diff --git a/lib/bundler/rubygems_ext.rb b/lib/bundler/rubygems_ext.rb
index 4ad2bdf46f04..6025fad97294 100644
--- a/lib/bundler/rubygems_ext.rb
+++ b/lib/bundler/rubygems_ext.rb
@@ -235,6 +235,20 @@ def extension_dir
       end
     end
 
+    alias_method :rg_build_info_dir, :build_info_dir
+    def build_info_dir
+      # A git checkout's build logs belong with that checkout's extension build.
+      # base_dir points at the directory holding every checkout, so logs keyed by
+      # full_name would collide between revisions of the same gem and would sit
+      # outside anything `bundle clean` prunes. extension_dir is unique per
+      # revision and goes away with the checkout.
+      if source.respond_to?(:extension_dir_name)
+        extension_dir
+      else
+        rg_build_info_dir
+      end
+    end
+
     # Can be removed once RubyGems 3.5.21 support is dropped
     remove_method :gem_dir if method_defined?(:gem_dir, false)
 
diff --git a/lib/rubygems/commands/install_command.rb b/lib/rubygems/commands/install_command.rb
index 2ebbc40a0308..288da7c31fee 100644
--- a/lib/rubygems/commands/install_command.rb
+++ b/lib/rubygems/commands/install_command.rb
@@ -93,7 +93,7 @@ def description # :nodoc:
   [build fails]
   Gem files will remain installed in \\
   /path/to/gems/some_extension_gem-1.0 for inspection.
-  Results logged to /path/to/gems/some_extension_gem-1.0/gem_make.out
+  Results logged to /path/to/build_info/some_extension_gem-1.0.gem_make.out
   $ gem install some_extension_gem -- --with-extension-lib=/path/to/lib
   [build succeeds]
   $ gem list some_extension_gem
@@ -110,7 +110,7 @@ def description # :nodoc:
   [build fails]
   Gem files will remain installed in \\
   /path/to/gems/some_extension_gem-1.0 for inspection.
-  Results logged to /path/to/gems/some_extension_gem-1.0/gem_make.out
+  Results logged to /path/to/build_info/some_extension_gem-1.0.gem_make.out
   $ [cd /path/to/gems/some_extension_gem-1.0]
   $ [edit files or what-have-you and run make]
   $ gem spec ../../cache/some_extension_gem-1.0.gem --ruby > \\
diff --git a/lib/rubygems/doctor.rb b/lib/rubygems/doctor.rb
index 4f26260d836a..bb20e9314961 100644
--- a/lib/rubygems/doctor.rb
+++ b/lib/rubygems/doctor.rb
@@ -21,7 +21,7 @@ class Gem::Doctor
 
   REPOSITORY_EXTENSION_MAP = [ # :nodoc:
     ["specifications", ".gemspec"],
-    ["build_info",     ".info"],
+    ["build_info",     ".info", ".mkmf.log", ".gem_make.out"],
     ["cache",          ".gem"],
     ["doc",            ""],
     ["extensions",     ""],
@@ -92,15 +92,15 @@ def doctor
   # Cleans up children of this gem repository
 
   def doctor_children # :nodoc:
-    REPOSITORY_EXTENSION_MAP.each do |sub_directory, extension|
-      doctor_child sub_directory, extension
+    REPOSITORY_EXTENSION_MAP.each do |sub_directory, *extensions|
+      doctor_child sub_directory, *extensions
     end
   end
 
   ##
-  # Removes files in +sub_directory+ with +extension+
+  # Removes files in +sub_directory+ with any of +extensions+
 
-  def doctor_child(sub_directory, extension) # :nodoc:
+  def doctor_child(sub_directory, *extensions) # :nodoc:
     directory = File.join(@gem_repository, sub_directory)
 
     Dir.entries(directory).sort.each do |ent|
@@ -109,7 +109,7 @@ def doctor_child(sub_directory, extension) # :nodoc:
       child = File.join(directory, ent)
       next unless File.exist?(child)
 
-      basename = File.basename(child, extension)
+      basename = strip_extension File.basename(child), extensions
       next if installed_specs.include? basename
       next if /^rubygems-\d/.match?(basename)
       next if sub_directory == "specifications" && basename == "default"
@@ -129,4 +129,15 @@ def doctor_child(sub_directory, extension) # :nodoc:
   rescue Errno::ENOENT
     # ignore
   end
+
+  ##
+  # Removes the first of +extensions+ that +name+ ends with
+
+  def strip_extension(name, extensions) # :nodoc:
+    extension = extensions.find do |ext|
+      !ext.empty? && name.end_with?(ext)
+    end
+
+    extension ? name.delete_suffix(extension) : name
+  end
 end
diff --git a/lib/rubygems/ext/builder.rb b/lib/rubygems/ext/builder.rb
index f1fce48823a8..b7e80d6b06bb 100644
--- a/lib/rubygems/ext/builder.rb
+++ b/lib/rubygems/ext/builder.rb
@@ -220,9 +220,11 @@ def build_error(output, backtrace = nil) # :nodoc:
     #{output}
 
 Gem files will remain installed in #{@gem_dir} for inspection.
-Results logged to #{gem_make_out}
 EOF
 
+    # Losing the log must not cost the user the build error itself.
+    message += "Results logged to #{gem_make_out}\n" if gem_make_out
+
     raise Gem::Ext::BuildError, message, backtrace
   end
 
