Polish Zeitwerk::NameError message

The current error for missing top-level constants is:

  expected file #{x_rb} to define constant X, but didn't

It could be argued that "constant X" is ambiguous,
because

    M::X = 1

defines a constant X.

By saying "the top-level constant X" we remove the
ambiguity, and the error message still reads well
for my taste.

lib/zeitwerk/loader/callbacks.rb

@@ -18,7 +18,8 @@ module Zeitwerk::Loader::Callbacks
       to_unload[cpath] = [file, cref] if reloading_enabled?
       run_on_load_callbacks(cpath, cget(*cref), file) unless on_load_callbacks.empty?
     else
-      msg = "expected file #{file} to define constant #{cpath}, but didn't"
+      the = cref[0] == Object ? "the top-level" : "the"
+      msg = "expected file #{file} to define #{the} constant #{cpath}"
       log(msg) if logger
 
       # Ruby still keeps the autoload defined, but we remove it because the

test/lib/zeitwerk/test_exceptions.rb

@@ -12,24 +12,44 @@ def assert_error_message(message, error)
     assert_equal message, error.message.lines.first.chomp
   end
 
-  test "raises NameError if the expected constant is not defined" do
+  test "raises NameError if the expected constant is not defined (top-level)" do
     on_teardown { remove_const :TyPo }
 
     files = [["typo.rb", "TyPo = 1"]]
     with_setup(files) do
       typo_rb = File.expand_path("typo.rb")
       error = assert_raises(Zeitwerk::NameError) { Typo }
-      assert_error_message "expected file #{typo_rb} to define constant Typo, but didn't", error
+      assert_error_message "expected file #{typo_rb} to define the top-level constant Typo", error
       assert_equal :Typo, error.name
     end
   end
 
-  test "eager loading raises NameError if files do not define the expected constants" do
+  test "raises NameError if the expected constant is not defined (namespace)" do
+    files = [["m/typo.rb", "M::TyPo = 1"]]
+    with_setup(files) do
+      typo_rb = File.expand_path("m/typo.rb")
+      error = assert_raises(Zeitwerk::NameError) { M::Typo }
+      assert_error_message "expected file #{typo_rb} to define the constant M::Typo", error
+      assert_equal :Typo, error.name
+    end
+  end
+
+  test "eager loading raises NameError if files do not define the expected constants (top-level)" do
     files = [["x.rb", ""]]
     with_setup(files) do
       x_rb = File.expand_path("x.rb")
       error = assert_raises(Zeitwerk::NameError) { loader.eager_load }
-      assert_error_message "expected file #{x_rb} to define constant X, but didn't", error
+      assert_error_message "expected file #{x_rb} to define the top-level constant X", error
+      assert_equal :X, error.name
+    end
+  end
+
+  test "eager loading raises NameError if files do not define the expected constants (namespace)" do
+    files = [["m/x.rb", ""]]
+    with_setup(files) do
+      x_rb = File.expand_path("m/x.rb")
+      error = assert_raises(Zeitwerk::NameError) { loader.eager_load }
+      assert_error_message "expected file #{x_rb} to define the constant M::X", error
       assert_equal :X, error.name
     end
   end
@@ -44,7 +64,7 @@ def assert_error_message(message, error)
     with_setup(files) do
       cli_x_rb = File.expand_path("cli/x.rb")
       error = assert_raises(Zeitwerk::NameError) { loader.eager_load }
-      assert_error_message "expected file #{cli_x_rb} to define constant Cli::X, but didn't", error
+      assert_error_message "expected file #{cli_x_rb} to define the constant Cli::X", error
       assert_equal :X, error.name
     end
   end

test/lib/zeitwerk/test_logging.rb

@@ -136,7 +136,7 @@ def logger.debug(message)
   test "logs failed autoloads, provided the require call succeeded" do
     files = [["x.rb", ""]]
     with_files(files) do
-      assert_logged(/expected file #{File.expand_path("x.rb")} to define constant X, but didn't/) do
+      assert_logged(/expected file #{File.expand_path("x.rb")} to define the top-level constant X/) do
         loader.push_dir(".")
         loader.setup
         assert_raises(Zeitwerk::NameError) { X }