Auto merge of rust-lang#132553 - EFanZh:zero-cost-rc-arc-deref, r=<try>

Make `Rc<T>::deref` and `Arc<T>::deref` zero-cost

Currently, `Rc<T>` and `Arc<T>` store pointers to `RcInner<T>` and `ArcInner<T>`. This PR changes the pointers so that they point to `T` directly instead.

This is based on the assumption that we access the `T` value more frequently than accessing reference counts. With this change, accessing the data can be done without offsetting pointers from `RcInner<T>` and `ArcInner<T>` to their contained data. This change might also enables some possibly useful future optimizations, such as:

- Convert `&[Rc<T>]` into `&[&T]` within O(1) time.
- Convert `&[Rc<T>]` into `Vec<&T>` utilizing `memcpy`.
- Convert `&Option<Rc<T>>` into `Option<&T>` without branching.
- Make `Rc<T>` and `Arc<T>` FFI compatible types where `T: Sized`.

Author: bors

library/alloc/src/lib.rs

@@ -212,6 +212,8 @@ mod testing;
 #[macro_use]
 mod macros;
 
+#[cfg(not(no_rc))]
+mod raw_rc;
 mod raw_vec;
 
 // Heaps provided for low-level allocation strategies