extendMG leaks memory

extendMG :: ModuleGraph -> [NodeKey] -> ModSummary -> ModuleGraph
extendMG ModuleGraph{..} deps ms = ModuleGraph
  { mg_mss = ModuleNode deps ms : mg_mss
  , mg_trans_deps = mkTransDeps (ModuleNode deps ms : mg_mss)
  , mg_non_boot = case isBootSummary ms of
      IsBoot -> mg_non_boot
      NotBoot -> extendModuleEnv mg_non_boot (ms_mod ms) ms
  }

The mg_non_boot field is lazy and retains a reference to the previous non-boot ModuleEnv.

Solution: make it a strict field.

Fixed by !8579 (closed)