feat: restore may_dangle drop support

Cargo.toml

@@ -27,6 +27,7 @@ strip = false
 allocator-api = []
 allocator-api2 = ["allocator-api", "dep:allocator-api2"]
 internals = []
+may-dangle = []
 proptest = ["dep:proptest", "std"]
 rayon = ["dep:rayon", "std"]
 serde = ["dep:serde_core"]
@@ -80,6 +81,10 @@ required-features = ["encase"]
 name = "equivalent"
 required-features = ["equivalent"]
 
+[[test]]
+name = "maydangle"
+required-features = ["may-dangle"]
+
 [[test]]
 name = "rayon"
 required-features = ["rayon"]

src/lib.rs

@@ -3,6 +3,7 @@
 #![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)
@@ -453,8 +454,8 @@ impl<Item, const INLINE: usize, Heap: Allocator> SmallVec<Item, INLINE, Heap> {
                 tail_start: end,
                 tail_len: length - end,
                 iter: range_slice.iter(),
-                // Since self is a &mut, passing it to a function would invalidate the slice
-                // iterator.
+                // Since self is a &mut, passing it to a function would
+                // invalidate the slice iterator.
                 vec: NonNull::new_unchecked(self as *mut _)
             }
         }
@@ -1501,7 +1502,10 @@ impl<Heap: Allocator> Drop for DropDealloc<'_, Heap> {
     }
 }
 
-impl<Item, const INLINE: usize, Heap: Allocator> Drop for SmallVec<Item, INLINE, Heap> {
+#[cfg_attr(feature = "may-dangle", syntax(unsafe))]
+impl<#[cfg_attr(feature = "may-dangle", 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

src/specialization.rs

@@ -257,10 +257,10 @@ where I: TrustedLen<Item = Item>
     fn spec_from_iter(iter: I) -> Self {
         let mut v = match iter.size_hint() {
             (_, Some(upper)) => SmallVec::with_capacity(upper),
-            // TrustedLen contract guarantees that `size_hint() == (_, None)` means that there
-            // are more than `usize::MAX` elements.
-            // Since the previous branch would eagerly panic if the capacity is too large
-            // (via `with_capacity`) we do the same here.
+            // TrustedLen contract guarantees that `size_hint() == (_, None)`
+            // means that there are more than `usize::MAX` elements.
+            // Since the previous branch would eagerly panic if the capacity is
+            // too large (via `with_capacity`) we do the same here.
             _ => panic!("capacity overflow")
         };
         // Reuse the extend specialization for TrustedLen.

tests/maydangle.rs

@@ -0,0 +1,13 @@
+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);
+}