"Irrefutable Pattern" Warnings in Let Chains

language design
40

I agree that the warning should be relaxed in some cases, especially since irrefutable patterns already are an accepted style in the middle of a let chain.

I think it should be relaxed for all cases when the first let in the chain (but not the last let in the chain) binds a variable, regardless of whether there is drop glue on its type.

Scoping a variable locally is convenient regardless of whether or not there is drop glue. It would be confusing if the warning allowed it for some types and not for others.

Example:

if let potential_answer = calculate() &&
        valid_answer(&potential_answer) {
    return potential_answer;
}

With your proposal, this would be a lint for i32 but not for Vec<i32>. I think if the lint is relaxed, it should be relaxed in this case for all types.

The alternative is to either keep potential_answer in scope longer than necessary, or use extra braces, similarly to your handle example:

{
    let potential_answer = calculate();
    if valid_answer(&potential_answer) {
        return potential_answer;
    }
}

This is already covered by the warning on the last let in the chain being irrefutable. That never makes sense, and shouldn't be relaxed.

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