Enable modifying constant shared values.

CHANGELOG.md

@@ -41,6 +41,7 @@ Enhancements
 * Originally, the debugger's custom state uses the same state as `EvalState::tag()` (which is the same as `NativeCallContext::tag()`).  It is now split into its own variable accessible under `Debugger::state()`.
 * Non-borrowed string keys can now be deserialized for object maps via `serde`.
 * `Scope::get` is added to get a reference to a variable's value.
+* Variable resolvers can now return a _shared_ value which can be mutated.
 
 
 Version 1.7.0

src/eval/stmt.rs

@@ -250,7 +250,15 @@ impl Engine {
 
                         let var_name = lhs.get_variable_name(false).expect("`Expr::Variable`");
 
-                        if !lhs_ptr.is_ref() {
+                        #[cfg(not(feature = "no_closure"))]
+                        // Temp results from expressions are flattened so should never be shared.
+                        // A shared value may be provided by a variable resolver, however.
+                        let is_temp_result = !lhs_ptr.is_ref() && !lhs_ptr.is_shared();
+                        #[cfg(feature = "no_closure")]
+                        let is_temp_result = !lhs_ptr.is_ref();
+
+                        // Cannot assign to temp result from expression
+                        if is_temp_result {
                             return Err(
                                 ERR::ErrorAssignmentToConstant(var_name.to_string(), pos).into()
                             );
@@ -950,7 +958,7 @@ impl Engine {
                         if !export.is_empty() {
                             if !module.is_indexed() {
                                 // Index the module (making a clone copy if necessary) if it is not indexed
-                                let mut m = crate::func::native::shared_take_or_clone(module);
+                                let mut m = crate::func::shared_take_or_clone(module);
                                 m.build_index();
                                 global.push_import(export.name.clone(), m);
                             } else {

src/types/dynamic.rs

@@ -1003,18 +1003,25 @@ impl Dynamic {
     ///
     /// Constant [`Dynamic`] values are read-only.
     ///
-    /// If a [`&mut Dynamic`][Dynamic] to such a constant is passed to a Rust function, the function
-    /// can use this information to return an error of
-    /// [`ErrorAssignmentToConstant`][crate::EvalAltResult::ErrorAssignmentToConstant] if its value
-    /// is going to be modified.
+    /// # Usage
     ///
-    /// This safe-guards constant values from being modified from within Rust functions.
+    /// If a [`&mut Dynamic`][Dynamic] to such a constant is passed to a Rust function, the function
+    /// can use this information to return the error
+    /// [`ErrorAssignmentToConstant`][crate::EvalAltResult::ErrorAssignmentToConstant] if its value
+    /// will be modified.
+    ///
+    /// This safe-guards constant values from being modified within Rust functions.
+    ///
+    /// # Shared Values
+    ///
+    /// If a [`Dynamic`] holds a _shared_ value, then it is read-only only if the shared value
+    /// itself is read-only.
     #[must_use]
     pub fn is_read_only(&self) -> bool {
         #[cfg(not(feature = "no_closure"))]
         match self.0 {
-            Union::Shared(.., ReadOnly) => return true,
-
+            // Shared values do not consider the current access mode
+            //Union::Shared(.., ReadOnly) => return true,
             Union::Shared(ref cell, ..) => {
                 return match locked_read(cell).access_mode() {
                     ReadWrite => false,

tests/var_scope.rs

@@ -1,4 +1,4 @@
-use rhai::{Engine, EvalAltResult, ParseErrorType, Position, Scope, INT};
+use rhai::{Dynamic, Engine, EvalAltResult, ParseErrorType, Position, Scope, INT};
 
 #[test]
 fn test_var_scope() -> Result<(), Box<EvalAltResult>> {
@@ -162,9 +162,16 @@ fn test_var_resolver() -> Result<(), Box<EvalAltResult>> {
     scope.push("chameleon", 123 as INT);
     scope.push("DO_NOT_USE", 999 as INT);
 
-    engine.on_var(|name, _, context| {
+    #[cfg(not(feature = "no_closure"))]
+    let mut base = Dynamic::ONE.into_shared();
+    #[cfg(not(feature = "no_closure"))]
+    let shared = base.clone();
+
+    engine.on_var(move |name, _, context| {
         match name {
             "MYSTIC_NUMBER" => Ok(Some((42 as INT).into())),
+            #[cfg(not(feature = "no_closure"))]
+            "HELLO" => Ok(Some(shared.clone())),
             // Override a variable - make it not found even if it exists!
             "DO_NOT_USE" => {
                 Err(EvalAltResult::ErrorVariableNotFound(name.to_string(), Position::NONE).into())
@@ -186,6 +193,18 @@ fn test_var_resolver() -> Result<(), Box<EvalAltResult>> {
         engine.eval_with_scope::<INT>(&mut scope, "MYSTIC_NUMBER")?,
         42
     );
+
+    #[cfg(not(feature = "no_closure"))]
+    {
+        assert_eq!(engine.eval_with_scope::<INT>(&mut scope, "HELLO")?, 1);
+        *base.write_lock::<INT>().unwrap() = 42;
+        assert_eq!(engine.eval_with_scope::<INT>(&mut scope, "HELLO")?, 42);
+        assert_eq!(
+            engine.eval_with_scope::<INT>(&mut scope, "HELLO = HELLO * 2; HELLO")?,
+            84
+        );
+    }
+
     assert_eq!(engine.eval_with_scope::<INT>(&mut scope, "chameleon")?, 1);
     assert!(
         matches!(*engine.eval_with_scope::<INT>(&mut scope, "DO_NOT_USE").expect_err("should error"),