ICE: `cycle detected when computing function signature of`

[matthiaskrgr]

auto-reduced (treereduce-rust):

#![feature(object_safe_for_dispatch)]
trait B {
    fn f(a: A) -> A;
}
trait A {
    fn concrete(b: B) -> B;
}
fn main() {}

original:

#![feature(object_safe_for_dispatch)]
trait B { fn f(a: A) -> A; }
trait A { fn concrete(b: B) -> B; }
fn main() {}

Version information

rustc 1.80.0-nightly (9e297bf54 2024-05-24)
binary: rustc
commit-hash: 9e297bf54d31eb3b30067208ff9af4416945a2ed
commit-date: 2024-05-24
host: x86_64-unknown-linux-gnu
release: 1.80.0-nightly
LLVM version: 18.1.6

Command:
/home/matthias/.rustup/toolchains/master/bin/rustc -Zunstable-options --edition=2024

Program output

warning: trait `B` is never used
 --> /tmp/icemaker_global_tempdir.Y2ZB7VqiZYxB/rustc_testrunner_tmpdir_reporting.n6c2niMcV32C/mvce.rs:2:7
  |
2 | trait B {
  |       ^
  |
  = note: `#[warn(dead_code)]` on by default

warning: trait `A` is never used
 --> /tmp/icemaker_global_tempdir.Y2ZB7VqiZYxB/rustc_testrunner_tmpdir_reporting.n6c2niMcV32C/mvce.rs:5:7
  |
5 | trait A {
  |       ^

warning: 2 warnings emitted

note: no errors encountered even though delayed bugs were created

note: those delayed bugs will now be shown as internal compiler errors

error: internal compiler error[E0391]: cycle detected when computing function signature of `B::f`
 --> /tmp/icemaker_global_tempdir.Y2ZB7VqiZYxB/rustc_testrunner_tmpdir_reporting.n6c2niMcV32C/mvce.rs:3:5
  |
3 |     fn f(a: A) -> A;
  |     ^^^^^^^^^^^^^^^^
  |
note: ...which requires checking if trait `A` is object safe...
 --> /tmp/icemaker_global_tempdir.Y2ZB7VqiZYxB/rustc_testrunner_tmpdir_reporting.n6c2niMcV32C/mvce.rs:5:1
  |
5 | trait A {
  | ^^^^^^^
note: ...which requires determining object safety of trait `A`...
 --> /tmp/icemaker_global_tempdir.Y2ZB7VqiZYxB/rustc_testrunner_tmpdir_reporting.n6c2niMcV32C/mvce.rs:5:1
  |
5 | trait A {
  | ^^^^^^^
note: ...which requires computing function signature of `A::concrete`...
 --> /tmp/icemaker_global_tempdir.Y2ZB7VqiZYxB/rustc_testrunner_tmpdir_reporting.n6c2niMcV32C/mvce.rs:6:5
  |
6 |     fn concrete(b: B) -> B;
  |     ^^^^^^^^^^^^^^^^^^^^^^^
note: ...which requires checking if trait `B` is object safe...
 --> /tmp/icemaker_global_tempdir.Y2ZB7VqiZYxB/rustc_testrunner_tmpdir_reporting.n6c2niMcV32C/mvce.rs:2:1
  |
2 | trait B {
  | ^^^^^^^
note: ...which requires determining object safety of trait `B`...
 --> /tmp/icemaker_global_tempdir.Y2ZB7VqiZYxB/rustc_testrunner_tmpdir_reporting.n6c2niMcV32C/mvce.rs:2:1
  |
2 | trait B {
  | ^^^^^^^
  = note: ...which again requires computing function signature of `B::f`, completing the cycle
note: cycle used when checking that `B` is well-formed
 --> /tmp/icemaker_global_tempdir.Y2ZB7VqiZYxB/rustc_testrunner_tmpdir_reporting.n6c2niMcV32C/mvce.rs:2:1
  |
2 | trait B {
  | ^^^^^^^
  = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information
note: delayed at compiler/rustc_query_system/src/query/job.rs:597:16 - disabled backtrace
 --> /tmp/icemaker_global_tempdir.Y2ZB7VqiZYxB/rustc_testrunner_tmpdir_reporting.n6c2niMcV32C/mvce.rs:3:5
  |
3 |     fn f(a: A) -> A;
  |     ^^^^^^^^^^^^^^^^

