Clarification on style for functions with very long return types

[davechallis]

Might be useful to include some guidance on how to deal with function return types that don't fit on a single line within the recommended line length limit, e.g.:

fn foo(
    &self,
    x: i32,
    y: i32
) -> fn(a: SomeLongComplexType, b: SomeOtherLongComplexType) -> Box<Future<Item = AnotherLongType, Error = ALongErrorType>> {
    ...
}

E.g. is it preferred to keep on a single line as in the example above? Or should it be split along similar lines to the outer function, i.e.

fn foo(
    &self,
    x: i32,
    y: i32
) -> fn(
    a: SomeLongComplexType,
    b: SomeOtherLongComplexType
) -> Box<Future<Item = AnotherLongType, Error = ALongErrorType>> {
    ...
}

[joshtriplett]

Ouch. Good example, and that seems painful for several reasons, not least of which making it easy to confuse the two function-syntax types and the return type.

As far as I can tell, we don't have any guidance for when the return type is too long to fit on one line.

I would personally be inclined to write that this way:

fn foo(
    &self,
    x: i32,
    y: i32
) ->
    fn(
        a: SomeLongComplexType,
        b: SomeOtherLongComplexType
    ) -> Box<Future<Item = AnotherLongType, Error = ALongErrorType>>
{
    ...
}

I also don't mind writing the fn( on the same line as the ) ->, but I'd prefer to add extra indentation on the return-type function-type arguments and on its return type, and then not put the { on the same line.