Re:Rusty Ownership and the Lifecycle’s Stone
by estebank on Sunday November 27, 2022 @12:54PM
(#33764934)
> There are some APIs like this but last time I checked they didn’t cover all usecases.
This refers to[1]
<&mut [T]>::split_at_mut(usize) -> (&mut [T], &mut [T])`
let (first_42, rest) = mutable_slice.split_at_mut(42);
let len = self.len();
let ptr = self.as_ptr();
// SAFETY: Caller has to check that `0 <= mid <= self.len()`
unsafe { (from_raw_parts(ptr, mid), from_raw_parts(ptr.add(mid), len - mid)) }
[2]: https://doc.rust-lang.org/src/core/slice/mod.rs.html#1641-16...
Re:Rusty Ownership and the Lifecycle’s Stone
by loeg on Sunday November 27, 2022 @02:01PM
(#33765565)
Yes, exactly. I was using this for zero-copy serialization into a buffer. At the time, I don’t believe that API had been stabilized yet.
Re:Rusty Ownership and the Lifecycle’s Stone
by estebank on Sunday November 27, 2022 @02:17PM
(#33765720)
That method has been around since 1.0 (as documented in the source code). Were you using Rust before 2015? If so, a lot has changed :)
Re:Rusty Ownership and the Lifecycle’s Stone
by loeg on Sunday November 27, 2022 @03:08PM
(#33766286)
No, post-1.0. I don't remember exactly why I had to roll my own with unsafe, but it was essentially the same thing.
Re:Rusty Ownership and the Lifecycle’s Stone
by bobbylarrybobby on Sunday November 27, 2022 @02:32PM
(#33765899)
For someone looking for an example of this, here's a SO post: https://stackoverflow.com/questions/74592882/extend-a-string...
Tl;dr Rust can't distinguish between a `Vec` whose length is mutable (which might need to reallocate) and one whose elements are mutable (which can provide mutable references to its elements but will never have to move them), so it prevents mutation in a case where it would be safe.
Two: splitting mut borrows. It’s reasonable to take a mut borrow of some resource and divide it into two independent borrows of exclusive components. Think non-overlapping subarrays, or independent struct members. The language does not really provide this for you, but it is safe. (There are some APIs like this but last time I checked they didn’t cover all usecases.)
Structs containing a borrow are just weird and annoying to work with. Lifetime parameterization everywhere is verbose and it’s difficult to determine where you’ve made a mistake. The syntax for describing lifetime relationships is non-obvious to me.