Show meaningful message when applying inapplicable suggestion

When applying a inapplicable suggestion, we currently show a
vague message: A suggestion is not applicable.

We have an existing method called `Suggestion#inapplicable_reason`
which will return a more meaningful message why suggestion can't
be applied.

changelogs/unreleased/225617-meaningful-inapplicable-error-message.yml

+---
+title: Show meaningful message when applying inapplicable suggestion
+merge_request: 37267
+author:
+type: changed

lib/gitlab/suggestions/suggestion_set.rb

@@ -83,7 +83,7 @@ module Gitlab
         end
 
         unless suggestion.appliable?(cached: false)
-          return _('A suggestion is not applicable.')
+          return suggestion.inapplicable_reason(cached: false)
         end
 
         unless latest_source_head?(suggestion)

locale/gitlab.pot

@@ -1139,9 +1139,6 @@ msgstr ""
 msgid "A subscription will trigger a new pipeline on the default branch of this project when a pipeline successfully completes for a new tag on the %{default_branch_docs} of the subscribed project."
 msgstr ""
 
-msgid "A suggestion is not applicable."
-msgstr ""
-
 msgid "A user with write access to the source branch selected this option"
 msgstr ""
 

spec/requests/api/suggestions_spec.rb

@@ -74,7 +74,7 @@ RSpec.describe API::Suggestions do
         put api(url, user)
 
         expect(response).to have_gitlab_http_status(:bad_request)
-        expect(json_response).to eq({ 'message' => 'A suggestion is not applicable.' })
+        expect(json_response).to eq({ 'message' => "Can't apply as this line was changed in a more recent version." })
       end
     end
 
@@ -133,7 +133,7 @@ RSpec.describe API::Suggestions do
             params: { ids: [suggestion.id, unappliable_suggestion.id] }
 
         expect(response).to have_gitlab_http_status(:bad_request)
-        expect(json_response).to eq({ 'message' => 'A suggestion is not applicable.' })
+        expect(json_response).to eq({ 'message' => "Can't apply as this line was changed in a more recent version." })
       end
     end
 

spec/services/suggestions/apply_service_spec.rb

@@ -609,40 +609,16 @@ RSpec.describe Suggestions::ApplyService do
       end
     end
 
-    context 'suggestion is eligible to be outdated' do
-      it 'returns error message' do
-        expect(suggestion).to receive(:outdated?) { true }
-
-        result = apply_service.new(user, suggestion).execute
-
-        expect(result).to eq(message: 'A suggestion is not applicable.',
-                             status: :error)
-      end
-    end
-
-    context 'note is outdated' do
-      before do
-        allow(diff_note).to receive(:active?) { false }
-      end
+    context 'suggestion is not appliable' do
+      let(:inapplicable_reason) { "Can't apply this suggestion." }
 
       it 'returns error message' do
-        result = apply_service.new(user, suggestion).execute
-
-        expect(result).to eq(message: 'A suggestion is not applicable.',
-                             status: :error)
-      end
-    end
+        expect(suggestion).to receive(:appliable?).and_return(false)
+        expect(suggestion).to receive(:inapplicable_reason).and_return(inapplicable_reason)
 
-    context 'suggestion was already applied' do
-      before do
-        suggestion.update!(applied: true, commit_id: 'sha')
-      end
-
-      it 'returns error message' do
         result = apply_service.new(user, suggestion).execute
 
-        expect(result).to eq(message: 'A suggestion is not applicable.',
-                             status: :error)
+        expect(result).to eq(message: inapplicable_reason, status: :error)
       end
     end