Consider changing to & for let bindings #40402

[gaurikholkar-zz]

This is a fix for #40402

For the example

fn main() {
    let v = vec![String::from("oh no")];
    
    let e = v[0];
}

It gives

error[E0507]: cannot move out of indexed content
 --> ex1.rs:4:13
  |
4 |     let e = v[0];
  |             ^^^^ cannot move out of indexed content
  |
  = help: consider changing to `&v[0]`

error: aborting due to previous error

Another alternative is

error[E0507]: cannot move out of indexed content
 --> ex1.rs:4:13
  |
4 |     let e = v[0];
  |             ^^^^ consider changing to `&v[0]`

error: aborting due to previous error

Also refer to #41564 for more details.

r? @nikomatsakis

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[gaurikholkar-zz]

@estebank @jonathandturner please review the format.

[nikomatsakis]

please review the format.

Specifically, as @gaurikholkar said, she and I were wondering what you guys thought might be best. Because the span to label for the suggestion is the same as the one for which we are reporting the error, the initial stab looked something like:

error[E0507]: cannot move out of indexed content
 --> ex1.rs:4:13
  |
4 |     let e = v[0];
  |             ^^^^
  |             |
  |             consider changing to `&v[0]`
  |             cannot move out of indexed content
  |

which didn't seem good. So we did the two other experiments you see in the main comment area. The main question is, should we repeat the "main error text" also in the label, or not?

[nikomatsakis]

The code looks good, only question is how to format the error.

[estebank]

What about using a span_suggestion?

error[E0507]: cannot move out of indexed content
 --> ex1.rs:4:13
  |
4 |     let e = v[0];
  |             ^^^^ cannot move out of indexed content
  |
help: consider using a reference instead:
  |     let e = &v[0];

It takes more vertical space, which I care little for, but it is the way these kind of suggestions are presented in other diagnostics.

[nikomatsakis]

@estebank hmm, I thought we were moving away from that format for some reason. I'm not a big fan but maybe we should just use it for consistency's sake, and then we can think about upgrading how all uses of it are rendered in one step at some point.

[estebank]

@nikomatsakis I'd opened #39152 some time ago as a catch all to improve the suggestion output.

It was my understanding that span suggestions should be reserved for cases where certainty is high so that they can be applied automatically by tools.

[nikomatsakis]

@estebank I've heard that line of thought. I find it hard to judge what an appropriate level of certainty is for a fictional tool -- but in general I do think people get quite frustrated when they apply a suggestion, only to find that it leads them to another error. And definitely presenting ill-formed code as a suggestion is very bad.

It seems to me that this suggestion is certainly "as good" as the ref suggestion we used to give. It will solve the immediate error, though it may not necessarily be the right move in the larger picture.

I'm trying to think if it's possible for the suggestion to be malformed. The only reason I can think for that to happen would be if () were required. However, I think that for this error to occur, the initializer must be an lvalue (a path, basically), and hence just inserting an & in front should always be valid.

[estebank]

@nikomatsakis I'm ok with the current output if we don't want to use span_suggestion. The help in label output doesn't look good IHMO. Also, I'm weary of having suggestions as the only the primary span in general, I get the feeling that the first line of the error is usually overlooked so I'd rather the primary span to always be a very short version of the error text.

[gaurikholkar-zz]

#40851

[gaurikholkar-zz]

@nikomatsakis @estebank have updated the PR.
Using span_suggestions the output is

error[E0507]: cannot move out of indexed content
 --> ex1.rs:4:13
  |
4 |     let e = v[0];
  |             ^^^^
  |             |
  |             help: consider using a reference instead `&v[0]`
  |             cannot move out of indexed content

error: aborting due to previous error

[nikomatsakis]

@bors r+

[bors]

📌 Commit 1c57bb4 has been approved by nikomatsakis