src/test/ui/union/union-nodrop.rs

   1 // run-pass
   2 // revisions: mirunsafeck thirunsafeck
   3 // [thirunsafeck]compile-flags: -Z thir-unsafeck
   4
   5 #![allow(dead_code)]
   6
   7 use std::mem::needs_drop;
   8 use std::mem::ManuallyDrop;
   9
  10 struct NeedDrop;
  11
  12 impl Drop for NeedDrop {
  13     fn drop(&mut self) {}
  14 }
  15
  16 // Constant expressios allow `NoDrop` to go out of scope,
  17 // unlike a value of the interior type implementing `Drop`.
  18 static X: () = (NoDrop { inner: ManuallyDrop::new(NeedDrop) }, ()).1;
  19
  20 const Y: () = (NoDrop { inner: ManuallyDrop::new(NeedDrop) }, ()).1;
  21
  22 const fn _f() { (NoDrop { inner: ManuallyDrop::new(NeedDrop) }, ()).1 }
  23
  24 // A union that scrubs the drop glue from its inner type
  25 union NoDrop<T> { inner: ManuallyDrop<T> }
  26
  27 // Copy currently can't be implemented on drop-containing unions,
  28 // this may change later
  29 // https://github.com/rust-lang/rust/pull/38934#issuecomment-271219289
  30
  31 // // We should be able to implement Copy for NoDrop
  32 // impl<T> Copy for NoDrop<T> {}
  33 // impl<T> Clone for NoDrop<T> {fn clone(&self) -> Self { *self }}
  34
  35 // // We should be able to implement Copy for things using NoDrop
  36 // #[derive(Copy, Clone)]
  37 struct Foo {
  38     x: NoDrop<Box<u8>>
  39 }
  40
  41 struct Baz {
  42     x: NoDrop<Box<u8>>,
  43     y: Box<u8>,
  44 }
  45
  46 union ActuallyDrop<T> { inner: ManuallyDrop<T> }
  47
  48 impl<T> Drop for ActuallyDrop<T> {
  49     fn drop(&mut self) {}
  50 }
  51
  52 fn main() {
  53     // NoDrop should not make needs_drop true
  54     assert!(!needs_drop::<Foo>());
  55     assert!(!needs_drop::<NoDrop<u8>>());
  56     assert!(!needs_drop::<NoDrop<Box<u8>>>());
  57     // presence of other drop types should still work
  58     assert!(needs_drop::<Baz>());
  59     // drop impl on union itself should work
  60     assert!(needs_drop::<ActuallyDrop<u8>>());
  61     assert!(needs_drop::<ActuallyDrop<Box<u8>>>());
  62 }