@@ -243,13 +245,72 @@ def build_extension(extension, dest_path) # :nodoc:
 
       verbose { results.join("\n") }
 
-      write_gem_make_out results.join "\n"
+      # Build logs are noisy, non-reproducible artifacts that are not meant to
+      # be installed. Drop the ones this build left behind, plus any written
+      # into the extension directory by a RubyGems old enough to put them there.
+      FileUtils.rm_f mkmf_log_candidates(extension_dir, dest_path)
+      FileUtils.rm_f File.join(dest_path, "gem_make.out")
+      FileUtils.rm_f [build_log_path("mkmf.log"), build_log_path("gem_make.out")]
+    rescue Gem::Ext::Builder::NoMakefileError => e
+      # extconf ran fine but produced no Makefile, so the extension was skipped
+      # rather than built and installing carries on. Keep the log that says why
+      # it was skipped, out of the installation tree but still reachable.
+      results << e.message
+      results << "Skipping make for #{extension} as no Makefile was found."
+
+      verbose { results.join("\n") }
+
+      preserve_mkmf_log extension_dir, dest_path
+      write_gem_make_out results.join("\n")
     rescue StandardError => e
       results << e.message
+
+      mkmf_log_dest = preserve_mkmf_log(extension_dir, dest_path)
+      if mkmf_log_dest
+        results << "To see why this extension failed to compile, please check the mkmf.log which can be found here:"
+        results << "  #{mkmf_log_dest}"
+      end
+
       build_error(results.join("\n"), $@)
     end
   end
 
+  ##
+  # Where a build log of +kind+ for this gem lives in the build_info directory.
+
+  def build_log_path(kind) # :nodoc:
+    File.join @spec.build_info_dir, "#{@spec.full_name}.#{kind}"
+  end
+
+  ##
+  # Moves the mkmf.log this build left behind into the build_info directory and
+  # returns its new path, or nil when there is none or it cannot be kept.
+  # Keeping a log must never replace the build error the caller is reporting,
+  # so a filesystem failure here is swallowed.
+
+  def preserve_mkmf_log(extension_dir, dest_path) # :nodoc:
+    mkmf_log = mkmf_log_candidates(extension_dir, dest_path).find {|log| File.exist?(log) }
+    return unless mkmf_log
+
+    destination = build_log_path "mkmf.log"
+
+    FileUtils.mkdir_p @spec.build_info_dir
+    FileUtils.mv mkmf_log, destination
+
+    destination
+  rescue SystemCallError
+    nil
+  end
+
+  ##
+  # Places a completed build may have left an mkmf.log, most specific first.
+  # Gem::Ext::ExtConfBuilder parks it in +dest_path+ so that the "clean" target
+  # cannot delete it; the other builders leave it where extconf ran.
+
+  def mkmf_log_candidates(extension_dir, dest_path) # :nodoc:
+    [File.join(dest_path, "mkmf.log"), File.join(extension_dir, "mkmf.log")]
+  end
+
   ##
   # Builds extensions.  Valid types of extensions are extconf.rb files,
   # configure scripts and rakefiles or mkrf_conf files.
@@ -277,17 +338,21 @@ def build_extensions
   end
 
   ##
-  # Writes +output+ to gem_make.out in the extension install directory.
+  # Writes +output+ to gem_make.out in the build_info directory and returns its
+  # path, or nil when it cannot be written. Only called when the extension was
+  # not built, to keep build logs out of the installation tree.
 
   def write_gem_make_out(output) # :nodoc:
-    destination = File.join @spec.extension_dir, "gem_make.out"
+    destination = build_log_path "gem_make.out"
 
-    FileUtils.mkdir_p @spec.extension_dir
+    FileUtils.mkdir_p @spec.build_info_dir
 
     File.open destination, "wb" do |io|
       io.puts output
     end
 
     destination
+  rescue SystemCallError
+    nil
   end
 end
diff --git a/lib/rubygems/ext/ext_conf_builder.rb b/lib/rubygems/ext/ext_conf_builder.rb
index 822454355d10..9cf7eafc5dc3 100644
--- a/lib/rubygems/ext/ext_conf_builder.rb
+++ b/lib/rubygems/ext/ext_conf_builder.rb
@@ -27,17 +27,14 @@ def self.build(extension, dest_path, results, args = [], lib_dir = nil, extensio
       cmd << "--target-rbconfig=#{target_rbconfig.path}" if target_rbconfig.path
       cmd.push(*args)
 
-      run(cmd, results, class_name, extension_dir) do |s, r|
-        mkmf_log = File.join(extension_dir, "mkmf.log")
-        if File.exist? mkmf_log
-          unless s.success?
-            r << "To see why this extension failed to compile, please check" \
-              " the mkmf.log which can be found here:\n"
-            r << "  " + File.join(dest_path, "mkmf.log") + "\n"
-          end
-          FileUtils.mv mkmf_log, dest_path
-        end
-      end
+      run(cmd, results, class_name, extension_dir)
+
+      # "clean" is the first make target, and mkmf puts mkmf.log in CLEANFILES,
+      # so park the log next to the built extension before make can delete it.
+      # Whether it is then dropped or kept for inspection is decided by
+      # Gem::Ext::Builder#build_extension.
+      mkmf_log = File.join(extension_dir, "mkmf.log")
+      FileUtils.mv mkmf_log, dest_path if File.exist?(mkmf_log)
 
