broken mir, trivial loop with -Zvalidate-mir

[matthiaskrgr]

Code

The code fails to pass -Zvalidate-mir

fn main() {
 for _ in &[0] {}
}

Meta

Repo is at c86039b

Error output

error: internal compiler error: broken MIR in Item(DefId(0:3 ~ main[317d]::main[0])) (input to phase Const) at bb13[0]:
encountered `Assign` statement with incompatible types:
left-hand side has type: !
right-hand side has type: ()
 --> main.rs:2:2
  |
2 |  for _ in &[0] {}
  |  ^^^^^^^^^^^^^^^^
  |
  = note: delayed at src/librustc_mir/transform/validate.rs:140:36

error: internal compiler error: broken MIR in Item(DefId(0:3 ~ main[317d]::main[0])) (after CheckPackedRef in phase Const) at bb13[0]:
encountered `Assign` statement with incompatible types:
left-hand side has type: !
right-hand side has type: ()
 --> main.rs:2:2
  |
2 |  for _ in &[0] {}
  |  ^^^^^^^^^^^^^^^^
  |
  = note: delayed at src/librustc_mir/transform/validate.rs:140:36

thread 'rustc' panicked at 'no errors encountered even though `delay_span_bug` issued', src/librustc_errors/lib.rs:367:17

Backtrace

error: internal compiler error: broken MIR in Item(DefId(0:3 ~ main[317d]::main[0])) (input to phase Const) at bb13[0]:
encountered `Assign` statement with incompatible types:
left-hand side has type: !
right-hand side has type: ()
 --> main.rs:2:2
  |
2 |  for _ in &[0] {}
  |  ^^^^^^^^^^^^^^^^
  |
  = note: delayed at src/librustc_mir/transform/validate.rs:140:36

error: internal compiler error: broken MIR in Item(DefId(0:3 ~ main[317d]::main[0])) (after CheckPackedRef in phase Const) at bb13[0]:
encountered `Assign` statement with incompatible types:
left-hand side has type: !
right-hand side has type: ()
 --> main.rs:2:2
  |
2 |  for _ in &[0] {}
  |  ^^^^^^^^^^^^^^^^
  |
  = note: delayed at src/librustc_mir/transform/validate.rs:140:36

