ICE: broken mir with -Zunsound-mir-opts: simplify arm identity

[matthiaskrgr]

Code

from ./src/test/ui/mir/issue-77359-simplify-arm-identity.rs

// run-pass

#![allow(dead_code)]

#[derive(Debug)]
enum MyEnum {
    Variant2,
    Variant3,
}

fn f(arg3: bool) -> MyStruct {
	match if arg3 { Some(MyEnum::Variant3) } else { None } {
		Some(t) => {
			let ah = t;
			return MyStruct(Some(ah));
		}
		_ => MyStruct(None)
	}
}

#[derive(Debug)]
struct MyStruct(Option<MyEnum>);

fn main() {
    f(true);
}

Meta

rustc --version --verbose:

rustc 1.49.0-nightly (1d5b7c3c9 2020-11-01)
binary: rustc
commit-hash: 1d5b7c3c96a86c96f1fbf1441475028df94d165a
commit-date: 2020-11-01
host: x86_64-unknown-linux-gnu
release: 1.49.0-nightly

Error output

error: internal compiler error: broken MIR in Item(WithOptConstParam { did: DefId(0:3 ~ issue_77359_simplify_arm_identity[317d]::f), const_param_did: None }) (end of phase Optimization) at bb5[0]:
use of local _5, which has no storage here
  --> ./src/test/ui/mir/issue-77359-simplify-arm-identity.rs:15:20
   |
15 |             return MyStruct(Some(ah));
   |                             ^^^^^^^^
   |
   = note: delayed at compiler/rustc_mir/src/transform/validate.rs:171:36

thread 'rustc' panicked at 'no errors encountered even though `delay_span_bug` issued', compiler/rustc_errors/src/lib.rs:958:13
note: run with `RUST_BACKTRACE=1` environment variable to display a 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/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.49.0-nightly (1d5b7c3c9 2020-11-01) running on x86_64-unknown-linux-gnu

note: compiler flags: -Z unsound-mir-opts

query stack during panic:
end of query stack

Backtrace

error: internal compiler error: broken MIR in Item(WithOptConstParam { did: DefId(0:3 ~ issue_77359_simplify_arm_identity[317d]::f), const_param_did: None }) (end of phase Optimization) at bb5[0]:
use of local _5, which has no storage here
  --> ./src/test/ui/mir/issue-77359-simplify-arm-identity.rs:15:20
   |
15 |             return MyStruct(Some(ah));
   |                             ^^^^^^^^
   |
   = note: delayed at compiler/rustc_mir/src/transform/validate.rs:171:36

