ICE from explicit tail call in defaulted trait method

[theemathas]

Code

#![feature(explicit_tail_calls)]
#![expect(incomplete_features)]

trait Trait {}

trait Trait {
    fn foo(&self) {
        become self.foo();
    }
}

@rustbot labels +F-explicit_tail_calls +requires-incomplete-features

Meta

Reproducible on the playground with version 1.91.0-nightly (2025-08-05 ec7c02612527d185c379)

Error output

error[E0428]: the name `Trait` is defined multiple times
 --> src/lib.rs:6:1
  |
4 | trait Trait {}
  | ----------- previous definition of the trait `Trait` here
5 |
6 | trait Trait {
  | ^^^^^^^^^^^ `Trait` redefined here
  |
  = note: `Trait` must be defined only once in the type namespace of this module

error: internal compiler error: compiler/rustc_middle/src/ty/instance.rs:633:21: failed to resolve instance for <Self as Trait>::foo
 --> src/lib.rs:7:5
  |
7 |     fn foo(&self) {
  |     ^^^^^^^^^^^^^

Backtrace

thread 'rustc' panicked at compiler/rustc_middle/src/ty/instance.rs:633:21:
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_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_mir_build::check_tail_calls::TailCallCkVisitor as rustc_middle::thir::visit::Visitor>::visit_expr
   9: <rustc_mir_build::check_tail_calls::TailCallCkVisitor as rustc_middle::thir::visit::Visitor>::visit_expr
  10: <rustc_mir_build::check_tail_calls::TailCallCkVisitor as rustc_middle::thir::visit::Visitor>::visit_block
  11: <rustc_mir_build::check_tail_calls::TailCallCkVisitor as rustc_middle::thir::visit::Visitor>::visit_expr
  12: <rustc_mir_build::check_tail_calls::TailCallCkVisitor as rustc_middle::thir::visit::Visitor>::visit_expr
  13: <rustc_mir_build::check_tail_calls::TailCallCkVisitor as rustc_middle::thir::visit::Visitor>::visit_expr
      [... omitted 1 frame ...]
  14: rustc_mir_build::builder::build_mir
  15: rustc_mir_transform::mir_built
      [... omitted 1 frame ...]
  16: rustc_mir_build::check_unsafety::check_unsafety
      [... omitted 1 frame ...]
  17: <rustc_middle::ty::context::TyCtxt>::par_hir_body_owners::<rustc_interface::passes::run_required_analyses::{closure#1}::{closure#0}>::{closure#0}
  18: rustc_interface::passes::analysis
      [... omitted 1 frame ...]
  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}
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-2025-08-06T07_27_59-27.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 [check_tail_calls] tail-call-checking `Trait::foo`
#1 [mir_built] building MIR for `Trait::foo`
#2 [check_unsafety] unsafety-checking `Trait::foo`
#3 [analysis] running analysis passes on this crate
end of query stack
For more information about this error, try `rustc --explain E0428`.

[theemathas]

Actually, the duplicated trait isn't needed for the ICE. This code causes the same ICE:

#![feature(explicit_tail_calls)]
#![expect(incomplete_features)]

trait Trait {
    fn foo(&self) {
        become self.foo();
    }
}