Fix unit_arg suggests wrongly for Default::default

[profetia]

Closes #14857

changelog: [unit_arg] fix wrong suggestion for Default::default

[rustbot]

r? @samueltardieu

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[samueltardieu]

I don't think it behaves as expected in the following cases:

    macro_rules! mac {
        (def) => { Default::default() };
        (nondef $e:expr) => { $e };
        (func $f:expr) => { $f() };
    }
    fn_take_unit(mac!(def));
    fn_take_unit(mac!(nondef Default::default()));
    fn_take_unit(mac!(func Default::default));

[profetia]

Turns out this lint cannot correctly handle macros at all:

    fn some_other_fn(_: &i32) {}

    macro_rules! another_mac {
        () => {
            some_other_fn(&Default::default());
        };
        ($e:expr) => {
            some_other_fn(&$e);
        };
    }

    fn_take_unit(another_mac!());
    fn_take_unit(another_mac!(1));

[profetia]

Thank you! Now both the Default::default and the macro problems are fixed.

[profetia]

@rustbot ready

[samueltardieu]

I'm not sure this is the right fix: we should not specialize it for Default::default(). Any expression whose type is not fully determined without being coerced to () is susceptible to trigger this.

For example, the following

fn def<T: Default>() -> T {
    Default::default()
}

fn take_unit(_: ()) {}

fn main() {
    take_unit(def());
}

exhibits the same problem as the original issue, without using Default::default().

clippy_utils::ty::expr_type_is_certain() might be useful here. If it returns false, let _: () = …; might be a good suggestion. And if the expression is default(), it can be totally removed from the suggestion, unless it comes from a macro (I don't think we should look inside macros).