1 files changed, 147 insertions, 0 deletions
diff --git a/third_party/rust/crossbeam-epoch/src/deferred.rs b/third_party/rust/crossbeam-epoch/src/deferred.rs
new file mode 100644
index 0000000000..2f3d79fdf1
--- /dev/null
+++ b/third_party/rust/crossbeam-epoch/src/deferred.rs
@@ -0,0 +1,147 @@
+use alloc::boxed::Box;
+use core::fmt;
+use core::marker::PhantomData;
+use core::mem::{self, MaybeUninit};
+use core::ptr;
+
+/// Number of words a piece of `Data` can hold.
+///
+/// Three words should be enough for the majority of cases. For example, you can fit inside it the
+/// function pointer together with a fat pointer representing an object that needs to be destroyed.
+const DATA_WORDS: usize = 3;
+
+/// Some space to keep a `FnOnce()` object on the stack.
+type Data = [usize; DATA_WORDS];
+
+/// A `FnOnce()` that is stored inline if small, or otherwise boxed on the heap.
+///
+/// This is a handy way of keeping an unsized `FnOnce()` within a sized structure.
+pub(crate) struct Deferred {
+    call: unsafe fn(*mut u8),
+    data: MaybeUninit<Data>,
+    _marker: PhantomData<*mut ()>, // !Send + !Sync
+}
+
+impl fmt::Debug for Deferred {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> Result<(), fmt::Error> {
+        f.pad("Deferred { .. }")
+    }
+}
+
+impl Deferred {
+    pub(crate) const NO_OP: Self = {
+        fn no_op_call(_raw: *mut u8) {}
+        Self {
+            call: no_op_call,
+            data: MaybeUninit::uninit(),
+            _marker: PhantomData,
+        }
+    };
+
+    /// Constructs a new `Deferred` from a `FnOnce()`.
+    pub(crate) fn new<F: FnOnce()>(f: F) -> Self {
+        let size = mem::size_of::<F>();
+        let align = mem::align_of::<F>();
+
+        unsafe {
+            if size <= mem::size_of::<Data>() && align <= mem::align_of::<Data>() {
+                let mut data = MaybeUninit::<Data>::uninit();
+                ptr::write(data.as_mut_ptr().cast::<F>(), f);
+
+                unsafe fn call<F: FnOnce()>(raw: *mut u8) {
+                    let f: F = ptr::read(raw.cast::<F>());
+                    f();
+                }
+
+                Deferred {
+                    call: call::<F>,
+                    data,
+                    _marker: PhantomData,
+                }
+            } else {
+                let b: Box<F> = Box::new(f);
+                let mut data = MaybeUninit::<Data>::uninit();
+                ptr::write(data.as_mut_ptr().cast::<Box<F>>(), b);
+
+                unsafe fn call<F: FnOnce()>(raw: *mut u8) {
+                    // It's safe to cast `raw` from `*mut u8` to `*mut Box<F>`, because `raw` is
+                    // originally derived from `*mut Box<F>`.
+                    let b: Box<F> = ptr::read(raw.cast::<Box<F>>());
+                    (*b)();
+                }
+
+                Deferred {
+                    call: call::<F>,
+                    data,
+                    _marker: PhantomData,
+                }
+            }
+        }
+    }
+
+    /// Calls the function.
+    #[inline]
+    pub(crate) fn call(mut self) {
+        let call = self.call;
+        unsafe { call(self.data.as_mut_ptr().cast::<u8>()) };
+    }
+}
+
+#[cfg(all(test, not(crossbeam_loom)))]
+mod tests {
+    #![allow(clippy::drop_copy)]
+
+    use super::Deferred;
+    use std::cell::Cell;
+
+    #[test]
+    fn on_stack() {
+        let fired = &Cell::new(false);
+        let a = [0usize; 1];
+
+        let d = Deferred::new(move || {
+            drop(a);
+            fired.set(true);
+        });
+
+        assert!(!fired.get());
+        d.call();
+        assert!(fired.get());
+    }
+
+    #[test]
+    fn on_heap() {
+        let fired = &Cell::new(false);
+        let a = [0usize; 10];
+
+        let d = Deferred::new(move || {
+            drop(a);
+            fired.set(true);
+        });
+
+        assert!(!fired.get());
+        d.call();
+        assert!(fired.get());
+    }
+
+    #[test]
+    fn string() {
+        let a = "hello".to_string();
+        let d = Deferred::new(move || assert_eq!(a, "hello"));
+        d.call();
+    }
+
+    #[test]
+    fn boxed_slice_i32() {
+        let a: Box<[i32]> = vec![2, 3, 5, 7].into_boxed_slice();
+        let d = Deferred::new(move || assert_eq!(*a, [2, 3, 5, 7]));
+        d.call();
+    }
+
+    #[test]
+    fn long_slice_usize() {
+        let a: [usize; 5] = [2, 3, 5, 7, 11];
+        let d = Deferred::new(move || assert_eq!(a, [2, 3, 5, 7, 11]));
+        d.call();
+    }
+}