From e963a7251cf734a527a1c5e0af0055305baf50f8 Mon Sep 17 00:00:00 2001
From: Stephen Chung <schungx@live.com>
Date: Mon, 27 Sep 2021 11:09:19 +0800
Subject: [PATCH] Check if empty string came from global instance.

---
 CHANGELOG.md            |  1 +
 src/engine.rs           | 27 +++++++++++++--------------
 src/immutable_string.rs | 23 +++++++++++++++++++++++
 3 files changed, 37 insertions(+), 14 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3015f82d..e3bc1101 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -43,6 +43,7 @@ Enhancements
 
 * `SmartString` now uses `LazyCompact` instead of `Compact` to minimize allocations.
 * Added `pop` for strings.
+* Added `ImmutableString::ptr_eq` to test if two strings point to the same allocation.
 
 ### `Scope` API
 
diff --git a/src/engine.rs b/src/engine.rs
index d0be3696..ac22b5be 100644
--- a/src/engine.rs
+++ b/src/engine.rs
@@ -3044,24 +3044,23 @@ impl Engine {
     }
 
     /// Check a result to ensure that the data size is within allowable limit.
-    fn check_return_value(&self, result: RhaiResult) -> RhaiResult {
-        match result {
+    fn check_return_value(&self, mut result: RhaiResult) -> RhaiResult {
+        if let Ok(ref mut r) = result {
             // Concentrate all empty strings into one instance to save memory
-            #[cfg(feature = "no_closure")]
-            Ok(r) if r.as_str_ref().map_or(false, &str::is_empty) => {
-                Ok(self.const_empty_string().into())
+            if let Dynamic(crate::dynamic::Union::Str(s, _, _)) = r {
+                if s.is_empty() {
+                    if !s.ptr_eq(&self.constants.empty_string) {
+                        *s = self.const_empty_string();
+                    }
+                    return result;
+                }
             }
-            // Concentrate all empty strings into one instance to save memory
-            #[cfg(not(feature = "no_closure"))]
-            Ok(r) if !r.is_shared() && r.as_str_ref().map_or(false, &str::is_empty) => {
-                Ok(self.const_empty_string().into())
-            }
-            // Check data sizes
+
             #[cfg(not(feature = "unchecked"))]
-            Ok(r) => self.check_data_size(&r).map(|_| r),
-            // Return all other results
-            _ => result,
+            self.check_data_size(&r)?;
         }
+
+        result
     }
 
     #[cfg(feature = "unchecked")]
diff --git a/src/immutable_string.rs b/src/immutable_string.rs
index 54e9bf88..8c146897 100644
--- a/src/immutable_string.rs
+++ b/src/immutable_string.rs
@@ -538,4 +538,27 @@ impl ImmutableString {
     pub(crate) fn make_mut(&mut self) -> &mut SmartString {
         shared_make_mut(&mut self.0)
     }
+    /// Returns `true` if the two [`ImmutableString`]'s point to the same allocation.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use rhai::ImmutableString;
+    ///
+    /// let s1: ImmutableString = "hello".into();
+    /// let s2 = s1.clone();
+    /// let s3: ImmutableString = "hello".into();
+    ///
+    /// assert_eq!(s1, s2);
+    /// assert_eq!(s1, s3);
+    /// assert_eq!(s2, s3);
+    ///
+    /// assert!(s1.ptr_eq(&s2));
+    /// assert!(!s1.ptr_eq(&s3));
+    /// assert!(!s2.ptr_eq(&s3));
+    /// ```
+    #[inline(always)]
+    pub fn ptr_eq(&self, other: &Self) -> bool {
+        Shared::ptr_eq(&self.0, &other.0)
+    }
 }