New lint: `duplicate_match_guards`

[ada4a]

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;
    }
    _ => {}
}

[ada4a]

I think the logic will be largely similar to eq_op

But there are some additional considerations:

1. the match body should be one of:
   • ExprKind::Block with no stmts and the if as the trailing_expr
   • ExprKind::If
2. the inner if may not have an else

[ada4a]

I might work on this myself, but I should probably finish #14895 first 😅