout);
-        Layout::Builtin(Builtin::EmptyList) => (
-            InPlace::InPlace,
-            // this pointer will never actually be dereferenced
-            Layout::Builtin(Builtin::Int64),
-        ),
-        Layout::Builtin(Builtin::List(memory_mode, elem_layout)) => (
-            match memory_mode {
-                MemoryMode::Unique => InPlace::InPlace,
-                MemoryMode::Refcounted => InPlace::Clone,
-            },
-            *elem_layout,
-        ),
-
-        _ => unreachable!("Invalid layout {:?} in List.drop", list_layout),
-    };
     call_bitcode_fn_returns_list(
         env,
         &[
@@ -353,6 +338,7 @@ pub fn list_drop<'a, 'ctx, 'env>(
             alignment_intvalue(env, &element_layout),
             layout_width(env, &element_layout),
             count.into(),
+            dec_element_fn.as_global_value().as_pointer_value().into(),
         ],
         &bitcode::LIST_DROP,
     )