error: internal compiler error[E0782]: trait objects must include the `dyn` keyword
 --> /tmp/icemaker_global_tempdir.Y2ZB7VqiZYxB/rustc_testrunner_tmpdir_reporting.n6c2niMcV32C/mvce.rs:6:20
  |
6 |     fn concrete(b: B) -> B;
  |                    ^
  |
  = note: `B` it is not object safe, so it can't be `dyn`
note: delayed at compiler/rustc_hir_analysis/src/hir_ty_lowering/lint.rs:63:17 - disabled backtrace
 --> /tmp/icemaker_global_tempdir.Y2ZB7VqiZYxB/rustc_testrunner_tmpdir_reporting.n6c2niMcV32C/mvce.rs:6:20
  |
6 |     fn concrete(b: B) -> B;
  |                    ^
help: use a new generic type parameter, constrained by `B`
  |
6 |     fn concrete<T: B>(b: T) -> B;
  |                ++++++    ~
help: you can also use an opaque type, but users won't be able to specify the type parameter when calling the `fn`, having to rely exclusively on type inference
  |
6 |     fn concrete(b: impl B) -> B;
  |                    ++++

error: internal compiler error[E0782]: trait objects must include the `dyn` keyword
 --> /tmp/icemaker_global_tempdir.Y2ZB7VqiZYxB/rustc_testrunner_tmpdir_reporting.n6c2niMcV32C/mvce.rs:6:26
  |
6 |     fn concrete(b: B) -> B;
  |                          ^
  |
note: delayed at compiler/rustc_hir_analysis/src/hir_ty_lowering/lint.rs:63:17 - disabled backtrace
 --> /tmp/icemaker_global_tempdir.Y2ZB7VqiZYxB/rustc_testrunner_tmpdir_reporting.n6c2niMcV32C/mvce.rs:6:26
  |
6 |     fn concrete(b: B) -> B;
  |                          ^
help: `B` is not object safe, use `impl B` to return an opaque type, as long as you return a single underlying type
  |
6 |     fn concrete(b: B) -> impl B;
  |                          ++++

error: internal compiler error[E0782]: trait objects must include the `dyn` keyword
 --> /tmp/icemaker_global_tempdir.Y2ZB7VqiZYxB/rustc_testrunner_tmpdir_reporting.n6c2niMcV32C/mvce.rs:3:13
  |
3 |     fn f(a: A) -> A;
  |             ^
  |
  = note: `A` it is not object safe, so it can't be `dyn`
note: delayed at compiler/rustc_hir_analysis/src/hir_ty_lowering/lint.rs:63:17 - disabled backtrace
 --> /tmp/icemaker_global_tempdir.Y2ZB7VqiZYxB/rustc_testrunner_tmpdir_reporting.n6c2niMcV32C/mvce.rs:3:13
  |
3 |     fn f(a: A) -> A;
  |             ^
help: use a new generic type parameter, constrained by `A`
  |
3 |     fn f<T: A>(a: T) -> A;
  |         ++++++    ~
help: you can also use an opaque type, but users won't be able to specify the type parameter when calling the `fn`, having to rely exclusively on type inference
  |
3 |     fn f(a: impl A) -> A;
  |             ++++

error: internal compiler error[E0782]: trait objects must include the `dyn` keyword
 --> /tmp/icemaker_global_tempdir.Y2ZB7VqiZYxB/rustc_testrunner_tmpdir_reporting.n6c2niMcV32C/mvce.rs:3:19
  |
3 |     fn f(a: A) -> A;
  |                   ^
  |
note: delayed at compiler/rustc_hir_analysis/src/hir_ty_lowering/lint.rs:63:17 - disabled backtrace
 --> /tmp/icemaker_global_tempdir.Y2ZB7VqiZYxB/rustc_testrunner_tmpdir_reporting.n6c2niMcV32C/mvce.rs:3:19
  |
3 |     fn f(a: A) -> A;
  |                   ^
help: `A` is not object safe, use `impl A` to return an opaque type, as long as you return a single underlying type
  |
3 |     fn f(a: A) -> impl A;
  |                   ++++

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: rustc 1.80.0-nightly (9e297bf54 2024-05-24) running on x86_64-unknown-linux-gnu

note: compiler flags: -Z unstable-options -Z dump-mir-dir=dir

query stack during panic:
end of query stack

@rustbot label +F-object_safe_for_dispatch