Destructuring irrefutable enums with ref inside match causes a panic

[mkroman]

When destructuring an irrefutable enum inside a destructured match pattern with references, the compiler crashes. I tried replicating the case with as little code as possible, but it seems to only happen when I use the enum from the irc crate.

This is the code I managed to narrow it down to:

extern crate irc;

use irc::client::data::command::Command;

pub enum Event<'a> {
    Command(&'a str, Vec<&'a str>)
}

struct Plugin;

impl Plugin {
    fn process(&self, command: Command, event: Event) {
        match event {
            Event::Command(ref cmd, ref args) => {
                let Command::PRIVMSG(ref target, ref msg) = command; // <--
            }
        }
    }
}

fn main() {
    let plugin = Plugin;
    let event = Event::Command("hello", vec!["world"]);
    let cmd = Command::PRIVMSG("foo".to_string(), "bar".to_string());

    plugin.process(cmd, event);
}

Note that it is destructuring from the command argument to the process function, and not the destructured reference cmd.

I expected cmd and args to both be references to the types in the Event::Command enum, and expected target and msg to be &strs referencing the Strings defined in the Command::PRIVMSG enum type in the irc crate.

Instead, the compiler panics with the following error:

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: run with `RUST_BACKTRACE=1` for a backtrace
thread 'rustc' panicked at 'assertion failed: `(left == right)` (left: `64`, right: `1`)', ../src/librustc/middle/check_match.rs:1051
note: Run with `RUST_BACKTRACE=1` for a backtrace.

Could not compile `plugins`.

Meta

Rust version:

rustc 1.8.0-nightly (c8fc481 2016-02-22)
binary: rustc
commit-hash: c8fc481
commit-date: 2016-02-22
host: x86_64-unknown-linux-gnu
release: 1.8.0-nightly

Backtrace:

   Compiling compiler-crash v0.1.0 (file:///home/mk/Projects/Rust/compiler-crash)
     Running `rustc src/main.rs --crate-name compiler_crash --crate-type bin -g --out-dir /home/mk/Projects/Rust/compiler-crash/target/debug --emit=dep-info,link -L dependency=/home/mk/Projects/Rust/compiler-crash/target/debug -L dependency=/home/mk/Projects/Rust/compiler-crash/target/debug/deps --extern irc=/home/mk/Projects/Rust/compiler-crash/target/debug/deps/libirc-cc59e736e2ba9456.rlib -L native=/home/mk/Projects/Rust/compiler-crash/target/debug/build/openssl-09576f2f9776fa80/out -L native=/home/mk/Projects/Rust/compiler-crash/target/debug/build/openssl-sys-extras-52d5315fb71d3c6d/out -L native=/usr/lib`
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
thread 'rustc' panicked at 'assertion failed: `(left == right)` (left: `64`, right: `1`)', ../src/librustc/middle/check_match.rs:1051
stack backtrace:
   1:     0x7f0f59186340 - sys::backtrace::tracing::imp::write::h2bb24970c1600d4fNyu
   2:     0x7f0f5918f6ab - panicking::default_handler::_$u7b$$u7b$closure$u7d$$u7d$::closure.43727
   3:     0x7f0f5918f203 - panicking::default_handler::hb760707c7feda3cdbez
   4:     0x7f0f59157d7c - sys_common::unwind::begin_unwind_inner::h78583c314295e3926mt
   5:     0x7f0f59158818 - sys_common::unwind::begin_unwind_fmt::h903be369b05a3ccccmt
   6:     0x7f0f564371d8 - middle::check_match::check_irrefutable::h8d1ba0bed3fbea2aQ4j
   7:     0x7f0f56407f42 - middle::check_match::check_local::h2d216d4e5ead2573K2j
   8:     0x7f0f56405571 - middle::check_match::check_expr::h52b2bafd753fd3e2ESi
   9:     0x7f0f5640a3b1 - intravisit::walk_arm::h17326268766873259457
  10:     0x7f0f5640529e - middle::check_match::check_expr::h52b2bafd753fd3e2ESi
  11:     0x7f0f564081ba - middle::check_match::check_fn::h082db3cfa9cb9bc8A3j
  12:     0x7f0f5640919d - middle::check_match::check_crate::hc28c1fe732f94679kSi
  13:     0x7f0f596ad42e - driver::phase_3_run_analysis_passes::_$u7b$$u7b$closure$u7d$$u7d$::closure.28446
  14:     0x7f0f596aa1e5 - middle::ty::context::ctxt<'tcx>::create_and_enter::h6911830796927350595
  15:     0x7f0f596a6d3f - driver::phase_3_run_analysis_passes::h10607465921210735652
  16:     0x7f0f596785b5 - driver::compile_input::h9417f7071f41b0b1Bca
  17:     0x7f0f59668907 - run_compiler::h1d04446f72d95b52HOc
  18:     0x7f0f596660b1 - sys_common::unwind::try::try_fn::h2224149726337910613
  19:     0x7f0f59183d9b - __rust_try
  20:     0x7f0f5917c34d - sys_common::unwind::inner_try::hdfdf33e7068c9cb28jt
  21:     0x7f0f59666900 - boxed::F.FnBox<A>::call_box::h8331008289514682961
  22:     0x7f0f5918dc79 - sys::thread::Thread::new::thread_start::h1272204999bddc95vby
  23:     0x7f0f518c5423 - start_thread
  24:     0x7f0f58e05cbc - clone
  25:                0x0 - <unknown>

