ICE: `internal error: entered unreachable code: Should have generated a fat pointer`

I tried this code:

```rust
// check-pass
// edition: 2021

// This test case is meant to demonstrate how close we can get to async
// functions in dyn traits with the current level of dyn* support.

#![feature(dyn_star)]
#![allow(incomplete_features)]

use std::future::Future;

trait DynAsyncCounter {
    fn increment<'a>(&'a mut self) -> dyn* Future<Output = usize> + 'a;
}

struct MyCounter {
    count: usize,
}

impl DynAsyncCounter for MyCounter {
    fn increment<'a>(&'a mut self) -> dyn* Future<Output = usize> + 'a {
        Box::pin(async {
            self.count += 1;
            self.count
        })
    }
}

async fn do_counter(counter: &mut dyn DynAsyncCounter) -> usize {
    counter.increment().await
}

#[kani::proof]
fn main() {
    let mut counter = MyCounter { count: 0 };
    let _ = do_counter(&mut counter);
}

```

using the following command line invocation:

```
RUSTFLAGS="--edition=2021" kani dyn-async-trait.rs
```

with Kani version: 0.22.0

I expected to see this happen: *explanation*


Instead, this happened: *explanation*

````
thread '<unnamed>' panicked at 'Type dyn* std::future::Future<Output = usize> must be a trait type (a dynamic type)', kani-compiler/src/codegen_cprover_gotoc/codegen/typ.rs:601:9
stack backtrace:
   0:     0x7f4657b667da - std::backtrace_rs::backtrace::libunwind::trace::hc84915c67f6fb1e5
                               at /rustc/c6fcdb690609769a240fc8ab0de0ce68d5ea7dba/library/std/src/../../backtrace/src/backtrace/libunwind.rs:93:5
   1:     0x7f4657b667da - std::backtrace_rs::backtrace::trace_unsynchronized::h74fef551999c3a2f
                               at /rustc/c6fcdb690609769a240fc8ab0de0ce68d5ea7dba/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x7f4657b667da - std::sys_common::backtrace::_print_fmt::h8acfd19c1a6f1d1d
                               at /rustc/c6fcdb690609769a240fc8ab0de0ce68d5ea7dba/library/std/src/sys_common/backtrace.rs:65:5
   3:     0x7f4657b667da - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::h9271b09f3576b7e0
                               at /rustc/c6fcdb690609769a240fc8ab0de0ce68d5ea7dba/library/std/src/sys_common/backtrace.rs:44:22
   4:     0x7f4657bc92ee - core::fmt::write::h27d0bbb767cff1d5
                               at /rustc/c6fcdb690609769a240fc8ab0de0ce68d5ea7dba/library/core/src/fmt/mod.rs:1208:17
   5:     0x7f4657b56c65 - std::io::Write::write_fmt::hc409ea2bb818fbea
                               at /rustc/c6fcdb690609769a240fc8ab0de0ce68d5ea7dba/library/std/src/io/mod.rs:1682:15
   6:     0x7f4657b665a5 - std::sys_common::backtrace::_print::hfa031a98cf1e4011
                               at /rustc/c6fcdb690609769a240fc8ab0de0ce68d5ea7dba/library/std/src/sys_common/backtrace.rs:47:5
   7:     0x7f4657b665a5 - std::sys_common::backtrace::print::he69ac0980f15620d
                               at /rustc/c6fcdb690609769a240fc8ab0de0ce68d5ea7dba/library/std/src/sys_common/backtrace.rs:34:9
   8:     0x7f4657b692ef - std::panicking::default_hook::{{closure}}::h014cf704a69dce2b
                               at /rustc/c6fcdb690609769a240fc8ab0de0ce68d5ea7dba/library/std/src/panicking.rs:267:22
   9:     0x7f4657b6902b - std::panicking::default_hook::h380e71f8d8d49df7
                               at /rustc/c6fcdb690609769a240fc8ab0de0ce68d5ea7dba/library/std/src/panicking.rs:286:9
  10:     0x55a9689aeb0d - kani_compiler::session::PANIC_HOOK::{{closure}}::{{closure}}::h10f33c4e395bf462
  11:     0x55a968904f67 - kani_compiler::codegen_cprover_gotoc::utils::debug::DEFAULT_HOOK::{{closure}}::{{closure}}::hf568cc16e5ffa08a
  12:     0x7f4657b69b2d - <alloc::boxed::Box<F,A> as core::ops::function::Fn<Args>>::call::hcfa7e50330911a79
                               at /rustc/c6fcdb690609769a240fc8ab0de0ce68d5ea7dba/library/alloc/src/boxed.rs:2032:9
  13:     0x7f4657b69b2d - std::panicking::rust_panic_with_hook::h483f1ef90c766581
