make sure to check raw capacity when decrementing refcounts

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

@@ -393,19 +393,18 @@ pub(crate) fn list_len<'ctx>(
         .into_int_value()
 }
 
-/// List.capacity : List * -> Nat
-pub(crate) fn list_capacity<'a, 'ctx, 'env>(
-    env: &Env<'a, 'ctx, 'env>,
+pub(crate) fn list_capacity_or_ref_ptr<'ctx>(
+    builder: &Builder<'ctx>,
     wrapper_struct: StructValue<'ctx>,
 ) -> IntValue<'ctx> {
-    call_list_bitcode_fn(
-        env,
-        &[wrapper_struct],
-        &[],
-        BitcodeReturns::Basic,
-        bitcode::LIST_CAPACITY,
-    )
-    .into_int_value()
+    builder
+        .build_extract_value(
+            wrapper_struct,
+            Builtin::WRAPPER_CAPACITY,
+            "list_capacity_or_ref_ptr",
+        )
+        .unwrap()
+        .into_int_value()
 }
 
 // Gets a pointer to just after the refcount for a list or seamless slice.

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

@@ -6,7 +6,7 @@ use crate::llvm::build::{
     FAST_CALL_CONV,
 };
 use crate::llvm::build_list::{
-    incrementing_elem_loop, list_capacity, list_refcount_ptr, load_list,
+    incrementing_elem_loop, list_capacity_or_ref_ptr, list_refcount_ptr, load_list,
 };
 use crate::llvm::convert::{basic_type_from_layout, zig_str_type, RocUnion};
 use bumpalo::collections::Vec;
@@ -693,7 +693,8 @@ fn modify_refcount_list_help<'a, 'ctx, 'env>(
     let parent = fn_val;
     let original_wrapper = arg_val.into_struct_value();
 
-    let capacity = list_capacity(env, original_wrapper);
+    // We use the raw capacity to ensure we always decrement the refcount of seamless slices.
+    let capacity = list_capacity_or_ref_ptr(builder, original_wrapper);
 
     let is_non_empty = builder.build_int_compare(
         IntPredicate::UGT,