closure_lifetime_binder with explicit lifetime annotation gives ICE: "cannot convert ReLateParam […] to a region vid"

[theemathas]

Code

#![feature(closure_lifetime_binder)]

fn fails() {
    callback(for<'a> |y: &'a mut i32| -> () {
        let _: &'a mut i32 = y;
    });
}

fn callback(_: impl for<'a> FnOnce(&'a mut i32)) {}

@rustbot labels +F-closure_lifetime_binder

This issue was discovered while experimenting with #141481

Meta

Reproducible on the playground with rustc 1.89.0-nightly (2025-05-23 3e674b06b5c74adea662)

Error output

Backtrace

error: internal compiler error: compiler/rustc_borrowck/src/universal_regions.rs:928:36: cannot convert `ReLateParam(DefId(0:4 ~ playground[7ed8]::fails::{closure#0}), LateNamed(DefId(0:5 ~ playground[7ed8]::fails::{closure#0}::'a), 'a))` to a region vid


thread 'rustc' panicked at compiler/rustc_borrowck/src/universal_regions.rs:928:36:
Box<dyn Any>
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_middle::util::bug::opt_span_bug_fmt::<rustc_span::span_encoding::Span>::{closure#0}
   3: rustc_middle::ty::context::tls::with_opt::<rustc_middle::util::bug::opt_span_bug_fmt<rustc_span::span_encoding::Span>::{closure#0}, !>::{closure#0}
   4: 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}, !>
   5: rustc_middle::util::bug::bug_fmt
   6: <rustc_borrowck::type_check::TypeChecker>::push_region_constraints
   7: <rustc_borrowck::type_check::TypeChecker>::ascribe_user_type
   8: rustc_borrowck::do_mir_borrowck
   9: <rustc_borrowck::root_cx::BorrowCheckRootCtxt>::get_or_insert_nested
  10: rustc_borrowck::mir_borrowck
      [... omitted 1 frame ...]
  11: <rustc_middle::ty::context::TyCtxt>::par_hir_body_owners::<rustc_interface::passes::run_required_analyses::{closure#2}::{closure#0}>::{closure#0}
  12: rustc_interface::passes::run_required_analyses
  13: rustc_interface::passes::analysis
      [... omitted 1 frame ...]
  14: 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}
  15: rustc_interface::interface::run_compiler::<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

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 `/playground/rustc-ice-2025-05-24T13_36_39-26.txt` to your bug report

note: compiler flags: --crate-type lib -C embed-bitcode=no -C codegen-units=1 -C debuginfo=2

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

query stack during panic:
#0 [mir_borrowck] borrow-checking `fails`
#1 [analysis] running analysis passes on this crate
end of query stack
error: could not compile `playground` (lib)

[theemathas]

This also gives the same ICE:

#![feature(closure_lifetime_binder)]

fn fails() {
    callback(for<'a> || -> () {
        let _: &'a i32 = &1;
    });
}

fn callback(_: impl FnOnce()) {}

[fmease]

#![feature(closure_lifetime_binder)]

fn main() {
    let _: fn() = for<'a> || -> () { let _: &'a (); };
}

[matthiaskrgr]

probably duplicate of #135845

[theemathas]

I'd argue that the code in this issue should compile, while the code in #135845 shouldn't.

[matthiaskrgr]

🤔 on nightly-2025-01-13, before the ice was introduced, this errored with

error: lifetime may not live long enough
 --> abc.rs:5:16
  |
4 |     callback(for<'a> |y: &'a mut i32| -> () {
  |                  --      - let's call the lifetime of this reference `'1`
  |                  |
  |                  lifetime `'a` defined here
5 |         let _: &'a mut i32 = y;
  |                ^^^^^^^^^^^ type annotation requires that `'1` must outlive `'a`