Add note when inherent impl for a alias type defined outside of the crate

compiler/rustc_hir_analysis/src/coherence/inherent_impls.rs

@@ -110,7 +110,22 @@ impl<'tcx> InherentCollect<'tcx> {
             Ok(())
         } else {
             let impl_span = self.tcx.def_span(impl_def_id);
-            Err(self.tcx.dcx().emit_err(errors::InherentTyOutsideNew { span: impl_span }))
+            let mut err = errors::InherentTyOutsideNew { span: impl_span, note: None };
+
+            if let hir::TyKind::Path(rustc_hir::QPath::Resolved(_, path)) =
+                self.tcx.hir_node_by_def_id(impl_def_id).expect_item().expect_impl().self_ty.kind
+                && let rustc_hir::def::Res::Def(DefKind::TyAlias, def_id) = path.res
+            {
+                let ty_name = self.tcx.def_path_str(def_id);
+                let alias_ty_name = self.tcx.type_of(def_id).skip_binder().to_string();
+                err.note = Some(errors::InherentTyOutsideNewAliasNote {
+                    span: self.tcx.def_span(def_id),
+                    ty_name,
+                    alias_ty_name,
+                });
+            }
+
+            Err(self.tcx.dcx().emit_err(err))
         }
     }
 

compiler/rustc_hir_analysis/src/errors.rs

@@ -1223,11 +1223,27 @@ pub(crate) struct InherentTyOutsideRelevant {
 
 #[derive(Diagnostic)]
 #[diag("cannot define inherent `impl` for a type outside of the crate where the type is defined", code = E0116)]
-#[note("define and implement a trait or new type instead")]
+#[help(
+    "consider defining a trait and implementing it for the type or using a newtype wrapper like `struct MyType(ExternalType);` and implement it"
+)]
+#[note(
+    "for more details about the orphan rules, see <https://doc.rust-lang.org/reference/items/implementations.html?highlight=orphan#orphan-rules>"
+)]
 pub(crate) struct InherentTyOutsideNew {
     #[primary_span]
     #[label("impl for type defined outside of crate")]
     pub span: Span,
+    #[subdiagnostic]
+    pub note: Option<InherentTyOutsideNewAliasNote>,
+}
+
+#[derive(Subdiagnostic)]
+#[note("`{$ty_name}` does not define a new type, only an alias of `{$alias_ty_name}` defined here")]
+pub(crate) struct InherentTyOutsideNewAliasNote {
+    #[primary_span]
+    pub span: Span,
+    pub ty_name: String,
+    pub alias_ty_name: String,
 }
 
 #[derive(Diagnostic)]