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Simplify if-let-Some-else-None using question mark operator #13626

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qsantos opened this issue Oct 29, 2024 · 2 comments
Open

Simplify if-let-Some-else-None using question mark operator #13626

qsantos opened this issue Oct 29, 2024 · 2 comments
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C-enhancement Category: Enhancement of lints, like adding more cases or adding help messages

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@qsantos
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qsantos commented Oct 29, 2024 •
edited by rustbot
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Consider the code below.

pub fn f(x: Option<u32>) -> Option<u32> {
    if let Some(x) = x {
        dbg!(x);
        Some(x * 2)
    } else {
        None
    }
}

Standard Clippy produces no warning. More exhaustive options suggest code that is no significantly better:

$ cargo clippy --all -- -W clippy::all -W clippy::nursery
error: use Option::map_or instead of an if let/else
 --> src/lib.rs:2:5
  |
2 | /     if let Some(x) = x {
3 | |         dbg!(x);
4 | |         Some(x * 2)
5 | |     } else {
6 | |         None
7 | |     }
  | |_____^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#option_if_let_else
  = note: `-D clippy::option-if-let-else` implied by `-D warnings`
  = help: to override `-D warnings` add `#[allow(clippy::option_if_let_else)]`
help: try
  |
2 ~     x.map_or(None, |x| {
3 +         dbg!(x);
4 +         Some(x * 2)
5 +     })
  |

However, the code can be written with less nesting, using the question mark operator:

pub fn f(x: Option<u32>) -> Option<u32> {
    let x = x?;
    dbg!(x);
    Some(x * 2)
}

Note that Clippy does suggest that for the code below:

pub fn f(x: Option<u32>) -> Option<u32> {
    let Some(x) = x else {
        return None;
    };
    dbg!(x);
    Some(x * 2)
}
error: this `let...else` may be rewritten with the `?` operator
 --> src/lib.rs:2:5
  |
2 | /     let Some(x) = x else {
3 | |         return None;
4 | |     };
  | |______^ help: replace it with: `let x = x?;`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#question_mark
  = note: `-D clippy::question-mark` implied by `-D warnings`
  = help: to override `-D warnings` add `#[allow(clippy::question_mark)]`

This pattern is particularly frequent when implementing iterators.
@samueltardieu samueltardieu added the C-enhancement Category: Enhancement of lints, like adding more cases or adding help messages label Oct 30, 2024
@DylanBulfin
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I'd like to try this one out, @rustbot claim

@DylanBulfin DylanBulfin mentioned this issue Dec 2, 2024
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@DylanBulfin
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The linked PR has some basic implementation code and a lot of very useful comments, but personal matters have been taking up my time and it'll be a while before I have time to continue working on it. So in case someone else is interested, @rustbot release-assignment

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Add new case for question_mark lint DylanBulfin/rust-clippy
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@samueltardieu @qsantos @DylanBulfin