feat: new lint for and_then when returning Option or Result

[aaron-ang]

close #6436

changelog: add new return_and_then lint

Additional src files changed to reflect the lint.

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[Manishearth]

r? @samueltardieu

bit swamped rn

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[Manishearth]

r? clippy

[samueltardieu]

You need to check that the expression you split in two parts doesn't use references to temporaries between those parts.

For example, the following code will fail

        String::from("<BOOM>")
            .strip_prefix("<")
            .and_then(|s| s.strip_suffix(">").map(String::from))

as the suggestion to use

        let s = String::from("<BOOM>")
            .strip_prefix("<")?;
        s.strip_suffix(">").map(String::from)

will not pass the borrow checker.

Maybe you could, after you check that the receiver is Option or Result, use something like:

    if let ty::Adt(_, args) = recv_type.kind()
        && args.iter().next().and_then(GenericArg::as_type).is_some_and(Ty::is_ref)
        && for_each_unconsumed_temporary(cx, recv, |_| ControlFlow::Break(())).is_break()
    {
        return;
    }

I've not checked if it would cover all the cases, as the lambda might not be called on non droppable types according to the comment on for_each_unconsumed_temporary(), but in practice it seems to work well.

[Centri3]

Happy to merge when Samuel approves 👍

[aaron-ang]

You need to check that the expression you split in two parts doesn't use references to temporaries between those parts.

For example, the following code will fail

        String::from("<BOOM>")
            .strip_prefix("<")
            .and_then(|s| s.strip_suffix(">").map(String::from))

as the suggestion to use

        let s = String::from("<BOOM>")
            .strip_prefix("<")?;
        s.strip_suffix(">").map(String::from)

will not pass the borrow checker.

Maybe you could, after you check that the receiver is Option or Result, use something like:

    if let ty::Adt(_, args) = recv_type.kind()
        && args.iter().next().and_then(GenericArg::as_type).is_some_and(Ty::is_ref)
        && for_each_unconsumed_temporary(cx, recv, |_| ControlFlow::Break(())).is_break()
    {
        return;
    }

I've not checked if it would cover all the cases, as the lambda might not be called on non droppable types according to the comment on for_each_unconsumed_temporary(), but in practice it seems to work well.

thanks for the comments! I've incorporated the suggestions :)

[samueltardieu]

This is starting to look very good. Could you please squash this? The ideal would be two commits (so that other people can easily review the lint code and its effects separately):

• one commit which implements the lint.
• one commit which applies it to Clippy sources.

I've started a FCP thread on Zulip.

[samueltardieu]

Also, this work could probably extend to such expressions placed after an explicit return as well, but this can be done in another PR.

[aaron-ang]

Also, this work could probably extend to such expressions placed after an explicit return as well, but this can be done in another PR.

wouldn't https://rust-lang.github.io/rust-clippy/master/index.html#needless_return catch that?

[samueltardieu]

wouldn't https://rust-lang.github.io/rust-clippy/master/index.html#needless_return catch that?

I'm specifically talking about a non-needless (early) return, i.e. a return which is not the last expression of a function.