New lint: `duplicate_match_guards`

What it does

Checks for the same condition being checked in a match guard and in the match body

Advantage

• Remove a likely typo or a copy and paste error

Drawbacks

• False negatives: if the condition is an impure function, it could've been called twice on purpose for its side effects
• removing a match guard might change the matching logic (so it might make sense to only suggest removing the inner if)

Example

match n {
    0 if a > b => {
        if a > b {
            return;
        }
    }
    _ => {}
}

Could be written as:

match n {
    0 if a > b => {
        return;
    }
    _ => {}
}

Start by reviewing the issue's Rust match example and the proposed duplicate condition check. Resolve whether the lint should remove the inner condition or the match guard, accounting for impure functions and matching semantics. Done means the lint behavior and its handling of these tradeoffs are clearly specified.