Rollup merge of #78144 - bugadani:elements-nodrop, r=oli-obk

Don't update `entries` in `TypedArena` if T does not need drop

As far as I can tell, `entries` is only used when dropping `TypedArenaChunk`s and their contents. It is already ignored there, if T is not `mem::needs_drop`, this PR just skips updating it's value.

You can see `TypedArenaChunk` ignoring the entry count in L71. The reasoning is similar to what you can find in `DroplessArena`.

r? @oli-obk

compiler/rustc_arena/src/lib.rs

@@ -217,8 +217,12 @@ impl<T> TypedArena<T> {
             let mut chunks = self.chunks.borrow_mut();
             let mut new_cap;
             if let Some(last_chunk) = chunks.last_mut() {
-                let used_bytes = self.ptr.get() as usize - last_chunk.start() as usize;
-                last_chunk.entries = used_bytes / mem::size_of::<T>();
+                // If a type is `!needs_drop`, we don't need to keep track of how many elements
+                // the chunk stores - the field will be ignored anyway.
+                if mem::needs_drop::<T>() {
+                    let used_bytes = self.ptr.get() as usize - last_chunk.start() as usize;
+                    last_chunk.entries = used_bytes / mem::size_of::<T>();
+                }
 
                 // If the previous chunk's len is less than HUGE_PAGE
                 // bytes, then this chunk will be least double the previous