ICE while compiling the ffi_helpers in 1.28 beta

[pietroalbini]

Building the ffi_helpers crate causes an ICE in beta 1.28. Looking at the backtrace seems like that's caused by the const evaluator.

• ffi_helpers regressed from stable to beta (build log) cc @Michael-F-Bryan

cc @oli-obk

[Mark-Simulacrum]

@oli-obk is fixing so not going to bisect this