Previously this:

fn main() {
    const STRING: &str = indoc! {"
        ...
    "};
}

would produce "...\n", while this:

const STRING: &str = indoc! {"
    ...
"};

would produce "...".

That doesn't make sense to me and was probably unintended.

The correct behavior is both should produce "...\n". If you want to get "...", it needs to be written as:

const STRING: &str = indoc! {"
    ..."
};