New Lint: Use question mark instead of Option::and_then

[camsteffen]

What it does

Detect functions that end with Option::and_then or Result::and_then and suggest using a question mark instead.

Categories

• Kind: style, maybe pedantic

It is simpler and more idiomatic.

Drawbacks

None.

Example

fn test(opt: Option<i32>) -> Option<i32> {
    opt.and_then(|n| {
        if n > 1 {
            Some(n + 1)
        } else {
            None
        }
    })
}

Could be written as:

fn test(opt: Option<i32>) -> Option<i32> {
    let n = opt?;
    if n > 1 {
        Some(n + 1)
    } else {
        None
    }
}

The and_then call should be the very last expression of the function (not inside an if block, for example).

More Questionable Cases

Single expression lambda (no block)

fn test(opt: Option<i32>) -> Option<i32> {
    opt.and_then(|n| foo(n))
}

At the end of a call chain

fn foo() -> Option<i32> {
    vec![1, 2, 3]
       .iter()
       .last()
       .and_then(|n| {
           // ...
       })
}

Personally I think those cases should still be linted. Maybe it can be configurable.

[hristotanev]

just checking if this is still good to be worked on? i am aware it has been a while since this was raised! if it is, i don't mind picking it up :)!

cc: @flip1995 @Manishearth @giraffate (sorry if i've overtagged someone)

[Manishearth]

Should be fine. We should figure out the name and the lint level but you can start work on it before we decide.

I'm open to the level being style or complexity as long as lintcheck isn't too noisy

[hristotanev]

yeah, that sounds good. as you said, both of those things shouldn't require a lot of re-work if i start on this now. will get a bit familiarised with the tool and we can continue the discussion either here or when i raise the pr (about naming and lint level). thanks again :).

[hristotanev]

@rustbot claim

[sbdchd]

I found myself rewriting:

let select = paren_select(p);
match select {
    Some(cm) => {
        if p.at_ts(COMPOUND_SELECT_FIRST) {
            opt_compound_select(p, cm)
        } else {
            Some(cm)
        }
    }
    None => None,
}

as

paren_select(p).and_then(|cm| {
    if p.at_ts(COMPOUND_SELECT_FIRST) {
        opt_compound_select(p, cm)
    } else {
        Some(cm)
    }
})

before switching to the question mark:

let cm = paren_select(p)?;
if p.at_ts(COMPOUND_SELECT_FIRST) {
    opt_compound_select(p, cm)
} else {
    Some(cm)
}

[aaron-ang]

@rustbot claim