ICE: `failed to resolve instance for <fn(FooImpl, ()) as FooFn>::bar`

[pufmat]

Code

pub trait Foo {
    type Bar<'a>;
}

pub struct FooImpl {}

impl Foo for FooImpl {
    type Bar<'a> = ();
}

pub trait FooFn {
    fn bar(&self);
}

impl<T: Foo> FooFn for fn(T, T::Bar<'_>) {
    fn bar(&self) {}
}

fn foo<T: Foo>(f: fn(T, T::Bar<'_>)) {
    let _: &dyn FooFn = &f;
}

fn main() {
    foo(|_: FooImpl, _| {});
}

https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=03ff68a5f607582e3aa3e5d1abb49f3d

Meta

rustc --version --verbose:

rustc 1.85.1 (4eb161250 2025-03-15)
binary: rustc
commit-hash: 4eb161250e340c8f48f66e2b929ef4a5bed7c181
commit-date: 2025-03-15
host: x86_64-unknown-linux-gnu
release: 1.85.1
LLVM version: 19.1.7

Error output

error: internal compiler error: compiler/rustc_middle/src/ty/instance.rs:585:21: failed to resolve instance for <fn(FooImpl, ()) as FooFn>::bar
  --> src/main.rs:12:5
   |
12 |     fn bar(&self);
   |     ^^^^^^^^^^^^^^


thread 'rustc' panicked at compiler/rustc_middle/src/ty/instance.rs:585:21:
Box<dyn Any>

Backtrace

stack backtrace:
   0: std::panicking::begin_panic::<rustc_errors::ExplicitBug>
   1: <rustc_errors::diagnostic::BugAbort as rustc_errors::diagnostic::EmissionGuarantee>::emit_producing_guarantee
   2: <rustc_errors::DiagCtxtHandle>::span_bug::<rustc_span::span_encoding::Span, alloc::string::String>
   3: rustc_middle::util::bug::opt_span_bug_fmt::<rustc_span::span_encoding::Span>::{closure#0}
   4: rustc_middle::ty::context::tls::with_opt::<rustc_middle::util::bug::opt_span_bug_fmt<rustc_span::span_encoding::Span>::{closure#0}, !>::{closure#0}
   5: rustc_middle::ty::context::tls::with_context_opt::<rustc_middle::ty::context::tls::with_opt<rustc_middle::util::bug::opt_span_bug_fmt<rustc_span::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
   6: rustc_middle::util::bug::span_bug_fmt::<rustc_span::span_encoding::Span>
   7: <rustc_middle::ty::instance::Instance>::expect_resolve
   8: <rustc_middle::ty::instance::Instance>::expect_resolve_for_vtable
   9: rustc_trait_selection::traits::vtable::vtable_entries::{closure#0}
  10: rustc_trait_selection::traits::vtable::vtable_entries
      [... omitted 3 frames ...]
  11: rustc_monomorphize::collector::create_mono_items_for_vtable_methods
  12: rustc_monomorphize::collector::items_of_instance
      [... omitted 1 frame ...]
  13: rustc_monomorphize::collector::collect_items_rec::{closure#0}
  14: rustc_monomorphize::collector::collect_items_rec
  15: rustc_monomorphize::collector::collect_items_rec
  16: rustc_monomorphize::partitioning::collect_and_partition_mono_items
      [... omitted 2 frames ...]
  17: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_ssa::traits::backend::CodegenBackend>::codegen_crate
  18: <rustc_interface::queries::Linker>::codegen_and_build_linker
  19: rustc_interface::passes::create_and_enter_global_ctxt::<core::option::Option<rustc_interface::queries::Linker>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2}>::{closure#2}::{closure#0}
  20: rustc_interface::interface::run_compiler::<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}

[compiler-errors]

This is an example of the "type inference + higher-ranked type" problem that should be fixed by the new trait solver. If you search through that tag you may be able to find another instnce of this issue.

[pufmat]

Okay, thanks.