                               at /rustc/c6fcdb690609769a240fc8ab0de0ce68d5ea7dba/library/std/src/panicking.rs:692:13
  14:     0x7f4657b698a9 - std::panicking::begin_panic_handler::{{closure}}::hd4c7d9116c0ef489
                               at /rustc/c6fcdb690609769a240fc8ab0de0ce68d5ea7dba/library/std/src/panicking.rs:579:13
  15:     0x7f4657b66c8c - std::sys_common::backtrace::__rust_end_short_backtrace::had27a083c7d7188b
                               at /rustc/c6fcdb690609769a240fc8ab0de0ce68d5ea7dba/library/std/src/sys_common/backtrace.rs:137:18
  16:     0x7f4657b695b2 - rust_begin_unwind
                               at /rustc/c6fcdb690609769a240fc8ab0de0ce68d5ea7dba/library/std/src/panicking.rs:575:5
  17:     0x7f4657bc5cd3 - core::panicking::panic_fmt::hbacb72817da3b060
                               at /rustc/c6fcdb690609769a240fc8ab0de0ce68d5ea7dba/library/core/src/panicking.rs:64:14
  18:     0x55a9688e35aa - kani_compiler::codegen_cprover_gotoc::codegen::typ::<impl kani_compiler::codegen_cprover_gotoc::context::goto_ctx::GotocCtx>::normalized_trait_name::h895d46c4fb1ec415
  19:     0x55a9688e2c2e - kani_compiler::codegen_cprover_gotoc::codegen::typ::<impl kani_compiler::codegen_cprover_gotoc::context::goto_ctx::GotocCtx>::codegen_trait_data::hd51c10437a5ae95e
  20:     0x55a9688e3dc8 - kani_compiler::codegen_cprover_gotoc::codegen::typ::<impl kani_compiler::codegen_cprover_gotoc::context::goto_ctx::GotocCtx>::codegen_ty::h3a1c4e105629c1cf
  21:     0x55a9688e29c6 - kani_compiler::codegen_cprover_gotoc::codegen::typ::<impl kani_compiler::codegen_cprover_gotoc::context::goto_ctx::GotocCtx>::trait_method_vtable_field_type::ha0d906bfaec8b77c
  22:     0x55a96897b0d3 - <alloc::vec::Vec<T> as alloc::vec::spec_from_iter::SpecFromIter<T,I>>::from_iter::h2a20199b30e6eed4
  23:     0x55a9688e936e - kani_compiler::codegen_cprover_gotoc::codegen::typ::<impl kani_compiler::codegen_cprover_gotoc::context::goto_ctx::GotocCtx>::codegen_ty_ref::h8f0e76378246970b
  24:     0x55a9688e4823 - kani_compiler::codegen_cprover_gotoc::codegen::typ::<impl kani_compiler::codegen_cprover_gotoc::context::goto_ctx::GotocCtx>::codegen_ty::h3a1c4e105629c1cf
  25:     0x55a968931934 - core::ops::function::impls::<impl core::ops::function::FnMut<A> for &mut F>::call_mut::h3d27524049e75026
  26:     0x55a968982876 - <alloc::vec::Vec<T> as alloc::vec::spec_from_iter::SpecFromIter<T,I>>::from_iter::he9d1f8227d49a086
  27:     0x55a96888041f - kani_compiler::codegen_cprover_gotoc::codegen::function::<impl kani_compiler::codegen_cprover_gotoc::context::goto_ctx::GotocCtx>::declare_function::hcb4ad696d24f1916
  28:     0x55a968909b60 - std::thread::local::LocalKey<T>::with::h8c541365775d3b68
  29:     0x55a968995f05 - <kani_compiler::codegen_cprover_gotoc::compiler_interface::GotocCodegenBackend as rustc_codegen_ssa::traits::backend::CodegenBackend>::codegen_crate::heaa499391d53b3f6
  30:     0x7f465a086fa1 - <rustc_session[19dfb3d05ff8562d]::session::Session>::time::<alloc[58cfa59ca61fafa9]::boxed::Box<dyn core[6d94cc961ac87456]::any::Any>, rustc_interface[aec886e93b1add3f]::passes::start_codegen::{closure#0}>
  31:     0x7f465a086ac9 - rustc_interface[aec886e93b1add3f]::passes::start_codegen
  32:     0x7f465a0847a6 - <rustc_interface[aec886e93b1add3f]::passes::QueryContext>::enter::<<rustc_interface[aec886e93b1add3f]::queries::Queries>::ongoing_codegen::{closure#0}::{closure#0}, core[6d94cc961ac87456]::result::Result<alloc[58cfa59ca61fafa9]::boxed::Box<dyn core[6d94cc961ac87456]::any::Any>, rustc_errors[56887b15c7ec6fbf]::ErrorGuaranteed>>
  33:     0x7f465a081a46 - <rustc_interface[aec886e93b1add3f]::queries::Queries>::ongoing_codegen
  34:     0x7f465a080f67 - <rustc_interface[aec886e93b1add3f]::interface::Compiler>::enter::<rustc_driver[94a2f0241de75afa]::run_compiler::{closure#1}::{closure#2}, core[6d94cc961ac87456]::result::Result<core[6d94cc961ac87456]::option::Option<rustc_interface[aec886e93b1add3f]::queries::Linker>, rustc_errors[56887b15c7ec6fbf]::ErrorGuaranteed>>
