Like a cell, but make lifetimes dynamic instead of ownership

Kalle Samuels

Last update: Dec 15, 2022

Related tags

Command-line lending-cell

Overview

LendingCell is a mutable container that allows you to get an owned reference to the same object. When the owned reference is dropped, ownership returns to the original container.

As opposed to a [std::cell::Cell] which moves Rust's ownership rules to runtime, a LendingCell moves Rust's lifetime rules to runtime.

The value of a LendingCell is present at construction-time, but you can convert it to a BorrowedCell<T> by calling [LendingCell::to_borrowed]. While that BorrowedCell lives (that is, until it is dropped), calling [LendingCell::try_get] will return None. The BorrowedCell has exclusive access to your type, as though you have a directly owned instance of the T, and so it is Send as long as T is Send. At last, when you drop the BorrowedCell, the T is returned to the LendingCell.

If you drop your LendingCell before dropping the BorrowedCell, then the T is dropped at the time you drop the BorrowedCell.

The invariants that Rust's memory safety rules enforce, like the single-mutable reference rule, are therefor partially ensured at compile time (you can't get two mutable references to the T), but also partially at runtime (while the BorrowedCell is active, the LendingCell behaves as though it is an Option containing None.

Comments

Validity of `const_ptr as *mut T` casts relies upon `Arc` implementation details

If it was okay to make these casts, there would have been Arc::as_mut_ptr. Thankfully there ~are other associated functions~ is UnsafeCell you could rely upon.

opened by ZetaNumbers 5