Rust compiler panics when trying to reference impl-ed constants with Self

[rlupton20]

EDITED: MCVE is here: #47814 (comment)

I tried to compile the following, which caused the rust compiler to panic.

...
�impl<'a> ArpIPv4<'a> {
    const LENGTH: usize = 20;
...
    pub fn to_buffer(&self) -> [u8; Self::LENGTH] {
        [0; ArpIPv4::LENGTH]
    }
}

The following compiles just fine:

...
�impl<'a> ArpIPv4<'a> {
    const LENGTH: usize = 20;
...
    pub fn to_buffer(&self) -> [u8; ArpIPv4::LENGTH] {
        [0; ArpIPv4::LENGTH]
    }
}

The error message:

[nix-shell:~/Code/nerd-stack]$ cargo run
   Compiling nerd-stack v0.1.0 (file:///home/richard/Code/nerd-stack)
warning: unused imports: `ipv4_to_string`, `mac_to_string`
 --> src/protocols/arp.rs:1:26
  |
1 | use address::{MAC, IPv4, mac_to_string, ipv4_to_string};
  |                          ^^^^^^^^^^^^^  ^^^^^^^^^^^^^^
  |
  = note: #[warn(unused_imports)] on by default

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 1.24.0-nightly (9389e23a8 2017-12-31) running on x86_64-unknown-linux-gnu

thread 'rustc' panicked at 'assertion failed: `(left == right)`
  left: `Some(NodeId(820))`,
 right: `None`: free_scope: DefId(0/0:83 ~ nerd_stack[3074]::protocols[0]::arp[0]::{{impl}}[2]) not recognized by the region scope tree for None / Some(DefId(0/0:86 ~ nerd_stack[3074]::protocols[0]::arp[0]::{{impl}}[2]::to_buffer[0]))', /checkout/src/librustc/middle/region.rs:729:13
note: Run with `RUST_BACKTRACE=1` for a backtrace.

error: Could not compile `nerd-stack`.

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

And running again with verbosity and backtrace:

[nix-shell:~/Code/nerd-stack]$ RUST_BACKTRACE=1 cargo run --verbose                                                                
       Fresh libc v0.2.36                                                                                                    
   Compiling nerd-stack v0.1.0 (file:///home/richard/Code/nerd-stack)                                                           
     Running `rustc --crate-name nerd_stack src/lib.rs --crate-type lib --emit=dep-info,link -C debuginfo=2 -C metadata=320d9acf78f2e38b -C extra-filename=-320d9acf78f2e38b --out-dir /home/richard/Code/nerd-stack
/target/debug/deps -C incremental=/home/richard/Code/nerd-stack/target/debug/incremental -L dependency=/home/richard/Code/nerd-stack/target/debug/deps --extern libc=/home/richard/Code/nerd-stack/target/debug/deps
/liblibc-1d53503271082800.rlib`        
warning: unused imports: `ipv4_to_string`, `mac_to_string`      
 --> src/protocols/arp.rs:1:26               
  |                                        
1 | use address::{MAC, IPv4, mac_to_string, ipv4_to_string};                         
  |                          ^^^^^^^^^^^^^  ^^^^^^^^^^^^^^
  |                                        
  = note: #[warn(unused_imports)] on by default
                                             
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 1.24.0-nightly (9389e23a8 2017-12-31) running on x86_64-unknown-linux-gnu
          
