add seamless slices for str

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

@@ -63,3 +63,19 @@ pub(crate) fn str_equal<'a, 'ctx, 'env>(
         bitcode::STR_EQUAL,
     )
 }
+
+// Gets a pointer to just after the refcount for a list or seamless slice.
+// The value is just after the refcount so that normal lists and seamless slices can share code paths easily.
+pub(crate) fn str_refcount_ptr<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    value: BasicValueEnum<'ctx>,
+) -> PointerValue<'ctx> {
+    call_str_bitcode_fn(
+        env,
+        &[value],
+        &[],
+        BitcodeReturns::Basic,
+        bitcode::STR_REFCOUNT_PTR,
+    )
+    .into_pointer_value()
+}