Dead Code Elimination
Nobody deliberately writes scripts with dead code (we hope).
They are, however, extremely common in template-based machine-generated scripts.
Rhai attempts to eliminate dead code.
“Dead code” is code that does nothing and has no side effects.
Example is an pure expression by itself as a statement (allowed in Rhai). The result of the expression is calculated then immediately discarded and not used.
{
let x = 999; // NOT eliminated: variable may be used later on (perhaps even an 'eval')
123; // eliminated: no effect
"hello"; // eliminated: no effect
[1, 2, x, 4]; // eliminated: no effect
if 42 > 0 { // '42 > 0' is replaced by 'true' and the first branch promoted
foo(42); // promoted, NOT eliminated: the function 'foo' may have side-effects
} else {
bar(x); // eliminated: branch is never reached
}
let z = x; // eliminated: local variable, no side-effects, and only pure afterwards
666 // NOT eliminated: this is the return value of the block,
// and the block is the last one so this is the return value of the whole script
}The above script optimizes to:
{
let x = 999;
foo(42);
666
}