       ENV["DESTDIR"] = nil
 
@@ -66,10 +63,6 @@ def self.build(extension, dest_path, results, args = [], lib_dir = nil, extensio
     end
 
     results
-  rescue Gem::Ext::Builder::NoMakefileError => error
-    results << error.message
-    results << "Skipping make for #{extension} as no Makefile was found."
-    # We are good, do not re-raise the error.
   ensure
     FileUtils.rm_rf tmp_dest if tmp_dest
   end
diff --git a/lib/rubygems/uninstaller.rb b/lib/rubygems/uninstaller.rb
index 28bf33ea7106..f87ee2f49841 100644
--- a/lib/rubygems/uninstaller.rb
+++ b/lib/rubygems/uninstaller.rb
@@ -272,6 +272,8 @@ def remove(spec)
 
     safe_delete { rm_r full_gem_path, exclusions: exclusions }
     safe_delete { FileUtils.rm_r spec.extension_dir }
+    safe_delete { FileUtils.rm_f File.join(spec.build_info_dir, "#{spec.full_name}.mkmf.log") }
+    safe_delete { FileUtils.rm_f File.join(spec.build_info_dir, "#{spec.full_name}.gem_make.out") }
 
     old_platform_name = spec.original_name
 
diff --git a/spec/bundler/installer/parallel_installer_spec.rb b/spec/bundler/installer/parallel_installer_spec.rb
index 20d39e885be3..29c661134b6f 100644
--- a/spec/bundler/installer/parallel_installer_spec.rb
+++ b/spec/bundler/installer/parallel_installer_spec.rb
@@ -152,34 +152,37 @@
     let(:gem_two) { definition.specs.find {|spec| spec.name == "two" } }
 
     it "takes all available slots" do
-      redefine_build_jobs do
+      acquired = track_build_jobs(rendezvous: true) do
         Bundler::ParallelInstaller.call(installer, definition.specs, 5, false, true)
       end
 
-      # Take 3 slots out of the 5 available.
-      expect(File.read(File.join(gem_one.extension_dir, "gem_make.out"))).to include("make -j3")
+      # Take 3 slots (capped per gem) out of the 5 available.
+      expect(acquired["one"]).to eq(3)
       # Take the remaining 2 slots.
-      expect(File.read(File.join(gem_two.extension_dir, "gem_make.out"))).to include("make -j2")
+      expect(acquired["two"]).to eq(2)
     end
 
     it "fallback to non parallel when no slots are available" do
-      redefine_build_jobs do
+      acquired = track_build_jobs(rendezvous: true) do
         Bundler::ParallelInstaller.call(installer, definition.specs, 3, false, true)
       end
 
       # Take 3 slots out of the 3 available.
-      expect(File.read(File.join(gem_one.extension_dir, "gem_make.out"))).to include("make -j3")
+      expect(acquired["one"]).to eq(3)
       # Fallback to one slot (non parallel).
-      expect(File.read(File.join(gem_two.extension_dir, "gem_make.out"))).to_not include("make -j")
+      expect(acquired["two"]).to eq(1)
     end
 
     it "uses one jobs when installing serially" do
+      acquired = nil
       Bundler.settings.temporary(jobs: 1) do
-        Bundler::ParallelInstaller.call(installer, definition.specs, 1, false, true)
+        acquired = track_build_jobs do
+          Bundler::ParallelInstaller.call(installer, definition.specs, 1, false, true)
+        end
       end
 
-      expect(File.read(File.join(gem_one.extension_dir, "gem_make.out"))).to_not include("make -j")
-      expect(File.read(File.join(gem_two.extension_dir, "gem_make.out"))).to_not include("make -j")
+      expect(acquired["one"]).to eq(1)
+      expect(acquired["two"]).to eq(1)
     end
 
     it "release the job slots" do
@@ -191,39 +194,52 @@
         end
       end
 
-      Bundler::ParallelInstaller.call(installer, definition.specs, 3, false, true)
+      acquired = track_build_jobs do
+        Bundler::ParallelInstaller.call(installer, definition.specs, 3, false, true)
+      end
 
       # Take 3 slots out of the 3 available.
-      expect(File.read(File.join(gem_one.extension_dir, "gem_make.out"))).to include("make -j3")
-      # Take 3 slots that were released.
-      expect(File.read(File.join(gem_two.extension_dir, "gem_make.out"))).to include("make -j3")
+      expect(acquired["one"]).to eq(3)
+      # Take 3 slots that were released by `one`.
+      expect(acquired["two"]).to eq(3)
     end
 
-    def redefine_build_jobs
+    # Records how many jobserver slots each gem's build acquired. RubyGems turns
+    # that count directly into `make -jN`, so asserting on it verifies slot
+    # allocation and release without reading a build log, which a successful
+    # build no longer writes. With +rendezvous+, "one" grabs its slots first and
+    # holds them until "two" has grabbed the rest, making the split deterministic.
+    def track_build_jobs(rendezvous: false)
+      acquired = {}
       old_method = Bundler::RubyGemsGemInstaller.instance_method(:build_jobs)
       Bundler::RubyGemsGemInstaller.remove_method(:build_jobs)
 
-      # Rendezvous so that "one" grabs its slots first and keeps holding them
-      # until "two" has grabbed the rest. Blocking on a queue avoids the
-      # busy-wait and makes the ordering deterministic.
       one_acquired = Thread::Queue.new
       two_acquired = Thread::Queue.new
 