Could not compile `compiler-crash`.

Caused by:
  Process didn't exit successfully: `rustc src/main.rs --crate-name compiler_crash --crate-type bin -g --out-dir /home/mk/Projects/Rust/compiler-crash/target/debug --emit=dep-info,link -L dependency=/home/mk/Projects/Rust/compiler-crash/target/debug -L dependency=/home/mk/Projects/Rust/compiler-crash/target/debug/deps --extern irc=/home/mk/Projects/Rust/compiler-crash/target/debug/deps/libirc-cc59e736e2ba9456.rlib -L native=/home/mk/Projects/Rust/compiler-crash/target/debug/build/openssl-09576f2f9776fa80/out -L native=/home/mk/Projects/Rust/compiler-crash/target/debug/build/openssl-sys-extras-52d5315fb71d3c6d/out -L native=/usr/lib` (exit code: 101)

Repository

I uploaded the code as a repository, with Cargo.lock for easier replication.
https://github.com/mkroman/compiler-crash

[durka]

Note that it's not actually expected to compile, because Command::PRIVMSG(ref target, ref msg) is refutable. But it shouldn't crash.

[joelself]

This looks to be the exact same compiler panic I encountered. I've made a branch with code that panics the compiler here. The specific line of code that is causing the panic is this one. Here's a minimal snippet that replicates the panic:

enum MakePanic {
  Panic(usize),
  Calm,
  Anxious,
}

#[test]
fn test_compile_panic() {
  let mkpnc = MakePanic::Panic(3);
  let MakePanic::Panic(val) = mkpnc;
  println!("Use val {}", val);
}

Note that it doesn't panic with fewer than 3 enum options. I tried Panic(usize), Panic(String), and Panic(Rc<RefCell<(String, usize)>>) and they all cause a panic. It also panics when the other options have values in them. It panics with a lifetime declared on the enum and used in one of the options. It'll panic if mkpnc is destructured into val by value and by reference.

Version beta:
rustc 1.8.0-beta.1 (facbfdd 2016-03-02)
binary: rustc
commit-hash: facbfdd
commit-date: 2016-03-02
host: x86_64-apple-darwin
release: 1.8.0-beta.1

Version nightly:
rustc 1.9.0-nightly (7b0b80a 2016-03-02)
binary: rustc
commit-hash: 7b0b80a
commit-date: 2016-03-02
host: x86_64-apple-darwin
release: 1.9.0-nightly

