Better String parameter error message.

src/engine.rs

@@ -708,7 +708,11 @@ impl Engine {
                 "{} ({})",
                 fn_name,
                 args.iter()
-                    .map(|name| self.map_type_name(name.type_name()))
+                    .map(|name| if name.is::<ImmutableString>() {
+                        "&str | ImmutableString"
+                    } else {
+                        self.map_type_name(name.type_name())
+                    })
                     .collect::<Vec<_>>()
                     .join(", ")
             ),

src/fn_register.rs

@@ -101,10 +101,10 @@ pub fn by_ref<T: Variant + Clone>(data: &mut Dynamic) -> &mut T {
 #[inline(always)]
 pub fn by_value<T: Variant + Clone>(data: &mut Dynamic) -> T {
     if TypeId::of::<T>() == TypeId::of::<&str>() {
-        // &str parameters are mapped to the underlying ImmutableString
+        // If T is &str, data must be ImmutableString, so map directly to it
-        let r = data.as_str().unwrap();
+        let ref_str = data.as_str().unwrap();
-        let x = unsafe { mem::transmute::<_, &T>(&r) };
+        let ref_T = unsafe { mem::transmute::<_, &T>(&ref_str) };
-        x.clone()
+        ref_T.clone()
     } else {
         // We consume the argument and then replace it with () - the argument is not supposed to be used again.
         // This way, we avoid having to clone the argument again, because it is already a clone when passed here.

tests/string.rs

@@ -168,7 +168,7 @@ fn test_string_fn() -> Result<(), Box<EvalAltResult>> {
 
     assert!(matches!(
         *engine.eval::<INT>(r#"foo3("hello")"#).expect_err("should error"),
-        EvalAltResult::ErrorFunctionNotFound(ref x, _) if x == "foo3 (string)"
+        EvalAltResult::ErrorFunctionNotFound(ref x, _) if x == "foo3 (&str | ImmutableString)"
     ));
 
     Ok(())