Subtle Semantic Changes After Optimization

Some optimizations can alter subtle semantics of the script.

For example:


#![allow(unused)]
fn main() {
if true {           // condition always true
123.456;        // eliminated
hello;          // eliminated, EVEN THOUGH the variable doesn't exist!
foo(42)         // promoted up-level
}

foo(42)             // <- the above optimizes to this
}


If the original script were evaluated instead, it would have been an error – the variable hello does not exist, so the script would have been terminated at that point with an error return.

In fact, any errors inside a statement that has been eliminated will silently disappear:


#![allow(unused)]
fn main() {
print("start!");
if my_decision { /* do nothing... */ }  // eliminated due to no effect
print("end!");

// The above optimizes to:

print("start!");
print("end!");
}


In the script above, if my_decision holds anything other than a boolean value, the script should have been terminated due to a type error.

However, after optimization, the entire if statement is removed (because an access to my_decision produces no side-effects), thus the script silently runs to completion without errors.

It is usually a Very Bad Idea™ to depend on a script failing or such kind of subtleties, but if it turns out to be necessary (why? I would never guess), turn script optimization off by setting the optimization level to OptimizationLevel::None.