How to destructure to reference to value instead of reference to borrow?

Question

How to rewrite what follows to have it all in one line, in function signature:

fn process(tup: &mut (u32,u32,&mut image::Luma<u8>)) {
  let &mut (x,y, _) = tup;
  let ref mut pixel = *tup.2;

I got as far as:

fn process(&mut (x,y, ref mut pixel): &mut (u32,u32,&mut image::Luma<u8>)) {

but that's not exact equivalent because I no longer can do:

*pixel = image::Luma([i as u8]);

inside the function, that I could do when I had temporary tup binding.

Fails with:

src\main.rs:43:14: 43:36 note: expected type `&mut image::Luma<u8>`
src\main.rs:43:14: 43:36 note:    found type `image::Luma<u8>`

I also tried:

process(&mut (x, y, pixel): &mut (u32,u32,&mut image::Luma<u8>))

but this fails with:

src\main.rs:23:12: 23:29 error: cannot move out of borrowed content [E0507]
src\main.rs:23 fn process(&mut (x,y, pixel): &mut (u32,u32,&mut image::Luma<u8>)) {
                          ^~~~~~~~~~~~~~~~~
src\main.rs:23 fn process(&mut (x,y, pixel): &mut (u32,u32,&mut image::Luma<u8>)) {
                                     ^~~~~

Basically what I need is pattern that can destructure reference to borrowed value from a borrow.

I'd like to point that in this specific case, you should be able to write `process((x, y, pixel): (u32, u32, &mut X))`. The outer `&mut` (on both sides) is useless here. — Matthieu M., Jun 20 '16 at 15:44
Does not compile without it due to type mismatch: `src\main.rs:72:33: 72:34 note: expected type (u32, u32, &mut image::Luma) src\main.rs:72:33: 72:34 note: found type &mut (u32, u32, &mut _)` I also can't deference at the call site because `cannot move out of borrowed content`. Would work if tuple third element wasn't a borrow. — Kamil Szot, Jun 20 '16 at 15:58
Just tried to use https://github.com/PistonDevelopers/image with their example 6.2 (drawing Julia fractal) and extract calculation to separate function. ... The more I fall, the more I learn. Anyways, thanks for help! — Kamil Szot, Jun 20 '16 at 17:14

Kamil Szot · Answer 1 · 2016-06-20T15:59:43.150

fn process(&mut (x,y, &mut ref mut pixel): &mut (u32,u32,&mut image::Luma<u8>)) {

This pattern &mut (x,y, &mut ref mut pixel) makes the pixel mutable reference to borrowed value.

&mut in &mut ref mut pixel unwraps the value from the borrow before ref mut makes the pixel reference to it.

I found this solution after looking here: http://rustbyexample.com/flow_control/match/destructuring/destructure_pointers.html

How to destructure to reference to value instead of reference to borrow?

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