Add feature to disable symbols.

doc/src/SUMMARY.md

@@ -95,17 +95,18 @@ The Rhai Scripting Language
    8. [Maximum Call Stack Depth](safety/max-call-stack.md)
    9. [Maximum Statement Depth](safety/max-stmt-depth.md)
 8. [Advanced Topics](advanced.md)
-   1. [Object-Oriented Programming (OOP)](language/oop.md)
-   2. [Serialization/Deserialization of `Dynamic` with `serde`](rust/serde.md)
-   3. [Script Optimization](engine/optimize/index.md)
+   1. [Disable Keywords and/or Operators](engine/disable.md)
+   2. [Object-Oriented Programming (OOP)](language/oop.md)
+   3. [Serialization/Deserialization of `Dynamic` with `serde`](rust/serde.md)
+   4. [Script Optimization](engine/optimize/index.md)
       1. [Optimization Levels](engine/optimize/optimize-levels.md)
       2. [Re-Optimize an AST](engine/optimize/reoptimize.md)
       3. [Eager Function Evaluation](engine/optimize/eager.md)
       4. [Side-Effect Considerations](engine/optimize/side-effects.md)
       5. [Volatility Considerations](engine/optimize/volatility.md)
       6. [Subtle Semantic Changes](engine/optimize/semantics.md)
-   4. [Eval Statement](language/eval.md)
-9. [Appendix](appendix/index.md)
-   1. [Keywords](appendix/keywords.md)
-   2. [Operators](appendix/operators.md)
-   3. [Literals](appendix/literals.md)
+   5. [Eval Statement](language/eval.md)
+9.  [Appendix](appendix/index.md)
+   6. [Keywords](appendix/keywords.md)
+   7. [Operators](appendix/operators.md)
+   8. [Literals](appendix/literals.md)

doc/src/about/features.md

@@ -64,3 +64,5 @@ Flexible
 * Support for [minimal builds] by excluding unneeded language [features].
 
 * Supports [most build targets](targets.md) including `no-std` and [WASM].
+
+* Surgically [disable keywords and operators] to restrict the language.

doc/src/engine/disable.md

@@ -0,0 +1,28 @@
+Disable Certain Keywords and/or Operators
+========================================
+
+{{#include ../links.md}}
+
+For certain embedded usage, it is sometimes necessary to restrict the language to a strict subset of Rhai
+to prevent usage of certain language features.
+
+Rhai supports surgically disabling a keyword or operator via the `Engine::disable_symbol` method.
+
+```rust
+use rhai::Engine;
+
+let mut engine = Engine::new();
+
+engine.disable_symbol("if");    // disable the 'if' keyword
+engine.disable_symbol("+=");    // disable the '+=' operator
+
+// The following all return parse errors.
+
+engine.compile("let x = if true { 42 } else { 0 };")?;
+//                        ^ missing ';' after statement end
+//                      ^ 'if' is parsed as a variable name
+
+engine.compile("let x = 40 + 2; x += 1;")?;
+//                                ^ '+=' is not recognized as an operator
+//                         ^ other operators are not affected
+```

doc/src/links.md

@@ -102,3 +102,5 @@
 [`OptimizationLevel::Full`]: {{rootUrl}}/engine/optimize/optimize-levels.md
 [`OptimizationLevel::Simple`]: {{rootUrl}}/engine/optimize/optimize-levels.md
 [`OptimizationLevel::None`]: {{rootUrl}}/engine/optimize/optimize-levels.md
+
+[disable keywords and operators]: {{rootUrl}}/engine/disable.md