Runtime selection of the active mutant.

The active mutant is constant for an entire suite run, so it is read once from the environment in the sandbox mix.exs prefix, before target project code, and stashed in :persistent_term (O(1) reads, built for write-once/read-many). Every selector site in the metamutant reads this key, normally hoisted to one read per function activation.

Eligible functions run their uninstrumented source when the active mutation belongs elsewhere: a lifted function through a copy of its original clauses, a function that stays in place through a second body chosen after its one read. Pure direct self-recursion may stay in a lifted function's clean clauses without rereading the selector; it relies on selection remaining stable during the computation. Ordinary entries and effectful recursion still read afresh, so put/1 can select another mutant between in-process calls.

A schema mutant is stored as {root_relative_file, local_id} in that single slot, the file as an atom (namespace_key/1): every instrumented function compares it on entry, and an atom compares in one instruction where a path string of equal length is read byte by byte. Mutare.Metamutant projects the slot to the local integer for the active file, 0 at baseline, or :inactive for other files. Thus only one file can be active and inactive files still short-circuit coverage before its tracking-flag read. put/1 and active/0 speak the string form. Standalone transforms continue to select by plain integer, including :start_id. The bootstrap combines MUTARE_ACTIVE_MUTANT (the local integer) with MUTARE_MUTANT_NAMESPACE (the file); the runner translates report ids before setting these. An absent namespace retains standalone integer selection.

This module defines both sides of that contract: Mutare.Metamutant uses its key and baseline when building selectors, while Mutare.Sandbox renders bootstrap_ast/0 into the target project's dependency-free project prefix, with an idempotent fallback in the test bootstrap.

Self-hosting: a private key for the suite-under-test

When the target is Mutare, the suite-under-test contains Mutare's own tests, many of which drive selection directly (put/1, building fixtures via Mutare.Transform) to exercise the machinery. If those tests wrote the same :persistent_term slot the harness uses to hold the mutant-under-test, they would clobber it mid-run — the active mutant would silently revert to baseline and every mutant whose killing test ran afterwards would register a false survivor (see NOTES.md, "Self-hosting").

So the runtime key is configurable: key/0 returns default_key/0 (:mutare_active, the harness key) unless override_env/0 names another, in which case the suite plays in that private slot. Mutare.Sandbox.Command sets that env var (to suite_key/0) on every sandbox mix it spawns, so the suite-under-test reads/writes :mutare_active__suite while the real metamutant — whose sites and bootstrap bake default_key/0 as literals at transform time, in the harness process where the override is unset — keeps reading :mutare_active. The two never collide. On a normal target there is no Mutare.Selector compiled in, so the env is inert.