Recover from accidental inclusion of if in let else

[Rageking8]

Given the following code: link

fn main() {
    let x = Some(123);
    if let Some(y) = x else {
        return;
    };
}

The current output is:

   Compiling playground v0.0.1 (/playground)
error: this `if` expression is missing a block after the condition
 --> src/main.rs:3:5
  |
3 |     if let Some(y) = x else {
  |     ^^
  |
help: add a block here
 --> src/main.rs:3:23
  |
3 |     if let Some(y) = x else {
  |                       ^

error: could not compile `playground` due to previous error

Ideally the output should suggest removing the additional if keyword in front of the let else. This mistake can occur due to muscle memory as if, if let, let else and in the future let chains are somewhat similar syntactically. Thanks.

@rustbot label +D-confusing +F-let-else

[faptc]

I think the current suggestion is valid.
Because one writes let y = Some(0); if let Some(x) = y else {} instead of
writing if let x = y else {}.

So the suggestion if changed maybe fall-negative.

[estebank]

We should likely add an alternative suggestion to remove the if, but keep both. I think that the current suggestion is likely going to be the more common situation, so it should still come first.

[ffmancera]

@rustbot claim

[hgrahamcs]

What would be the best way to go about adding an alternate suggestion? I'd like to try my hand at this.

What I think needs to be done is adding help text to compiler/rustc_error_messages/locales/en-US/parser.ftl,
add a variant to compiler/rustc_parse/src/errors.rs IfExpressionMissingThenBlockSub,
and then add something to compiler/rustc_parse/src/parser/expr.rs in recover_block_from_condition to add the alternate after the primary suggestion.

Is this a good starting point?

[estebank]

@hgrahamcs it sounds like you are in the right path.

[HintringerFabian]

I looked a bit into this issue and have already found where to add text to the parser.ftl and the errors.rs as @hgrahamcs mentioned, but i am a bit overwhelmed how and where to start in the recover_block_from_condition in the expr.rs.

Does anyone have a hint for me?

[HintringerFabian]

@rustbot claim

[sladyn98]

@rustbot claim

[estebank]

but i am a bit overwhelmed how and where to start in the recover_block_from_condition in the expr.rs.

You can add info!(?binding) instructions in the parser to track your progress.

I'm not sure about the specifics out of the top of my head here of what type each step in the parse is, but you will have successfully parsed the if, the condition and then you'll encounter an else token when a body/block was expected. That's where the new recovery should kick in, pointing at the if span saying that you might want to remove it.

[edward-shen]

@rustbot claim