New lint idea: `matches_with_unrelated_if`

What it does

Warns against code where the if inside matches! doesn't use any of the variables introduced by the pattern, and suggest moving the if out of the matches!

Advantage

• Removes the illusory connection between the pattern-matching and the if-check
• If the user did intend to use the variable(s) in the if, the lint could hint at their oversight.
• Makes for less complex matches!, which rustfmt tends to appreciate

Drawbacks

• Sometimes this grouping might be deliberate?

Example

struct Foo;

impl Foo {
    fn foo(&self) -> Option<i32> {
        todo!()
    }
    fn is_bar(&self) -> bool {
        todo!()
    }
}

fn main() {
    let f = Foo;
    let _ = matches!(f.foo(), Some(_) if l.is_bar());
}

Could be written as:

struct Foo;

impl Foo {
    fn foo(&self) -> Option<i32> {
        todo!()
    }
    fn is_bar(&self) -> bool {
        todo!()
    }
}

fn main() {
    let f = Foo;
    let _ = matches!(f.foo(), Some(_)) && l.is_bar();
}

Comparison with existing lints

Could not find any similar lints

Additional Context

Inspired by https://github.com/rust-lang/rust-analyzer/blob/c6b7da6e5d36bea554ab16db2d1be700ef79cf97/crates/ide-completion/src/context/analysis.rs#L947-L950