Disallow ZST allocations with TypedArena.

`DroplessArena::alloc` already disallows ZST allocation.
`TypedArena::alloc` allows it but:
- (a) it's never used, and
- (b) writing to `NonNull::dangling()` seems dubious, even if the write
  is zero-sized.

This commit just changes it to panic on a ZST. This eliminates an
untested code path, and we shouldn't be allocating ZSTs anyway.
It also eliminates an unused ZST code path in `clear_last_chunk`.
Tests are also updated accordingly.

compiler/rustc_arena/src/lib.rs

@@ -140,25 +140,19 @@ impl<T> TypedArena<T> {
     /// Allocates an object in the `TypedArena`, returning a reference to it.
     #[inline]
     pub fn alloc(&self, object: T) -> &mut T {
+        assert!(size_of::<T>() != 0);
+
         if self.ptr == self.end {
             self.grow(1)
         }
 
         unsafe {
-            if size_of::<T>() == 0 {
-                self.ptr.set(self.ptr.get().wrapping_byte_add(1));
-                let ptr = ptr::NonNull::<T>::dangling().as_ptr();
-                // Don't drop the object. This `write` is equivalent to `forget`.
-                ptr::write(ptr, object);
-                &mut *ptr
-            } else {
-                let ptr = self.ptr.get();
-                // Advance the pointer.
-                self.ptr.set(self.ptr.get().add(1));
-                // Write into uninitialized memory.
-                ptr::write(ptr, object);
-                &mut *ptr
-            }
+            let ptr = self.ptr.get();
+            // Advance the pointer.
+            self.ptr.set(self.ptr.get().add(1));
+            // Write into uninitialized memory.
+            ptr::write(ptr, object);
+            &mut *ptr
         }
     }
 
@@ -302,16 +296,10 @@ fn clear_last_chunk(&self, last_chunk: &mut ArenaChunk<T>) {
         let end = self.ptr.get().addr();
         // We then calculate the number of elements to be dropped in the last chunk,
         // which is the filled area's length.
-        let diff = if size_of::<T>() == 0 {
-            // `T` is ZST. It can't have a drop flag, so the value here doesn't matter. We get
-            // the number of zero-sized values in the last and only chunk, just out of caution.
-            // Recall that `end` was incremented for each allocated value.
-            end - start
-        } else {
-            // FIXME: this should *likely* use `offset_from`, but more
-            // investigation is needed (including running tests in miri).
-            (end - start) / size_of::<T>()
-        };
+        assert_ne!(size_of::<T>(), 0);
+        // FIXME: this should *likely* use `offset_from`, but more
+        // investigation is needed (including running tests in miri).
+        let diff = (end - start) / size_of::<T>();
         // Pass that to the `destroy` method.
         unsafe {
             last_chunk.destroy(diff);

compiler/rustc_arena/src/tests.rs

@@ -22,7 +22,6 @@ fn clear(&mut self) {
             if let Some(last_chunk) = chunks_borrow.last_mut() {
                 self.clear_last_chunk(last_chunk);
                 let len = chunks_borrow.len();
-                // If `T` is ZST, code below has no effect.
                 for mut chunk in chunks_borrow.drain(..len - 1) {
                     chunk.destroy(chunk.entries);
                 }
@@ -117,18 +116,6 @@ fn test_noncopy() {
     }
 }
 
-#[test]
-fn test_typed_arena_zero_sized() {
-    let arena = TypedArena::default();
-    #[cfg(not(miri))]
-    const N: usize = 100000;
-    #[cfg(miri)]
-    const N: usize = 1000;
-    for _ in 0..N {
-        arena.alloc(());
-    }
-}
-
 #[test]
 fn test_typed_arena_clear() {
     let mut arena = TypedArena::default();
@@ -207,7 +194,8 @@ fn test_typed_arena_drop_on_clear() {
     static DROP_COUNTER: Cell<u32> = Cell::new(0)
 }
 
-struct SmallDroppable;
+#[allow(unused)]
+struct SmallDroppable(u8);
 
 impl Drop for SmallDroppable {
     fn drop(&mut self) {
@@ -222,7 +210,7 @@ fn test_typed_arena_drop_small_count() {
         let arena: TypedArena<SmallDroppable> = TypedArena::default();
         for _ in 0..100 {
             // Allocate something with drop glue to make sure it doesn't leak.
-            arena.alloc(SmallDroppable);
+            arena.alloc(SmallDroppable(0));
         }
         // dropping
     };