Script Optimization

Rhai includes an optimizer that tries to optimize a script after parsing. This can reduce resource utilization and increase execution speed.

Script optimization can be turned off via the no_optimize feature.

Optimization Levels

There are three levels of optimization: None, Simple and Full. The default is Simple.

An Engine’s optimization level is set via Engine::set_optimization_level.

// Turn on aggressive optimizations


None is obvious – no optimization on the AST is performed.

Simple (Default)

Simple performs only relatively safe optimizations without causing side-effects (i.e. it only relies on static analysis and built-in operators for constant standard types, and will not perform any external function calls).


After constants propagation is performed, if the constants are then modified (yes, it is possible, via Rust functions), the modified values will not show up in the optimized script.

Only the initialization values of constants are ever retained.


Overriding a built-in operator in the Engine afterwards has no effect after the optimizer replaces an expression with its calculated value.


Full is much more aggressive, including calling external functions on constant arguments to determine their results.

One benefit to this is that many more optimization opportunities arise, especially with regards to comparison operators.