Detect when `mut arg: &Type` should have been `arg: &mut Type` #112357

estebank · 2023-06-06T19:03:40Z

Code

struct Object;

fn change_object(mut object: &Object) {
    let object2 = Object;
    object = object2;
    // and as follow up object = &object2;
}

fn main() {
    let mut object = Object;
    change_object(&object);
}

Current output

error[E0308]: mismatched types
 --> src/main.rs:5:14
  |
3 | fn change_object(mut object: &Object) {
  |                              ------- expected due to this parameter type
4 |     let object2 = Object;
5 |     object = object2;
  |              ^^^^^^^
  |              |
  |              expected `&Object`, found `Object`
  |              help: consider borrowing here: `&object2`


------


error[E0597]: `object2` does not live long enough
 --> src/main.rs:5:14
  |
3 | fn change_object(mut object: &Object) {
  |                              - let's call the lifetime of this reference `'1`
4 |     let object2 = Object;
  |         ------- binding `object2` declared here
5 |     object = &object2;
  |     ---------^^^^^^^^
  |     |        |
  |     |        borrowed value does not live long enough
  |     assignment requires that `object2` is borrowed for `'1`
6 | }
  | - `object2` dropped here while still borrowed

Desired output

error[E0308]: mismatched types
 --> src/main.rs:5:14
  |
3 | fn change_object(mut object: &Object) {
  |                              ------- expected due to this parameter type
4 |     let object2 = Object;
5 |     object = object2;
  |              ^^^^^^^ expected `&Object`, found `Object`
help: you might have meant to change the `Object` that `object` points to
  |
3 ~ fn change_object(object: &mut Object) {
4 |     let object2 = Object;
5 ~     *object = object2;
  |



----


error[E0597]: `object2` does not live long enough
 --> src/main.rs:5:14
  |
3 | fn change_object(mut object: &Object) {
  |                              - let's call the lifetime of this reference `'1`
4 |     let object2 = Object;
  |         ------- binding `object2` declared here
5 |     object = &object2;
  |     ---------^^^^^^^^
  |     |        |
  |     |        borrowed value does not live long enough
  |     assignment requires that `object2` is borrowed for `'1`
6 | }
  | - `object2` dropped here while still borrowed
help: you might have meant to change the `Object` that `object` points to
  |
3 ~ fn change_object(object: &mut Object) {
4 |     let object2 = Object;
5 ~     *object = object2;
  |

Rationale and extra context

I've seen this case done by a couple of newcomers. For the first case we might want to give both suggestions, depending on how targeted we can actually make this.

Other cases

No response

Anything else?

No response

estebank linked a pull request Dec 31, 2024 that will close this issue

Detect mut arg: &Ty meant to be arg: &mut Ty and provide structured suggestion #134977

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Detect when `mut arg: &Type` should have been `arg: &mut Type` #112357

Detect when `mut arg: &Type` should have been `arg: &mut Type` #112357

estebank commented Jun 6, 2023

Detect when mut arg: &Type should have been arg: &mut Type #112357

Detect when mut arg: &Type should have been arg: &mut Type #112357

Comments

estebank commented Jun 6, 2023

Code

Current output

Desired output

Rationale and extra context

Other cases

Anything else?

Detect when `mut arg: &Type` should have been `arg: &mut Type` #112357

Detect when `mut arg: &Type` should have been `arg: &mut Type` #112357