  35:     0x7f465a07bf88 - rustc_span[ae2df7aa7c6c667b]::with_source_map::<core[6d94cc961ac87456]::result::Result<(), rustc_errors[56887b15c7ec6fbf]::ErrorGuaranteed>, rustc_interface[aec886e93b1add3f]::interface::run_compiler<core[6d94cc961ac87456]::result::Result<(), rustc_errors[56887b15c7ec6fbf]::ErrorGuaranteed>, rustc_driver[94a2f0241de75afa]::run_compiler::{closure#1}>::{closure#0}::{closure#0}>
  36:     0x7f465a07ba75 - <scoped_tls[db87656d258d017b]::ScopedKey<rustc_span[ae2df7aa7c6c667b]::SessionGlobals>>::set::<rustc_interface[aec886e93b1add3f]::interface::run_compiler<core[6d94cc961ac87456]::result::Result<(), rustc_errors[56887b15c7ec6fbf]::ErrorGuaranteed>, rustc_driver[94a2f0241de75afa]::run_compiler::{closure#1}>::{closure#0}, core[6d94cc961ac87456]::result::Result<(), rustc_errors[56887b15c7ec6fbf]::ErrorGuaranteed>>
  37:     0x7f465a07b062 - std[acd1f573e90225f]::sys_common::backtrace::__rust_begin_short_backtrace::<rustc_interface[aec886e93b1add3f]::util::run_in_thread_pool_with_globals<rustc_interface[aec886e93b1add3f]::interface::run_compiler<core[6d94cc961ac87456]::result::Result<(), rustc_errors[56887b15c7ec6fbf]::ErrorGuaranteed>, rustc_driver[94a2f0241de75afa]::run_compiler::{closure#1}>::{closure#0}, core[6d94cc961ac87456]::result::Result<(), rustc_errors[56887b15c7ec6fbf]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[6d94cc961ac87456]::result::Result<(), rustc_errors[56887b15c7ec6fbf]::ErrorGuaranteed>>
  38:     0x7f465a6ec49e - <<std[acd1f573e90225f]::thread::Builder>::spawn_unchecked_<rustc_interface[aec886e93b1add3f]::util::run_in_thread_pool_with_globals<rustc_interface[aec886e93b1add3f]::interface::run_compiler<core[6d94cc961ac87456]::result::Result<(), rustc_errors[56887b15c7ec6fbf]::ErrorGuaranteed>, rustc_driver[94a2f0241de75afa]::run_compiler::{closure#1}>::{closure#0}, core[6d94cc961ac87456]::result::Result<(), rustc_errors[56887b15c7ec6fbf]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[6d94cc961ac87456]::result::Result<(), rustc_errors[56887b15c7ec6fbf]::ErrorGuaranteed>>::{closure#1} as core[6d94cc961ac87456]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  39:     0x7f465bbda2d3 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::he955d3e33f115328
                               at /rustc/c6fcdb690609769a240fc8ab0de0ce68d5ea7dba/library/alloc/src/boxed.rs:2000:9
  40:     0x7f465bbda2d3 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h32515d9eaa012a19
                               at /rustc/c6fcdb690609769a240fc8ab0de0ce68d5ea7dba/library/alloc/src/boxed.rs:2000:9
  41:     0x7f465bbda2d3 - std::sys::unix::thread::Thread::new::thread_start::h5af9cdd9a5a58d64
                               at /rustc/c6fcdb690609769a240fc8ab0de0ce68d5ea7dba/library/std/src/sys/unix/thread.rs:108:17
  42:     0x7f4657905bb5 - <unknown>
  43:     0x7f4657987d90 - <unknown>
  44:                0x0 - <unknown>

Kani unexpectedly panicked during compilation.
Please file an issue here: https://github.com/model-checking/kani/issues/new?labels=bug&template=bug_report.md

[Kani] current codegen item: declare_function: do_counter
[Kani] current codegen location: Loc { file: "/tmp/kani/dyn-async-trait.rs", function: None, start_line: 29, start_col: Some(1), end_line: 29, end_col: Some(64) }
error: /home/matthias/.kani/kani-0.22.0/bin/kani-compiler exited with status exit status: 101
````

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ICE: `internal error: entered unreachable code: Should have generated a fat pointer` #2266

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ICE: internal error: entered unreachable code: Should have generated a fat pointer #2266

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ICE: `internal error: entered unreachable code: Should have generated a fat pointer` #2266