fix: Only special case single line item attribute suggestions
`rustc` currently special cases suggestions to add [`#[derive(_)]\n` and other attributes](dc1feabef2/compiler/rustc_errors/src/emitter.rs (L2288C36-L2288C72)), 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.
```rust
#[derive(Clone)]
```
and
```rust
#[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`.
e3dbcdf753