auto-fix for redundant_else lint

[lapla-cogito]

redundant_else can be fixed automatically.

changelog: [redundant_else]: auto-fix when appropriate

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[samueltardieu]

What if the else block declares variables? Not only will they be dragged into the outer scope, but also their dropping will be deferred until the end of the outer scope, which may be problematic in case of a guard.

For example, the PR fails with this code:

fn with_drop(t: bool) -> u32 {
    let a = 42;
    if t {
        return 0;
    } else {
        let a = "foobar";
        println!("this a = {a}");
    }
    a + 1
}

because let a = "foobar"; has been dragged into the outer scope and hides the outer a. Here the example is constructed specially to exhibit the failure, but with a mutex guard we might have real issues.

Maybe the suggestion should apply only when no bindings are introduced inside the else block, even though the lint could warn about the redundant else.

[samueltardieu]

Another (very minor) issue is that if the fix belongs to a macro, the indentation shown will be the one of the macro call, not the proper one, e.g.,

macro_rules! mac {
    ($t:expr) => {{
        if $t {
            return 0;
        } else {
            return 42;
        }
    }}
}

fn call_macro() -> u32 {
                       mac!(true)
}

will suggest using

macro_rules! mac {
    ($t:expr) => {{
        if $t {
            return 0;
        }
                       return 42;
    }}
}

[lapla-cogito]

@samueltardieu As you point out, the indentation of the suggestion would be the same as the indentation of the macro invocation in this implementation. However, I wonder if there is a solution for this since Clippy can only know the structure after the macro has been expanded...

[samueltardieu]

This is why I said this was a very minor nit: I don't think it is important to fix, and I'm not sure it is easy to do so.