1 files changed, 9 insertions, 1 deletions
diff --git a/test/ruby/test_extlibs.rb b/test/ruby/test_extlibs.rb
index 5016692abe..7f8f033eca 100644
--- a/test/ruby/test_extlibs.rb
+++ b/test/ruby/test_extlibs.rb
@@ -1,11 +1,19 @@
 require "envutil"
 
 class TestExtLibs < Test::Unit::TestCase
+  @extdir = $".grep(/\/rbconfig\.rb\z/) {break "#$`/ext"}
+
   def self.check_existence(ext, add_msg = nil)
     add_msg = ".  #{add_msg}" if add_msg
+    log = "#{@extdir}/#{ext}/mkmf.log"
     define_method("test_existence_of_#{ext}") do
       assert_separately([], <<-"end;", ignore_stderr: true) # do
-        assert_nothing_raised("extension library `#{ext}' is not found#{add_msg}") do
+        log = #{log.dump}
+        msg = proc {
+          "extension library `#{ext}' is not found#{add_msg}\n" <<
+            (File.exist?(log) ? File.read(log) : "\#{log} not found")
+        }
+        assert_nothing_raised(msg) do
           require "#{ext}"
         end
       end;