Expand description

This library provides wrapper types that permit sending non Send types to other threads and use runtime checks to ensure safety.

It provides three types: Fragile and Sticky which are similar in nature but have different behaviors with regards to how destructors are executed and the extra SemiSticky type which uses Sticky if the value has a destructor and Fragile if it does not.

All three types wrap a value and provide a Send bound. Neither of the types permit access to the enclosed value unless the thread that wrapped the value is attempting to access it. The difference between the types starts playing a role once destructors are involved.

A Fragile will actually send the T from thread to thread but will only permit the original thread to invoke the destructor. If the value gets dropped in a different thread, the destructor will panic.

A Sticky on the other hand does not actually send the T around but keeps it stored in the original thread’s thread local storage. If it gets dropped in the originating thread it gets cleaned up immediately, otherwise it leaks until the thread shuts down naturally. Sticky because it borrows into the TLS also requires you to “prove” that you are not doing any funny business with the borrowed value that lives for longer than the current stack frame which results in a slightly more complex API.

There is a third typed called SemiSticky which shares the API with Sticky but internally uses a boxed Fragile if the type does not actually need a dtor in which case Fragile is preferred.

Fragile Usage

Fragile is the easiest type to use. It works almost like a cell.

use std::thread;
use fragile::Fragile;

// creating and using a fragile object in the same thread works
let val = Fragile::new(true);
assert_eq!(*val.get(), true);

// once send to another thread it stops working
thread::spawn(move || {

Sticky Usage

Sticky is similar to Fragile but because it places the value in the thread local storage it comes with some extra restrictions to make it sound. The advantage is it can be dropped from any thread but it comes with extra restrictions. In particular it requires that values placed in it are 'static and that StackTokens are used to restrict lifetimes.

use std::thread;
use fragile::Sticky;

// creating and using a fragile object in the same thread works
let val = Sticky::new(true);
assert_eq!(*val.get(tok), true);

// once send to another thread it stops working
thread::spawn(move || {


Most of the time trying to use this crate is going to indicate some code smell. But there are situations where this is useful. For instance you might have a bunch of non Send types but want to work with a Send error type. In that case the non sendable extra information can be contained within the error and in cases where the error did not cross a thread boundary yet extra information can be obtained.

Drop / Cleanup Behavior

All types will try to eagerly drop a value if they are dropped on the right thread. Sticky and SemiSticky will however temporarily leak memory until a thread shuts down if the value is dropped on the wrong thread. The benefit however is that if you have that type of situation, and you can live with the consequences, the type is not panicking. A Fragile dropped in the wrong thread will not just panic, it will effectively also tear down the process because panicking in destructors is non recoverable.


By default the crate has no dependencies. Optionally the slab feature can be enabled which optimizes the internal storage of the Sticky type to make it use a slab instead.


Crates a token on the stack with a certain name for semi-sticky.


A Fragile<T> wraps a non sendable T to be safely send to other threads.
Returned when borrowing fails.
A SemiSticky<T> keeps a value T stored in a thread if it has a drop.
A token that is placed to the stack to constrain lifetimes.
A Sticky<T> keeps a value T stored in a thread.