Merge pull request #3985 from roc-lang/pass-string-byval-host

allow passing of RocStr by-value from a host

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

@@ -3718,6 +3718,37 @@ fn expose_function_to_host_help_c_abi_v2<'a, 'ctx, 'env>(
         Linkage::External,
     );
 
+    // a temporary solution to be able to pass RocStr by-value from a host language.
+    {
+        let extra = match cc_return {
+            CCReturn::Return => 0,
+            CCReturn::ByPointer => 1,
+            CCReturn::Void => 0,
+        };
+
+        for (i, layout) in arguments.iter().enumerate() {
+            if let Layout::Builtin(Builtin::Str) = layout {
+                // Indicate to LLVM that this argument is semantically passed by-value
+                // even though technically (because of its size) it is passed by-reference
+                let byval_attribute_id = Attribute::get_named_enum_kind_id("byval");
+                debug_assert!(byval_attribute_id > 0);
+
+                // if ret_typ is a pointer type. We need the base type here.
+                let ret_typ = c_function.get_type().get_param_types()[i + extra];
+                let ret_base_typ = if ret_typ.is_pointer_type() {
+                    ret_typ.into_pointer_type().get_element_type()
+                } else {
+                    ret_typ.as_any_type_enum()
+                };
+
+                let byval_attribute = env
+                    .context
+                    .create_type_attribute(byval_attribute_id, ret_base_typ);
+                c_function.add_attribute(AttributeLoc::Param((i + extra) as u32), byval_attribute);
+            }
+        }
+    }
+
     let subprogram = env.new_subprogram(c_function_name);
     c_function.set_subprogram(subprogram);