fix: Only special case single line item attribute suggestions

[Muscraft]

rustc currently special cases suggestions to add #[derive(_)]\n and other attributes, to add more context to the suggestions.

// The suggestion adds an entire line of code, ending on a newline, so we'll also
// print the following line, to provide context of what we're advising people to
// do. Otherwise you would only see contextless code that can be confused for
// already existing code, despite the colors and UI elements.
// We special case #[derive(_)]\n and other attribute suggestions, because those
// are the ones where context is most useful.

This special case is a bit broad at the moment and applies to suggestions just to add an attribute, as well as suggestions that contain an attribute and other code, i.e.

#[derive(Clone)]

and

#[cfg(not(test))]
impl Default for NewWithCfg {
    fn default() -> Self {
        Self::new()
    }
}

In the latter case, adding a line for context after the suggestion doesn't provide much benefit. Example:

This PR makes it so that this special case only applies to suggestions that just add an attribute and nothing else. This will also make rustc's output match annotate-snippets.

[rustbot]

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

[rustbot]

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[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.