use capacity instead of length in list deinit

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

@@ -76,7 +76,7 @@ pub const RocList = extern struct {
     }
 
     pub fn deinit(self: RocList, comptime T: type) void {
-        utils.decref(self.bytes, self.len(), @alignOf(T));
+        utils.decref(self.bytes, self.capacity, @alignOf(T));
     }
 
     pub fn elements(self: RocList, comptime T: type) ?[*]T {
@@ -84,14 +84,21 @@ pub const RocList = extern struct {
     }
 
     pub fn isUnique(self: RocList) bool {
-        // the empty list is unique (in the sense that copying it will not leak memory)
+        return self.refcountMachine() == utils.REFCOUNT_ONE;
-        if (self.isEmpty()) {
+    }
-            return true;
+
+    fn refcountMachine(self: RocList) usize {
+        if (self.capacity == 0) {
+            // the zero-capacity is Clone, copying it will not leak memory
+            return utils.REFCOUNT_ONE;
         }
 
-        // otherwise, check if the refcount is one
         const ptr: [*]usize = @ptrCast([*]usize, @alignCast(@alignOf(usize), self.bytes));
-        return (ptr - 1)[0] == utils.REFCOUNT_ONE;
+        return (ptr - 1)[0];
+    }
+
+    fn refcountHuman(self: RocList) usize {
+        return self.refcountMachine() - utils.REFCOUNT_ONE + 1;
     }
 
     pub fn makeUniqueExtra(self: RocList, alignment: u32, element_width: usize, update_mode: UpdateMode) RocList {
@@ -740,10 +747,14 @@ fn swapElements(source_ptr: [*]u8, element_width: usize, index_1: usize, index_2
 pub fn listConcat(list_a: RocList, list_b: RocList, alignment: u32, element_width: usize) callconv(.C) RocList {
     // NOTE we always use list_a! because it is owned, we must consume it, and it may have unused capacity
     if (list_b.isEmpty()) {
+        if (list_a.capacity == 0) {
+            return list_b;
+        } else {
             // we must consume this list. Even though it has no elements, it could still have capacity
             list_b.deinit(usize);
 
             return list_a;
+        }
     } else if (list_a.isUnique()) {
         const total_length: usize = list_a.len() + list_b.len();
 

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

@@ -324,6 +324,10 @@ pub const RocStr = extern struct {
     }
 
     fn refcountMachine(self: RocStr) usize {
+        if (self.getCapacity() == 0 or self.isSmallStr()) {
+            return utils.REFCOUNT_ONE;
+        }
+
         const ptr: [*]usize = @ptrCast([*]usize, @alignCast(@alignOf(usize), self.str_bytes));
         return (ptr - 1)[0];
     }

crates/valgrind/src/lib.rs

@@ -193,6 +193,22 @@ fn list_concat_consumes_first_argument() {
     valgrind_test("List.concat (List.withCapacity 1024) [1,2,3] |> List.len |> Num.toStr");
 }
 
+#[test]
+fn list_concat_consumes_second_argument() {
+    valgrind_test(indoc!(
+        r#"
+        (
+            a : List U8
+            a = []
+            b = List.reserve [] 11
+            List.concat a b
+            |> List.len
+            |> Num.toStr
+        )
+        "#
+    ));
+}
+
 #[test]
 fn str_capacity_concat() {
     valgrind_test(r#"Str.withCapacity 42 |> Str.concat "foobar""#);