Maximum Expression Nesting Depth

Rhai by default limits statement and expression nesting to a maximum depth of 64 (which should be plenty) when they are at global level, but only a depth of 32 when they are within function bodies.

For debug builds, these limits are set further downwards to 32 and 16 respectively.

That is because it is possible to overflow the Engine’s stack when it tries to recursively parse an extremely deeply-nested code stream.

// The following, if long enough, can easily cause stack overflow during parsing.
let a = (1+(1+(1+(1+(1+(1+(1+(1+(1+(1+(...)+1)))))))))));

This limit may be changed via Engine::set_max_expr_depths.

There are two limits to set, one for the maximum depth at global level, and the other for function bodies.

A script exceeding the maximum nesting depths will terminate with a parse error. The malicious AST will not be able to get past parsing in the first place.

This check can be disabled via the unchecked feature for higher performance (but higher risks as well).

let mut engine = Engine::new();

engine.set_max_expr_depths(50, 5);  // allow nesting up to 50 layers of expressions/statements
                                    // at global level, but only 5 inside functions


Multiple layers of expressions may be generated for a simple language construct, even though it may correspond to only one AST node.

That is because the Rhai parser internally runs a recursive chain of function calls and it is important that a malicious script does not panic the parser in the first place.

Beware of recursion

Functions are placed under stricter limits because of the multiplicative effect of recursion.

A function can effectively call itself while deep inside an expression chain within the function body, thereby overflowing the stack even when the level of recursion is within limit.

fn deep_calc(a, n) {
    (a+(a+(a+(a+(a+(a+(a+(a+(a+ ... (a+deep_calc(a,n+1)) ... )))))))))
    //                                 ^^^^^^^^^^^^^^^^ recursive call!

let a = 42;

let result = (a+(a+(a+(a+(a+(a+(a+(a+(a+ ... (a+deep_calc(a,0)) ... )))))))));

In the contrived example above, each recursive call to the function deep_calc adds the total number of nested expression layers to Rhai’s evaluation stack. Sooner or later (most likely sooner than the limit for maximum depth of function calls is reached), a stack overflow can be expected.

In general, make sure that C x ( 5 + F ) + S layered calls do not cause a stack overflow, where:

  • C = maximum call stack depth,
  • F = maximum statement depth for functions,
  • S = maximum statement depth at global level.