Add more checks for pointers with vtable meta

The rules for casting `*mut X<dyn A>` -> `*mut Y<dyn B>` are as follows:
- If `B` has a principal
  - `A` must have exactly the same principal (including generics)
  - Auto traits of `B` must be a subset of autotraits in `A`

Note that `X<_>` and `Y<_>` can be identity, or arbitrary structs with last field being the dyn type.
The lifetime of the trait object itself (`dyn ... + 'a`) is not checked.

This prevents a few soundness issues with `#![feature(arbitrary_self_types)]` and trait upcasting.
Namely, these checks make sure that vtable is always valid for the pointee.

Author: WaffleLapkin

alloc/src/boxed.rs

@@ -2374,7 +2374,7 @@ impl dyn Error + Send {
         let err: Box<dyn Error> = self;
         <dyn Error>::downcast(err).map_err(|s| unsafe {
             // Reapply the `Send` marker.
-            Box::from_raw(Box::into_raw(s) as *mut (dyn Error + Send))
+            mem::transmute::<Box<dyn Error>, Box<dyn Error + Send>>(s)
         })
     }
 }
@@ -2387,8 +2387,8 @@ impl dyn Error + Send + Sync {
     pub fn downcast<T: Error + 'static>(self: Box<Self>) -> Result<Box<T>, Box<Self>> {
         let err: Box<dyn Error> = self;
         <dyn Error>::downcast(err).map_err(|s| unsafe {
-            // Reapply the `Send + Sync` marker.
-            Box::from_raw(Box::into_raw(s) as *mut (dyn Error + Send + Sync))
+            // Reapply the `Send + Sync` markers.
+            mem::transmute::<Box<dyn Error>, Box<dyn Error + Send + Sync>>(s)
         })
     }
 }