       Bundler::RubyGemsGemInstaller.define_method(:build_jobs) do
-        if spec.name == "one"
-          value = old_method.bind(self).call
-          one_acquired << true
-          two_acquired.pop
-        elsif spec.name == "two"
-          one_acquired.pop
-          value = old_method.bind(self).call
-          two_acquired << true
-        end
+        value =
+          if rendezvous && spec.name == "one"
+            v = old_method.bind(self).call
+            one_acquired << true
+            two_acquired.pop
+            v
+          elsif rendezvous && spec.name == "two"
+            one_acquired.pop
+            v = old_method.bind(self).call
+            two_acquired << true
+            v
+          else
+            old_method.bind(self).call
+          end
 
+        acquired[spec.name] = value
         value
       end
 
       yield
+
+      acquired
     ensure
       Bundler::RubyGemsGemInstaller.remove_method(:build_jobs)
       Bundler::RubyGemsGemInstaller.define_method(:build_jobs, old_method)
diff --git a/spec/commands/install_spec.rb b/spec/commands/install_spec.rb
index a0b56a3d2970..d4b88902e21e 100644
--- a/spec/commands/install_spec.rb
+++ b/spec/commands/install_spec.rb
@@ -1369,10 +1369,18 @@ def run
           s.extensions = extension
 
           s.write(extension, extconf_code)
+          # A successful build no longer leaves gem_make.out behind. Force the
+          # build to fail at the make stage so the make command line, including
+          # the jobserver `-j`, is recorded in build_info for these assertions.
+          s.write("ext/mypsych/mypsych.c", "#error forced build failure for test")
         end
       end
     end
 
+    def gem_make_out
+      File.read(File.join(@gemspec.build_info_dir, "#{@gemspec.full_name}.gem_make.out"))
+    end
+
     after do
       if @old_makeflags
         ENV["MAKEFLAGS"] = @old_makeflags
@@ -1381,42 +1389,36 @@ def run
       end
     end
 
-    it "doesn't pass down -j to make when MAKEFLAGS is set" do
+    it "doesn't pass down -j to make when MAKEFLAGS is set", rubygems: ">= 4.1.0.dev" do
       ENV["MAKEFLAGS"] = "-j1"
 
-      install_gemfile(<<~G, env: { "BUNDLE_JOBS" => "8" })
+      install_gemfile(<<~G, env: { "BUNDLE_JOBS" => "8" }, raise_on_error: false)
         source "https://gem.repo4"
         gem "mypsych"
       G
 
-      gem_make_out = File.read(File.join(@gemspec.extension_dir, "gem_make.out"))
-
       expect(gem_make_out).not_to include("make -j8")
     end
 
     it "uses 3 slots from the available pool when running the compilation of an extension", rubygems: ">= 4.1.0.dev" do
       ENV.delete("MAKEFLAGS")
 
-      install_gemfile(<<~G, env: { "BUNDLE_JOBS" => "8" })
+      install_gemfile(<<~G, env: { "BUNDLE_JOBS" => "8" }, raise_on_error: false)
         source "https://gem.repo4"
         gem "mypsych"
       G
 
-      gem_make_out = File.read(File.join(@gemspec.extension_dir, "gem_make.out"))
-
       expect(gem_make_out).to include("make -j3")
     end
 
     it "consumes 3 slots from the pool when BUNDLE_JOBS isn't set", rubygems: ">= 4.1.0.dev" do
       ENV.delete("MAKEFLAGS")
 
-      install_gemfile(<<~G)
+      install_gemfile(<<~G, raise_on_error: false)
         source "https://gem.repo4"
         gem "mypsych"
       G
 
-      gem_make_out = File.read(File.join(@gemspec.extension_dir, "gem_make.out"))
-
       expect(gem_make_out).to include("make -j3")
     end
   end
diff --git a/spec/install/gemfile/git_spec.rb b/spec/install/gemfile/git_spec.rb
index 2b74aa849ac8..847e5fa35efc 100644
--- a/spec/install/gemfile/git_spec.rb
+++ b/spec/install/gemfile/git_spec.rb
@@ -536,4 +536,33 @@
       end
     end
   end
+
+  describe "a git gem whose extension fails to build" do
+    # Where a build log goes is decided by the RubyGems running the install, and
+    # older ones write a bare gem_make.out into the extension directory.
+    it "keeps the build log with that checkout instead of the shared repository", rubygems: ">= 4.1.0.dev" do
+      build_git "foo", "1.0" do |s|
+        s.add_c_extension
+        # Overwrite the source add_c_extension wrote, before the checkout is
+        # committed, so that building it fails.
+        s.write "ext/foo.c", "#error forced build failure for test\n"
+      end
+
+      install_gemfile <<~G, raise_on_error: false
+        source "https://gem.repo1"
+        gem "foo", :git => "#{lib_path("foo-1.0")}"
+      G
+
+      # The log lands in the checkout's own extension directory, which is unique
+      # per revision and which `bundle clean` prunes along with the checkout.
+      logs = Dir.glob("#{Gem.dir}/bundler/gems/extensions/*/*/*/*.gem_make.out")
+
+      expect(logs.size).to eq(1)
+      expect(File.basename(File.dirname(logs.first))).to start_with("foo-1.0-")
+      expect(File.read(logs.first)).to include("forced build failure for test")
+
+      # Nothing is left directly under bundler/gems, which holds checkouts.
+      expect(Pathname.new("#{Gem.dir}/bundler/gems/build_info")).not_to exist
+    end
+  end
 end
diff --git a/test/rubygems/test_gem_commands_install_command.rb b/test/rubygems/test_gem_commands_install_command.rb
index a99afbfef74e..1127d0a85967 100644
--- a/test/rubygems/test_gem_commands_install_command.rb
+++ b/test/rubygems/test_gem_commands_install_command.rb
@@ -1721,6 +1721,9 @@ def test_pass_down_the_job_option_to_make
 
