Release completed statement temporaries before exit-hook workloads

bench/README.md

@@ -391,6 +391,32 @@ changed from 0.352 seconds to 0.021 seconds. Peak mark-table entries in these
 runs changed from 301,767 to 1,772. Both builds print the same result. These
 numbers measure this collector workload, not the duration of the CI workflow.
 
+## `exit_gc_workload.rb`
+
+```sh
+ruby bench/exit_gc_workload.rb [temporary_errors] [collections]
+./mere-ruby bench/exit_gc_workload.rb [temporary_errors] [collections]
+```
+
+The final class body creates 20,000 temporary conversion errors by default.
+An exit hook collects them, reports the remaining count, and times 20 further
+collections. Stdout checks that the captured result survives. The interpreter
+uses conservative stack roots, so its retained count need not be exactly zero.
+
+On Apple silicon at O2 with Mere `991f299b`, against mere-ruby `23ce853`
+(2026-11-07), two runs retained all 20,000 errors before this fix and 126 after.
+The timed collections took 0.99–1.35 seconds before and 0.033–0.042 seconds
+after. Stdout matched Ruby 4.0.9.
+
+The driver now ends the completed top-level statement's temporary allocation
+interval before running exit hooks. Its result remains explicitly rooted in
+`gc_hold`, and captured values retain their usual roots. This particularly
+affects test runners that execute their suites from an exit hook. A local
+`test_UTF-8_upcase_ascii` comparison went from about 121 to 7.5 seconds in the
+test body, with all 40,569 assertions passing; initialization adds roughly a
+minute and is not included in those body times. Concurrent validation limits
+timing precision.
+
 ## `file_read_checks.rb`
 
 ```sh

bench/exit_gc_workload.rb

@@ -0,0 +1,29 @@
+# A final class body creates temporary conversion errors, then exit hooks
+# perform repeated collections. Count retained errors and time the actual GC.
+count = Integer(ARGV.fetch(0, '20000'))
+rounds = Integer(ARGV.fetch(1, '20'))
+at_exit do
+  GC.enable
+  GC.start
+  warn "retained_errors=#{ObjectSpace.each_object(Encoding::UndefinedConversionError).count}"
+  start = Process.clock_gettime(Process::CLOCK_MONOTONIC)
+  rounds.times { GC.start }
+  warn "collections=#{rounds} seconds=#{Process.clock_gettime(Process::CLOCK_MONOTONIC) - start}"
+  p ExitGCWorkload.new.kept
+end
+GC.disable
+Object.const_set(:EXIT_GC_INPUT, count)
+class ExitGCWorkload
+  def self.build
+    codepoints = Array.new(EXIT_GC_INPUT) { |i| (0x1000 + i % 0x1000).chr('UTF-8') }.select do |code|
+      begin
+        code.encode('US-ASCII')
+        true
+      rescue Encoding::UndefinedConversionError
+        false
+      end
+    end
+    define_method(:kept) { codepoints }
+  end
+  build
+end

corpus/504_completed_statement_exit_roots.rb

@@ -0,0 +1,26 @@
+# Exit hooks must keep captured values, but not every temporary allocated by
+# the program's final statement. A conservative collector may retain a few
+# incidental handles; retaining most of this completed workload is a leak.
+at_exit do
+  GC.enable
+  GC.start
+  p ObjectSpace.each_object(Encoding::UndefinedConversionError).count < 1000
+  p ExitRootFixture.new.kept
+end
+GC.disable
+
+class ExitRootFixture
+  def self.build
+    kept = ['kept', {answer: [42]}]
+    selected = (0..4000).map { |i| (i + 0x1000).chr('UTF-8') }.select do |code|
+      begin
+        code.encode('US-ASCII')
+        true
+      rescue Encoding::UndefinedConversionError
+        false
+      end
+    end
+    define_method(:kept) { [kept, selected] }
+  end
+  build
+end

m_driver.mere

@@ -12533,6 +12533,10 @@ let run_src = fn (src0: str) -> fn (print_result: bool) -> fn (name: str) ->
       else gc_drive (gc_total + 2) lastv)
     else
       // ---- finish: reporting, at_exit, optional print ----
+      // The last top-level statement has finished. Its temporary allocation
+      // interval must not become a root for the entire at_exit workload.
+      // gc_drive already pinned lastv in gc_hold for optional result printing.
+      let _ = fb_wm_stmt () in
       let (meths, sup, ocls, ivars) = world in
       // (=exit prints them after the at_exit handlers instead: all of a
       // test/unit file runs inside one, so before them it reads nothing)