impl Trait for .. {} does not require a feature gate

[alexcrichton]

I suspect that we may want more experience with this syntax before freezing it, but I may also be misremembering!

cc @nikomatsakis
cc @flaper87

[alexcrichton]

triage: I-nominated

[flaper87]

@alexcrichton I believe you're right. I'll work on a fix for this. Here's a test case:

use std::marker::MarkerTrait;

trait Test: MarkerTrait {}

impl Test for .. {}

fn main() {}

[pnkfelix]

1.0 beta, P-backcompat-lang.