Conditional Zero-Cost RefCell

The RefCell structure is often used in conjunction with the Rc in order to allow multiple reference-counted references to mutable data in Rust, with the borrow checking pushed into run-time, using a bit of memory and performing run-time checks.

With RefCell, as long as the try_borrow and try_borrow_mut functions are not used, and as long as sufficiently done testing has empirically verified that the borrow and borrow_mut function don't panic on run-time, then the overhead of RefCell can be reduced to zero using a feature enablement in the compilation stage, treating it as an UnsafeCell, its underlying implementation, without changing program semantics.

Thus, the czc-refcell module makes a proxy to the standard library RefCell that allows it to function as an UnsafeCell, if the unsafe_refcell_as_safe feature is enabled, using a different type for RefCell. Otherwise, and by default, the standard std::cell::RefCell is proxied as it is with all the guarantees it provides.

This provided compile-time conditional type has the same overhead of UnsafeCell, but mimics the interface of RefCell. Its implementation is based on a stripped-down RefCell from the standard library, where the borrow and borrow_mut are in-lined to a direct proxy into the UnsafeCell without any checking.

Note that under the non-default unchecked RefCell build mode, code paths that would originally result in RefCell-led panics will instead lead to undefined behavior, which can be exploitable, from a security perspective, and introduce program instabilities from a correctness perspective. This is a tradeoff between risk and performance, and users of unchecked RefCell builds should be aware of that.

Usage

Import RefCell from czc_refcell::cell instead of std::cell and use it, barring the try_borrow and try_borrow_mut methods.

extern czc_refcell;

use czc_refcell::cell::RefCell;

...

You should allow the users of your library to control whether you'd be giving them a checked or an unchecked RefCell types. At any case, if there is a mismatch of expectation, Rust's compiler will present a meaningful error.

[features]
unsafe_refcell_as_safe = ["czc-refcell/unsafe_refcell_as_safe"]

You can test the performance of a unchecked RefCell with a build feature flag, e.g:

cargo build --features 'unsafe_refcell_as_safe'

TODO

• For the exported stub type, think whether to export the try_borrow and try_borrow_mut as versions that always return Err. Currently, these methods are not implemented.