src/tools/rustfmt/tests/source/issue-3651.rs - toolchain/rustc - Git at Google

 fn f() -> Box<
     dyn FnMut() -> Thing< WithType = LongItemName, Error = LONGLONGLONGLONGLONGONGEvenLongerErrorNameLongerLonger>,
 >{
 }
	fn f() -> Box<
	dyn FnMut() -> Thing< WithType = LongItemName, Error = LONGLONGLONGLONGLONGONGEvenLongerErrorNameLongerLonger>,
	>{
	}