         write_file(extconf_path) do |io|
           io.puts "require 'mkmf'"
+          # Force the build to fail at the make stage so the build log is
+          # written. The make command line (including -j) is recorded there.
+          io.puts "File.write('a.c', '#error forced build failure for test')"
           io.puts "create_makefile '#{spec.name}'"
         end
 
@@ -1729,12 +1732,12 @@ def test_pass_down_the_job_option_to_make
     end
 
     use_ui @ui do
-      assert_raise Gem::MockGemUi::SystemExitException, @ui.error do
+      assert_raise Gem::MockGemUi::TermError, @ui.error do
         @cmd.invoke "a", "-j4"
       end
     end
 
-    gem_make_out = File.read(File.join(gemspec.extension_dir, "gem_make.out"))
+    gem_make_out = File.read(File.join(gemspec.build_info_dir, "#{gemspec.full_name}.gem_make.out"))
     if vc_windows? && nmake_found?
       refute_includes(gem_make_out, " -j4")
     else
diff --git a/test/rubygems/test_gem_commands_update_command.rb b/test/rubygems/test_gem_commands_update_command.rb
index 9d15406bd13f..1056b4985ba3 100644
--- a/test/rubygems/test_gem_commands_update_command.rb
+++ b/test/rubygems/test_gem_commands_update_command.rb
@@ -856,6 +856,9 @@ def test_pass_down_the_job_option_to_make
 
         write_file(extconf_path) do |io|
           io.puts "require 'mkmf'"
+          # Force the build to fail at the make stage so the build log is
+          # written. The make command line (including -j) is recorded there.
+          io.puts "File.write('a.c', '#error forced build failure for test')"
           io.puts "create_makefile '#{spec.name}'"
         end
 
@@ -869,7 +872,7 @@ def test_pass_down_the_job_option_to_make
       @cmd.invoke("a", "-j2")
     end
 
-    gem_make_out = File.read(File.join(gemspec.extension_dir, "gem_make.out"))
+    gem_make_out = File.read(File.join(gemspec.build_info_dir, "#{gemspec.full_name}.gem_make.out"))
     if vc_windows? && nmake_found?
       refute_includes(gem_make_out, " -j2")
     else
diff --git a/test/rubygems/test_gem_doctor.rb b/test/rubygems/test_gem_doctor.rb
index 1bcdc39022a8..705bcf09b729 100644
--- a/test/rubygems/test_gem_doctor.rb
+++ b/test/rubygems/test_gem_doctor.rb
@@ -121,6 +121,33 @@ def test_doctor_dry_run
     assert_equal Gem.path, [@gemhome, @userhome]
   end
 
+  def test_doctor_keeps_build_logs_of_installed_gems
+    a = gem "a"
+
+    Gem.use_paths @userhome, @gemhome
+
+    build_info_dir = File.join @gemhome, "build_info"
+    FileUtils.mkdir_p build_info_dir
+
+    kept = ["#{a.full_name}.mkmf.log", "#{a.full_name}.gem_make.out"].map do |name|
+      File.join build_info_dir, name
+    end
+    stale = File.join build_info_dir, "b-2.gem_make.out"
+
+    FileUtils.touch kept + [stale]
+
+    doctor = Gem::Doctor.new @gemhome
+
+    capture_output do
+      use_ui @ui do
+        doctor.doctor
+      end
+    end
+
+    kept.each {|path| assert_path_exist path }
+    assert_path_not_exist stale
+  end
+
   def test_doctor_non_gem_home
     other_dir = File.join @tempdir, "other", "dir"
 
diff --git a/test/rubygems/test_gem_ext_builder.rb b/test/rubygems/test_gem_ext_builder.rb
index 8f90687ede30..6b4eed2cf211 100644
--- a/test/rubygems/test_gem_ext_builder.rb
+++ b/test/rubygems/test_gem_ext_builder.rb
@@ -244,7 +244,9 @@ def test_build_extensions
 
       assert_path_exist @spec.extension_dir
       assert_path_exist @spec.gem_build_complete_path
-      assert_path_exist File.join @spec.extension_dir, "gem_make.out"
+      assert_path_not_exist File.join @spec.extension_dir, "gem_make.out"
+      assert_path_not_exist File.join @spec.extension_dir, "mkmf.log"
+      assert_path_not_exist File.join @spec.gem_dir, "ext", "mkmf.log"
       assert_path_exist File.join @spec.extension_dir, "a.rb"
       assert_path_exist File.join @spec.gem_dir, "lib", "a.rb"
       assert_path_exist File.join @spec.gem_dir, "lib", "a", "b.rb"
@@ -298,7 +300,9 @@ def test_build_extensions_install_ext_only
 
