rust-lang / rust-lang/rust

ICE：Expected to find a suitable `Fn`/`FnMut`/`FnOnce` implementation for ..

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C-bug I-ICE S-has-mcve T-compiler
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Rust
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Description

Code
trait Trait {
    type Assoc;
}

fn foo<F,T>(mut f: F) -> i32 
where F: FnMut(<T as Trait>::Assoc) -> i32 {
    (|x| f(x))(foo::<F, T>(f))
}

fn main() {}
Meta

rustc --version --verbose:

rustc 1.92.0-nightly (a9d0a6f15 2025-09-16)
binary: rustc
commit-hash: a9d0a6f15533a364816c4d81e2192009ef601d33
commit-date: 2025-09-16
host: x86_64-unknown-linux-gnu
release: 1.92.0-nightly
LLVM version: 21.1.1
Error output
error[E0277]: the trait bound `T: Trait` is not satisfied
 --> <source>:5:1
  |
5 | / fn foo<F,T>(mut f: F) -> i32 
6 | | where F: FnMut(<T as Trait>::Assoc) -> i32 {
  | |__________________________________________^ the trait `Trait` is not implemented for `T`
  |
help: consider further restricting type parameter `T` with trait `Trait`
  |
6 | where F: FnMut(<T as Trait>::Assoc) -> i32, T: Trait {
  |                                           ++++++++++

error[E0277]: the trait bound `T: Trait` is not satisfied
 --> <source>:6:10
  |
6 | where F: FnMut(<T as Trait>::Assoc) -> i32 {
  |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Trait` is not implemented for `T`
  |
help: consider further restricting type parameter `T` with trait `Trait`
  |
6 | where F: FnMut(<T as Trait>::Assoc) -> i32, T: Trait {
  |                                           ++++++++++

error[E0277]: the trait bound `T: Trait` is not satisfied
 --> <source>:7:10
  |
7 |     (|x| f(x))(foo::<F, T>(f))
  |          ^^^^ the trait `Trait` is not implemented for `T`
  |
help: consider further restricting type parameter `T` with trait `Trait`
  |
6 | where F: FnMut(<T as Trait>::Assoc) -> i32, T: Trait {
  |                                           ++++++++++

error[E0277]: the trait bound `T: Trait` is not satisfied
 --> <source>:7:7
  |
7 |     (|x| f(x))(foo::<F, T>(f))
  |       ^ the trait `Trait` is not implemented for `T`
  |
  = note: all function arguments must have a statically known size
  = help: unsized fn params are gated as an unstable feature
help: consider further restricting type parameter `T` with trait `Trait`
  |
6 | where F: FnMut(<T as Trait>::Assoc) -> i32, T: Trait {
  |                                           ++++++++++

error[E0277]: the trait bound `T: Trait` is not satisfied
 --> <source>:7:16
  |
7 |     (|x| f(x))(foo::<F, T>(f))
  |                ^^^^^^^^^^^^^^ the trait `Trait` is not implemented for `T`
  |
help: consider further restricting type parameter `T` with trait `Trait`
  |
6 | where F: FnMut(<T as Trait>::Assoc) -> i32, T: Trait {
  |                                           ++++++++++

error[E0308]: mismatched types
 --> <source>:7:16
  |
7 |     (|x| f(x))(foo::<F, T>(f))
  |     ---------- ^^^^^^^^^^^^^^ expected associated type, found `i32`
  |     |
  |     arguments to this function are incorrect
  |
  = note: expected associated type `<T as Trait>::Assoc`
                        found type `i32`
  = help: consider constraining the associated type `<T as Trait>::Assoc` to `i32` or calling a method that returns `<T as Trait>::Assoc`
  = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
note: closure parameter defined here
 --> <source>:7:7
  |
7 |     (|x| f(x))(foo::<F, T>(f))
  |       ^

error[E0277]: the trait bound `T: Trait` is not satisfied
 --> <source>:7:6
  |
7 |     (|x| f(x))(foo::<F, T>(f))
  |      ^^^ the trait `Trait` is not implemented for `T`
  |
help: consider further restricting type parameter `T` with trait `Trait`
  |
6 | where F: FnMut(<T as Trait>::Assoc) -> i32, T: Trait {
  |                                           ++++++++++

error: internal compiler error: compiler/rustc_hir_typeck/src/callee.rs:1032:17: Expected to find a suitable `Fn`/`FnMut`/`FnOnce` implementation for `{closure@<source>:7:6: 7:9}`
 --> <source>:7:5
  |
7 |     (|x| f(x))(foo::<F, T>(f))
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^


