Re-Optimize an AST

Sometimes it is more efficient to store one single, large script with delimited code blocks guarded by constant variables. This script is compiled once to an AST.

Then, depending on the execution environment, constants are passed into the Engine and the AST is re-optimized based on those constants via the Engine::optimize_ast method, effectively pruning out unused code sections.

The final, optimized AST is then used for evaluations.

#![allow(unused)]
fn main() {
// Compile master script to AST
let master_ast = engine.compile(
r"
    if SCENARIO == 1 {
        do_work();
    } else if SCENARIO == 2 {
        do_something();
    } else if SCENARIO == 3 {
        do_something_else();
    } else {
        do_nothing();
    }
")?;

// Create a new 'Scope' - put constants in it to aid optimization
let mut scope = Scope::new();
scope.push_constant("SCENARIO", 1_i64);

// Re-optimize the AST
let new_ast = engine.optimize_ast(&scope, master_ast.clone(), OptimizationLevel::Simple);

// 'new_ast' is essentially: 'do_work()'
}