Crate drop_tracker

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Expand description

Crate to check if a variable got correctly dropped. This crate is mostly useful in unit tests for code involving ManuallyDrop, MaybeUninit, unsafe memory management, custom containers, and more.

Concepts

The main struct of this crate is DropTracker. Once you initialize a tracker, you call DropTracker::track on it to get a DropItem. Each drop item is identified by a key; the key can be used at any time to check the state of the item and see if it’s alive or if it has been dropped.

Examples

This is how you would test that a container like Vec drops all its items when the container is dropped:

use drop_tracker::DropTracker;

let mut tracker = DropTracker::new();

// Create a new vector and add a bunch of elements to it. The elements in this case are
// identified by integer keys (1, 2, 3), but any hashable type would work.
let v = vec![tracker.track(1),
             tracker.track(2),
             tracker.track(3)];

// Assert that all elements in the vector are alive
tracker.all_alive(1..=3)
       .expect("expected all elements to be alive");

// Once the vector is dropped, all items should be dropped with it
drop(v);
tracker.all_dropped(1..=3)
       .expect("expected all elements to be dropped");

This is how you would test a struct that involves MaybeUninit:

ⓘ
use std::mem::MaybeUninit;

struct MyOption<T> {
    set: bool,
    data: MaybeUninit<T>,
}

impl<T> MyOption<T> {
    fn none() -> Self {
        Self { set: false, data: MaybeUninit::uninit() }
    }

    fn some(x: T) -> Self {
        Self { set: true, data: MaybeUninit::new(x) }
    }
}

// BUG: MyOption<T> does not implement Drop!
// BUG: The instance inside `data` may be initialized but not be properly destructed!

// BUG: The following code will silently leak memory:
let opt = MyOption::some(String::from("hello"));
drop(opt); // the String does not get deallocated

// DropTracker is able to catch this sort of bugs:
use drop_tracker::DropTracker;

let mut tracker = DropTracker::new();
let opt = MyOption::some(tracker.track("item"));

tracker.state(&"item")
       .alive()
       .expect("item is expected to be alive"); // works

drop(opt);

tracker.state(&"item")
       .dropped()
       .expect("item is expected to be dropped"); // panics, meaning that the bug was detected

If you want to write more succint code and don’t care about the error message, you can also use the following assert methods:

Here is how the first example above could be rewritten more consisely using assert methods:

use drop_tracker::DropTracker;

let mut tracker = DropTracker::new();
let v = vec![tracker.track(1),
             tracker.track(2),
             tracker.track(3)];

tracker.assert_all_alive(1..=3);
drop(v);
tracker.assert_all_dropped(1..=3);

Double drop

DropItem will panic if it gets dropped twice or more, as this is generally a bug and may cause undefined behavior. This feature can be used to identify bugs with code using ManuallyDrop, MaybeUninit or std::ptr::drop_in_place, like in the following example:

ⓘ
use std::ptr;
use drop_tracker::DropTracker;

let mut tracker = DropTracker::new();
let mut item = tracker.track("something");

unsafe { ptr::drop_in_place(&mut item); } // ok
unsafe { ptr::drop_in_place(&mut item); } // panic!

Use in collections

The DropItem instances returned by DropTracker::track hold a clone of the key passed to track. The DropItems are comparable and hashable if the underlying key is. This makes DropItem instances usable directly in collections like HashMap, BTreeMap, HashSet and many more.

Here is an example involving HashSet:

use drop_tracker::DropTracker;
use std::collections::HashSet;

let mut tracker = DropTracker::new();

let mut set = HashSet::from([
    tracker.track(1),
    tracker.track(2),
    tracker.track(3),
]);

set.remove(&3);

tracker.state(&1).alive().expect("first item should be alive");
tracker.state(&2).alive().expect("second item should be alive");
tracker.state(&3).dropped().expect("third item should be dropped");

Keys are required to be hashable and unique. If you need DropItem to hold a non-hashable value, or a repeated value, you can construct a DropItem with an arbitrary value using DropTracker::track_with_value:

use drop_tracker::DropTracker;

let mut tracker = DropTracker::new();

// Construct items identified by integers and holding floats (which are not hashable)
let item1 = tracker.track_with_value(1, 7.52);
let item2 = tracker.track_with_value(2, 3.89);

// Items compare according to their value
assert!(item1 > item2); // 7.52 > 3.89

// Items that support comparison can be put in a vector and sorted
let mut v = vec![item1, item2];
v.sort_by(|x, y| x.partial_cmp(y).unwrap());

Structs

Enums

  • State
    A type that represents the state of a DropItem: either alive or dropped.