error: "can't handle type: TyClosure"

[dwrensha]

Consider the following program:

// field_ty.rs

fn main() {
    let mut y = 0;
    let mut box_maybe_closure = Box::new(None);
    *box_maybe_closure = Some(|| { y += 1; });
}

I expect miri to be able to run this program without any errors. Instead, I get:

$ RUST_BACKTRACE=1 cargo run --bin miri field_ty.rs 
    Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
     Running `target/debug/miri field_ty.rs`
ERROR:rustc_miri::interpret::eval_context: 

An error occurred in miri:
2: rustc_miri::interpret::error::{{impl}}::from
        at src/librustc_mir/interpret/error.rs:24
3: core::convert::{{impl}}::into<rustc_miri::interpret::error::EvalErrorKind,rustc_miri::interpret::error::EvalError>
        at /checkout/src/libcore/convert.rs:398
4: rustc_miri::interpret::eval_context::{{impl}}::get_field_ty<miri::Evaluator>
        at /home/dwrensha/src/miri/src/librustc_mir/interpret/mod.rs:5
5: rustc_miri::interpret::eval_context::{{impl}}::field_path_offset_and_ty<miri::Evaluator,core::iter::Map<core::iter::Skip<core::slice::Iter<u32>>, closure>>
        at /home/dwrensha/src/miri/src/librustc_mir/interpret/eval_context.rs:1041
6: rustc_miri::interpret::eval_context::{{impl}}::nonnull_offset_and_ty<miri::Evaluator>
        at /home/dwrensha/src/miri/src/librustc_mir/interpret/eval_context.rs:1021
7: rustc_miri::interpret::eval_context::{{impl}}::eval_rvalue_into_lvalue<miri::Evaluator>
        at /home/dwrensha/src/miri/src/librustc_mir/interpret/eval_context.rs:752
8: rustc_miri::interpret::step::{{impl}}::statement<miri::Evaluator>
        at /home/dwrensha/src/miri/src/librustc_mir/interpret/step.rs:95
9: rustc_miri::interpret::step::{{impl}}::step<miri::Evaluator>
        at /home/dwrensha/src/miri/src/librustc_mir/interpret/step.rs:61
10: miri::eval_main::run_main
        at miri/lib.rs:121
11: miri::eval_main
        at miri/lib.rs:134

error: can't handle type: [closure@field_ty.rs:6:31: 6:45 y:&mut i32], TyClosure(DefId { krate: CrateNum(0), node: DefIndex(2147483659) => field_ty/8cd878b::main[0]::{{closure}}[0] }, ClosureSubsts { substs: Slice([&mut i32]) })
 --> field_ty.rs:5:42
  |
5 |     let mut box_maybe_closure = Box::new(None);
  |                                          ^^^^
  |
note: inside call to main
 --> field_ty.rs:3:1
  |
3 | / fn main() {
4 | |     let mut y = 0;
5 | |     let mut box_maybe_closure = Box::new(None);
6 | |     *box_maybe_closure = Some(|| { y += 1; });
7 | | }
  | |_^

error: aborting due to previous error

thread 'main' panicked at 'Box<Any>', /checkout/src/librustc_errors/lib.rs:524:8
stack backtrace:
   0: std::sys::imp::backtrace::tracing::imp::unwind_backtrace
             at /checkout/src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1: std::sys_common::backtrace::_print
             at /checkout/src/libstd/sys_common/backtrace.rs:71
   2: std::panicking::default_hook::{{closure}}
             at /checkout/src/libstd/sys_common/backtrace.rs:60
             at /checkout/src/libstd/panicking.rs:380
   3: std::panicking::default_hook
             at /checkout/src/libstd/panicking.rs:396
   4: std::panicking::rust_panic_with_hook
             at /checkout/src/libstd/panicking.rs:610
   5: std::panicking::begin_panic
   6: rustc_errors::Handler::abort_if_errors
   7: miri::after_analysis
             at miri/bin/miri.rs:139
   8: core::ops::function::Fn::call
             at /checkout/src/libcore/ops/function.rs:48
   9: rustc_driver::driver::compile_input::{{closure}}
  10: rustc::ty::context::TyCtxt::create_and_enter
  11: rustc_driver::driver::compile_input
  12: rustc_driver::run_compiler
  13: miri::main
             at miri/bin/miri.rs:258
  14: __rust_maybe_catch_panic
             at /checkout/src/libpanic_unwind/lib.rs:98
  15: std::rt::lang_start
             at /checkout/src/libstd/panicking.rs:458
             at /checkout/src/libstd/panic.rs:361
             at /checkout/src/libstd/rt.rs:61
  16: main
  17: __libc_start_main
  18: _start

[RalfJung]

This will be fixed by #306. However, currently CI is broken until this Rust PR lands, so we can't merge right now.