Change MemCx::alloc_bytes to return NonNull (pgcentralfoundation#2214)

Postgres will throw if it gets NULL here, so roll with that.

Author: workingjubilee

pgrx/src/list/flat_list.rs

@@ -112,7 +112,7 @@ impl<'cx, T: Enlist> List<'cx, T> {
                 // No silly reasoning, simply allocate ~2 cache lines for a list
                 let list_size = 128;
                 unsafe {
-                    let list: *mut pg_sys::List = mcx.alloc_bytes(list_size).cast();
+                    let list: *mut pg_sys::List = mcx.alloc_bytes(list_size).cast().as_ptr();
                     assert!(list.is_non_null());
                     (*list).type_ = T::LIST_TAG;
                     (*list).max_length = ((list_size - mem::size_of::<pg_sys::List>())

pgrx/src/memcx.rs

@@ -29,8 +29,10 @@ impl<'mcx> MemCx<'mcx> {
 
     /// Allocate a raw byte buffer `size` bytes in length
     /// and returns a pointer to the new allocation.
-    pub fn alloc_bytes(&self, size: usize) -> *mut u8 {
-        unsafe { pg_sys::MemoryContextAlloc(self.ptr.as_ptr(), size).cast() }
+    pub fn alloc_bytes(&self, size: usize) -> NonNull<u8> {
+        let ptr = unsafe { pg_sys::MemoryContextAlloc(self.ptr.as_ptr(), size).cast() };
+        // SAFETY: `pg_sys::MemoryContextAlloc` is not allowed to return NULL, it must error.
+        unsafe { NonNull::new_unchecked(ptr) }
     }
 
     /// Stores the current memory context, switches to *this* memory context,