Function std::ptr::drop_in_place

#[lang = "drop_in_place"]
pub unsafe fn drop_in_place<T>(to_drop: *mut T) where
    T: ?Sized, 

Executes the destructor (if any) of the pointed-to value.

This is semantically equivalent to calling ptr::read and discarding the result, but has the following advantages:

• It is required to use drop_in_place to drop unsized types like trait objects, because they can't be read out onto the stack and dropped normally.

• It is friendlier to the optimizer to do this over ptr::read when dropping manually allocated memory (e.g. when writing Box/Rc/Vec), as the compiler doesn't need to prove that it's sound to elide the copy.

Safety

Behavior is undefined if any of the following conditions are violated:

• to_drop must be valid.

• to_drop must be properly aligned.

Additionally, if T is not Copy, using the pointed-to value after calling drop_in_place can cause undefined behavior. Note that *to_drop = foo counts as a use because it will cause the the value to be dropped again. write can be used to overwrite data without causing it to be dropped.

Examples

Manually remove the last item from a vector:

use std::ptr;
use std::rc::Rc;

let last = Rc::new(1);
let weak = Rc::downgrade(&last);

let mut v = vec![Rc::new(0), last];

unsafe {
    // Without a call `drop_in_place`, the last item would never be dropped,
    // and the memory it manages would be leaked.
    ptr::drop_in_place(&mut v[1]);
    v.set_len(1);
}

assert_eq!(v, &[0.into()]);

// Ensure that the last item was dropped.
assert!(weak.upgrade().is_none());