ICE: unstable fingerprints for evaluate_obligation during incremental compilation

Summary

Clippy panics with an internal compiler error due to unstable fingerprints in incremental compilation when checking code that uses futures_util stream combinators (filter_map + buffer_unordered + collect) with async closures.

Version

rustc 1.95.0 (59807616e 2026-04-14)
clippy 0.1.95 (59807616e1 2026-04-14)
x86_64-unknown-linux-gnu

Error output

error: internal compiler error: encountered incremental compilation error with evaluate_obligation(756fcc40b4fb2203-2f9389f8417c9990)
  |
  = note: please follow the instructions below to create a bug report with the provided information
  = note: for incremental compilation bugs, having a reproduction is vital

thread 'rustc' panicked at compiler/rustc_middle/src/verify_ich.rs:80:9:
Found unstable fingerprints for evaluate_obligation(756fcc40b4fb2203-2f9389f8417c9990): Ok(EvaluatedToOk)

Query stack during panic

#0 [evaluate_obligation] evaluating trait selection obligation `{async closure@src/fs/watcher.rs:712:17: 712:34}: futures_util::fns::FnMut1<core::option::Option<(String, PathBuf, u64, SystemTime)>>`
#1 [type_op_ascribe_user_type] evaluating `type_op_ascribe_user_type` (StreamExt::collect with FilterMap<BufferUnordered<Iter<Map<IntoIter<String>, CoroutineClosure>>>, CoroutineClosure>)
... and 13 other queries

Dep node stack (try_mark_green)

#0 check_match(backend_rs::create_app_from_endpoint)
#1 mir_built(backend_rs::create_app_from_endpoint)
#2 has_ffi_unwind_calls(backend_rs::create_app_from_endpoint)
#3 mir_promoted(backend_rs::create_app_from_endpoint)
#4 mir_borrowck(backend_rs::create_app_from_endpoint)

Reproduction notes

• Occurs during incremental compilation only. A clean build (cargo clean -p backend-rs) succeeds.
• The code uses futures_util stream combinators: buffer_unordered → filter_map (with an async closure) → collect::<Vec<_>>().
• Compiler flags: --crate-type lib -C embed-bitcode=no -C debuginfo=2 -C incremental=[REDACTED]
• Triggered by cargo clippy -- -D warnings on a subsequent build after source changes.

Workaround

cargo clean -p backend-rs clears the corrupted incremental cache and allows compilation to succeed.

Start at compiler/rustc_middle/src/verify_ich.rs:80 and trace the evaluate_obligation query described in the query and dep-node stacks. Reproduce with cargo clippy -- -D warnings after a source change and incremental rebuild, then verify that the futures_util stream-combinator case no longer produces an unstable-fingerprint ICE without requiring cargo clean.