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You can already kind of do this with `pub fn x_mut(Self { x, .. }: &mut Self) -> &mut i32`, but 1. Your function is no longer callable like p.x_mut(); you have

You can already kind of do this with `pub fn x_mut(Self { x, .. }: &mut Self) -> &mut i32`, but 1. Your function is no longer callable like p.x_mut(); you have to go through `Point::x_mut(p)` (seems kind of dumb that the first argument has to literally be called `self` and can't just be of type `Self`) 2. The compiler doesn't actually use the fact that you're ignoring the unspecified fields of `Self`, although it seems like that analysis should be trivial, given that rust already understands that `calculate(&mut point.x, &mut point.y)` isn't overlapping?

roarcher 4y ago
Now that I'm actually putting this code into an editor to try your suggestion, I'm noticing the embarrassing nested `impl Point`, which is sadly no longer editable. Oh, well. Test your code, kids. Is this what you had in mind? struct Point { x: i32, y: i32, } impl Point { pub fn x_mut(Self { x, .. }: &mut Self) -> &mut i32 { x } pub fn y_mut(Self { y, .. }: &mut Self) -> &mut i32 { y } pub fn calculate(&mut self) -> i32 { let x = Self::x_mut(self); let y = Self::y_mut(self); *x * *y } } The `self` on the second line of the `calculate` function still gets flagged with "cannot borrow `*self` as mutable more than once at a time", which I think is what you were talking about in your second point. I had no idea that the syntax above was actually valid in argument lists. Learned something new about Rust today.
- bobbylarrybobby 4y ago
  Yep, that's the code I meant. And yeah I know it doesn't work, but it should! We've already done the partial borrow analysis for the compiler! Unfortunately the destructuring is only sugar and doesn't change what the borrow checker sees: two functions that each take a `&mut Self`.

roarcher 4y ago

Now that I'm actually putting this code into an editor to try your suggestion, I'm noticing the embarrassing nested `impl Point`, which is sadly no longer editable. Oh, well. Test your code, kids. Is this what you had in mind? struct Point { x: i32, y: i32, } impl Point { pub fn x_mut(Self { x, .. }: &mut Self) -> &mut i32 { x } pub fn y_mut(Self { y, .. }: &mut Self) -> &mut i32 { y } pub fn calculate(&mut self) -> i32 { let x = Self::x_mut(self); let y = Self::y_mut(self); *x * *y } } The `self` on the second line of the `calculate` function still gets flagged with "cannot borrow `*self` as mutable more than once at a time", which I think is what you were talking about in your second point. I had no idea that the syntax above was actually valid in argument lists. Learned something new about Rust today.

bobbylarrybobby 4y ago
Yep, that's the code I meant. And yeah I know it doesn't work, but it should! We've already done the partial borrow analysis for the compiler! Unfortunately the destructuring is only sugar and doesn't change what the borrow checker sees: two functions that each take a `&mut Self`.