ICE: OutputTypeParameterMismatch error under -Clink-dead-code

[Manishearth]

Code

Must be compiled with -C link-dead-code

Minimized test

use std::rc::Rc;
use std::borrow::{Cow, ToOwned};
use std::{mem, ptr};


pub unsafe trait Yokeable<'a>: 'static {
    type Output: 'a;
}

pub struct Yoke<Y: for<'a> Yokeable<'a>, C> {
    // must be the first field for drop order
    // this will have a 'static lifetime parameter, that parameter is a lie
    yokeable: Y,
    cart: C,
}


impl<Y: for<'a> Yokeable<'a>> Yoke<Y, Rc<[u8]>> {
    pub fn project<'this, P>(
        &'this self,
        f: for<'a> fn(&'this <Y as Yokeable<'a>>::Output, &'a ()) -> <P as Yokeable<'a>>::Output,
    ) -> Yoke<P, Rc<[u8]>>
    where
        P: for<'a> Yokeable<'a>,
    {
        unimplemented!()
    }
}

pub fn slice(y: Yoke<&'static str, Rc<[u8]>>) -> Yoke<&'static [u8], Rc<[u8]>> {
    y.project(move |yk, _| yk.as_bytes())
}


unsafe impl<'a, T: 'static + ToOwned + ?Sized> Yokeable<'a> for Cow<'static, T>
where
    <T as ToOwned>::Owned: Sized,
{
    type Output = Cow<'a, T>;

}

unsafe impl<'a, T: 'static + ?Sized> Yokeable<'a> for &'static T {
    type Output = &'a T;
}

Nonminimized original failure

For the nonminimized example, the tests under https://github.com/Manishearth/icu4x/blob/yoke-ice/utils/yoke/src/yoke.rs#L474-L551 fail when run under -Clink-dead-code

Steps:

• check out the yoke-ice branch of https://github.com/Manishearth/icu4x
• cd utils/yoke
• RUSTDOCFLAGS="-Clink-dead-code" cargo +nightly test

Adding code that makes use of the functions defined in these tests does not change anything, e.g. the function in the first test is not being called, but if you add code that calls it like below, the ICE still happens. I'm not sure as to what role -Clink-dead-code has here.

let rc: Rc<[u8]> = Rc::from("hello".as_bytes());
let yoke: Yoke<&'static str, Rc<[u8]>> = Yoke::attach_to_cart_badly(rc, |rc| std::str::from_utf8(&rc).unwrap());

Meta

rustc --version --verbose:

rustc 1.55.0-nightly (e8cb1a4a5 2021-06-27)
binary: rustc
commit-hash: e8cb1a4a567ce88e459ffd431207eff2e0f0ffa5
commit-date: 2021-06-27
host: x86_64-unknown-linux-gnu
release: 1.55.0-nightly
LLVM version: 12.0.1

Error output

---- src/yoke.rs - yoke::Yoke<Y, C>::project (line 500) stdout ----
error: internal compiler error: compiler/rustc_trait_selection/src/traits/codegen.rs:78:17: Encountered error `OutputTypeParameterMismatch(Binder(<[closure@src/yoke.rs:16:17: 16:38] as std::ops::FnMut<(&<main::_doctest_main_src_yoke_rs_500_0::Bar as yoke::Yokeable<'_>>::Output, &())>>, [Region(BrAnon(0))]), Binder(<[closure@src/yoke.rs:16:17: 16:38] as std::ops::FnMut<(&main::_doctest_main_src_yoke_rs_500_0::Bar, &())>>, []), Sorts(ExpectedFound { expected: main::_doctest_main_src_yoke_rs_500_0::Bar, found: <main::_doctest_main_src_yoke_rs_500_0::Bar as yoke::Yokeable<'_>>::Output }))` selecting `Binder(<[closure@src/yoke.rs:16:17: 16:38] as std::ops::FnMut<(&main::_doctest_main_src_yoke_rs_500_0::Bar, &())>>, [])` during codegen

thread 'rustc' panicked at 'Box<dyn Any>', compiler/rustc_errors/src/lib.rs:1007:9
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 1.55.0-nightly (e8cb1a4a5 2021-06-27) running on x86_64-unknown-linux-gnu

note: compiler flags: -C codegen-units=1 -C embed-bitcode=no -C link-dead-code --crate-type bin

query stack during panic:
#0 [codegen_fulfill_obligation] checking if `std::ops::FnMut` fulfills its obligations
#1 [resolve_instance] resolving instance `<[closure@src/yoke.rs:16:17: 16:38] as std::ops::FnMut<(&main::_doctest_main_src_yoke_rs_500_0::Bar, &())>>::call_mut`
end of query stack
error: aborting due to previous error