thread 'rustc' panicked at 'no errors encountered even though `delay_span_bug` issued', src/librustc_errors/lib.rs:367:17
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /home/matthias/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/libunwind.rs:86
   1: backtrace::backtrace::trace_unsynchronized
             at /home/matthias/.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: <alloc::boxed::Box<F> as core::ops::function::Fn<A>>::call
             at ./src/liballoc/boxed.rs:1092
  11: rustc_driver::report_ice
             at src/librustc_driver/lib.rs:1160
  12: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:524
  13: std::panicking::begin_panic
             at ./src/libstd/panicking.rs:450
  14: <rustc_errors::HandlerInner as core::ops::drop::Drop>::drop
             at src/librustc_errors/lib.rs:367
  15: core::ptr::drop_in_place
             at ./src/libcore/ptr/mod.rs:184
  16: core::ptr::drop_in_place
             at ./src/libcore/ptr/mod.rs:184
  17: core::ptr::drop_in_place
             at ./src/libcore/ptr/mod.rs:184
  18: core::ptr::drop_in_place
             at ./src/libcore/ptr/mod.rs:184
  19: core::ptr::drop_in_place
             at ./src/libcore/ptr/mod.rs:184
  20: core::ptr::drop_in_place
             at ./src/libcore/ptr/mod.rs:184
  21: core::ptr::drop_in_place
             at ./src/libcore/ptr/mod.rs:184
  22: core::ptr::drop_in_place
             at ./src/libcore/ptr/mod.rs:184
  23: <alloc::rc::Rc<T> as core::ops::drop::Drop>::drop
             at ./src/liballoc/rc.rs:1159
  24: core::ptr::drop_in_place
             at ./src/libcore/ptr/mod.rs:184
  25: core::ptr::drop_in_place
             at ./src/libcore/ptr/mod.rs:184
  26: core::mem::drop
             at ./src/libcore/mem/mod.rs:877
  27: rustc_interface::interface::run_compiler_in_existing_thread_pool::{{closure}}::{{closure}}
             at ./src/librustc_interface/interface.rs:199
  28: rustc_data_structures::profiling::TimingGuard::run
             at ./src/librustc_data_structures/profiling.rs:555
  29: rustc_interface::interface::run_compiler_in_existing_thread_pool::{{closure}}
             at ./src/librustc_interface/interface.rs:199
  30: rustc_span::with_source_map
             at ./src/librustc_span/lib.rs:727
  31: rustc_interface::interface::run_compiler_in_existing_thread_pool
             at ./src/librustc_interface/interface.rs:189
  32: rustc_interface::interface::run_compiler::{{closure}}
             at ./src/librustc_interface/interface.rs:211
  33: rustc_interface::util::spawn_thread_pool::{{closure}}::{{closure}}::{{closure}}
             at ./src/librustc_interface/util.rs:149
  34: scoped_tls::ScopedKey<T>::set
             at /home/matthias/.cargo/registry/src/github.com-1ecc6299db9ec823/scoped-tls-1.0.0/src/lib.rs:137
  35: rustc_interface::util::spawn_thread_pool::{{closure}}::{{closure}}
             at ./src/librustc_interface/util.rs:145
  36: scoped_tls::ScopedKey<T>::set
             at /home/matthias/.cargo/registry/src/github.com-1ecc6299db9ec823/scoped-tls-1.0.0/src/lib.rs:137
  37: rustc_ast::attr::with_globals::{{closure}}
             at ./src/librustc_ast/attr/mod.rs:44
  38: scoped_tls::ScopedKey<T>::set
             at /home/matthias/.cargo/registry/src/github.com-1ecc6299db9ec823/scoped-tls-1.0.0/src/lib.rs:137
  39: rustc_ast::attr::with_globals
             at ./src/librustc_ast/attr/mod.rs:44
  40: rustc_interface::util::spawn_thread_pool::{{closure}}
             at ./src/librustc_interface/util.rs:144
  41: rustc_interface::util::scoped_thread::{{closure}}
             at ./src/librustc_interface/util.rs:119
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose 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 1.46.0-dev running on x86_64-unknown-linux-gnu

note: compiler flags: -Z validate-mir

query stack during panic:
end of query stack

[RalfJung]

Oh that is interesting:

left-hand side has type: !
right-hand side has type: ()

@nikomatsakis @eddyb is that expected? The other way around would make sense (assigning a ! into anything, really), but with a ! on the left, that seems odd? Or maybe the message just gets the sides wrong.

[RalfJung]

Yeah, that MIR looks really broken:

    let mut _14: !;                      // in scope 0 at test.rs:2:2: 2:18
[...]
        _14 = const ();                  // scope 2 at test.rs:2:2: 2:18

Cc @matthewjasper

[matthewjasper]

This is probably due to this line:

rust/src/librustc_mir_build/build/expr/into.rs

Line 395 in 665190b

this.cfg.push_assign_unit(block, source_info, destination, this.hir.tcx());

Break, Continueand Return shouldn't have this, since they have type !, not ().

[nagisa]

is that expected? The other way around would make sense (assigning a ! into anything, really), but with a ! on the left, that seems odd? Or maybe the message just gets the sides wrong.

This is a super old issue going back all the way to the initial MIR implementation and I’m somewhat confident is documented somewhere, though I cannot find an issue for this.

IIRC these invalid assignments only happen in unreachable basic blocks, so however wrong it looks in isolation, it is dead code and I think it never was a priority to fix either generation or the MIR validation we had back then for exactly that reason.

Looks like -Zvalidate-mir is a new take on validation? Perhaps the easy fix would be to not validate unreachable blocks.