Make `ptr_cast_add_auto_to_object` lint into hard error

In Rust 1.81, we added a FCW lint (including linting in dependencies)
against pointer casts that add an auto trait to dyn bounds.  This was
part of work making casts of pointers involving trait objects stricter
which was needed to restabilize trait upcasting.

We considered just making this a hard error at the time, but opted
against it due to breakage found by crater.  This breakage was mostly
due to the `anymap` crate which has been a persistent problem for us.

It's now a year later, and the fact that this is not yet a hard error
is giving us pause about stabilizing arbitrary self types and
`derive(CoercePointee)`.  So let's now make a hard error of this.

1 files changed, 1 insertions, 0 deletions

diff --git a/compiler/rustc_error_codes/src/lib.rs b/compiler/rustc_error_codes/src/lib.rs
index 098ca42be2b..d53d5678832 100644
--- a/compiler/rustc_error_codes/src/lib.rs
+++ b/compiler/rustc_error_codes/src/lib.rs
@@ -547,6 +547,7 @@ E0800: 0800,
 E0801: 0801,
 E0802: 0802,
 E0803: 0803,
+E0804: 0804,
         );
     )
 }