It can sometimes occur in the course of designing an API that it would be convenient, or even necessary, to allow fields within a struct to hold references to other fields within that same struct. Rust‘s concept of ownership and borrowing is powerful, but can’t express such a scenario yet. //! Creating such a struct manually would require unsafe code to erase lifetime parameters from the field types. Accessing the fields directly would be completely unsafe as a result. This library addresses that issue by allowing access to the internal fields only under carefully controlled circumstances, through closures that are bounded by generic lifetimes to prevent infiltration or exfiltration of any data with an incorrect lifetime. In short, while the struct internally uses unsafe code to store the fields, the interface exposed to the consumer of the struct is completely safe. The implementation of this interface is subtle and verbose, hence the macro to automate the process.
The API of this crate consists of the rental
macro that generates safe self-referential structs, a few example instantiations to demonstrate the API provided by such structs (see examples
), and a module of premade instantiations to cover common use cases (see common
).
One instance where this crate is useful is when working with libloading
. That crate provides a Library
struct that defines methods to borrow Symbol
s from it. These symbols are bounded by the lifetime of the library, and are thus considered a borrow. Under normal circumstances, one would be unable to store both the library and the symbols within a single struct, but the macro defined in this crate allows you to define a struct that is capable of storing both simultaneously, like so:
rental! { pub mod rent_libloading { use libloading; #[rental(deref_suffix)] // This struct will deref to the Deref::Target of Symbol. pub struct RentSymbol<S: 'static> { lib: Box<libloading::Library>, // Library is boxed for StableDeref. sym: libloading::Symbol<'lib, S>, // The 'lib lifetime borrows lib. } } } fn main() { let lib = libloading::Library::new("my_lib.so").unwrap(); // Open our dylib. if let Ok(rs) = rent_libloading::RentSymbol::try_new( Box::new(lib), |lib| unsafe { lib.get::<extern "C" fn()>(b"my_symbol") }) // Loading symbols is unsafe. { (*rs)(); // Call our function }; }
In this way we can store both the Library
and the Symbol
that borrows it in a single struct. We can even tell our struct to deref to the function pointer itself so we can easily call it. This is legal because the function pointer does not contain any of the special lifetimes introduced by the rental struct in its type signature, which means reborrowing will not expose them to the outside world. As an aside, the unsafe
block for loading the symbol is necessary because the act of loading a symbol from a dylib is unsafe, and is unrelated to rental.
There are a few limitations with the current implementation due to bugs or pending features in rust itself. These will be lifted once the underlying language allows it.
'static
must still produce legal types, otherwise it will not compile. In most situations this is fine, since most of the use cases for this library involve erasing all of the lifetimes anyway, but there‘s no reason why the head element of a rental struct shouldn’t be able to take arbitrary lifetime params. This is currently impossible to fully support due to lack of an 'unsafe
lifetime or equivalent feature.Foo<T>
where Foo
is some StableDeref
container, or rental will not be able to correctly guess the Deref::Target
of the type. If you are using a custom type that does not fit this pattern, you can use the target_ty
attribute on the field to manually specify the target type. If the head field is NOT a subrental, then it may have any form as long as it is StableDeref
.