_recoverOnePassJoin(function, guardsOnly = false) matches the existing canonical invocation-root profile: one loop result is returned immediately by its owning function, and loop entry values follow the function's established input bindings. It reuses the invocation resolver to restore statement-level breaks as owning returns. The recognized prefix contains only reads of established input/capture aliases; those aliases and immutable iteration inputs become ordered required entry snapshots. Arbitrary lazy definitions remain outside this profile. Parameters, rest and captures retain their original function metadata.

The initial recovery profile requires iteration binding identities to be fresh at the function root. A binding already declared as a function parameter, capture, rest input or prefix alias keeps its original loop scope. Equal debug indices do not affect this decision; only binding identity does. The same restriction applies to body or false-arm declarations that would move to the function root: they may neither shadow an existing root binding nor collide with another declaration moved there. Their existing loop scopes remain in place until a broader scope-preserving reconstruction is implemented.

The resolver introduces a fresh self binding for any restored backedge. A use of that binding keeps the original loop, preserving genuine iteration and tail transitions. Functions already using self also retain their existing plan.

Recovery completes only when the existing single-result immutable-join pass accepts the restored body. Lazy definitions, broader joins, nested loops and hidden transfer/value regions keep the original function and loop target. No second transfer walker or owner/result-arity admission pass is introduced. The reused invocation-root helpers consume normalized FAST only; their existing edag/from_fast location introduces no graph analysis here. Relocating that shared API belongs to the compiler architecture work.

Common preparation uses this profile for JavaScript, Rust and public FJS source output. The JavaScript renderer spells empty alternatives as guards so the canonical joins parse through the existing FJS grammar. Genuine source loops keep their existing state/result layouts. This extends the supplied source output profile without extending source syntax or graph/text identity guarantees.

Source preparation selects the guard-only subset: every moved decision has an empty normally completing alternative and a then block ending in an explicit return or throw, with the same rule for nested guards. Normal inner decisions, inverted guards and nonempty alternatives retain the original invocation loop. Default JavaScript and Rust keep the broader existing join profile. This check chooses whether to remove the source loop; it does not introduce another join lowering or revalidate producer-established owners and result arities.