Backtrace

stack backtrace:
   0: std::panicking::begin_panic
   1: std::panic::panic_any
   2: rustc_errors::HandlerInner::bug
   3: rustc_errors::Handler::bug
   4: rustc_middle::ty::context::tls::with_opt
   5: rustc_middle::util::bug::opt_span_bug_fmt
   6: rustc_middle::util::bug::bug_fmt
   7: rustc_infer::infer::InferCtxtBuilder::enter
   8: rustc_trait_selection::traits::codegen::codegen_fulfill_obligation
   9: rustc_query_system::query::plumbing::get_query_impl
  10: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::codegen_fulfill_obligation
  11: rustc_ty_utils::instance::inner_resolve_instance
  12: rustc_ty_utils::instance::resolve_instance
  13: rustc_query_system::query::plumbing::get_query_impl
  14: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::resolve_instance
  15: rustc_middle::ty::instance::Instance::resolve_opt_const_arg
  16: rustc_middle::ty::instance::Instance::resolve
  17: <rustc_mir::monomorphize::collector::MirNeighborCollector as rustc_middle::mir::visit::Visitor>::visit_terminator
  18: rustc_mir::monomorphize::collector::collect_neighbours
  19: rustc_mir::monomorphize::collector::collect_items_rec
  20: rustc_mir::monomorphize::collector::collect_items_rec
  21: rustc_session::utils::<impl rustc_session::session::Session>::time
  22: rustc_mir::monomorphize::collector::collect_crate_mono_items
  23: rustc_mir::monomorphize::partitioning::collect_and_partition_mono_items
  24: rustc_query_system::query::plumbing::get_query_impl
  25: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::collect_and_partition_mono_items
  26: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_ssa::traits::backend::CodegenBackend>::codegen_crate
  27: rustc_interface::passes::QueryContext::enter
  28: rustc_interface::queries::Queries::ongoing_codegen
  29: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  30: rustc_span::with_source_map
  31: rustc_interface::interface::create_compiler_and_run
  32: scoped_tls::ScopedKey<T>::set

[Manishearth]

This may be related to #86702

[Manishearth]

The weird thing is that even if I make sure all of the functions are used, I still do not get the error without -C link-dead-code, and still get the error with -C link-dead-code

I cannot get this ICE to occur without -C link-dead-code

[Manishearth]

Jonas says this might be #62529

[Manishearth]

The ICE here is fixed by #85499, however it gets replaced with the following (incorrect) error:

error: implementation of `Yokeable` is not general enough
  --> foo.rs:31:7
   |
31 |     y.project(move |yk, _| yk.as_bytes())
   |       ^^^^^^^ implementation of `Yokeable` is not general enough
   |
   = note: `&[u8]` must implement `Yokeable<'0>`, for any lifetime `'0`...
   = note: ...but `Yokeable<'_>` is actually implemented for the type `&'static [u8]`

[hellow554]

Reduced to this:

trait Yokeable<'a> {
    type Output: 'a;
}

fn project(_: for<'a> fn(<() as Yokeable<'a>>::Output)) {}

impl<'a> Yokeable<'a> for () {
    type Output = ();
}

fn main() {
    project(|_| {});
}

looks related at least

[lqd]

The subtlety with reducing this is that it quickly falls into an actual duplicate of #62529, losing what made it slightly different under #85499. That PR will successfully compile this reduction, but not the OP.

[hellow554]

@lqd you're right. Here's what needs to be done, to be able to trigger the ICE with -C link-dead-code again:

fn crash() {
    project(|_| {});
}

fn main() {}

which seems plausible, because that's what link-dead-code is supposed to do, right?
Is that enough or am I missing something?

[lqd]

I guess it depends on your reduction goals :)

If you want the minimal ICE regardless of the intent of the OP, that's valuable for tests and debugging.

Equally valuable, we could reduce to try to understand why the OP doesn't compile, even without the ICE: there could be

• an existing rustc bug that manifests today as this ICE but is surfaced as an error with Normalize projections under binders #85499
• or something unclear in documentation or diagnostics about why this shouldn't compile
• or an issue in Normalize projections under binders #85499 itself.

This would likely require reducing using the CI artifacts of that PR.