The lifetime of the parameter is the lifetime of references in the pool. This allows us to bound lifetimes on autoreleased objects, thereby making them safe to return.

A nightly-only feature that adds the auto trait `AutoreleaseSafe` and uses it to prevent calling `autoreleasepool` with closures that capture an outer `AutoreleasePool`.

This fixes on nightly the unsoundness hole preventing `autoreleasepool` from being used to safely construct references to autoreleased objects.

Prevent using the lifetime from an outer pool when an inner pool is active.