Trying to use `impl Trait` in a trait implementation causes ICE

[LunaBorowska]

Trying the following on beta:

use std::hash::{Hash, Hasher};

struct X;

impl Hash for X {
    fn hash(&self, hasher: &mut impl Hasher) {}
}

fn main() {}

I get the following error message.

   Compiling playground v0.0.1 (file:///playground)
thread 'rustc' panicked at 'called `Option::unwrap()` on a `None` value', libcore/option.rs:335:21
note: Run with `RUST_BACKTRACE=1` for a backtrace.

error: internal compiler error: unexpected panic

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.26.0-beta.3 (8a75d2b50 2018-04-08) running on x86_64-unknown-linux-gnu

note: compiler flags: -C codegen-units=1 -C debuginfo=2 --crate-type bin

note: some of the compiler flags provided by cargo are hidden

error: Could not compile `playground`.

To learn more, run the command again with --verbose.

Playpen link: https://play.rust-lang.org/?gist=5f2404761b1e9eef3faca5ab5bd92167&version=beta