#[auto_enum] on non-statement expression does not work without unstable features

[taiki-e]

#[auto_enum]
fn match_(x: bool) -> Option<impl Iterator<Item = u8>> {
    Some(
        #[auto_enum(Iterator)]
        match x {
            true => std::iter::once(0),
            _ => std::iter::repeat(1),
        },
    )
}

#[auto_enum]
fn if_(x: bool) -> Option<impl Iterator<Item = u8>> {
    Some(
        #[auto_enum(Iterator)]
        if x { std::iter::once(0) } else { std::iter::repeat(1) },
    )
}

Related: #86

[taiki-e]

pin-project using the same logic should also have this problem: taiki-e/pin-project#189