Introduce a way to construct Range from start + length

toc:

(start..)[..count]

I think this is impossible, since indexing operator can only return values in places that already exist, given the signature

fn index(&self, index: Idx) -> &Self::Output;

(note the connected lifetime of &Self::Output to &self from elision)

and (start..)[..count] would then desugar to *Index::index(&(start..), ..count)). But start.. of course does not happen to already contain the result of calculating start..start+count (for every possible value of count) inside of it

CodesInChaos:

However it's still ambiguous with a.. + b.

a.. + b doesn’t actually parse successfully, since + has higher precedence than .. - so the only place this sequence of tokens can currently come up is in macros that never end up parsing them as an expression at all.