syn/
drops.rs

1// SPDX-License-Identifier: Apache-2.0 OR MIT
2
3use std::iter;
4use std::mem::ManuallyDrop;
5use std::ops::{Deref, DerefMut};
6use std::option;
7use std::slice;
8
9#[repr(transparent)]
10pub(crate) struct NoDrop<T: ?Sized>(ManuallyDrop<T>);
11
12impl<T> NoDrop<T> {
13    pub(crate) fn new(value: T) -> Self
14    where
15        T: TrivialDrop,
16    {
17        NoDrop(ManuallyDrop::new(value))
18    }
19}
20
21impl<T: ?Sized> Deref for NoDrop<T> {
22    type Target = T;
23    fn deref(&self) -> &Self::Target {
24        &self.0
25    }
26}
27
28impl<T: ?Sized> DerefMut for NoDrop<T> {
29    fn deref_mut(&mut self) -> &mut Self::Target {
30        &mut self.0
31    }
32}
33
34pub(crate) trait TrivialDrop {}
35
36impl<T> TrivialDrop for iter::Empty<T> {}
37impl<T> TrivialDrop for slice::Iter<'_, T> {}
38impl<T> TrivialDrop for slice::IterMut<'_, T> {}
39impl<T> TrivialDrop for option::IntoIter<&T> {}
40impl<T> TrivialDrop for option::IntoIter<&mut T> {}
41
42#[test]
43fn test_needs_drop() {
44    use std::mem::needs_drop;
45
46    struct NeedsDrop;
47
48    impl Drop for NeedsDrop {
49        fn drop(&mut self) {}
50    }
51
52    assert!(needs_drop::<NeedsDrop>());
53
54    // Test each of the types with a handwritten TrivialDrop impl above.
55    assert!(!needs_drop::<iter::Empty<NeedsDrop>>());
56    assert!(!needs_drop::<slice::Iter<NeedsDrop>>());
57    assert!(!needs_drop::<slice::IterMut<NeedsDrop>>());
58    assert!(!needs_drop::<option::IntoIter<&NeedsDrop>>());
59    assert!(!needs_drop::<option::IntoIter<&mut NeedsDrop>>());
60}
syn/drops.rs

syn/
drops.rs
