Post-mono cycle ICE with dyn and RPIT

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Code

This is a variant of #150508. Unlike that issue, this issue ICEs in both the old solver and new solver.

trait Apply {
    type Output<T: Trait>: Trait;
}
struct Identity;
impl Apply for Identity {
    type Output<T: Trait> = T;
}

struct Thing<A: Apply>(A);

trait Trait {}

impl<A: Apply> Trait for Thing<A> where <A as Apply>::Output<Self>: Trait {}

/*
effectively:
impl Trait for Thing<Identity> where Thing<Identity>: Trait {}
*/

fn weird<A: Apply>(x: A) -> impl Trait {
    Thing(x)
}

fn main() {
    let _ = Box::new(weird(Identity)) as Box<dyn Trait>;
}

In the above code, each individual trait bound holds pre-monomorphization. However, after monomorphization, the required trait bound is Thing<Identity>: Trait. Trying to prove this bound runs into an inductive cycle.

cc @lcnr

Error output

error: internal compiler error: /rustc-dev/e22c616e4e87914135c1db261a03e0437255335e/compiler/rustc_mir_transform/src/validate.rs:82:25: broken MIR in Item(DefId(0:21 ~ playground[c925]::main)) (after phase change to runtime-optimized) at bb2[0]:
                                Unsize coercion, but `std::boxed::Box<Thing<Identity>>` isn't coercible to `std::boxed::Box<dyn Trait>`
  --> src/main.rs:25:13
   |
25 |     let _ = Box::new(weird(Identity)) as Box<dyn Trait>;
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^


thread 'rustc' (26) panicked at /rustc-dev/e22c616e4e87914135c1db261a03e0437255335e/compiler/rustc_mir_transform/src/validate.rs:82:25:
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_mir_transform::validate::CfgChecker>::fail::<alloc::string::String>
   8: <rustc_mir_transform::validate::Validator as rustc_mir_transform::pass_manager::MirPass>::run_pass
   9: rustc_mir_transform::run_optimization_passes
  10: rustc_mir_transform::optimized_mir
      [... omitted 1 frame ...]
  11: <rustc_middle::ty::context::TyCtxt>::instance_mir
  12: rustc_monomorphize::collector::items_of_instance
      [... omitted 1 frame ...]
  13: rustc_monomorphize::collector::collect_items_rec
  14: rustc_monomorphize::collector::collect_crate_mono_items::{closure#1}::{closure#0}
  15: rustc_monomorphize::partitioning::collect_and_partition_mono_items
      [... omitted 1 frame ...]
  16: rustc_codegen_ssa::base::codegen_crate::<rustc_codegen_llvm::LlvmCodegenBackend>
  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::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-2026-04-20T04_48_17-24.txt` to your bug report

note: rustc 1.97.0-nightly (e22c616e4 2026-04-19) running on x86_64-unknown-linux-gnu

note: compiler flags: --crate-type bin -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 [optimized_mir] optimizing MIR for `main`
#1 [items_of_instance] collecting items used by `main`
#2 [collect_and_partition_mono_items] collect_and_partition_mono_items
end of query stack

Reproduce the supplied example on the reported nightly, then inspect rustc_mir_transform/src/validate.rs around the reported validator failure and the monomorphization path named in the query stack. Trace how the post-monomorphization trait cycle reaches MIR validation; done means the example no longer causes an internal compiler error and is handled consistently by the compiler.

Written by the indexing model from the issue text.

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Post-mono cycle ICE with dyn and RPIT

Description

Code

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Error output

Contributor guide

First steps

Research direction

Assessment

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