Prevent rustfmt from adding impl Trait definitions to types

[ytmimi]

Fixes #5086

[ytmimi]

Please let me know what additional test cases I could include to cover more type declarations!

[calebcartwright]

Thanks so @ytmimi for the quick fix! This would indeed have addressed the immediate issue, but I'm going to close in favor of #5093. I'd been doing some cleanup on aliases recently so I'd started looking at this yesterday evening and want to go that direction since that addresses some things more holistically.

I'd really like to address the underlying indentation issue with ImplTrait kinds when using version Two of formatting, but have opted to continue leveraging the existing hack to avoid running into that issue on type aliases.

I suspect that issue could be a fairly tricky fix, but if you wanted to take a look at it I suspect it's either due to the usage of a suboptimal shape and/or a bug in join_bounds_inner that attempts to determine whether or not to block indent

rustfmt/src/types.rs

Lines 812 to 825 in eee8f04

	ast::TyKind::ImplTrait(_, ref it) => {
	// Empty trait is not a parser error.
	if it.is_empty() {
	return Some("impl".to_owned());
	}
	let rw = if context.config.version() == Version::One {
	it.rewrite(context, shape)
	} else {
	join_bounds(context, shape, it, false)
	};
	rw.map(|it_str| {
	let space = if it_str.is_empty() { "" } else { " " };
	format!("impl{}{}", space, it_str)
	})