Suggestion to make a binding mutable could lead beginners astray

[Nadrieril]

Code

fn main() {
    let y = Some(0);
    if let Some(x) = y {
        x = 2;
    }
}

Current output

error[E0384]: cannot assign twice to immutable variable `x`
 --> src/main.rs:4:9
  |
3 |     if let Some(x) = y {
  |                 -
  |                 |
  |                 first assignment to `x`
  |                 help: consider making this binding mutable: `mut x`
4 |         x = 2;
  |         ^^^^^ cannot assign twice to immutable variable

Desired output

error[E0384]: cannot assign twice to immutable variable `x`
 --> src/main.rs:4:9
  |
3 |     if let Some(x) = y {
  |                 -
  |                 |
  |                 first assignment to `x`
  |                 help: consider making this binding mutable: `mut x`
  |                 help: to modify the original value, take a borrow instead: `ref mut x`
4 |         x = 2;
  |         ^^^^^ cannot assign twice to immutable variable

Rationale and extra context

In the original code, it's unclear whether the intention is to modify the contents of y or only the copy contained in x. The suggestion assumes the second case, which gives:

fn main() {
    let y = Some(0);
    if let Some(mut x) = y {
        x = 2;
    }
}

This doesn't modify y yet compiles fine, which I expect could be surprising to a beginner. This meaning of mut is far from obvious. With the updated error message, subsequent errors guide the user to actually modifying y.

Other cases

No response

Anything else?

No response

[eggyal]

Given that y was not declared mut, the suggestion to use ref mut x alone won't be sufficient here. Perhaps the mutability of y's binding should be considered and/or mentioned in the diagnostic.

[Nadrieril]

If you apply the ref mut x suggestion you get another error etc until eventually you get the suggestion to declare y mutable. That feels clear enough to me and a cool series of learnings for my hypothetical beginner.