       assert_path_exist @spec.extension_dir
       assert_path_exist @spec.gem_build_complete_path
-      assert_path_exist File.join @spec.extension_dir, "gem_make.out"
+      assert_path_not_exist File.join @spec.extension_dir, "gem_make.out"
+      assert_path_not_exist File.join @spec.extension_dir, "mkmf.log"
+      assert_path_not_exist File.join @spec.gem_dir, "ext", "mkmf.log"
       assert_path_exist File.join @spec.extension_dir, "a.rb"
       assert_path_not_exist File.join @spec.gem_dir, "lib", "a.rb"
       assert_path_not_exist File.join @spec.gem_dir, "lib", "a", "b.rb"
@@ -351,13 +355,213 @@ def test_build_multiple_extensions
       assert_path_exist @spec.extension_dir
       assert_path_exist @spec.gem_build_complete_path
       assert_path_exist File.join @spec.gem_dir, "ext", "foo"
-      assert_path_exist File.join @spec.extension_dir, "gem_make.out"
+      assert_path_not_exist File.join @spec.extension_dir, "gem_make.out"
+      assert_path_not_exist File.join @spec.extension_dir, "mkmf.log"
+      assert_path_not_exist File.join @spec.gem_dir, "ext", "mkmf.log"
       assert_path_exist File.join @spec.extension_dir, "a.rb"
       assert_path_exist File.join @spec.gem_dir, "lib", "a.rb"
       assert_path_exist File.join @spec.gem_dir, "lib", "a", "b.rb"
     end
   end
 
+  def test_build_extensions_does_not_install_logs_on_success
+    pend "terminates on mswin" if vc_windows? && ruby_repo?
+
+    @spec.extensions << "ext/extconf.rb"
+
+    ext_dir = File.join @spec.gem_dir, "ext"
+    FileUtils.mkdir_p ext_dir
+
+    File.open File.join(ext_dir, "extconf.rb"), "w" do |f|
+      f.write <<-'RUBY'
+        require 'mkmf'
+
+        create_makefile 'a'
+      RUBY
+    end
+
+    use_ui @ui do
+      @builder.build_extensions
+    end
+
+    assert_path_exist @spec.gem_build_complete_path
+
+    # No build logs are left anywhere in the installation tree.
+    assert_path_not_exist File.join @spec.extension_dir, "gem_make.out"
+    assert_path_not_exist File.join @spec.extension_dir, "mkmf.log"
+    assert_path_not_exist File.join ext_dir, "mkmf.log"
+    assert_path_not_exist File.join ext_dir, "gem_make.out"
+    assert_path_not_exist File.join @spec.build_info_dir, "#{@spec.full_name}.mkmf.log"
+    assert_path_not_exist File.join @spec.build_info_dir, "#{@spec.full_name}.gem_make.out"
+  end
+
+  def test_build_extensions_logs_to_build_info_on_make_failure
+    pend "terminates on mswin" if vc_windows? && ruby_repo?
+
+    @spec.extensions << "ext/extconf.rb"
+
+    ext_dir = File.join @spec.gem_dir, "ext"
+    FileUtils.mkdir_p ext_dir
+
+    # have_header makes mkmf actually write an mkmf.log. extconf then succeeds,
+    # so "make clean" runs before the build and would delete that log unless it
+    # has been parked out of the way first.
+    File.open File.join(ext_dir, "extconf.rb"), "w" do |f|
+      f.write <<-'RUBY'
+        require 'mkmf'
+
+        have_header 'stdio.h'
+
+        File.write 'a.c', "#error forced build failure for test\n"
+
+        create_makefile 'a'
+      RUBY
+    end
+
+    e = assert_raise Gem::Ext::BuildError do
+      use_ui @ui do
+        @builder.build_extensions
+      end
+    end
+
+    mkmf_log = File.join @spec.build_info_dir, "#{@spec.full_name}.mkmf.log"
+
+    assert_path_exist mkmf_log
+    assert_includes e.message, mkmf_log
+
+    assert_path_not_exist File.join @spec.extension_dir, "mkmf.log"
+    assert_path_not_exist File.join ext_dir, "mkmf.log"
+  end
+
+  def test_build_extensions_removes_stale_build_info_logs_on_success
+    pend "terminates on mswin" if vc_windows? && ruby_repo?
+
+    @spec.extensions << "ext/extconf.rb"
+
+    ext_dir = File.join @spec.gem_dir, "ext"
+    FileUtils.mkdir_p ext_dir
+
+    File.open File.join(ext_dir, "extconf.rb"), "w" do |f|
+      f.write <<-'RUBY'
+        require 'mkmf'
+
+        create_makefile 'a'
+      RUBY
+    end
+
+    # Logs left in build_info by an earlier failed build, and in the extension
+    # directory by a RubyGems old enough to install them there.
+    FileUtils.mkdir_p @spec.build_info_dir
+    FileUtils.mkdir_p @spec.extension_dir
+    stale = [
+      File.join(@spec.build_info_dir, "#{@spec.full_name}.mkmf.log"),
+      File.join(@spec.build_info_dir, "#{@spec.full_name}.gem_make.out"),
+      File.join(@spec.extension_dir, "gem_make.out"),
+    ]
+    FileUtils.touch stale
+
+    use_ui @ui do
+      @builder.build_extensions
+    end
+
+    assert_path_exist @spec.gem_build_complete_path
+
+    stale.each {|path| assert_path_not_exist path }
+  end
+
+  def test_build_extensions_keeps_logs_when_no_makefile_is_generated
+    pend "terminates on mswin" if vc_windows? && ruby_repo?
+
+    @spec.extensions << "ext/extconf.rb"
+
+    ext_dir = File.join @spec.gem_dir, "ext"
+    FileUtils.mkdir_p ext_dir
+
+    # extconf exits cleanly but generates no Makefile, the way one that bails out
+    # early on an unsupported platform does. The extension is then skipped rather
+    # than built, and the log is the only record of why.
+    File.open File.join(ext_dir, "extconf.rb"), "w" do |f|
+      f.write "# nothing to build on this platform\n"
+    end
+
+    use_ui @ui do
+      @builder.build_extensions
+    end
+
+    gem_make_out = File.join @spec.build_info_dir, "#{@spec.full_name}.gem_make.out"
+
+    assert_path_exist gem_make_out
+    assert_includes File.read(gem_make_out), "no Makefile was found"
+
+    # Skipping still leaves nothing behind in the installation tree.
+    assert_path_not_exist File.join @spec.extension_dir, "mkmf.log"
+    assert_path_not_exist File.join @spec.extension_dir, "gem_make.out"
+    assert_path_not_exist File.join ext_dir, "mkmf.log"
+  end
+
+  def test_build_extensions_reports_build_error_when_logs_cannot_be_written
+    @spec.extensions << "extconf.rb"
+
+    FileUtils.mkdir_p @spec.gem_dir
+
+    # A plain file where build_info belongs makes every log write fail.
+    FileUtils.rm_rf @spec.build_info_dir
+    File.write @spec.build_info_dir, ""
+
+    e = assert_raise Gem::Ext::BuildError do
+      use_ui @ui do
+        @builder.build_extensions
+      end
+    end
+
+    # The build failure survives instead of being replaced by the log failure.
+    assert_match(/\AERROR: Failed to build gem native extension.$/, e.message)
+    assert_match(/: No such file/, e.message)
+    refute_includes e.message, "Results logged to"
+  end
+
+  def test_build_extensions_logs_to_build_info_on_failure
+    pend "terminates on mswin" if vc_windows? && ruby_repo?
+
+    @spec.extensions << "ext/extconf.rb"
+
+    ext_dir = File.join @spec.gem_dir, "ext"
+    FileUtils.mkdir_p ext_dir
+
+    File.open File.join(ext_dir, "extconf.rb"), "w" do |f|
+      f.write <<-'RUBY'
+        require 'mkmf'
+
+        have_library 'nonexistent' or abort 'need libnonexistent'
+
+        create_makefile 'a'
+      RUBY
+    end
+
+    e = assert_raise Gem::Ext::BuildError do
+      use_ui @ui do
+        @builder.build_extensions
+      end
+    end
+
+    mkmf_log = File.join @spec.build_info_dir, "#{@spec.full_name}.mkmf.log"
+    gem_make_out = File.join @spec.build_info_dir, "#{@spec.full_name}.gem_make.out"
+
+    assert_path_exist mkmf_log
+    assert_path_exist gem_make_out
+
+    # Logs are not left in the installation tree.
+    assert_path_not_exist File.join @spec.extension_dir, "mkmf.log"
+    assert_path_not_exist File.join @spec.extension_dir, "gem_make.out"
+    assert_path_not_exist File.join ext_dir, "mkmf.log"
+
+    # The error message points at the new build_info paths.
+    assert_includes e.message, gem_make_out
+    assert_includes e.message, mkmf_log
+
+    assert_path_not_exist @spec.gem_build_complete_path
+  end
+
   def test_build_extensions_none
     use_ui @ui do
       @builder.build_extensions
@@ -401,12 +605,14 @@ def test_build_extensions_extconf_bad
     assert_equal "Building native extensions. This could take a while...\n", @ui.output
     assert_equal "", @ui.error
 
