ICE: compiling binary against lib with `feature(generic_const_exprs)`

[gkanwar]

I am finding an ICE when splitting a single binary with #![feature(generic_const_exprs)] into a binary plus a library crate that implements the structs and traits. This only occurs when I fail to add #![feature(generic_const_exprs)] to the binary crate as well. (Am I using the terminology crate correctly? I am still new to the whole rust environment...)

I think it is reasonable to expect the feature to be marked in the binary as well, but I would not expect an ICE when it is left out.

Code

This code compiles correctly as a standalone binary:

#![feature(generic_const_exprs)]

pub trait TraitWithAssocConst {
  const SIZE: usize;
}

pub struct Test;
impl TraitWithAssocConst for Test {
  const SIZE: usize = 8;
}

pub trait TraitDependingOnAssocConst {
  fn dummy<T>(&self, u: &T) -> ()
  where T : TraitWithAssocConst,
  [(); T::SIZE]: ;
}

pub struct Test2;
impl TraitDependingOnAssocConst for Test2 {
  fn dummy<T>(&self, u: &T) -> ()
  where T : TraitWithAssocConst,
  [(); T::SIZE]:
  {
  }
}

fn main() -> () {
  let t1 = Test;
  let t2 = Test2;
  t2.dummy(&t1);
}

Now I split this into a library crate mylib and a binary depending on it, but fail to include #![feature(generic_const_exprs)] in the binary:

// Binary crate:
// I "forget" to enable the feature here.
// #![feature(generic_const_exprs)]

use mylib::*;

fn main() -> () {
  let t1 = Test;
  let t2 = Test2;
  t2.dummy(&t1);
}

// Library crate mylib:
// I do enable the feature here.
#![feature(generic_const_exprs)]

pub trait TraitWithAssocConst {
  const SIZE: usize;
}

pub struct Test;
impl TraitWithAssocConst for Test {
  const SIZE: usize = 8;
}

pub trait TraitDependingOnAssocConst {
  fn dummy<T>(&self, u: &T) -> ()
  where T : TraitWithAssocConst,
  [(); T::SIZE]: ;
}

pub struct Test2;
impl TraitDependingOnAssocConst for Test2 {
  fn dummy<T>(&self, u: &T) -> ()
  where T : TraitWithAssocConst,
  [(); T::SIZE]:
  {
  }
}

The binary then fails to compile with the ICE trace given below.

Meta

rustc --version --verbose:

rustc 1.57.0-nightly (8c2b6ea37 2021-09-11)
binary: rustc
commit-hash: 8c2b6ea37d7719a0370bd404030eef9702c1752c
commit-date: 2021-09-11
host: x86_64-apple-darwin
release: 1.57.0-nightly
LLVM version: 13.0.0

Error output

   Compiling tmp v0.1.0 (/Users/gkanwar/tmp)
warning: the feature `generic_const_exprs` is incomplete and may not be safe to use and/or cause compiler crashes
 --> src/lib/lib.rs:1:12
  |
1 | #![feature(generic_const_exprs)]
  |            ^^^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(incomplete_features)]` on by default
  = note: see issue #76560 <https://github.com/rust-lang/rust/issues/76560> for more information

warning: unused variable: `u`
  --> src/lib/lib.rs:20:22
   |
20 |   fn dummy<T>(&self, u: &T) -> ()
   |                      ^ help: if this is intentional, prefix it with an underscore: `_u`
   |
   = note: `#[warn(unused_variables)]` on by default

warning: `tmp` (lib) generated 2 warnings
warning: Error finalizing incremental compilation session directory `/Users/gkanwar/tmp/target/debug/incremental/test-1wbxzdr6ui2on/s-g2aqzqcst1-1hgx4sl-working`: No such file or directory (os error 2)

error: internal compiler error: Encountered error `Unimplemented` selecting `Binder(<^0 as mylib::TraitWithAssocConst>, [Ty(Anon)])` during codegen
  |
  = note: delayed at compiler/rustc_trait_selection/src/traits/codegen.rs:68:32

error: internal compiler error: ty::ConstKind::Error constructed but no error reported.
  |
  = note: delayed at /rustc/8c2b6ea37d7719a0370bd404030eef9702c1752c/compiler/rustc_middle/src/ty/consts.rs:183:43

error: internal compiler error: `ErrorReported` without an error
 --> src/bin/test.rs:9:6
  |
9 |   t2.dummy(&t1);
  |      ^^^^^
  |
  = note: delayed at compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs:843:31

thread 'rustc' panicked at 'no errors encountered even though `delay_span_bug` issued', compiler/rustc_errors/src/lib.rs:1165:13
stack backtrace:
   0: _rust_begin_unwind
   1: std::panicking::begin_panic_fmt
   2: rustc_errors::HandlerInner::flush_delayed
   3: <rustc_errors::HandlerInner as core::ops::drop::Drop>::drop
   4: core::ptr::drop_in_place<rustc_session::parse::ParseSess>
   5: <alloc::rc::Rc<T> as core::ops::drop::Drop>::drop
   6: core::ptr::drop_in_place<rustc_interface::interface::Compiler>
   7: rustc_span::with_source_map
   8: scoped_tls::ScopedKey<T>::set
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose 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 1.57.0-nightly (8c2b6ea37 2021-09-11) running on x86_64-apple-darwin

note: compiler flags: -C embed-bitcode=no -C split-debuginfo=unpacked -C debuginfo=2 -C incremental --crate-type bin

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

query stack during panic:
end of query stack
warning: `tmp` (bin "test") generated 1 warning
error: could not compile `tmp`; 1 warning emitted

[hellow554]

Indeed, I only can trigger the ICE if I have a lib and a binary.

main.rs:

use abc::dummy;

fn main() -> () {
    dummy(());
}

lib.rs:

#![allow(incomplete_features)]
#![feature(generic_const_exprs)]

pub trait TraitWithAssocConst {
    const SIZE: usize;
}

impl TraitWithAssocConst for () {
    const SIZE: usize = 8;
}

pub fn dummy<T>(_: T)
where
    T: TraitWithAssocConst,
    [(); T::SIZE]: ,
{
}

Before:

error: constant expression depends on a generic parameter
  --> src/lib.rs:15:5
   |
15 |     [(); T::SIZE]: ,
   |     ^^^^^^^^^^^^^
   |
   = note: this may fail depending on what value the parameter takes

Regressed somewhere between e2be5f5...a1947b3

[BoxyUwU]

Probably a duplicate of #79018 see this comment

[gkanwar]

Thanks, I think you are right that this bug is an instance of that issue. Should this one be closed?