Refactor dyn-compatibility error and suggestions

This CL makes a number of small changes to dyn compatibility errors:
- "object safety" has been renamed to "dyn-compatibility" throughout
- "Convert to enum" suggestions are no longer generated when there
  exists a type-generic impl of the trait or an impl for `dyn OtherTrait`
- Several error messages are reorganized for user readability

Additionally, the dyn compatibility error creation code has been
split out into functions.

cc #132713
cc #133267

Author: cramertj

compiler/rustc_feature/src/removed.rs

@@ -159,7 +159,7 @@ declare_features! (
     /// then removed. But there was no utility storing it separately, so now
     /// it's in this list.
     (removed, no_stack_check, "1.0.0", None, None),
-    /// Allows making `dyn Trait` well-formed even if `Trait` is not dyn-compatible (object safe).
+    /// Allows making `dyn Trait` well-formed even if `Trait` is not dyn compatible (object safe).
     /// Renamed to `dyn_compatible_for_dispatch`.
     (removed, object_safe_for_dispatch, "1.83.0", Some(43561),
      Some("renamed to `dyn_compatible_for_dispatch`")),

compiler/rustc_feature/src/unstable.rs

@@ -271,7 +271,7 @@ declare_features! (
     (unstable, doc_notable_trait, "1.52.0", Some(45040)),
     /// Allows using the `may_dangle` attribute (RFC 1327).
     (unstable, dropck_eyepatch, "1.10.0", Some(34761)),
-    /// Allows making `dyn Trait` well-formed even if `Trait` is not dyn-compatible[^1].
+    /// Allows making `dyn Trait` well-formed even if `Trait` is not dyn compatible[^1].
     /// In that case, `dyn Trait: Trait` does not hold. Moreover, coercions and
     /// casts in safe Rust to `dyn Trait` for such a `Trait` is also forbidden.
     ///