Fix builds.

src/engine.rs

@@ -4,8 +4,11 @@ use crate::api::custom_syntax::CustomSyntax;
 use crate::func::native::{OnDebugCallback, OnParseTokenCallback, OnPrintCallback, OnVarCallback};
 use crate::packages::{Package, StandardPackage};
 use crate::tokenizer::Token;
-use crate::types::dynamic::map_std_type_name;
-use crate::{Identifier, ImmutableString, Module, Position, RhaiError, Shared, StaticVec, ERR};
+use crate::types::dynamic::{map_std_type_name, Union};
+use crate::{
+    Dynamic, Identifier, ImmutableString, Module, Position, RhaiError, RhaiResult, Shared,
+    StaticVec, ERR,
+};
 #[cfg(feature = "no_std")]
 use std::prelude::v1::*;
 use std::{
@@ -287,6 +290,31 @@ impl Engine {
         self.empty_string.clone()
     }
 
+    /// Check a result to ensure that it is valid.
+    pub(crate) fn check_return_value(&self, mut result: RhaiResult, pos: Position) -> RhaiResult {
+        let _pos = pos;
+
+        match result {
+            Ok(ref mut r) => {
+                // Concentrate all empty strings into one instance to save memory
+                if let Dynamic(Union::Str(s, _, _)) = r {
+                    if s.is_empty() {
+                        if !s.ptr_eq(&self.empty_string) {
+                            *s = self.const_empty_string();
+                        }
+                        return result;
+                    }
+                }
+
+                #[cfg(not(feature = "unchecked"))]
+                self.check_data_size(&r, _pos)?;
+            }
+            _ => (),
+        }
+
+        result
+    }
+
     /// Pretty-print a type name.
     ///
     /// If a type is registered via [`register_type_with_name`][Engine::register_type_with_name],