How to get memory occupied by an object in rust from the code itself

Question

I have a requirement to read the memory occupied by an object based on some events. Is there's a simple method that I can use from the standard library ? I need to access the memory usage of the following object which contains nested structs.

HashMap<String, HashMap<AppIdentifier, HashMap<String, u64>>>

Answer 1

I recommend you create a trait with a method that returns the size of self's transitively-owned allocations:

trait AllocatedSize {
    fn allocated_size(&self) -> usize;
}

And then implement that for everything involved:

impl AllocatedSize for u64 {
    fn allocated_size(&self) -> usize {
        0
    }
}

impl AllocatedSize for String {
    fn allocated_size(&self) -> usize {
        self.capacity()
    }
}

impl AllocatedSize for AppIdentifier {
    fn allocated_size(&self) -> usize {
        todo!()
    }
}

impl<K: AllocatedSize, V: AllocatedSize> AllocatedSize for HashMap<K, V> {
    fn allocated_size(&self) -> usize {
        // every element in the map directly owns its key and its value
        const ELEMENT_SIZE: usize = std::mem::size_of::<K>() + std::mem::size_of::<V>();

        // directly owned allocation
        // NB: self.capacity() may be an underestimate, see its docs
        // NB: also ignores control bytes, see hashbrown implementation
        let directly_owned = self.capacity() * ELEMENT_SIZE;

        // transitively owned allocations
        let transitively_owned = self
            .iter()
            .map(|(key, val)| key.allocated_size() + val.allocated_size())
            .sum();

        directly_owned + transitively_owned
    }
}