Suggest if let when a failed if block could be parsed as one #44990

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1. estebank

   added

   A-diagnosticsArea: Messages for errors, warnings, and lintsArea: Messages for errors, warnings, and lints

   C-enhancementCategory: An issue proposing an enhancement or a PR with one.Category: An issue proposing an enhancement or a PR with one.

   WG-diagnosticsWorking group: DiagnosticsWorking group: Diagnostics

   on Oct 3, 2017
2. arielb1

   added

   E-needs-mentor

   on Oct 3, 2017
3. cramertj commented on Oct 3, 2017

   

   cramertj

   on Oct 3, 2017

   Last edited by cramertj

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   Is if let the right suggestion here? Often the code would be fixed by just adding an extra =. This code is perfectly valid, for example:

   fn main() {
       let x = 0;
       let foo = Some(0);
       if Some(x) == foo {}
   }
4. estebank commented on Oct 3, 2017

   

   estebank

   on Oct 3, 2017

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   I think it would if x wasn't in the scope. We can't know that during parsing, but the errors for

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

   are

   error[E0425]: cannot find value `x` in this scope
    --> src/main.rs:3:13
     |
   3 |     if Some(x) = foo {}
     |             ^ not found in this scope
   
   error[E0308]: mismatched types
    --> src/main.rs:3:8
     |
   3 |     if Some(x) = foo {}
     |        ^^^^^^^^^^^^^
     |        |
     |        help: did you mean to compare equality?: `Some(x) == foo`
     |        expected bool, found ()
     |
     = note: expected type `bool`
                found type `()`
   

   Which tells me we could catch not found in this scope errors that occur on if conditions and try them as if let. If it succeeds to parse the rest of the block, then emit the original error with an extra suggestion.

   For the case where let x = 0 is in the code, the output is only the mismatched types error.

   👍React with 👍1Reacted by Taylor Cramer
5. estebank commented on May 22, 2019

   

   estebank

   on May 22, 2019

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   cc @Centril, given the recent it let desugaring work.
6. Centril commented on May 23, 2019

   

   Centril

   on May 23, 2019

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   @estebank Added to the tracking issue for list of potential issues to work on; tho since it is preexisting, not as a blocker but a nice-to-have.

   👍React with 👍1Reacted by Esteban Kuber
7. Centril

   added

   A-let-chainsArea: let chains (if-let, while-let, ...)Area: let chains (if-let, while-let, ...)

   on Oct 10, 2019
8. estebank

   added

   T-compilerRelevant to the compiler team, which will review and decide on the PR/issue.Relevant to the compiler team, which will review and decide on the PR/issue.

   D-papercutDiagnostics: An error or lint that needs small tweaks.Diagnostics: An error or lint that needs small tweaks.

   on Jan 22, 2020
9. bors

   added a commit that references this issue on Sep 1, 2020

   Auto merge of rust-lang#75931 - estebank:suggest-if-let, r=petrochenkov

   ...

   This issue will close once commit d824b23 is merged into the 'master' branch.

   d824b23
10. bors

    closed this as completedin 07112caon Sep 1, 2020