Instances of ProgramLifetime classes (@scriptc/types) are never freed in scriptc builds. A LifetimeRegion gives a long-running program a way to free a whole generation of them at once, for example the previous program of a compiler's watch mode.

import { LifetimeRegion } from "@scriptc/experimental";

let previous: LifetimeRegion | undefined;
function rebuild(): Program {
  const region = new LifetimeRegion();
  const program = region.run(() => buildProgram());
  previous?.release();
  previous = region;
  return program;
}

run(fn) calls fn with the region as the current region and returns its result. ProgramLifetime instances created during the call belong to the region; calls nest.
Instances created outside every run call belong to the default region, which is never freed.
Work that resumes after an await inside fn runs outside the region.
release() requests that the region be freed. scriptc frees it once nothing outside the region refers to any of its instances; until then the region stays allocated. Later run calls on a released region allocate in the default region.
A region that holds a published object (@scriptc/threads) is never freed.

Under Node.js, run(fn) returns fn() and release() does nothing.

In scriptc builds, SCRIPTC_REGION_STATS=1 reports every release attempt on stderr.