From f0b4f036504a823ca673fee8f3bae47e1ad30894 Mon Sep 17 00:00:00 2001
From: HampusM <hampus@hampusmat.com>
Date: Wed, 26 Mar 2025 19:04:32 +0100
Subject: feat: make unsized fields not cause UB

---
 src/lib.rs | 37 ++++++++++++++++++++++++++++++++++---
 1 file changed, 34 insertions(+), 3 deletions(-)

(limited to 'src')

diff --git a/src/lib.rs b/src/lib.rs
index cdbb7b0..a8d5e82 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -42,6 +42,10 @@ impl Drop for OwnedAnyPtr
 {
     fn drop(&mut self)
     {
+        if self.size() == 0 {
+            return;
+        }
+
         unsafe {
             dealloc(
                 self.ptr.cast::<u8>(),
@@ -153,6 +157,8 @@ impl MultiVec
     )
     {
         if self.capacity != 0 {
+            assert_eq!(fields.as_ref().len(), self.field_arr_byte_offsets.len());
+
             if self.capacity == self.length {
                 self.grow_amortized(1, &fields);
             }
@@ -245,8 +251,10 @@ impl MultiVec
         let (new_layout, new_field_arr_byte_offsets) =
             Self::create_layout(new_capacity, &fields);
 
-        let Some(new_ptr) = NonNull::new(unsafe {
-            realloc(self.ptr.as_ptr().cast::<u8>(), *layout, new_layout.size())
+        let Some(new_ptr) = NonNull::new(if layout.size() == 0 {
+            std::ptr::dangling_mut()
+        } else {
+            unsafe { realloc(self.ptr.as_ptr().cast::<u8>(), *layout, new_layout.size()) }
         }) else {
             handle_alloc_error(new_layout);
         };
@@ -280,7 +288,11 @@ impl MultiVec
     {
         let (layout, field_arr_byte_offsets) = Self::create_layout(capacity, fields);
 
-        let Some(ptr) = NonNull::new(unsafe { alloc(layout) }) else {
+        let Some(ptr) = NonNull::new(if layout.size() == 0 {
+            std::ptr::dangling_mut()
+        } else {
+            unsafe { alloc(layout) }
+        }) else {
             handle_alloc_error(layout);
         };
 
@@ -386,6 +398,10 @@ impl Drop for MultiVec
 
         for index in 0..self.length {
             for (field_index, field_size) in self.field_sizes.iter().enumerate() {
+                if *field_size == 0 {
+                    continue;
+                }
+
                 let field_arr_byte_offset = self.field_arr_byte_offsets[field_index];
 
                 let field_arr_ptr = unsafe { self.ptr.byte_add(field_arr_byte_offset) };
@@ -403,6 +419,10 @@ impl Drop for MultiVec
         }
 
         if let Some(layout) = self.layout {
+            if layout.size() == 0 {
+                return;
+            }
+
             unsafe {
                 std::alloc::dealloc(self.ptr.as_ptr().cast::<u8>(), layout);
             }
@@ -530,6 +550,17 @@ mod tests
         );
     }
 
+    #[test]
+    fn push_all_unsized_fields_work()
+    {
+        struct UnsizedThing;
+
+        let mut multi_vec = MultiVec::new();
+
+        multi_vec.push([OwnedAnyPtr::new(()), OwnedAnyPtr::new(UnsizedThing)]);
+        multi_vec.push([OwnedAnyPtr::new(()), OwnedAnyPtr::new(UnsizedThing)]);
+    }
+
     //#[test]
     //fn multiple_pushes_in_preallocated_works()
     //{
-- 
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