Add Collapsible match lint

[camsteffen]

changelog: Add collapsible_match lint

Closes #1252
Closes #2521

This lint finds nested match or if let patterns that can be squashed together. It is designed to be very conservative to only find cases where merging the patterns would most likely reduce cognitive complexity.

Example:

match result {
    Ok(opt) => match opt {
        Some(x) => x,
        _ => return,
    }
    _ => return,
}

to

match result {
    Ok(Some(x)) => x,
    _ => return,
}

These criteria must be met for the lint to fire:

• The inner match has exactly 2 branches.
• Both the outer and inner match have a "wild" branch like _ => ... There is a special case for None => .. to also be considered "wild-like".
• The contents of the wild branches are identical.
• The binding which "links" the matches is never used elsewhere.

Thanks to the hir, if let's are easily included with this lint since they are desugared into equivalent match'es.

I think this would fit into the style category, but I would also understand changing it to pedantic.

[rust-highfive]

r? @llogiq

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[camsteffen]

Hmm my stderr is too long? I'm not sure what to do about that.

[giraffate]

How about splitting a test file into two?

[camsteffen]

Done.

[llogiq]

Good stuff! I especially like the SpanlessEq extension. I would start in the style category until we get reports about it swamping the output. @bors r+

[bors]

📌 Commit 0e20788 has been approved by llogiq

[bors]

⌛ Testing commit 0e20788 with merge 6d847c4...

[bors]

💔 Test failed - checks-action_test

[flip1995]

@bors retry rollup