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(", ")
             ),

tests/decrement.rs

@@ -8,7 +8,7 @@ fn test_decrement() -> Result<(), Box<EvalAltResult>> {
 
     assert!(matches!(
         *engine.eval::<String>(r#"let s = "test"; s -= "ing"; s"#).expect_err("expects error"),
-        EvalAltResult::ErrorFunctionNotFound(err, _) if err == "- (string, string)"
+        EvalAltResult::ErrorFunctionNotFound(err, _) if err == "- (&str | ImmutableString, &str | ImmutableString)"
     ));
 
     Ok(())

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(err, _) if err == "foo3 (&str | ImmutableString)"
     ));
 
     Ok(())