Add new lint stacked_if_match

[promptkp]

fixes #12483

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changelog: new [stacked_if_match] lint against if if and match match

[rustbot]

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[bors]

☔ The latest upstream changes (presumably #11476) made this pull request unmergeable. Please resolve the merge conflicts.

[alex-semenyuk]

In theory cases with match if and if match should work? I would be nice to have examples what happens in this cases

[promptkp]

Hi @alex-semenyuk, thank you for the comment. To prevent overlinting, stacked_if_match only lints against if if and match match. I'll add additional test cases for match if and if match.

Note: this is my first contribution to Clippy, so if there's a faster/cleaner way to implement this, please let me know!

[bors]

☔ The latest upstream changes (presumably #13247) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #13322) made this pull request unmergeable. Please resolve the merge conflicts.

[Alexendoo]

A few times I've looked at this again trying to figure out what the rustfmt rules are but they seem difficult to predict, e.g. here's an if in the middle of a condition that gets formatted in the undesired way

    if 1 == if xxxxxxxxxxxxxxxxxx {
        111111111111
    } else {
        222222222222222
    } && x == y
    {
        3
    } else {
        4
    };

Maybe instead of trying to recreate the behaviour we should check the actual format directly, something like calling SourceMap::lookup_char_pos to check if the inner if's hi is one above the outer if's lo