Update List.dropAt to no longer use Nat

crates/compiler/builtins/bitcode/src/list.zig

@@ -698,28 +698,33 @@ pub fn listDropAt(
     list: RocList,
     alignment: u32,
     element_width: usize,
-    drop_index: usize,
+    drop_index_u64: u64,
     dec: Dec,
 ) callconv(.C) RocList {
     const size = list.len();
+    const size_u64 = @as(u64, @intCast(size));
     // If droping the first or last element, return a seamless slice.
     // For simplicity, do this by calling listSublist.
     // In the future, we can test if it is faster to manually inline the important parts here.
-    if (drop_index == 0) {
+    if (drop_index_u64 == 0) {
         return listSublist(list, alignment, element_width, 1, size -| 1, dec);
-    } else if (drop_index == size -| 1) {
+    } else if (drop_index_u64 == size_u64 - 1) { // It's fine if (size - 1) wraps on size == 0 here,
+        // because if size is 0 then it's always fine for this branch to be taken; no
+        // matter what drop_index was, we're size == 0, so empty list will always be returned.
         return listSublist(list, alignment, element_width, 0, size -| 1, dec);
     }
 
     if (list.bytes) |source_ptr| {
-        if (drop_index >= size) {
+        if (drop_index_u64 >= size_u64) {
             return list;
         }
 
-        if (drop_index < size) {
-            const element = source_ptr + drop_index * element_width;
-            dec(element);
-        }
+        // This cast must be lossless, because we would have just early-returned if drop_index
+        // were >= than `size`, and we know `size` fits in usize.
+        const drop_index = @as(usize, @intCast(drop_index_u64));
+
+        const element = source_ptr + drop_index * element_width;
+        dec(element);
 
         // NOTE
         // we need to return an empty list explicitly,

crates/compiler/gen_dev/src/lib.rs

@@ -1991,7 +1991,7 @@ trait Backend<'a> {
                 //    list: RocList,
                 //    alignment: u32,
                 //    element_width: usize,
-                //    drop_index: usize,
+                //    drop_index: u64,
                 //    dec: Dec,
 
                 self.build_fn_call(
@@ -2008,7 +2008,7 @@ trait Backend<'a> {
                         list_layout,
                         Layout::U32,
                         layout_usize,
-                        layout_usize,
+                        Layout::U64,
                         layout_usize,
                     ],
                     ret_layout,

crates/compiler/gen_llvm/src/llvm/build_list.rs

@@ -299,7 +299,7 @@ pub(crate) fn list_sublist<'a, 'ctx>(
     )
 }
 
-/// List.dropAt : List elem, Nat -> List elem
+/// List.dropAt : List elem, U64 -> List elem
 pub(crate) fn list_drop_at<'a, 'ctx>(
     env: &Env<'a, 'ctx, '_>,
     layout_interner: &STLayoutInterner<'a>,

crates/compiler/gen_wasm/src/low_level.rs

@@ -722,7 +722,7 @@ impl<'a> LowLevelCall<'a> {
                 //  list: RocList,             i32
                 //  element_width: usize,      i32
                 //  alignment: u32,            i32
-                //  drop_index: usize,         i32
+                //  drop_index: u64,           i64
                 //  dec: Dec,                  i32
 
                 // Load the return pointer and the list