# Struct std::panic::AssertUnwindSafe 1.9.0 [−] [src]

    pub struct AssertUnwindSafe<T>(pub T);

A simple wrapper around a type to assert that it is unwind safe.

When using `catch_unwind` it may be the case that some of the closed over variables are not unwind safe. For example if `&mut T` is captured the compiler will generate a warning indicating that it is not unwind safe. It may not be the case, however, that this is actually a problem due to the specific usage of `catch_unwind` if unwind safety is specifically taken into account. This wrapper struct is useful for a quick and lightweight annotation that a variable is indeed unwind safe.

# Examples

One way to use `AssertUnwindSafe` is to assert that the entire closure itself is unwind safe, bypassing all checks for all variables:

    use std::panic::{self, AssertUnwindSafe};

    let mut variable = 4;

    // This code will not compile because the closure captures `&mut variable`
    // which is not considered unwind safe by default.

    // panic::catch_unwind(|| {
    //     variable += 3;
    // });

    // This, however, will compile due to the `AssertUnwindSafe` wrapper
    let result = panic::catch_unwind(AssertUnwindSafe(|| {
        variable += 3;
    }));
    // ...Run

Wrapping the entire closure amounts to a blanket assertion that all captured variables are unwind safe. This has the downside that if new captures are added in the future, they will also be considered unwind safe. Therefore, you may prefer to just wrap individual captures, as shown below. This is more annotation, but it ensures that if a new capture is added which is not unwind safe, you will get a compilation error at that time, which will allow you to consider whether that new capture in fact represent a bug or not.

    use std::panic::{self, AssertUnwindSafe};

    let mut variable = 4;
    let other_capture = 3;

    let result = {
        let mut wrapper = AssertUnwindSafe(&mut variable);
        panic::catch_unwind(move || {
            **wrapper += other_capture;
        })
    };
    // ...Run

## Trait Implementations

### `impl<T> UnwindSafe for AssertUnwindSafe<T>`
[src]

### `impl<T> RefUnwindSafe for AssertUnwindSafe<T>`
[src]

### `impl<T> Deref for AssertUnwindSafe<T>`
[src]

#### `type Target = T`

The resulting type after dereferencing

#### `fn deref(&self) -> &T`

The method called to dereference a value

### `impl<T> DerefMut for AssertUnwindSafe<T>`
[src]

#### `fn deref_mut(&mut self) -> &mut T`

The method called to mutably dereference a value

### `impl<R, F: FnOnce() -> R> FnOnce<()> for AssertUnwindSafe<F>`
[src]

#### `type Output = R`

The returned type after the call operator is used.

#### `extern "rust-call" fn call_once(self, _args: ()) -> R`

🔬 This is a nightly-only experimental API. (`fn_traits` #29625)

This is called when the call operator is used.

### `impl<T: Debug> Debug for AssertUnwindSafe<T>`
1.16.0
[src]

#### `fn fmt(&self, f: &mut Formatter) -> Result`

Formats the value using the given formatter.

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