-    gem_make_out = File.join @spec.extension_dir, "gem_make.out"
+    gem_make_out = File.join @spec.build_info_dir, "#{@spec.full_name}.gem_make.out"
     cmd_make_out = File.read(gem_make_out)
 
     assert_match %r{#{Regexp.escape Gem.ruby} .* extconf\.rb}, cmd_make_out
     assert_match(/: No such file/, cmd_make_out)
 
+    assert_path_not_exist File.join @spec.extension_dir, "gem_make.out"
+
     assert_path_not_exist @spec.gem_build_complete_path
 
     assert_equal cwd, Dir.pwd
@@ -414,7 +620,7 @@ def test_build_extensions_extconf_bad
 
   def test_build_extensions_unsupported
     FileUtils.mkdir_p @spec.gem_dir
-    gem_make_out = File.join @spec.extension_dir, "gem_make.out"
+    gem_make_out = File.join @spec.build_info_dir, "#{@spec.full_name}.gem_make.out"
     @spec.extensions << nil
 
     e = assert_raise Gem::Ext::BuildError do
diff --git a/test/rubygems/test_gem_ext_ext_conf_builder.rb b/test/rubygems/test_gem_ext_ext_conf_builder.rb
index bc383e5540a9..b776883f4557 100644
--- a/test/rubygems/test_gem_ext_ext_conf_builder.rb
+++ b/test/rubygems/test_gem_ext_ext_conf_builder.rb
@@ -26,16 +26,18 @@ def test_class_build
 
     output = []
 
