Fix RawTable::allocation_info for empty table

Fixes rust-lang#376

src/raw/mod.rs

@@ -531,7 +531,12 @@ impl<T, A: Allocator + Clone> RawTable<T, A> {
     #[inline]
     #[cfg(feature = "raw")]
     pub fn allocation_info(&self) -> (NonNull<u8>, Layout) {
-        self.table.allocation_info(Self::TABLE_LAYOUT)
+        if self.table.is_empty_singleton() {
+            // `self.table.allocation_info` can only be called when the table is not empty.
+            return (NonNull::dangling(), Layout::new::<()>());
+        } else {
+            self.table.allocation_info(Self::TABLE_LAYOUT)
+        }
     }
 
     /// Returns the index of a bucket from a `Bucket`.
@@ -1589,6 +1594,11 @@ impl<A: Allocator + Clone> RawTableInner<A> {
 
     #[inline]
     fn allocation_info(&self, table_layout: TableLayout) -> (NonNull<u8>, Layout) {
+        debug_assert!(
+            !self.is_empty_singleton(),
+            "this function can only be called on non-empty tables"
+        );
+
         // Avoid `Option::unwrap_or_else` because it bloats LLVM IR.
         let (layout, ctrl_offset) = match table_layout.calculate_layout_for(self.buckets()) {
             Some(lco) => lco,

tests/raw.rs

@@ -0,0 +1,11 @@
+#![cfg(feature = "raw")]
+
+use hashbrown::raw::RawTable;
+use std::mem;
+
+#[test]
+fn test_allocation_info() {
+    assert_eq!(RawTable::<()>::new().allocation_info().1.size(), 0);
+    assert_eq!(RawTable::<u32>::new().allocation_info().1.size(), 0);
+    assert!(RawTable::<u32>::with_capacity(1).allocation_info().1.size() > mem::size_of::<u32>());
+}