I would like to implement some method like struct S { i: i32, j: i32, } impl S { fn i(&mut self) -> &i32 { // Possibly do something with the mutable reference here // ... // Finally return an immutable reference &self.i } } Unfortunately that is no…

Are the field types i32, or more generally Copy, in the real case?

I am afraid without something like "borrow regions" we can't use partial borrows in such way, as compiler currently unable to tell from method signature that it returns reference only to fixed part of the struct.

You can sort of fake a "split" borrow by exposing the guts: fn split(&mut self) -> (&i32, &i32) { // do some mutation, and then return all relevant bits to the caller as immutable. // these borrows keep `self` borrowed mutably until they go away, // so all data needs to be …

Sad to hear that. The partial borrowing RFC is already in the pile of RFCs and issues I'm subscribed to. Thanks for that split pattern option. I guess I'll go with some pattern where one method takes a mutable reference and does the work and another one takes an immutable reference and returns the…

[image] cschramm: That panic seems to violate Rust principles but it looks like the borrowing system isn’t mature enough to be able to stick to them yet… The pattern you're trying to use here (take &mut self and return a &T, then use self again while the &T is alive) feels kinda odd for Rust.…

To @Michael-F-Bryan ’s point about illegal states being unrepresentable, here is a contrived example using type state (aka session types) to delineate the API of a type based on its state, at compile time. You can represent different states using non-generic types as well, if you want. There may be…

Thanks again for another option to tackle the problem. That type state pattern definitely makes sense in some cases, however, in other's I'm undecided if it feels more like a better solution or just a workaround. Let me get the problem more specific in two steps: Step 1: Cases where I tripped over…

The lazy cache is the poster child for interior mutability. You did mention cells, so I take it you’re aware of interior mutation. One thought worth bearing in mind is that &T vs &mut T isn’t so much that the former is immutable and latter mutable, but rather whether you want to allow aliasing or …

Wow, not thinking about mutability when thinking about &self vs. &mut self is worth a lot. I didn't really recognize that before. I'd even say the term mut is kind of misleading, now that I'm aware of it. :smile: With that mindset, wrapping the stream needed to get the body in the example with a Re…

Return immutable reference, taking mutable reference to self

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Cannot return mut reference to field	9	1230	August 5, 2022
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Return immutable reference, taking mutable reference to self

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