Fix top level namespace declaration indexing (#2297)

Co-authored-by: Emily Samp <egiurleo@users.noreply.github.com>

lib/ruby_indexer/lib/ruby_indexer/declaration_listener.rb

@@ -71,7 +71,7 @@ def on_class_node_enter(node)
 
       superclass = node.superclass
 
-      nesting = name.start_with?("::") ? [name.delete_prefix("::")] : @stack + [name.delete_prefix("::")]
+      nesting = actual_nesting(name)
 
       parent_class = case superclass
       when Prism::ConstantReadNode, Prism::ConstantPathNode
@@ -119,8 +119,7 @@ def on_module_node_enter(node)
 
       comments = collect_comments(node)
 
-      nesting = name.start_with?("::") ? [name.delete_prefix("::")] : @stack + [name.delete_prefix("::")]
-      entry = Entry::Module.new(nesting, @file_path, node.location, constant_path.location, comments)
+      entry = Entry::Module.new(actual_nesting(name), @file_path, node.location, constant_path.location, comments)
 
       @owner_stack << entry
       @index.add(entry)
@@ -709,5 +708,19 @@ def parameter_name(node)
         :"(#{names_with_commas})"
       end
     end
+
+    sig { params(name: String).returns(T::Array[String]) }
+    def actual_nesting(name)
+      nesting = @stack + [name]
+      corrected_nesting = []
+
+      nesting.reverse_each do |name|
+        corrected_nesting.prepend(name.delete_prefix("::"))
+
+        break if name.start_with?("::")
+      end
+
+      corrected_nesting
+    end
   end
 end

lib/ruby_indexer/test/classes_and_modules_test.rb

@@ -546,5 +546,43 @@ def bar; end
       assert_equal(7, name_location.start_column)
       assert_equal(10, name_location.end_column)
     end
+
+    def test_indexing_namespaces_inside_top_level_references
+      index(<<~RUBY)
+        module ::Foo
+          class Bar
+          end
+        end
+      RUBY
+
+      # We want to explicitly verify that we didn't introduce the leading `::` by accident, but `Index#[]` deletes the
+      # prefix when we use `refute_entry`
+      entries = @index.instance_variable_get(:@entries)
+      refute(entries.key?("::Foo"))
+      refute(entries.key?("::Foo::Bar"))
+      assert_entry("Foo", Entry::Module, "/fake/path/foo.rb:0-0:3-3")
+      assert_entry("Foo::Bar", Entry::Class, "/fake/path/foo.rb:1-2:2-5")
+    end
+
+    def test_indexing_namespaces_inside_nested_top_level_references
+      index(<<~RUBY)
+        class Baz
+          module ::Foo
+            class Bar
+            end
+
+            class ::Qux
+            end
+          end
+        end
+      RUBY
+
+      refute_entry("Baz::Foo")
+      refute_entry("Baz::Foo::Bar")
+      assert_entry("Baz", Entry::Class, "/fake/path/foo.rb:0-0:8-3")
+      assert_entry("Foo", Entry::Module, "/fake/path/foo.rb:1-2:7-5")
+      assert_entry("Foo::Bar", Entry::Class, "/fake/path/foo.rb:2-4:3-7")
+      assert_entry("Qux", Entry::Class, "/fake/path/foo.rb:5-4:6-7")
+    end
   end
 end