Would you not call this uncomfortably explicit:

https://github.com/rust-lang/rfcs/pull/105#issuecomment-6410...

?

Type inference does make such things easy inside function bodies, but there are a lot of functions so it's not a complete solution.

Even some of the uncomposed iterator signatures are uncomfortable. An example:

    fn partition<B, F>(self, mut f: F) -> (B, B) where
        Self: Sized,
        B: Default + Extend<Self::Item>,
        F: FnMut(&Self::Item) -> bool

D gives you

    Range partition(alias predicate, SwapStrategy ss = SwapStrategy.unstable, Range)(Range r)
        if ((ss == SwapStrategy.stable && isRandomAccessRange!(Range))
                || (ss != SwapStrategy.stable && isForwardRange!(Range)))

which, if changed to only support stable partitions (like Rust), would be

    Range partition(alias predicate, Range)(Range r)
        if (isRandomAccessRange!(Range))

Note that I'm _not_ saying D's solution is better. I'm saying I find it more comfortable.

So it's lack of abstract return types. Thanks for clarifying.