Backtrace beta:

thread 'rustc' panicked at 'assertion failed: `(left == right)` (left: `2`, right: `1`)', ../src/librustc/middle/check_match.rs:1051
stack backtrace:
   1:        0x107ebe3a8 - sys::backtrace::tracing::imp::write::h8c3fbdc55f1abdcag4u
   2:        0x107ec6d15 - panicking::default_handler::_$u7b$$u7b$closure$u7d$$u7d$::closure.44007
   3:        0x107ec6817 - panicking::default_handler::h6775aa517c5d030aFvz
   4:        0x107e89fc6 - sys_common::unwind::begin_unwind_inner::h9a1b0d9dd0c062e122t
   5:        0x107e8aabe - sys_common::unwind::begin_unwind_fmt::h99d6622b969d640e81t
   6:        0x104d7731e - middle::check_match::check_irrefutable::h037ce85fe9e0719fP4j
   7:        0x104d46440 - middle::check_match::check_local::h57d1a825afa7d07fJ2j
   8:        0x104d466a7 - middle::check_match::check_fn::h02fdb8e29648c14az3j
   9:        0x104d46e8e - middle::check_match::check_crate::h5c6040bd71f7b985jSi
  10:        0x103e8090f - driver::phase_3_run_analysis_passes::_$u7b$$u7b$closure$u7d$$u7d$::closure.28271
  11:        0x103e7d869 - middle::ty::context::ctxt<'tcx>::create_and_enter::h8855598638456051733
  12:        0x103e7a2e2 - driver::phase_3_run_analysis_passes::h7337521525707930622
  13:        0x103e4e0bd - driver::compile_input::h467e81e458addce4Hca
  14:        0x103e3d788 - run_compiler::h971a7f9ccb1ca35e6Oc
  15:        0x103e3ade5 - sys_common::unwind::try::try_fn::h8709370279840092323
  16:        0x107ebbd7b - __rust_try
  17:        0x107eb3fa3 - sys_common::unwind::inner_try::h8630ed5c10186d0a4Zt
  18:        0x103e3b699 - boxed::F.FnBox<A>::call_box::h4993598626702227198
  19:        0x107ec5d0c - sys::thread::Thread::new::thread_start::h2721df3f64ee52b36Jy
  20:     0x7fff81f6f059 - _pthread_body
  21:     0x7fff81f6efd6 - _pthread_start

Backtrace nightly:

thread 'rustc' panicked at 'assertion failed: `(left == right)` (left: `2`, right: `1`)', ../src/librustc/middle/check_match.rs:1051
stack backtrace:
   1:        0x113bc43a8 - sys::backtrace::tracing::imp::write::h7add7fdda97786a1s4u
   2:        0x113bccd15 - panicking::default_handler::_$u7b$$u7b$closure$u7d$$u7d$::closure.44030
   3:        0x113bcc817 - panicking::default_handler::h72a4f6df9637bc80Rvz
   4:        0x113b8ffc6 - sys_common::unwind::begin_unwind_inner::hf798769ad97c097fe3t
   5:        0x113b90abe - sys_common::unwind::begin_unwind_fmt::hb2ee19b4aa59ba8ak2t
   6:        0x1105cd7be - middle::check_match::check_irrefutable::hb3dbd95dda6de55c92j
   7:        0x11059c980 - middle::check_match::check_local::h61b952193c791ff530j
   8:        0x11059cbe7 - middle::check_match::check_fn::hab6c47340d646ea4T1j
   9:        0x11059d3ce - middle::check_match::check_crate::h4de6707235db8a8dDQi
  10:        0x10f6dea1f - driver::phase_3_run_analysis_passes::_$u7b$$u7b$closure$u7d$$u7d$::closure.28283
  11:        0x10f6db979 - middle::ty::context::ctxt<'tcx>::create_and_enter::h17676359018619823038
  12:        0x10f6d83f2 - driver::phase_3_run_analysis_passes::h9214455933593547357
  13:        0x10f6ac19d - driver::compile_input::h26b63276678a06cfHca
  14:        0x10f699ce8 - run_compiler::hd54b7109a035c12b6Oc
  15:        0x10f697345 - sys_common::unwind::try::try_fn::h18306268176533686141
  16:        0x113bc1d7b - __rust_try
  17:        0x113bb9fa3 - sys_common::unwind::inner_try::hdaa7ae415a983f2ag0t
  18:        0x10f697bf9 - boxed::F.FnBox<A>::call_box::h14172178064022666043
  19:        0x113bcbd0c - sys::thread::Thread::new::thread_start::h45441f586afbe2dciKy
  20:     0x7fff81f6f059 - _pthread_body
  21:     0x7fff81f6efd6 - _pthread_start

[mitaa]

I think this should be fixed (#31561)