fix: collapsible_match suggests ref/derefs when needed

[vishruth-thimmaiah]

Fixes: #14155

If an expression is borrowed/dereferenced, suggest using .as_ref() or .copied() respectively in the outer conditional statement.

changelog: [collapsible_match] suggests ref/dereferencing when needed.

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[nk9]

Checking in here. It looks to me like the state is wrong now. Aren't we actually waiting on review?

[samueltardieu]

We are expecting both a review and a rebase. The bot settings have been changed a few days ago to let the PR in review state, so I'll reset it.

@rustbot review

[samueltardieu]

It looks like you ignore the fact that the reference might be mutable. The following code will suggest using .as_ref():

    let mut arr = ["a", "b", "c"];
    if let Some(mut last) = arr.last_mut() {
        match &mut last {
            &mut &mut "a" | &mut &mut "b" => {
                unimplemented!()
            },
            _ => (),
        }
    }

The suggestion might also be incorrect in case of raw references, where the types may not match (untested):

    const NULL_PTR: *const &'static str = std::ptr::null();
    if let Some(last) = arr.last() {
        match &raw const *last {
            NULL_PTR => unimplemented!(),
            _ => (),
        }
    }

[vishruth-thimmaiah]

I couldn't figure out any way to handle &raw refs, so I just disabled the lint for &raw refs. If you have any suggestions on how to handle them, I'd appreciate it!