nightly compiler panic with type-annotated array of `impl Generator`

[anderspapitto]

playground link:

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=c5cabc7e014cd71996c8bcb60acab446

Code:

#![feature(never_type)]
#![feature(impl_trait_in_bindings)]
#![feature(generators,generator_trait)]

use std::ops::Generator;

fn mk_gen() -> impl Generator<Return=!, Yield=()> {
    || { loop { yield; } }
}

fn main() {
    let gens: [impl Generator<Return=!, Yield=()>;2] = [ mk_gen(), mk_gen() ];
}

Error:

   Compiling playground v0.0.1 (/playground)
warning: unused variable: `gens`
  --> src/main.rs:12:9
   |
12 |     let gens: [impl Generator<Return=!, Yield=()>;2] = [ mk_gen(), mk_gen() ];
   |         ^^^^ help: consider prefixing with an underscore: `_gens`
   |
   = note: #[warn(unused_variables)] on by default

error: internal compiler error: broken MIR in DefId(0/0:5 ~ playground[c9b6]::main[0]) (_1 = [move _2, move _3]): bad assignment ([impl std::ops::Generator; 2] = [impl std::ops::Generator; 2]): NoSolution
  --> src/main.rs:12:56
   |
12 |     let gens: [impl Generator<Return=!, Yield=()>;2] = [ mk_gen(), mk_gen() ];
   |                                                        ^^^^^^^^^^^^^^^^^^^^^^

thread 'rustc' panicked at 'no errors encountered even though `delay_span_bug` issued', src/librustc_errors/lib.rs:354:17
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:39
   1: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:70
   2: std::panicking::default_hook::{{closure}}
             at src/libstd/sys_common/backtrace.rs:58
             at src/libstd/panicking.rs:200
   3: std::panicking::default_hook
             at src/libstd/panicking.rs:215
   4: rustc::util::common::panic_hook
   5: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:482
   6: std::panicking::begin_panic
   7: <rustc_errors::Handler as core::ops::drop::Drop>::drop
   8: core::ptr::real_drop_in_place
   9: core::ptr::real_drop_in_place
  10: core::ptr::real_drop_in_place
  11: <scoped_tls::ScopedKey<T>>::set
query stack during panic:
end of query stack

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 1.34.0-nightly (f47ec2ad5 2019-02-14) 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.

Note that there is no compiler panic if the array is of size 1, or if the explicit type annotation on gens is omitted

[estebank]

Triage: no change.