miri: ICE when calling mem::uninitialized for an enum with 256 variants.

Running MIRI on the following code causes an ICE:

error: internal compiler error: /rustc/a8cf3991177f30694200002cd9479ffbbe6d9a1a/src/librustc_middle/macros.rs:7:9: Unexpected error during validation: using uninitialized data, but this operation requires initialized memory

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:916:9
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/libunwind.rs:86
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print_fmt
             at src/libstd/sys_common/backtrace.rs:78
   3: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
             at src/libstd/sys_common/backtrace.rs:59
   4: core::fmt::write
             at src/libcore/fmt/mod.rs:1076
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1537
   6: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:62
   7: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:49
   8: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:198
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:217
  10: rustc_driver::report_ice
  11: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:477
  12: std::panicking::begin_panic
  13: rustc_errors::HandlerInner::bug
  14: rustc_errors::Handler::bug
  15: rustc_middle::util::bug::opt_span_bug_fmt::{{closure}}
  16: rustc_middle::ty::context::tls::with_opt::{{closure}}
  17: rustc_middle::ty::context::tls::with_opt
  18: rustc_middle::util::bug::opt_span_bug_fmt
  19: rustc_middle::util::bug::bug_fmt
  20: rustc_mir::interpret::validity::<impl rustc_mir::interpret::eval_context::InterpCx<M>>::validate_operand_internal
  21: rustc_mir::interpret::step::<impl rustc_mir::interpret::eval_context::InterpCx<M>>::eval_rvalue_into_place
  22: rustc_mir::interpret::step::<impl rustc_mir::interpret::eval_context::InterpCx<M>>::statement
  23: miri::eval::eval_main
  24: rustc_middle::ty::context::tls::enter_global
  25: <miri::MiriCompilerCalls as rustc_driver::Callbacks>::after_analysis
  26: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  27: rustc_span::with_source_map
  28: rustc_interface::interface::run_compiler_in_existing_thread_pool
  29: scoped_tls::ScopedKey<T>::set

code:

#![allow(unused)]

enum A {
    A0,
    A1,
    A2,
    A3,
    A4,
    A5,
    A6,
    A7,
    A8,
    A9,
    A10,
    A11,
    A12,
    A13,
    A14,
    A15,
    A16,
    A17,
    A18,
    A19,
    A20,
    A21,
    A22,
    A23,
    A24,
    A25,
    A26,
    A27,
    A28,
    A29,
    A30,
    A31,
    A32,
    A33,
    A34,
    A35,
    A36,
    A37,
    A38,
    A39,
    A40,
    A41,
    A42,
    A43,
    A44,
    A45,
    A46,
    A47,
    A48,
    A49,
    A50,
    A51,
    A52,
    A53,
    A54,
    A55,
    A56,
    A57,
    A58,
    A59,
    A60,
    A61,
    A62,
    A63,
    A64,
    A65,
    A66,
    A67,
    A68,
    A69,
    A70,
    A71,
    A72,
    A73,
    A74,
    A75,
    A76,
    A77,
    A78,
    A79,
    A80,
    A81,
    A82,
    A83,
    A84,
    A85,
    A86,
    A87,
    A88,
    A89,
    A90,
    A91,
    A92,
    A93,
    A94,
    A95,
    A96,
    A97,
    A98,
    A99,
    A100,
    A101,
    A102,
    A103,
    A104,
    A105,
    A106,
    A107,
    A108,
    A109,
    A110,
    A111,
    A112,
    A113,
    A114,
    A115,
    A116,
    A117,
    A118,
    A119,
    A120,
    A121,
    A122,
    A123,
    A124,
    A125,
    A126,
    A127,
    A128,
    A129,
    A130,
    A131,
    A132,
    A133,
    A134,
    A135,
    A136,
    A137,
    A138,
    A139,
    A140,
    A141,
    A142,
    A143,
    A144,
    A145,
    A146,
    A147,
    A148,
    A149,
    A150,
    A151,
    A152,
    A153,
    A154,
    A155,
    A156,
    A157,
    A158,
    A159,
    A160,
    A161,
    A162,
    A163,
    A164,
    A165,
    A166,
    A167,
    A168,
    A169,
    A170,
    A171,
    A172,
    A173,
    A174,
    A175,
    A176,
    A177,
    A178,
    A179,
    A180,
    A181,
    A182,
    A183,
    A184,
    A185,
    A186,
    A187,
    A188,
    A189,
    A190,
    A191,
    A192,
    A193,
    A194,
    A195,
    A196,
    A197,
    A198,
    A199,
    A200,
    A201,
    A202,
    A203,
    A204,
    A205,
    A206,
    A207,
    A208,
    A209,
    A210,
    A211,
    A212,
    A213,
    A214,
    A215,
    A216,
    A217,
    A218,
    A219,
    A220,
    A221,
    A222,
    A223,
    A224,
    A225,
    A226,
    A227,
    A228,
    A229,
    A230,
    A231,
    A232,
    A233,
    A234,
    A235,
    A236,
    A237,
    A238,
    A239,
    A240,
    A241,
    A242,
    A243,
    A244,
    A245,
    A246,
    A247,
    A248,
    A249,
    A250,
    A251,
    A252,
    A253,
    A254,
    A255,
}

fn main() {
    let _: A = unsafe { std::mem::uninitialized() };
}