do not implement unsafe auto traits for types with unsafe fields

If a type has unsafe fields, its safety invariants are not simply
the conjunction of its field types' safety invariants. Consequently,
it's invalid to reason about the safety properties of these types
in a purely structural manner — i.e., the manner in which `auto`
traits are implemented.

Makes progress towards #132922.

compiler/rustc_next_trait_solver/src/solve/trait_goals.rs

@@ -169,6 +169,14 @@ where
             return result;
         }
 
+        // Only consider auto impls of unsafe traits when there are no unsafe
+        // fields.
+        if ecx.cx().trait_is_unsafe(goal.predicate.def_id())
+            && goal.predicate.self_ty().has_unsafe_fields()
+        {
+            return Err(NoSolution);
+        }
+
         // We only look into opaque types during analysis for opaque types
         // outside of their defining scope. Doing so for opaques in the
         // defining scope may require calling `typeck` on the same item we're