#unsafe #wrapper #drop #into-inner

no-std into_inner_drop

A helper library for implementing into_inner method for drop types safely

1 unstable release

0.1.0 Jan 13, 2020

#1053 in Rust patterns

MITNFA license

73 lines

Into inner drop

Safe helper for types that should implement both Drop and .into_inner() method.


There's a repeating pattern where people want to have a type that does something on drop, but also want to be able to avoid dropping and destructure the type using some method that takes self.

Furhter, sometimes people want to destructure the type in drop implementation, which isn't safe due to it being passed by mutable refernece.

Hand-rolling unsafe code was neccessary until this crate existed. This crate takes the responsibility for ensuring that the drop impl is sound. More eyes, less bugs.

This crate is no_std.


Let's say you want to have a special type that prints a string on drop, but with ability to take out the string without printing. This is how you would approach it using this crate:

// Not strictly neccessary, but helps encapsulate the inner representation.
mod inner {
    // In fact, you could even avoid this newtype!
    // But in case you have something more complicated, you might need a newtype.
    pub(super) struct PrintOnDrop(pub(super) String);

    // You need this helper type to implement Drop.
    pub(super) enum PrintOnDropImpl {}

    // Drop is implemented a bit differently
    impl into_inner_drop::DetachedDrop for PrintOnDropImpl {
        // This type will be passed to your drop function by value (move).
        type Implementor = PrintOnDrop;

        // The drop implementation. The main difference is passing inner representation by-value.
        fn drop(value: Self::Implementor) {
            // You can destructucutre your type here if you want!
            // E.g. let string = value.0;
            println!("Dropping: {}", value.0);

use into_inner_drop::IntoInnerHelper;

// Public representation

/// A String that is printed when dropped.
pub struct PrintOnDrop(IntoInnerHelper<inner::PrintOnDrop, inner::PrintOnDropImpl>);

impl PrintOnDrop {
    /// Crates a string that is printed on drop.
    fn new(string: String) -> Self {

    /// Takes out the string, preventing printing on drop.
    fn into_string(self) -> String {

fn main() {
    let print_on_drop = PrintOnDrop::new("Hello world!".to_owned());
    let dont_print_on_drop = PrintOnDrop::new("Hello Rustceans!".to_owned());

    let string = dont_print_on_drop.into_string();
    println!("NOT on drop: {}", string);

As you can see, the code has some boilerplate, but no unsafe. I'm already trying to come up with a macro to make it much easier. See the appropriate issue on GitHub to participate.



No runtime deps