Normalizing <T as Trait>::AssocType in a union causes an ICE

[guswynn]

https://github.com/rust-lang/rust/pull/81172/files
was reading this and trying to test how unions work with generics, and got an ICE

removing the union causes the ICE to NOT trigger

Code

#[repr(C)]
union PtrRepr<T: ?Sized> {
    const_ptr: *const T,
    mut_ptr: *mut T,
    components: PtrComponents<T>,
}

#[repr(C)]
struct PtrComponents<T: Pointee + ?Sized> {
    data_address: *const (),
    metadata: <T as Pointee>::Metadata,
}



pub trait Pointee {
   type Metadata;
}

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=b3888d392c9bf1a2ee1bf1cfb961b84b

Meta

rustc --version --verbose:

1.49 (playground

Error output

thread 'rustc' panicked at 'Box<Any>', compiler/rustc_errors/src/lib.rs:958:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

note: the compiler unexpectedly panicked. this is a bug.

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: rustc 1.49.0 (e1884a8e3 2020-12-29) running on x86_64-unknown-linux-gnu

note: compiler flags: -C embed-bitcode=no -C codegen-units=1 -C debuginfo=2 --crate-type lib

note: some of the compiler flags provided by cargo are hidden

query stack during panic:
#0 [normalize_generic_arg_after_erasing_regions] normalizing `<T as Pointee>::Metadata`
#1 [needs_drop_raw] computing whether `PtrComponents<T>` needs drop
end of query stack
error: aborting due to previous error

error: could not compile `playground`

To learn more, run the command again with --verbose.

Backtrace

thread 'rustc' panicked at 'Box<Any>', compiler/rustc_errors/src/lib.rs:958:9
stack backtrace:
   0: std::panicking::begin_panic
   1: rustc_errors::HandlerInner::bug
   2: rustc_errors::Handler::bug
   3: rustc_middle::util::bug::opt_span_bug_fmt::{{closure}}
   4: rustc_middle::ty::context::tls::with_opt::{{closure}}
   5: rustc_middle::ty::context::tls::with_opt
   6: rustc_middle::util::bug::opt_span_bug_fmt
   7: rustc_middle::util::bug::bug_fmt
   8: rustc_infer::infer::InferCtxtBuilder::enter
   9: rustc_traits::normalize_erasing_regions::normalize_generic_arg_after_erasing_regions
  10: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  11: rustc_data_structures::stack::ensure_sufficient_stack
  12: rustc_query_system::query::plumbing::get_query_impl
  13: <rustc_middle::ty::normalize_erasing_regions::NormalizeAfterErasingRegionsFolder as rustc_middle::ty::fold::TypeFolder>::fold_ty
  14: rustc_ty::needs_drop::needs_drop_raw
  15: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::needs_drop_raw>::compute
  16: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  17: rustc_data_structures::stack::ensure_sufficient_stack
  18: rustc_query_system::query::plumbing::get_query_impl
  19: <rustc_passes::stability::Checker as rustc_hir::intravisit::Visitor>::visit_item
  20: rustc_middle::hir::map::Map::visit_item_likes_in_module
  21: rustc_passes::stability::check_mod_unstable_api_usage
  22: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::check_mod_unstable_api_usage>::compute
  23: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  24: rustc_data_structures::stack::ensure_sufficient_stack
  25: rustc_query_system::query::plumbing::get_query_impl
  26: rustc_query_system::query::plumbing::ensure_query_impl
  27: <std::panic::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once
  28: rustc_session::utils::<impl rustc_session::session::Session>::time
  29: rustc_interface::passes::analysis
  30: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::analysis>::compute
  31: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  32: rustc_data_structures::stack::ensure_sufficient_stack
  33: rustc_query_system::query::plumbing::get_query_impl
  34: rustc_interface::passes::QueryContext::enter
  35: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  36: rustc_span::with_source_map
  37: rustc_interface::interface::create_compiler_and_run
  38: rustc_span::with_session_globals

[guswynn]

Minimization:

union PtrRepr<T: ?Sized> {
    const_ptr: *const T,
    mut_ptr: *mut T,
    components: <T as Pointee>::Metadata
}

pub trait Pointee {
    type Metadata;
}

ICE's

and

union PtrRepr<T: Pointee + ?Sized> {
    const_ptr: *const T,
    mut_ptr: *mut T,
    components: <T as Pointee>::Metadata
}

pub trait Pointee {
    type Metadata;
}

Errors with a unions may not contain fields that need dropping

[guswynn]

Appears to only be unions:

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=963f3ae6d30dd91fdd7ee5c30857ddc1

[YangKeao]

I have run cargo bisect between 2020-07-01 and 2020-12-29 and found that this regression was brought by e3051d8

Here are more information:

error: internal compiler error: compiler/rustc_traits/src/normalize_erasing_regions.rs:43:32: could not fully normalize `<T as Pointee>::Metadata` 

query stack during panic:
#0 [normalize_generic_arg_after_erasing_regions] normalizing `<T as Pointee>::Metadata`
#1 [check_mod_unstable_api_usage] checking for unstable API usage in top-level module
#2 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to previous error

When the generic parameter has enough constraints, e.g.

pub trait Pointee {
   type Metadata;
}

// minimization
union PtrRepr<T: Pointee> {
    components: <T as Pointee>::Metadata,
}

This error disappears.

[guswynn]

@YangKeao was it this PR? https://github.com/rust-lang/rust/pull/77547/files

[guswynn]

Yeah with the code "fixed" with the constraint it disappears, ideally rustc would tell the user that (like it does in the struct case i think I linked above), and not ICE

[YangKeao]

@YangKeao was it this PR? https://github.com/rust-lang/rust/pull/77547/files

Yes. It calls normalize_erasing_regions before the type-check, which will cause ICE. A quick fix could be delaying the union unstable api check until the end of type-check, but I don't know where is the most suitable place for the check 😿