Update List.reserve to no longer use Nat

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

@@ -482,16 +482,22 @@ pub fn listWithCapacity(
 pub fn listReserve(
     list: RocList,
     alignment: u32,
-    spare: usize,
+    spare: u64,
     element_width: usize,
     update_mode: UpdateMode,
 ) callconv(.C) RocList {
-    const old_length = list.len();
+    const original_len = list.len();
-    if ((update_mode == .InPlace or list.isUnique()) and list.getCapacity() >= list.len() + spare) {
+    const cap = @as(u64, @intCast(list.getCapacity()));
+    const desired_cap = @as(u64, @intCast(original_len)) +| spare;
+
+    if ((update_mode == .InPlace or list.isUnique()) and cap >= desired_cap) {
         return list;
     } else {
-        var output = list.reallocate(alignment, old_length + spare, element_width);
+        // Make sure on 32-bit targets we don't accidentally wrap when we cast our U64 desired capacity to U32.
-        output.length = old_length;
+        const reserve_size: u64 = @min(desired_cap, @as(u64, @intCast(std.math.maxInt(usize))));
+
+        var output = list.reallocate(alignment, @as(usize, @intCast(reserve_size)), element_width);
+        output.length = original_len;
         return output;
     }
 }

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

@@ -169,7 +169,7 @@ pub(crate) fn list_get_unsafe<'a, 'ctx>(
     )
 }
 
-/// List.reserve : List elem, Nat -> List elem
+/// List.reserve : List elem, U64 -> List elem
 pub(crate) fn list_reserve<'a, 'ctx>(
     env: &Env<'a, 'ctx, '_>,
     layout_interner: &STLayoutInterner<'a>,

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

@@ -688,7 +688,7 @@ pub(crate) fn run_low_level<'a, 'ctx>(
             list_prepend(env, layout_interner, original_wrapper, elem, elem_layout)
         }
         ListReserve => {
-            // List.reserve : List elem, Nat -> List elem
+            // List.reserve : List elem, U64 -> List elem
             debug_assert_eq!(args.len(), 2);
 
             let (list, list_layout) = scope.load_symbol_and_layout(&args[0]);

crates/compiler/gen_wasm/src/low_level.rs

@@ -514,7 +514,7 @@ impl<'a> LowLevelCall<'a> {
                 //  (return pointer)           i32
                 //  list: RocList              i32
                 //  alignment: u32             i32
-                //  spare: usize               i32
+                //  spare: u64                 i64
                 //  element_width: usize       i32
                 //  update_mode: UpdateMode    i32