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: param_cache.rs
    path of file: ./repos/rusqlite/src/util
    
    the code of the file until where you have to start completion: use super::SmallCString;
use std::cell::RefCell;
use std::collections::BTreeMap;

/// Maps parameter names to parameter indices.
#[derive(Default, Clone, Debug)]
// BTreeMap seems to do better here unless we want to pull in a custom hash
// function.
pub(crate) struct ParamIndexCache(RefCell<BTreeMap<SmallCString, usize>>);

impl ParamIndexCache {
    pub fn get_or_insert_with<F>(&self, s: &str, func: F) -> Option<usize>
    where
        F: FnOnce(&std::ffi::CStr) -> Option<usize>,
    {
        let mut cache = self.0.borrow_mut()
    
    fn test_cache() {
        let p = ParamIndexCache::default();
        let v = p.get_or_insert_with("foo", |cstr| {
            assert_eq!(cstr.to_str().unwrap(), "foo");
            Some(3)
        });
        assert_eq!(v, Some(3));
        let v = p.get_or_insert_with("foo", |_| {
            panic!("shouldn't be called this time");
        });
        assert_eq!(v, Some(3));
        let v = p.get_or_insert_with("gar\0bage", |_| {
            panic!("shouldn't be called here either");
        });
        assert_eq!(v, None);
        let v = p.get_or_insert_with("bar", |cstr| {
            assert_eq!(cstr.to_str().unwrap(), "bar");
            None
        });
        assert_eq!(v, None);
        let v = p.get_or_insert_with("bar", |cstr| {
            assert_eq!(cstr.to_str().unwrap(), "bar");
            Some(30)
        });
        assert_eq!(v, Some(30));
    }