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#680 in Rust patterns

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MIT license

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crates.io documentation

🔎 loupe is a set of tools to analyse and to profile Rust code. For the moment, it only provides tools about memory usage. It's mostly driven by Wasmer's needs, but feel free to propose new features!

loupe is a French word to express magnifying glass, and can be pronounced exactly like loop. The bird above is a Fauvette à lunettes (Curruca conspicillata, Spectacled Warbler).


The classical Cargo step! Add the following line to your Cargo.toml file:

loupe = "0.1"

Memory Usage

loupe provides the MemoryUsage trait. It allows to know the size of a value in bytes, recursively. So it traverses most of the types (some are missing, feel free to contribute!), and its fields or variants as deep as possible. Hopefully, it tracks already visited values so that it doesn't enter an infinite loupe loop. The trait looks like this:

pub trait MemoryUsage {
    fn size_of_val(&self, tracker: &mut dyn MemoryUsageTracker) -> usize;

loupe provides a size_of_val function that is a close sibling of std::mem::size_of_val. It can be used the same way.

loupe exports its best companion: handful procedural macros from the loupe-derive crate, to automatically implement the MemoryUsage trait if possible, on structs and enums.

Thus, one only needs to write:

use loupe::MemoryUsage;
use std::mem;

struct S {
    x: Vec<i32>,
    y: Vec<i32>,

fn main() {
    let s = S {
        x: vec![1, 2, 3],
        y: vec![1, 2, 3],
    assert_eq!(48, mem::size_of_val(&s));
    assert_eq!(72, loupe::size_of_val(&s));

In the example above, we see that each elements of Vec<i32> has been counted in the size of the value s. In Wasmer, it is possible to get the size of an Instance, which traverses Module, Store, Engine, Compiler etc. It's an entire tree of values that is traversed and the size of each value is summed.

Opinionated implementations

Even if MemoryUsage is already implemented for common types, some types are missing. We happily welcome more implementations! However, implementations of MemoryUsage:

  • must never alter the values,
  • must never panic of fail,
  • must be deterministic as much as possible (ideally, everytime).

In the same spirit, our implementation of MemoryUsage for *const T or *mut T (and other pointer types, like NonNull, UnsafeCell etc.) just returns the size of the pointer, but it doesn't dereference the pointer as it's unsafe. It doesn't mean one must not do that: It's totally possible if it's sure that the pointer can be safely dereferenced.

Remember that a user can implement MemoryUsage by hand; no need to try to have a default implementation for all the standard types.

Finally, our implementations are certainly not perfect! Feel free to challenge it and come to discuss!


MIT License, see LICENSE.