xref: /third_party/rust/crates/syn/src/drops.rs (revision fad3a1d3) 
1use std::iter;
2use std::mem::ManuallyDrop;
3use std::ops::{Deref, DerefMut};
4use std::option;
5use std::slice;
6
7#[repr(transparent)]
8pub(crate) struct NoDrop<T: ?Sized>(ManuallyDrop<T>);
9
10impl<T> NoDrop<T> {
11    pub(crate) fn new(value: T) -> Self
12    where
13        T: TrivialDrop,
14    {
15        NoDrop(ManuallyDrop::new(value))
16    }
17}
18
19impl<T: ?Sized> Deref for NoDrop<T> {
20    type Target = T;
21    fn deref(&self) -> &Self::Target {
22        &self.0
23    }
24}
25
26impl<T: ?Sized> DerefMut for NoDrop<T> {
27    fn deref_mut(&mut self) -> &mut Self::Target {
28        &mut self.0
29    }
30}
31
32pub(crate) trait TrivialDrop {}
33
34impl<T> TrivialDrop for iter::Empty<T> {}
35impl<'a, T> TrivialDrop for slice::Iter<'a, T> {}
36impl<'a, T> TrivialDrop for slice::IterMut<'a, T> {}
37impl<'a, T> TrivialDrop for option::IntoIter<&'a T> {}
38impl<'a, T> TrivialDrop for option::IntoIter<&'a mut T> {}
39
40#[test]
41fn test_needs_drop() {
42    use std::mem::needs_drop;
43
44    struct NeedsDrop;
45
46    impl Drop for NeedsDrop {
47        fn drop(&mut self) {}
48    }
49
50    assert!(needs_drop::<NeedsDrop>());
51
52    // Test each of the types with a handwritten TrivialDrop impl above.
53    assert!(!needs_drop::<iter::Empty<NeedsDrop>>());
54    assert!(!needs_drop::<slice::Iter<NeedsDrop>>());
55    assert!(!needs_drop::<slice::IterMut<NeedsDrop>>());
56    assert!(!needs_drop::<option::IntoIter<&NeedsDrop>>());
57    assert!(!needs_drop::<option::IntoIter<&mut NeedsDrop>>());
58}
59