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ivan-aksamentov opened this issue May 11, 2025 · 1 comment · Fixed by #14783
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return_and_then: suggests using ? operator on Option in function which returns Result #14781

ivan-aksamentov opened this issue May 11, 2025 · 1 comment · Fixed by #14783
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@samueltardieu
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C-bug Category: Clippy is not doing the correct thing I-false-positive Issue: The lint was triggered on code it shouldn't have I-suggestion-causes-bug Issue: The suggestion compiles but changes the code to behave in an unintended way

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@ivan-aksamentov
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ivan-aksamentov commented May 11, 2025 •
edited by rustbot
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Summary

Related: #14051
CC: @aaron-ang

It seems that return_and_then does not take into account the surrounding context when suggesting a transformation.

The transformation suggested, changes meaning of the code (early return) and is only correct inside blocks returning an Option. Anywhere else it produces code which does not compile, and if it did compile, it would behave differently.

I discovered this by running

cargo clippy --fix

So this fix is marked for autofix (or whatever it is called) but is not currently safe to use.

Really good lint I'd like to enable, but needs some more work.

I think 2 parts are missing here (the list is not exhaustive)

  • check that we are about to return from the parent block (my code does not return; it's a let binding)
  • check that the parent block returns an Option (my block does not; it returns Result)

(I don't know the terminology, but by "parent block" I mean the block that the suggested ? operator will return from)

Lint Name

return_and_then

Reproducer

https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=ed73a58ce166b40d8c9567a5843aa773

I tried this code (a bit contrived, but derived from some real code):

#![warn(clippy::return_and_then)]
#![allow(dead_code, unused_variables)]

fn foo(_: &str, _: (u32, u32)) -> Result<(u32, u32), ()> {
  Ok((1, 1))
}

fn bug(_: Option<&str>) -> Result<(), ()> {
  let year: Option<&str> = None;
  let month: Option<&str> = None;
  let day: Option<&str> = None;

  let _day = if let (Some(year), Some(month)) = (year, month) {
    day.and_then(|day| foo(day, (1, 31)).ok())
  } else {
    None
  };

  Ok(())
}

fn main() {
    println!("Hello, world!");
}

I expected to see this happen:

No warning, or warning with the suggested code being correct.

Instead, this happened:

warning: use the question mark operator instead of an `and_then` call
  --> src/main.rs:14:5
   |
14 |     day.and_then(|day| foo(day, (1, 31)).ok())
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#return_and_then
note: the lint level is defined here
  --> src/main.rs:1:9
   |
1  | #![warn(clippy::return_and_then)]
   |         ^^^^^^^^^^^^^^^^^^^^^^^
help: try
   |
14 ~     let day = day?;
15 +     foo(day, (1, 31)).ok()
   |

After applying the transform, the code becomes:

#![warn(clippy::return_and_then)]
#![allow(dead_code, unused_variables)]

fn foo(_: &str, _: (u32, u32)) -> Result<(u32, u32), ()> {
  Ok((1, 1))
}

fn bug(_: Option<&str>) -> Result<(), ()> {
  let year: Option<&str> = None;
  let month: Option<&str> = None;
  let day: Option<&str> = None;

  let _day = if let (Some(year), Some(month)) = (year, month) {
    let day = day?;
    foo(day, (1, 31)).ok()
  } else {
    None
  };

  Ok(())
}

fn main() {
    println!("Hello, world!");
}

Not only it changes meaning (early return, which original code has no intention to do), but also triggering compiler error:

error[E0277]: the `?` operator can only be used on `Result`s, not `Option`s, in a function that returns `Result`
  --> src/main.rs:14:18
   |
8  | fn bug(_: Option<&str>) -> Result<(), ()> {
   | ----------------------------------------- this function returns a `Result`
...
14 |     let day = day?;
   |                  ^ use `.ok_or(...)?` to provide an error compatible with `Result<(), ()>`
   |
   = help: the trait `FromResidual<Option<Infallible>>` is not implemented for `Result<(), ()>`
   = help: the trait `FromResidual<Result<Infallible, E>>` is implemented for `Result<T, F>`

Version

rustc 1.86.0 (05f9846f8 2025-03-31)
binary: rustc
commit-hash: 05f9846f893b09a1be1fc8560e33fc3c815cfecb
commit-date: 2025-03-31
host: x86_64-unknown-linux-gnu
release: 1.86.0
LLVM version: 19.1.7

clippy 0.1.86 (05f9846f89 2025-03-31)

Additional Labels

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@ivan-aksamentov ivan-aksamentov added C-bug Category: Clippy is not doing the correct thing I-false-positive Issue: The lint was triggered on code it shouldn't have labels May 11, 2025
@samueltardieu
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@rustbot label +I-suggestion-causes-bug
@rustbot claim

@rustbot rustbot added the I-suggestion-causes-bug Issue: The suggestion compiles but changes the code to behave in an unintended way label May 11, 2025
@samueltardieu samueltardieu mentioned this issue May 11, 2025
Merged
github-merge-queue bot pushed a commit that referenced this issue May 12, 2025
@llogiq
return_and_then: only lint returning expressions (#14783)
459364b 
If an expression is not going to return from the current function of
closure, it should not get linted.

This also allows `return` expression to be linted, in addition to the
final expression. Those require blockification and proper indentation.

changelog: [`return_and_then`]: only lint returning expressions

Fixes #14781
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Labels
C-bug Category: Clippy is not doing the correct thing I-false-positive Issue: The lint was triggered on code it shouldn't have I-suggestion-causes-bug Issue: The suggestion compiles but changes the code to behave in an unintended way
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return_and_then: only lint returning expressions samueltardieu/rust-clippy
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