Special case 'generic param from outer item' message for Self

compiler/rustc_resolve/messages.ftl

@@ -114,8 +114,14 @@ resolve_forward_declared_generic_param =
     .label = defaulted generic parameters cannot be forward declared
 
 resolve_generic_params_from_outer_item =
-    can't use generic parameters from outer item
-    .label = use of generic parameter from outer item
+    can't use {$is_self ->
+        [true] `Self`
+        *[false] generic parameters
+    } from outer item
+    .label = use of {$is_self ->
+        [true] `Self`
+        *[false] generic parameter
+    } from outer item
     .refer_to_type_directly = refer to the type directly here instead
     .suggestion = try introducing a local generic parameter here
 

compiler/rustc_resolve/src/diagnostics.rs

@@ -568,12 +568,14 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
                     DefKind::Const => Some(errs::GenericParamsFromOuterItemStaticOrConst::Const),
                     _ => None,
                 };
+                let is_self = matches!(outer_res, Res::SelfTyParam { .. } | Res::SelfTyAlias { .. });
                 let mut err = errs::GenericParamsFromOuterItem {
                     span,
                     label: None,
                     refer_to_type_directly: None,
                     sugg: None,
                     static_or_const,
+                    is_self,
                 };
 
                 let sm = self.tcx.sess.source_map();

compiler/rustc_resolve/src/errors.rs

@@ -46,6 +46,7 @@ pub(crate) struct GenericParamsFromOuterItem {
     pub(crate) sugg: Option<GenericParamsFromOuterItemSugg>,
     #[subdiagnostic]
     pub(crate) static_or_const: Option<GenericParamsFromOuterItemStaticOrConst>,
+    pub(crate) is_self: bool,
 }
 
 #[derive(Subdiagnostic)]