feat: restore may_dangle drop support

Cargo.toml

@@ -32,6 +32,7 @@ strip = false
 allocator-api = []
 allocator-api2 = ["dep:allocator-api2", "allocator-api"]
 internals = []
+may_dangle = []
 proptest = ["dep:proptest", "std"]
 rayon = ["std", "dep:rayon"]
 serde = ["dep:serde_core"]

src/lib.rs

@@ -9,6 +9,7 @@
 #![cfg_attr(docsrs, feature(doc_cfg))]
 #![cfg_attr(feature = "specialization", allow(incomplete_features))]
 #![cfg_attr(feature = "specialization", feature(specialization, trusted_len))]
+#![cfg_attr(feature = "may_dangle", feature(dropck_eyepatch))]
 #![cfg_attr(
     all(feature = "allocator-api", not(feature = "allocator-api2")),
     feature(allocator_api)
@@ -1492,6 +1493,33 @@ impl<Heap: Allocator> Drop for DropDealloc<'_, Heap> {
     }
 }
 
+#[cfg(feature = "may_dangle")]
+unsafe impl<#[may_dangle] Item, const INLINE: usize, Heap: Allocator> Drop
+    for SmallVec<Item, INLINE, Heap>
+{
+    fn drop(&mut self) {
+        let (length, on_heap) = self.length.parts();
+        // SAFETY: the tag tells which member is active
+        let ptr = unsafe { self.raw.as_mut_ptr(on_heap) };
+        // SAFETY: see above
+        unsafe {
+            let _drop_dealloc = if on_heap {
+                let capacity = self.raw.heap.1;
+                Some(DropDealloc {
+                    ptr: NonNull::new_unchecked(ptr as *mut u8),
+                    size_bytes: capacity * size_of::<Item>(),
+                    align: align_of::<Item>(),
+                    allocator: &self.allocator
+                })
+            } else {
+                None
+            };
+            core::ptr::slice_from_raw_parts_mut(ptr, length).drop_in_place();
+        }
+    }
+}
+
+#[cfg(not(feature = "may_dangle"))]
 impl<Item, const INLINE: usize, Heap: Allocator> Drop for SmallVec<Item, INLINE, Heap> {
     fn drop(&mut self) {
         let (length, on_heap) = self.length.parts();

tests/may_dangle.rs

@@ -0,0 +1,15 @@
+#![cfg(feature = "may_dangle")]
+
+use smallvec::SmallVec;
+
+#[derive(Default)]
+struct MyData<'a> {
+    arr: SmallVec<&'a u8, 5>,
+    dat: u8
+}
+
+#[test]
+fn may_dangle_allows_borrow_of_another_field() {
+    let mut dat = MyData::default();
+    dat.arr.push(&dat.dat);
+}