Bug in fulfillment_errors.rs when compiling

[DavideCanton]

Code

use cgroups_rs;
use nix::mount::{mount, MsFlags};

use std::path::Path;

fn main() {
    mount(None, Path::new("foo"), None, MsFlags::empty(), None).unwrap();
}

Meta

rustc --version --verbose:

rustc 1.84.0 (9fc6b4312 2025-01-07)
binary: rustc
commit-hash: 9fc6b43126469e3858e2fe86cafb4f0fd5068869
commit-date: 2025-01-07
host: x86_64-unknown-linux-gnu
release: 1.84.0
LLVM version: 19.1.5

Error output

thread 'rustc' panicked at compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs:1824:44:
called `Option::unwrap()` on a `None` value

Backtrace

stack backtrace:
   0: rust_begin_unwind
   1: core::panicking::panic_fmt
   2: core::panicking::panic
   3: core::option::unwrap_failed
   4: <rustc_trait_selection::error_reporting::TypeErrCtxt>::report_similar_impl_candidates
   5: <rustc_trait_selection::error_reporting::TypeErrCtxt>::report_fulfillment_error
   6: <rustc_trait_selection::error_reporting::TypeErrCtxt>::report_fulfillment_errors
   7: rustc_hir_typeck::typeck
      [... omitted 1 frame ...]
   8: <rustc_middle::hir::map::Map>::par_body_owners::<rustc_hir_analysis::check_crate::{closure#4}>::{closure#0}
   9: rustc_hir_analysis::check_crate
  10: rustc_interface::passes::run_required_analyses
  11: rustc_interface::passes::analysis
      [... omitted 1 frame ...]
  12: rustc_interface::interface::run_compiler::<core::result::Result<(), rustc_span::ErrorGuaranteed>, rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

error: 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 1.84.0 (9fc6b4312 2025-01-07) running on x86_64-unknown-linux-gnu

note: compiler flags: --crate-type bin -C embed-bitcode=no -C debuginfo=2 -C incremental=[REDACTED]

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

query stack during panic:
#0 [typeck] type-checking `main`
#1 [analysis] running analysis passes on this crate
end of query stack

Content of Cargo.toml

[package]
name = "rustc_bug"
version = "0.1.0"
edition = "2021"

[dependencies]
cgroups-rs = "0.3.4"
nix = { version = "0.29.0", features = ["mount"] }

Additional information

The bug seems related to the use cgroups_rs; line, if I remove it the bug goes away.

Compiling with cargo +nightly build works fine.

Please note that the code does not actually compile, with the following patch it does compile without panics.

-    mount(None, Path::new("foo"), None, MsFlags::empty(), None).unwrap();
+    mount::<Path, Path, Path, Path>(None, Path::new("foo"), None, MsFlags::empty(), None).unwrap();

[lqd]

There were some fixed ICEs in diagnostics in the last cycle, and we may have missed a backport for this one. I'll bisect and look for the PR that fixed it.

[lqd]

I found the culprit: it was me! #133304

[lqd]

The good news is that should be fixed when the next stable is released, the bad news is it's in 5 weeks. I guess we can keep this open for now.

I'll also mark this as a stable regression...

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-medium