thread 'rustc' (3) panicked at compiler/rustc_hir_typeck/src/callee.rs:1032:17:
Box<dyn Any>
Backtrace 

stack backtrace:
   0:     0x7704d8c05953 - <std::sys::backtrace::BacktraceLock::print::DisplayBacktrace as core::fmt::Display>::fmt::h1d9a860939142303
   1:     0x7704d9401fc4 - core::fmt::write::h4cde2f6e6eca889f
   2:     0x7704d8bba5f1 - std::io::Write::write_fmt::hbbb02a45894db6e0
   3:     0x7704d8bcb5d2 - std::sys::backtrace::BacktraceLock::print::hfcdaff63564fa703
   4:     0x7704d8bd1539 - std::panicking::default_hook::{{closure}}::hcc588fdbd9e0eddf
   5:     0x7704d8bd1063 - std::panicking::default_hook::h21841b6cba7fa436
   6:     0x7704d7bf5bd7 - std[6c96415bc1a38016]::panicking::update_hook::<alloc[c6e2df4d00e1f0b6]::boxed::Box<rustc_driver_impl[459800387efcc559]::install_ice_hook::{closure#1}>>::{closure#0}
   7:     0x7704d8bd195f - std::panicking::panic_with_hook::hedf05123aa8b61bb
   8:     0x7704d7c32471 - std[6c96415bc1a38016]::panicking::begin_panic::<rustc_errors[af568b6d67acca2f]::ExplicitBug>::{closure#0}
   9:     0x7704d7c25c86 - std[6c96415bc1a38016]::sys::backtrace::__rust_end_short_backtrace::<std[6c96415bc1a38016]::panicking::begin_panic<rustc_errors[af568b6d67acca2f]::ExplicitBug>::{closure#0}, !>
  10:     0x7704d7c23729 - std[6c96415bc1a38016]::panicking::begin_panic::<rustc_errors[af568b6d67acca2f]::ExplicitBug>
  11:     0x7704d7c3db61 - <rustc_errors[af568b6d67acca2f]::diagnostic::BugAbort as rustc_errors[af568b6d67acca2f]::diagnostic::EmissionGuarantee>::emit_producing_guarantee
  12:     0x7704d81aaa2c - <rustc_errors[af568b6d67acca2f]::DiagCtxtHandle>::span_bug::<rustc_span[9408016a182fdcc0]::span_encoding::Span, alloc[c6e2df4d00e1f0b6]::string::String>
  13:     0x7704d81cfd97 - rustc_middle[e11099e36afb5411]::util::bug::opt_span_bug_fmt::<rustc_span[9408016a182fdcc0]::span_encoding::Span>::{closure#0}
  14:     0x7704d81cff7a - rustc_middle[e11099e36afb5411]::ty::context::tls::with_opt::<rustc_middle[e11099e36afb5411]::util::bug::opt_span_bug_fmt<rustc_span[9408016a182fdcc0]::span_encoding::Span>::{closure#0}, !>::{closure#0}
  15:     0x7704d81beb4b - rustc_middle[e11099e36afb5411]::ty::context::tls::with_context_opt::<rustc_middle[e11099e36afb5411]::ty::context::tls::with_opt<rustc_middle[e11099e36afb5411]::util::bug::opt_span_bug_fmt<rustc_span[9408016a182fdcc0]::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
  16:     0x7704d6210d67 - rustc_middle[e11099e36afb5411]::util::bug::span_bug_fmt::<rustc_span[9408016a182fdcc0]::span_encoding::Span>
  17:     0x7704da2014c4 - <rustc_hir_typeck[ec5df76106ecc9bb]::fn_ctxt::FnCtxt>::analyze_closure
  18:     0x7704d986460a - <rustc_hir_typeck[ec5df76106ecc9bb]::upvar::InferBorrowKindVisitor as rustc_hir[f64a1fb3a7caff8d]::intravisit::Visitor>::visit_expr
  19:     0x7704d9864780 - <rustc_hir_typeck[ec5df76106ecc9bb]::upvar::InferBorrowKindVisitor as rustc_hir[f64a1fb3a7caff8d]::intravisit::Visitor>::visit_expr
  20:     0x7704d98681b1 - rustc_hir_typeck[ec5df76106ecc9bb]::typeck_with_inspect::{closure#0}
  21:     0x7704d98662b6 - rustc_query_impl[8ceca73aa722394f]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[8ceca73aa722394f]::query_impl::typeck::dynamic_query::{closure#2}::{closure#0}, rustc_middle[e11099e36afb5411]::query::erase::Erased<[u8; 8usize]>>
  22:     0x7704d95e74c4 - rustc_query_system[44f2cb83942a7389]::query::plumbing::try_execute_query::<rustc_query_impl[8ceca73aa722394f]::DynamicConfig<rustc_data_structures[e568e5fb65a8620a]::vec_cache::VecCache<rustc_span[9408016a182fdcc0]::def_id::LocalDefId, rustc_middle[e11099e36afb5411]::query::erase::Erased<[u8; 8usize]>, rustc_query_system[44f2cb83942a7389]::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl[8ceca73aa722394f]::plumbing::QueryCtxt, false>