-    result = Gem::Ext::ExtConfBuilder.build "extconf.rb", @dest_path, output, [], nil, @ext
-
-    assert_same result, output
-
-    assert_match(/^current directory:/, output[0])
-    assert_match(/^#{Regexp.quote(Gem.ruby)}.* extconf.rb/, output[1])
-
     if Gem.java_platform?
-      assert_includes(output, "Skipping make for extconf.rb as no Makefile was found.")
+      # extconf returns before creating a Makefile, so the extension is skipped.
+      # Deciding what that means is Gem::Ext::Builder#build_extension's job now,
+      # so the error reaches it instead of being swallowed here.
+      assert_raise Gem::Ext::Builder::NoMakefileError do
+        Gem::Ext::ExtConfBuilder.build "extconf.rb", @dest_path, output, [], nil, @ext
+      end
     else
+      result = Gem::Ext::ExtConfBuilder.build "extconf.rb", @dest_path, output, [], nil, @ext
+
+      assert_same result, output
+
       assert_equal "creating Makefile\n", output[2]
       assert_match(/^current directory:/, output[3])
       assert_contains_make_command "clean", output[4]
@@ -43,6 +45,9 @@ def test_class_build
       assert_contains_make_command "install", output[10]
     end
 
+    assert_match(/^current directory:/, output[0])
+    assert_match(/^#{Regexp.quote(Gem.ruby)}.* extconf.rb/, output[1])
+
     assert_empty Dir.glob(File.join(@ext, "siteconf*.rb"))
     assert_empty Dir.glob(File.join(@ext, ".gem.*"))
   end
@@ -110,10 +115,12 @@ def test_class_build_extconf_fail
     assert_equal "extconf failed, exit code 1", error.message
 
     assert_match(/^#{Regexp.quote(Gem.ruby)}.* extconf.rb/, output[1])
-    assert_match(File.join(@dest_path, "mkmf.log"), output[4])
-    assert_includes(output, "To see why this extension failed to compile, please check the mkmf.log which can be found here:\n")
+    refute_includes(output, "To see why this extension failed to compile, please check the mkmf.log which can be found here:\n")
 
-    assert_path_exist File.join @dest_path, "mkmf.log"
+    # mkmf.log is left in the extension directory; deciding where it ends up is
+    # left to Gem::Ext::Builder#build_extension.
+    assert_path_exist File.join @ext, "mkmf.log"
+    assert_path_not_exist File.join @dest_path, "mkmf.log"
   end
 
   def test_class_build_extconf_success_without_warning
@@ -133,6 +140,9 @@ def test_class_build_extconf_success_without_warning
 
     refute_includes(output, "To see why this extension failed to compile, please check the mkmf.log which can be found here:\n")
 
+    # mkmf.log is parked in dest_path so that "make clean" cannot delete it.
+    # Dropping it is Gem::Ext::Builder#build_extension's job, not this one's.
+    assert_path_not_exist File.join @ext, "mkmf.log"
     assert_path_exist File.join @dest_path, "mkmf.log"
   end
 
diff --git a/test/rubygems/test_gem_specification.rb b/test/rubygems/test_gem_specification.rb
index c63e68be47dd..3c5d22ba7b55 100644
--- a/test/rubygems/test_gem_specification.rb
+++ b/test/rubygems/test_gem_specification.rb
@@ -1562,8 +1562,9 @@ def test_build_extensions_preview
 
     @ext.build_extensions
 
+    # A successful build no longer leaves gem_make.out in the install tree.
     gem_make_out = File.join @ext.extension_dir, "gem_make.out"
-    assert_path_exist gem_make_out
+    assert_path_not_exist gem_make_out
   end
 
   def test_contains_requirable_file_eh
diff --git a/test/rubygems/test_gem_uninstaller.rb b/test/rubygems/test_gem_uninstaller.rb
index d1aacb40536b..2e526e157757 100644
--- a/test/rubygems/test_gem_uninstaller.rb
+++ b/test/rubygems/test_gem_uninstaller.rb
@@ -391,6 +391,39 @@ def test_uninstall_extension
     assert_path_not_exist @spec.extension_dir
   end
 
+  def test_uninstall_removes_build_info_logs
+    @spec.extensions << "extconf.rb"
+    write_file File.join(@tempdir, "extconf.rb") do |io|
+      io.write <<-RUBY
+require 'mkmf'
+create_makefile '#{@spec.name}'
+      RUBY
+    end
+
+    @spec.files += %w[extconf.rb]
+
+    use_ui @ui do
+      path = Gem::Package.build @spec
+
+      installer = Gem::Installer.at path, force: true
+      installer.install
+    end
+
+    # Build logs left behind in build_info by a previous failed build.
+    FileUtils.mkdir_p @spec.build_info_dir
+    mkmf_log = File.join @spec.build_info_dir, "#{@spec.full_name}.mkmf.log"
+    gem_make_out = File.join @spec.build_info_dir, "#{@spec.full_name}.gem_make.out"
+    FileUtils.touch mkmf_log
+    FileUtils.touch gem_make_out
+
+    uninstaller = Gem::Uninstaller.new @spec.name, executables: true
+    uninstaller.uninstall
+
+    assert_path_not_exist @spec.extension_dir
+    assert_path_not_exist mkmf_log
+    assert_path_not_exist gem_make_out
+  end
+
   def test_uninstall_nonexistent
     uninstaller = Gem::Uninstaller.new "bogus", executables: true