Forbid unusable type aliases

[Stebalien]

As far as I can tell, the following type alias is unusable:

type I<'a> = &'a (Iterator + 'a);

This isn't a complete type because Iterator::Item is unspecified and isn't usable because there is no way to specify Iterator::Item (as far as I know) after the fact.

Personally, I feel type aliases like this should be compile time errors. I'm bringing this nitpick up now because fixing it is technically a breaking change .

[jonas-schievink]

type I<'a> = &'a (Iterator + 'a);

fn main() {}

now gives:

<anon>:1:19: 1:32 error: the value of the associated type `Item` (from the trait `core::iter::Iterator`) must be specified [E0191]
<anon>:1 type I<'a> = &'a (Iterator + 'a);
                           ^~~~~~~~~~~~~
<anon>:1:19: 1:32 help: see the detailed explanation for E0191
error: aborting due to previous error

So this looks fixed!