Add let_else_ok_or lint

[sylvestre]

New style lint that detects let...else statements binding the payload of an Option whose else branch only does return Err(<expr>), and suggests the more concise Option::ok_or/ok_or_else with the ? operator:

let Some(value) = opt else {
    return Err("missing");
};
// ->
let value = opt.ok_or("missing")?;

ok_or_else is suggested when the error value is non-trivial, to preserve the laziness of the original else branch. The lint bails on ref bindings, comments or #[cfg] in the else branch, and return Err(..) expressions produced by a macro expansion (e.g. anyhow::bail!), where the error value cannot be rendered into a sound suggestion.

changelog: Add let_else_ok_or lint

[rustbot]

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[github-actions]

Lintcheck changes for 05c9949

Lint	Added	Removed	Changed
clippy::let_else_ok_or	3	0	0

This comment will be updated if you push new changes

[samueltardieu]

The lint will suggest code that doesn't compile in const contexts, such as:

const fn f(a: Option<u32>) -> Result<u32, ()> {
    let Some(a) = a else {
        return Err(());
    };
    Ok(a)
}

You should check that:

• either you are not in a const context
• or the three features const_try, const_trait_impl and const_option_ops are enabled (you can do the test once and store the result in the LetElseOkOr struct, and use impl_lint_pass! instead of declare_lint_pass!)

View changes since this review

[rustbot]

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[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.