You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: purifier.rs
    path of file: ./repos/prusti-dev/prusti-viper/src/encoder
    
    the code of the file until where you have to start completion: // This file should be in `prusti-common/src/vir/optimizations/purification/`,
// but it depends on encoderâ€¦
// TODO: add a vir::Type::SnapOf(t: Box<vir::Type>) variant to vir::Type to
// represent types like "snapshot of X". Resolve SnapOf in snapshot patcher.

use rustc_hash::{FxHashMap, FxHashSet};
use vir_crate::polymorp
    
    fn compute_borrowing_variables_fixpoint(&mut self) {
        let mut changed = true;
        while changed {
            let mut add_queue = Vec::new();
            for var in &self.borrowing_variables {
                if let Some(dependents) = self.dependents.remove(var) {
                    add_queue.push(dependents);
                }
            }
            changed = !add_queue.is_empty();
            for dependents in add_queue {
                self.borrowing_variables.extend(dependents);
            }
        }
    }