Repository: rust
Issue: Internal compiler error: find_vtable_types_for_unsizing: invalid coercion
Labels: I-ICE,T-compiler,A-specialization,C-bug,requires-nightly,F-specialization,glacier,F-coerce_unsized,S-bug-has-test

I tried this code (modified version of `src/test/ui/specialization/issue-44861.rs`):

```rust
#![feature(specialization)]
#![feature(unsize, coerce_unsized)]

use std::ops::CoerceUnsized;

pub struct SmartassPtr<A: Smartass+?Sized>(A::Data);

pub trait Smartass {
    type Data;
    type Data2: CoerceUnsized<*const [u8]>;
}

pub trait MaybeObjectSafe {}

impl MaybeObjectSafe for () {}

impl<T> Smartass for T {
    type Data = <Self as Smartass>::Data2;
    default type Data2 = *const [u8; 0];
}

impl Smartass for () {
    type Data2 = *const [u8; 1];
}

impl Smartass for dyn MaybeObjectSafe {
    type Data = *const [u8];
    type Data2 = *const [u8; 0];
}

impl<U: Smartass+?Sized, T: Smartass+?Sized> CoerceUnsized<SmartassPtr<T>> for SmartassPtr<U>
    where <U as Smartass>::Data: std::ops::CoerceUnsized<<T as Smartass>::Data>
{}

pub fn conv(s: SmartassPtr<()>) -> SmartassPtr<dyn MaybeObjectSafe> {
    s // This shouldn't coerce
}
```

I expected to see this happen: mismatched types error in conv due to mismatched types

Instead, this happened: ICE

[Playground](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=d507a3c12e29d16097b94e13ef6401dd)
### Meta

`rustc --version --verbose`:
```
rustc 1.46.0-nightly (346aec9b0 2020-07-11)
binary: rustc
commit-hash: 346aec9b02f3c74f3fce97fd6bda24709d220e49
commit-date: 2020-07-11
host: x86_64-unknown-linux-gnu
release: 1.46.0-nightly
LLVM version: 10.0
```

<details><summary>Backtrace</summary>
<p>

```
warning: the feature `specialization` is incomplete and may not be safe to use and/or cause compiler crashes
 --> tmp/coerce-unsize-spec.rs:2:12
  |
2 | #![feature(specialization)]
  |            ^^^^^^^^^^^^^^
  |
  = note: `#[warn(incomplete_features)]` on by default
  = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information

error: internal compiler error: src/librustc_mir/monomorphize/collector.rs:898:14: find_vtable_types_for_unsizing: invalid coercion <() as Smartass>::Data -> <dyn MaybeObjectSafe as Smartass>::Data

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:916:9
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/libunwind.rs:86
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print_fmt
             at src/libstd/sys_common/backtrace.rs:78
   3: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
             at src/libstd/sys_common/backtrace.rs:59
   4: core::fmt::write
             at src/libcore/fmt/mod.rs:1076
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1537
   6: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:62
   7: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:49
   8: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:198
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:217
  10: rustc_driver::report_ice
  11: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:530
  12: std::panicking::begin_panic
  13: rustc_errors::HandlerInner::bug
  14: rustc_errors::Handler::bug
  15: rustc_middle::util::bug::opt_span_bug_fmt::{{closure}}
  16: rustc_middle::ty::context::tls::with_opt::{{closure}}
  17: rustc_middle::ty::context::tls::with_opt
  18: rustc_middle::util::bug::opt_span_bug_fmt
  19: rustc_middle::util::bug::bug_fmt
  20: rustc_mir::monomorphize::collector::find_vtable_types_for_unsizing
  21: rustc_mir::monomorphize::collector::find_vtable_types_for_unsizing
  22: <rustc_mir::monomorphize::collector::MirNeighborCollector as rustc_middle::mir::visit::Visitor>::visit_rvalue
  23: rustc_mir::monomorphize::collector::collect_neighbours
  24: rustc_mir::monomorphize::collector::collect_items_rec
  25: rustc_session::utils::<impl rustc_session::session::Session>::time
  26: rustc_mir::monomorphize::collector::collect_crate_mono_items
  27: rustc_mir::monomorphize::partitioning::collect_and_partition_mono_items
  28: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::collect_and_partition_mono_items>::compute
  29: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  30: rustc_data_structures::stack::ensure_sufficient_stack
  31: rustc_query_system::query::plumbing::get_query_impl
  32: rustc_codegen_ssa::back::symbol_export::exported_symbols_provider_local
  33: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::exported_symbols>::compute
  34: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  35: rustc_data_structures::stack::ensure_sufficient_stack
  36: rustc_query_system::query::plumbing::get_query_impl
  37: rustc_metadata::rmeta::encoder::encode_metadata_impl
  38: rustc_data_structures::sync::join
  39: rustc_metadata::rmeta::decoder::cstore_impl::<impl rustc_middle::middle::cstore::CrateStore for rustc_metadata::creader::CStore>::encode_metadata
  40: rustc_middle::ty::context::TyCtxt::encode_metadata
  41: rustc_interface::passes::start_codegen
  42: rustc_middle::ty::context::tls::enter_global
  43: rustc_interface::queries::Queries::ongoing_codegen
  44: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  45: rustc_span::with_source_map
  46: rustc_interface::interface::create_compiler_and_run
  47: scoped_tls::ScopedKey<T>::set
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.46.0-nightly (346aec9b0 2020-07-11) running on x86_64-unknown-linux-gnu

note: compiler flags: --crate-type lib

query stack during panic:
#0 [collect_and_partition_mono_items] collect_and_partition_mono_items
#1 [exported_symbols] exported_symbols
end of query stack
error: aborting due to previous error; 1 warning emitted
```

</p>
</details>