Making overwrite opt-in #crazyideas · baby steps

submited by

Style Pass

2024-09-26 23:30:03

What would you say if I told you that it was possible to (a) eliminate a lot of “inter-method borrow conflicts” without introducing something like view types and (b) make pinning easier even than boats’s pinned places proposal, all without needing pinned fields or even a pinned keyword? You’d probably say “Sounds great… what’s the catch?” The catch it requires us to change Rust’s fundamental assumption that, given x: &mut T, you can always overwrite *x by doing *x = /* new value */, for any type T: Sized. This kind of change is tricky, but not impossible, to do over an edition.

We can reduce inter-procedural borrow check errors, increase clarity, and make pin vastly simpler to work with if we limit when it is possible to overwrite an &mut reference. The idea is that if you have a mutable reference x: &mut T, it should only be possible to overwrite x via *x = /* new value */ or to swap its value via std::mem::swap if T: Overwrite. To start with, most structs and enums would implement Overwrite, and it would be a default bound, like Sized; but we would transition in a future edition to have structs/enums be !Overwrite by default and to have T: Overwrite bounds written explicitly.

People often ask me to name something I would change about Rust if I could. One of the items on my list is the fact that, given a mutable reference x: &mut SomeStruct to some struct, I can overwrite the entire value of x by doing *x = /* new value */, versus only modifying individual fields like x.field = /* new value */.