note: run with `RUST_BACKTRACE=1` for a backtrace                                                                                                                                                                                                                                                                                                                                                                                       thread 'rustc' panicked at 'assertion failed: `(left == right)`        
  left: `Some(NodeId(820))`,
 right: `None`: free_scope: DefId(0/0:83 ~ nerd_stack[3074]::protocols[0]::arp[0]::{{impl}}[2]) not recognized by the region scope tree for None / Some(DefId(0/0:86 ~ nerd_stack[3074]::protocols[0]::arp[0]::{{imp
l}}[2]::to_buffer[0]))', /checkout/src/librustc/middle/region.rs:729:13
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
   1: std::sys_common::backtrace::print
   2: std::panicking::default_hook::{{closure}}
   3: std::panicking::default_hook
   4: std::panicking::rust_panic_with_hook
   5: std::panicking::begin_panic
   6: std::panicking::begin_panic_fmt
   7: rustc::middle::region::ScopeTree::early_free_scope
   8: rustc::middle::free_region::RegionRelations::is_subregion_of
   9: rustc::infer::lexical_region_resolve::resolve
  10: rustc::infer::InferCtxt::resolve_regions_and_report_errors_inner
  11: rustc_typeck::check::regionck::<impl rustc_typeck::check::FnCtxt<'a, 'gcx, 'tcx>>::regionck_expr
  12: <std::thread::local::LocalKey<T>>::with
  13: rustc::infer::InferCtxtBuilder::enter
  14: rustc_typeck::check::typeck_tables_of
  15: rustc::ty::maps::<impl rustc::ty::maps::queries::typeck_tables_of<'tcx>>::compute_result
  16: rustc::dep_graph::graph::DepGraph::with_task_impl
  17: rustc_errors::Handler::track_diagnostics
  18: rustc::ty::maps::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::cycle_check
  19: rustc::ty::maps::<impl rustc::ty::maps::queries::typeck_tables_of<'tcx>>::force
  20: rustc::ty::maps::plumbing::force_from_dep_node
  21: rustc::dep_graph::graph::DepGraph::try_mark_green
  22: rustc::ty::maps::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::try_mark_green_and_read
  23: rustc::ty::maps::<impl rustc::ty::maps::queries::const_eval<'tcx>>::try_get
  24: rustc::ty::maps::TyCtxtAt::const_eval
  25: <rustc::traits::project::AssociatedTypeNormalizer<'a, 'b, 'gcx, 'tcx> as rustc::ty::fold::TypeFolder<'gcx, 'tcx>>::fold_const
  26: rustc::ty::structural_impls::<impl rustc::ty::fold::TypeFoldable<'tcx> for &'tcx rustc::ty::TyS<'tcx>>::super_fold_with
  27: <rustc::traits::project::AssociatedTypeNormalizer<'a, 'b, 'gcx, 'tcx> as rustc::ty::fold::TypeFolder<'gcx, 'tcx>>::fold_ty
  28: <rustc_data_structures::accumulate_vec::AccumulateVec<A> as core::iter::traits::FromIterator<<A as rustc_data_structures::array_vec::Array>::Element>>::from_iter
  29: rustc::ty::fold::TypeFoldable::fold_with
  30: rustc::traits::project::normalize
  31: rustc_typeck::check::FnCtxt::normalize_associated_types_in
  32: <std::thread::local::LocalKey<T>>::with
  33: rustc::infer::InferCtxtBuilder::enter
  34: rustc_typeck::check::wfcheck::CheckTypeWellFormedVisitor::check_associated_item
  35: rustc::hir::Crate::visit_all_item_likes
  36: rustc::session::Session::track_errors
  37: rustc_typeck::check_crate
  38: <std::thread::local::LocalKey<T>>::with
  39: <std::thread::local::LocalKey<T>>::with
  40: rustc::ty::context::TyCtxt::create_and_enter
  41: rustc_driver::driver::compile_input
  42: rustc_driver::run_compiler

error: Could not compile `nerd-stack`.

Caused by:
  process didn't exit successfully: `rustc --crate-name nerd_stack src/lib.rs --crate-type lib --emit=dep-info,link -C debuginfo=2 -C metadata=320d9acf78f2e38b -C extra-filename=-320d9acf78f2e38b --out-dir /home/richard/Code/nerd-stack/target/debug/deps -C incremental=/home/richard/Code/nerd-stack/target/debug/incremental -L dependency=/home/richard/Code/nerd-stack/target/debug/deps --extern libc=/home/richard/Code/nerd-stack/target/debug/deps/liblibc-1d53503271082800.rlib` (exit code: 101)

Running nixos 17.09 using rust nightly installed using the mozilla nixpkgs overlay. It's about a month out of date, so I'll check the updates.

[dtolnay]

This still crashes as of the most recent nightly. Self-contained repro:

struct ArpIPv4<'a> {
    s: &'a u8
}

impl<'a> ArpIPv4<'a> {
    const LENGTH: usize = 20;

    pub fn to_buffer() -> [u8; Self::LENGTH] {
        unimplemented!()
    }
}

Mentioning @eddyb who added this assertion in #41914.

[eddyb]

@dtolnay That was confusing at first, heh - the multi-line assert_eq message which doesn't split custom messages on their own line doesn't help at all (can we split it off?).

So what's happening is the length expression is using a generic parameter in its outer scope. This hasn't ever been supported, but it's extremely obnoxious to prevent - we could ban Self in rustc_resolve but that already works in some specific situations, so it'd be backwards-incompatible.

This problem would get properly fixed once we actually start supporting generics-in-scope for array expressions, and that can't happen without lazy normalization (cc @nikomatsakis).

[internally-combusted]

I'm having the same problem still.

let new_quad: [VertexData; Renderer::QUAD_VERTICES().len()] = vec![];

succeeds in making the compiler cry about the function call in the array size, but with

let new_quad: [VertexData; Self::QUAD_VERTICES().len()] = vec![];

the compiler panics.

Here's my rustc --version --verbose:

rustc 1.33.0 (2aa4c46cf 2019-02-28)
binary: rustc
commit-hash: 2aa4c46cfdd726e97360c2734835aa3515e8c858
commit-date: 2019-02-28
host: x86_64-apple-darwin
release: 1.33.0
LLVM version: 8.0

[Centril]

@dtolnay's exampe ICEs at rustc_mir::borrow_check::universal_regions::UniversalRegionIndices::to_region_vid::{{closure}}, cc @matthewjasper