thread 'rustc' panicked at 'no errors encountered even though `delay_span_bug` issued', compiler/rustc_errors/src/lib.rs:958:13
stack backtrace:
   0:     0x7ffaf14ba5f0 - std::backtrace_rs::backtrace::libunwind::trace::h448c56741b6011d3
                               at /rustc/1d5b7c3c96a86c96f1fbf1441475028df94d165a/library/std/src/../../backtrace/src/backtrace/libunwind.rs:100:5
   1:     0x7ffaf14ba5f0 - std::backtrace_rs::backtrace::trace_unsynchronized::h2d26397c5720fdbb
                               at /rustc/1d5b7c3c96a86c96f1fbf1441475028df94d165a/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x7ffaf14ba5f0 - std::sys_common::backtrace::_print_fmt::h720a2f61f75b9f58
                               at /rustc/1d5b7c3c96a86c96f1fbf1441475028df94d165a/library/std/src/sys_common/backtrace.rs:67:5
   3:     0x7ffaf14ba5f0 - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::h8d62bbfda6d5c836
                               at /rustc/1d5b7c3c96a86c96f1fbf1441475028df94d165a/library/std/src/sys_common/backtrace.rs:46:22
   4:     0x7ffaf152a52c - core::fmt::write::h1857a60b204f1b6a
                               at /rustc/1d5b7c3c96a86c96f1fbf1441475028df94d165a/library/core/src/fmt/mod.rs:1078:17
   5:     0x7ffaf14ac252 - std::io::Write::write_fmt::h16a9fe7680ac245c
                               at /rustc/1d5b7c3c96a86c96f1fbf1441475028df94d165a/library/std/src/io/mod.rs:1518:15
   6:     0x7ffaf14bf5a5 - std::sys_common::backtrace::_print::h9ba51a6db618de7d
                               at /rustc/1d5b7c3c96a86c96f1fbf1441475028df94d165a/library/std/src/sys_common/backtrace.rs:49:5
   7:     0x7ffaf14bf5a5 - std::sys_common::backtrace::print::h5f20e41c85e91716
                               at /rustc/1d5b7c3c96a86c96f1fbf1441475028df94d165a/library/std/src/sys_common/backtrace.rs:36:9
   8:     0x7ffaf14bf5a5 - std::panicking::default_hook::{{closure}}::h5b7294ca19e8edab
                               at /rustc/1d5b7c3c96a86c96f1fbf1441475028df94d165a/library/std/src/panicking.rs:208:50
   9:     0x7ffaf14bf248 - std::panicking::default_hook::hb3948d1f74b6ff4c
                               at /rustc/1d5b7c3c96a86c96f1fbf1441475028df94d165a/library/std/src/panicking.rs:227:9
  10:     0x7ffaf1d8e158 - rustc_driver::report_ice::h1ddbc5609a6e1f37
  11:     0x7ffaf14bfea6 - std::panicking::rust_panic_with_hook::h55d23fef0ad751bc
                               at /rustc/1d5b7c3c96a86c96f1fbf1441475028df94d165a/library/std/src/panicking.rs:597:17
  12:     0x7ffaf14bf9c7 - std::panicking::begin_panic_handler::{{closure}}::h56ae74c6dea6e141
                               at /rustc/1d5b7c3c96a86c96f1fbf1441475028df94d165a/library/std/src/panicking.rs:499:13
  13:     0x7ffaf14baa8c - std::sys_common::backtrace::__rust_end_short_backtrace::h3c4721f56689fae4
                               at /rustc/1d5b7c3c96a86c96f1fbf1441475028df94d165a/library/std/src/sys_common/backtrace.rs:141:18
  14:     0x7ffaf14bf929 - rust_begin_unwind
                               at /rustc/1d5b7c3c96a86c96f1fbf1441475028df94d165a/library/std/src/panicking.rs:495:5
  15:     0x7ffaf14bf8db - std::panicking::begin_panic_fmt::h447a9fe10c3ac155
                               at /rustc/1d5b7c3c96a86c96f1fbf1441475028df94d165a/library/std/src/panicking.rs:437:5
  16:     0x7ffaf4dff1c1 - rustc_errors::HandlerInner::flush_delayed::hcdae04493e012a6e
  17:     0x7ffaf4dfbb91 - <rustc_errors::HandlerInner as core::ops::drop::Drop>::drop::h27a485c12bfd5a8a
  18:     0x7ffaf1d9b606 - core::ptr::drop_in_place::hac2c86d61f6a657d
  19:     0x7ffaf1da0796 - <alloc::rc::Rc<T> as core::ops::drop::Drop>::drop::h60c965ef9381f1d3
  20:     0x7ffaf1d46b2d - core::ptr::drop_in_place::h11c56312c481deac
  21:     0x7ffaf1d36a2c - rustc_span::with_source_map::h4e49cbd9a39d01b4
  22:     0x7ffaf1da70c1 - scoped_tls::ScopedKey<T>::set::hca96dcfe1d32eebf
  23:     0x7ffaf1daed75 - std::sys_common::backtrace::__rust_begin_short_backtrace::h21cfd4855c5058aa
  24:     0x7ffaf1d17188 - core::ops::function::FnOnce::call_once{{vtable.shim}}::h2df6ccf8e166462a
  25:     0x7ffaf14ceeaa - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h9e7afb7a0a438236
                               at /rustc/1d5b7c3c96a86c96f1fbf1441475028df94d165a/library/alloc/src/boxed.rs:1307:9
  26:     0x7ffaf14ceeaa - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h70c646c4271337a1
                               at /rustc/1d5b7c3c96a86c96f1fbf1441475028df94d165a/library/alloc/src/boxed.rs:1307:9
  27:     0x7ffaf14ceeaa - std::sys::unix::thread::Thread::new::thread_start::h35d2b8d36f210d02
                               at /rustc/1d5b7c3c96a86c96f1fbf1441475028df94d165a/library/std/src/sys/unix/thread.rs:89:17
  28:     0x7ffaf13ca3e9 - start_thread
  29:     0x7ffaf12e7293 - __GI___clone
  30:                0x0 - <unknown>

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/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.49.0-nightly (1d5b7c3c9 2020-11-01) running on x86_64-unknown-linux-gnu

note: compiler flags: -Z unsound-mir-opts

query stack during panic:
end of query stack

[jonas-schievink]

This is probably #77359

[matthiaskrgr]

Hm, the issue is already a couple of weeks old while I was not able to reproduce this one a few days ago.

[matthiaskrgr]

This happens since a6403b0 🤔

[tmiasko]

Yes, this is a duplicate of #77359. The changes from #78147 fixed the validator, so the bug is now correctly identified when incorrect transform is enabled with -Zunsound-mir-opts.

[Alexendoo]

No longer ICEs on latest nightly, bisect shows since #78360

[bjorn3]

Does it still ICE if you enable optimizations?

[Alexendoo]

It does, looks like it's still an ICE

[Alexendoo]

This (and #77359) no longer ICE since #95125, seems to be the case for any opt level

cc @JakobDegen is that intentional or is #94177 still pending?

[JakobDegen]

This is only the case because the changes to MIR make that opt completely broken and just never fire under any circumstances. #94177 needs to be revised because of that change, but the plan is still for it to fix these bugs for real