When running a script like:

// This benchmark stresses method invocation by batches

class Toggle {
    init(startState) {
        this.state = startState;
    }

    value() { return this.state; }

    activate() {
        this.state = !this.state;
        return this;
    }
}

var start = clock();
var val = true;
var toggle = Toggle(val);
var batch = 0;

while (clock() - start < 10) {
    for (var i = 0; i < 10000; i = i + 1) {
        val = toggle.activate().value();
    }

    batch = batch + 1;
}

Golox encounters a golang runtime error and panics with: panic: runtime error: hash of unhashable type ast.Call.

The ast.Call expression contains a slice of arguments. Golang does not allow the hashing of objects that contain slices and this is what I believe to be the cause of the bug.

The required hashing exists in VisitAssignExpr. The hash key is currently the interface type Expression so that different kinds of expressions can be hashed locally. Currently all of the expression nodes are passed by value so a possible solution could be to start passing nodes by reference instead of value. Alternatively, we could assign a new function to each node that provides a string for Golang to consistently hash.