Recursion and tail calls

A source-tail call supplies the function’s result without further work. This includes final expressions, explicit returns, selected conditional-expression branches, tail if/match arms, the arms of return if, the cases of return match, and result branches of and/or. An if or match block bound to a local or used as an assignment’s value is not a tail position. Arithmetic, wrapping in Ok or Some, conversion, or ? after a call makes it non-tail. Evaluating arguments before the call does not make it non-tail.

For a direct source-tail call within its caller’s direct recursion cycle, compilation requires an approved native tail transfer. Otherwise it reports the callee and the lifetime, resource-scope exit, or representation restriction at that call. Cycles are computed after concrete generic specialization and include both tail and non-tail direct edges. Different specializations are different nodes. A cycle that also has non-tail edges has no constant-stack guarantee.

A transfer evaluates arguments left to right, then drops pending temporaries and the caller’s remaining owned parameters and locals in ordinary exit order, then enters the callee. Moved arguments belong to the callee. Destructors on unmoved locals run before the callee body and do not by themselves prevent a transfer. An argument’s ? takes an ordinary error exit instead.

These restrictions retain the caller for an ordinary call:

• Any reference argument into the caller’s locals, owned parameters, or temporaries, including a receiver in that storage.
• A pending with exit action after the call.
• An owned inline argument, such as a range, record, closure, or generator frame.
• A result requiring conversion, wrapping, propagation, or incompatible result storage.

Nonrecursive ordinary fallbacks are legal. Reference arguments originating in the caller’s reference parameters may be forwarded, including reborrows, projections, borrowed-match payloads, borrowed iteration items, and forwarded reference results. Their owners belong to an older caller. Exact-compatible inline results can also transfer, including ranges, records, nested sums, and concrete generator frames. No hidden heap allocation extends argument lifetime.

Indirect Callable calls follow the same transfer restrictions but are outside the direct recursion guarantee. Concrete closure invocation, worker starts, generator construction, and generator bodies are outside that graph. Ordinary recursive calls during generator resumption may consume stack. Explicit direct-call edges in generated adapters participate; an indirect call into an adapter does not create a direct edge. See the tail-call ABI for implementation.