Rollup merge of rust-lang#120248 - WaffleLapkin:bonk-ptr-object-casts…

…, r=compiler-errors,oli-obk,lnicola

Make casts of pointers to trait objects stricter

This is an attempt to `fix` rust-lang#120222 and rust-lang#120217.

This is done by adding restrictions on casting pointers to trait objects.

Before this PR the rules were as follows:

> When casting `*const X<dyn A>` -> `*const Y<dyn B>`, principal traits in `A` and `B` must refer to the same trait definition (or no trait).

With this PR the rules are changed to

> When casting `*const X<dyn Src>` -> `*const Y<dyn Dst>`
> - if `Dst` has a principal trait `DstP`,
>   - `Src` must have a principal trait `SrcP`
>   - `dyn SrcP` and `dyn DstP` must be the same type (modulo the trait object lifetime, `dyn T+'a` -> `dyn T+'b` is allowed)
>   - Auto traits in `Dst` must be a subset of auto traits in `Src`
>     - Not adhering to this is currently a FCW (warn-by-default + `FutureReleaseErrorReportInDeps`), instead of an error
> - if `Src` has a principal trait `Dst` must as well
>   - this restriction will be removed in a follow up PR

This ensures that
1. Principal trait's generic arguments match (no `*const dyn Tr<A>` -> `*const dyn Tr<B>` casts, which are a problem for [rust-lang#120222](rust-lang#120222))
2. Principal trait's lifetime arguments match (no `*const dyn Tr<'a>` -> `*const dyn Tr<'b>` casts, which are a problem for [rust-lang#120217](rust-lang#120217))
3. No auto traits can be _added_ (this is a problem for arbitrary self types, see [this comment](rust-lang#120248 (comment)))

Some notes:
 - We only care about the metadata/last field, so you can still cast `*const dyn T` to `*const WithHeader<dyn T>`, etc
- The lifetime of the trait object itself (`dyn A + 'lt`) is not checked, so you can still cast `*mut FnOnce() + '_` to `*mut FnOnce() + 'static`, etc
  - This feels fishy, but I couldn't come up with a reason it must be checked

The diagnostics are currently not great, to say the least, but as far as I can tell this correctly fixes the issues.

cc `@oli-obk` `@compiler-errors` `@lcnr`

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)
         })
     }
 }