Compiler panic when using repr(packed) associated constant in a match arm

[Dillonu]

I recently updated my nightly toolchain for rust, and compiled a project I was working on. Previously I was on the nightly-2018-04-19-x86_64-pc-windows-msvc version, and updated to the nightly-2018-04-29-x86_64-pc-windows-msvc version. I encountered a compiler error when dealing with match statements and Associated Constants, centered around alignment issues of the associated constant.

After some testing, I managed to create a simple example with the code below:

#[derive(PartialEq, Eq)] // Needed for pattern matching
#[repr(packed)] // Commenting out this line removes the error
pub struct Num(u64);

impl Num {
    const ZERO: Self = Num(0); // Error points to this line as the alignment issue
}

fn decrement(a: Num) -> Num {
    match a {
        // Matching Num against Num::Zero with repr(packed) causes alignment error:
        Num::ZERO => Num::ZERO, // Commenting out this line removes the error
        a => Num(a.0 - 1)
    }
}

fn main() {
}

cargo build:

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: ConstEvalErr { span: src\main.rs:6:5: 6:31, kind: Miri(EvalError { kind: AlignmentCheckFailed { required: 8, has: 1 }, backtrace: None }, []) }', libcore\result.rs:945:5
stack backtrace:
   0: <std::collections::hash::map::DefaultHasher as core::fmt::Debug>::fmt
   1: <std::sys::windows::os_str::Buf as std::sys_common::IntoInner<std::sys_common::wtf8::Wtf8Buf>>::into_inner
   2: std::panicking::take_hook
   3: std::panicking::take_hook
   4: rustc::ty::structural_impls::<impl rustc::ty::context::Lift<'tcx> for rustc::middle::const_val::ErrKind<'a>>::lift_to_tcx
   5: std::panicking::rust_panic_with_hook
   6: std::panicking::begin_panic_fmt
   7: rust_begin_unwind
   8: core::panicking::panic_fmt
   9: <rustc_mir::transform::inline::CallSite<'tcx> as core::fmt::Debug>::fmt
  10: rustc_mir::hair::pattern::PatternContext::lower_pattern
  11: <unknown>
  12: rustc_mir::hair::pattern::PatternContext::lower_pattern
  13: rustc_mir::hair::pattern::PatternContext::lower_pattern
  14: rustc_mir::hair::pattern::PatternContext::lower_pattern
  15: <rustc_mir::transform::inline::CallSite<'tcx> as core::fmt::Debug>::fmt
  16: <unknown>
  17: <unknown>
  18: <rustc_mir::hair::pattern::check_match::MatchVisitor<'a, 'tcx> as rustc::hir::intravisit::Visitor<'tcx>>::visit_expr
  19: <rustc_mir::hair::pattern::check_match::MatchVisitor<'a, 'tcx> as rustc::hir::intravisit::Visitor<'tcx>>::visit_expr
  20: <rustc_mir::monomorphize::collector::MonoItemCollectionMode as core::fmt::Debug>::fmt
  21: rustc_mir::hair::pattern::check_match::check_crate
  22: rustc::ty::context::tls::track_diagnostic
  23: rustc::dep_graph::graph::DepGraph::assert_ignored
  24: rustc::ty::context::tls::track_diagnostic
  25: rustc::ty::maps::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::try_mark_green_and_read
  26: rustc::ty::maps::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::try_mark_green_and_read
  27: rustc::ty::maps::<impl rustc::ty::context::TyCtxt<'a, 'tcx, 'lcx>>::check_match
  28: <rustc_mir::transform::inline::CallSite<'tcx> as core::fmt::Debug>::fmt
  29: <rustc_mir::monomorphize::collector::MonoItemCollectionMode as core::fmt::Debug>::fmt
  30: rustc_mir::hair::pattern::check_match::check_crate
  31: <env_logger::filter::inner::Filter as core::fmt::Display>::fmt
  32: <env_logger::filter::inner::Filter as core::fmt::Display>::fmt
  33: <unknown>
  34: rustc_driver::driver::compile_input
  35: rustc_driver::run_compiler
  36: rustc_driver::target_features::add_configuration
  37: <unknown>
  38: rustc_driver::target_features::add_configuration
  39: _rust_maybe_catch_panic
  40: rustc_driver::profile::trace::write_style
  41: rustc_driver::main
  42: <unknown>
  43: std::panicking::update_panic_count
  44: _rust_maybe_catch_panic
  45: std::rt::lang_start_internal
  46: <unknown>
  47: <unknown>
  48: BaseThreadInitThunk
  49: RtlUserThreadStart
query stack during panic:
#0 [check_match] processing `decrement`
end of query stack

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 1.27.0-nightly (79252ff4e 2018-04-29) running on x86_64-pc-windows-msvc

note: compiler flags: -C debuginfo=2 -C incremental --crate-type bin

note: some of the compiler flags provided by cargo are hidden

error: Could not compile `error-example`.

To learn more, run the command again with --verbose.

Meta

rustc --version --verbose:

rustc 1.27.0-nightly (79252ff4e 2018-04-29)
binary: rustc
commit-hash: 79252ff4e25d82f9fe856cb66f127b79cdace163
commit-date: 2018-04-29
host: x86_64-pc-windows-msvc
release: 1.27.0-nightly
LLVM version: 6.0

I've also tested and encountered this error on the following versions:

• nightly-2018-04-27-x86_64-pc-windows-msvc
• nightly-2018-04-28-x86_64-pc-windows-msvc
• nightly-2018-04-29-x86_64-pc-windows-msvc
• nightly-2018-04-30-x86_64-pc-windows-msvc

[oli-obk]

Sounds like some alignment check is getting the alignment from the wrong side of an assignment.

I hope it wasn't introduced in a beta backported bugfix, we need to check that.

[oli-obk]

Ok, It's just in nightly, not in beta! I'll try to bisect