False positive for clippy::question_mark #13417

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1. nojima

   added

   C-bugCategory: Clippy is not doing the correct thingCategory: Clippy is not doing the correct thing

   I-false-positiveIssue: The lint was triggered on code it shouldn't haveIssue: The lint was triggered on code it shouldn't have

   on Sep 19, 2024
2. nojima commented on Sep 19, 2024

   

   nojima

   on Sep 19, 2024

   Last edited by nojima

   Author

   More actions

   Replacing the code as clippy suggests will result in the following compile error.

   #![allow(clippy::disallowed_names)]
   
   struct Foo {
       opt_x: Option<String>,
   }
   
   fn bar(foo: &mut Foo) -> Option<String> {
       let ref x = foo.opt_x()?; // ← replaced with ? operator
       let opt_y = Some(x.clone());
       std::mem::replace(&mut foo.opt_x, opt_y)
   }

   error[E0507]: cannot move out of `foo.opt_x` which is behind a mutable reference
      --> src/main.rs:8:17
       |
   8   |     let ref x = foo.opt_x?;
       |                 ^^^^^^^^^-
       |                 |
       |                 `foo.opt_x` moved due to this method call
       |                 move occurs because `foo.opt_x` has type `Option<String>`, which does not implement the `Copy` trait
       |
   

   This is because the ? operator takes ownership of opt_x. Therefore, we can avoid this error by inserting as_ref in the middle. The following code compiles:

   fn bar(foo: &mut Foo) -> Option<String> {
       let x = foo.opt_x.as_ref()?; // ← inserted as_ref() here
       ...
   }

   In general, a sentence of the following form

   let Some(ref var) = expr else { return None };

   should be replaced with

   let var = expr.as_ref()?;
3. sanpii commented on Oct 26, 2024

   

   sanpii

   on Oct 26, 2024

   More actions

   And:

   let Some(ref mut var) = expr else { return None };

   with:

   let var = expr.as_mut()?;
4. github-merge-queue

   added a commit that references this issue on Mar 16, 2025

   question_mark: avoid incorrect suggestion when `ref` binding used (#1…

   ...

   This issue will close once commit 4800557 is merged into the 'master' branch.

   4800557
5. blyxyas

   closed this as completedin #14158on Mar 16, 2025