generativity | Generativity refers to the creation of a unique lifetime

generativity is a Rust library. generativity has no bugs, it has no vulnerabilities and it has low support. However generativity has a Non-SPDX License. You can download it from GitHub.

Generativity refers to the creation of a unique lifetime: one that the Rust borrow checker will not unify with any other lifetime. This can be used to brand types such that you know that you, and not another copy of you, created them. This is required for sound unchecked indexing and similar tricks. The first step of achieving generativity is an invariant lifetime. Then the consumer must not be able to unify that lifetime with any other lifetime. Traditionally, this is achieved with a closure. If you have the user write their code in a for<'a> fn(Invariant<'a>) -> _ callback, the local typechecking within this callback is forced to assume that it can be handed any lifetime (the for<'a> bound), and that it cannot possibly use another lifetime, even 'static, in its place (the invariance). This crate implements a different approach using macros and a Drop-based scope guard. When you call generativity::make_guard! to make a unique lifetime guard, the macro first creates an Id holding an invariant lifetime. It then puts within a Drop type a &'id Id<'id> reference. A different invocation of the macro has a different end timing to its tag lifetime, so the lifetimes cannot be unified. These types have no safe way to construct them other than via make_guard!, thus the lifetime is guaranteed unique. This effectively does the same thing as wrapping the rest of the function in an immediately invoked closure. We do some pre-work (create the tag and give the caller the guard), run the user code (the code after here, the closure previously), and then run some cleanup code after (in the drop implementation). This same technique of a macro hygiene hidden impl Drop can be used by any API that would normally use a closure argument, such as crossbeam's scoped threads, to make the containing scope the safe, wrapped scope if they so desire the trade-off of macro versus closure indentation.