  23:     0x7704d95e6e49 - rustc_query_impl[8ceca73aa722394f]::query_impl::typeck::get_query_non_incr::__rust_end_short_backtrace
  24:     0x7704d9ec96c5 - <rustc_middle[e11099e36afb5411]::ty::context::TyCtxt>::par_hir_body_owners::<rustc_hir_analysis[1618d3df19afa2f8]::check_crate::{closure#2}>::{closure#0}
  25:     0x7704d9ec8a4d - rustc_hir_analysis[1618d3df19afa2f8]::check_crate
  26:     0x7704d95e8bb0 - rustc_interface[f4ad0248271d71d8]::passes::analysis
  27:     0x7704d95e886d - rustc_query_impl[8ceca73aa722394f]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[8ceca73aa722394f]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[e11099e36afb5411]::query::erase::Erased<[u8; 0usize]>>
  28:     0x7704da465526 - rustc_query_system[44f2cb83942a7389]::query::plumbing::try_execute_query::<rustc_query_impl[8ceca73aa722394f]::DynamicConfig<rustc_query_system[44f2cb83942a7389]::query::caches::SingleCache<rustc_middle[e11099e36afb5411]::query::erase::Erased<[u8; 0usize]>>, false, false, false>, rustc_query_impl[8ceca73aa722394f]::plumbing::QueryCtxt, false>
  29:     0x7704da46517c - rustc_query_impl[8ceca73aa722394f]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
  30:     0x7704da672d4f - rustc_interface[f4ad0248271d71d8]::passes::create_and_enter_global_ctxt::<core[88dcab6addd45e6c]::option::Option<rustc_interface[f4ad0248271d71d8]::queries::Linker>, rustc_driver_impl[459800387efcc559]::run_compiler::{closure#0}::{closure#2}>::{closure#2}::{closure#0}
  31:     0x7704da5e96be - rustc_interface[f4ad0248271d71d8]::interface::run_compiler::<(), rustc_driver_impl[459800387efcc559]::run_compiler::{closure#0}>::{closure#1}
  32:     0x7704da5482f8 - std[6c96415bc1a38016]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[f4ad0248271d71d8]::util::run_in_thread_with_globals<rustc_interface[f4ad0248271d71d8]::util::run_in_thread_pool_with_globals<rustc_interface[f4ad0248271d71d8]::interface::run_compiler<(), rustc_driver_impl[459800387efcc559]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  33:     0x7704da547fdc - <<std[6c96415bc1a38016]::thread::Builder>::spawn_unchecked_<rustc_interface[f4ad0248271d71d8]::util::run_in_thread_with_globals<rustc_interface[f4ad0248271d71d8]::util::run_in_thread_pool_with_globals<rustc_interface[f4ad0248271d71d8]::interface::run_compiler<(), rustc_driver_impl[459800387efcc559]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[88dcab6addd45e6c]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  34:     0x7704da54e68d - std::sys::thread::unix::Thread::new::thread_start::hb28ad79a6a69b48c
  35:     0x7704d3c94ac3 - <unknown>
  36:     0x7704d3d26850 - <unknown>
  37:                0x0 - <unknown>

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: please make sure that you have updated to the latest nightly

note: please attach the file at `/app/rustc-ice-2025-09-19T08_13_00-1.txt` to your bug report

note: compiler flags: -C debuginfo=2 --crate-type bin -C linker=/opt/compiler-explorer/gcc-15.2.0/bin/gcc

query stack during panic:
#0 [typeck] type-checking `foo`
#1 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to 8 previous errors

Some errors have detailed explanations: E0277, E0308.
For more information about an error, try `rustc --explain E0277`.

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start by reproducing the minimal Rust example with the reported nightly compiler and inspect the type-checking query for foo. The panic is reported at compiler/rustc_hir_typeck/src/callee.rs:1032, in closure analysis; done means the example produces diagnostics without an internal compiler error.

Written by the indexing model from the issue text.

Assessment

Tech stack
rust
Domain
compilers
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
35/100

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