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ICE on "computing whether GenFuture<_> is Copy" #95034
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- changed the title
[-]ICE on "computing whether CustomFuture is `Copy`"[/-][+]ICE on "computing whether GenFuture<_> is `Copy`"[/+]on Mar 17, 2022 @rustbot claim
Reacted by Ali MJ Al-NasrawyWow this ICE is super weird. I believe it has to do something with the Generalizer in rustc_infer, the fact that generator interiors are represented with higher-kinded types, the fact that the
Eparameter in the example above is bivariant, and the way that universes work.Gonna keep working on it for a couple of hours, but I think it might be impossible to fix currently. Or at least it requires someone a lot smarter than me.
the fact that the E parameter in the example above is bivariant
Is this a thing in current rust? 🤯
Honestly, I was surprised that rustc accepts
CustomFutand not complains about unused type params.trait Tr<A> { type B; type C; } struct S<A, B, T: Tr<A, B = B>>(<T as Tr<A>>::C);
Does this pattern exist in practice? People would use PhantomData and this should avoid this problem (I hope).
Is this a thing in current rust?
Yep, see https://doc.rust-lang.org/nightly/nightly-rustc/src/rustc_infer/infer/combine.rs.html#506-530. This wf trick just ends up not working for higher-kinded types due to strange caveats...
Reacted by Ali MJ Al-NasrawyWell, either a caveat or just a bug I cannot find.
I honestly don't think I understand enough about variance and the Generalizer type relation to fix this issue. I think this would technically be solved by lazy projection normalization, since this is due to a inference variable created by normalizing a projection type.
I will at least add a known-bug test for this. Sorry!
@rustbot release-assignment
Reacted by Ali MJ Al-Nasrawy6 remaining items
Assigning priority as discussed in the Zulip thread of the Prioritization Working Group.
@rustbot label -I-prioritize +P-medium
I was able to minimize it further so that
CustomFutno longer needs to implFuturenor to be awaited. However, The ICE now happens at codegen with the messageassertion failed: !instance.substs.needs_infer(), but can still be fixed by the known workarounds and it regressed in the same nightly version.I hope the new ICE provides further insight.
pub trait Object<'a> { // <- can't remove `'a` type Error; // <- can't remove because then `E`s are no longer constrained below type Future; } impl Object<'static> for u8 { type Error = (); type Future = (); } impl<'a, E, A: Object<'a, Error = E>> Object<'a> for (A,) { type Error = (); type Future = CustomFut<'a, E, A>; // <- Replacing `E` with `A::Error` compiles } pub struct CustomFut<'a, E, A: Object<'a, Error = E>> { ph: std::marker::PhantomData<(A::Future,)>, // <- adding `E` to the tuple compiles } pub trait AsyncFn { fn call_async(&self); } impl<F: Fn() -> Fut, Fut> AsyncFn for F { fn call_async(&self) {} } async fn create<T: Object<'static>>() { let _fut: T::Future = unimplemented!(); std::future::ready(()).await; // <- any await point should work } pub fn test() { let _ = create::<(u8,)>.call_async(); // <- .call_async() is important (can't remove or replace with `(...)()`; so is `(u8,)` (`u8` doesn't work) }
Error output
Output with backtrace
thread 'rustc' panicked at 'assertion failed: !instance.substs.needs_infer()', compiler/rustc_codegen_llvm/src/mono_item.rs:53:9 stack backtrace: 0: 0x7f711a89ba9c - std::backtrace_rs::backtrace::libunwind::trace::hf6a6dfd7da937cb0 at /rustc/f1edd0429582dd29cccacaf50fd134b05593bd9c/library/std/src/../../backtrace/src/backtrace/libunwind.rs:90:5 1: 0x7f711a89ba9c - std::backtrace_rs::backtrace::trace_unsynchronized::hc596a19e4891f7f3 at /rustc/f1edd0429582dd29cccacaf50fd134b05593bd9c/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5 2: 0x7f711a89ba9c - std::sys_common::backtrace::_print_fmt::hb16700db31584325 at /rustc/f1edd0429582dd29cccacaf50fd134b05593bd9c/library/std/src/sys_common/backtrace.rs:67:5 3: 0x7f711a89ba9c - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::h231c4190cfa75162 at /rustc/f1edd0429582dd29cccacaf50fd134b05593bd9c/library/std/src/sys_common/backtrace.rs:46:22 4: 0x7f711a8f8fdc - core::fmt::write::h2a1462b5f8eea807 at /rustc/f1edd0429582dd29cccacaf50fd134b05593bd9c/library/core/src/fmt/mod.rs:1163:17 5: 0x7f711a88bc05 - std::io::Write::write_fmt::h71ddfebc68685972 at /rustc/f1edd0429582dd29cccacaf50fd134b05593bd9c/library/std/src/io/mod.rs:1696:15 6: 0x7f711a89ef60 - std::sys_common::backtrace::_print::hcc197d4bebf2b369 at /rustc/f1edd0429582dd29cccacaf50fd134b05593bd9c/library/std/src/sys_common/backtrace.rs:49:5 7: 0x7f711a89ef60 - std::sys_common::backtrace::print::h335a66af06738c7c at /rustc/f1edd0429582dd29cccacaf50fd134b05593bd9c/library/std/src/sys_common/backtrace.rs:36:9 8: 0x7f711a89ef60 - std::panicking::default_hook::{{closure}}::h6fac9ac9c8b79e52 at /rustc/f1edd0429582dd29cccacaf50fd134b05593bd9c/library/std/src/panicking.rs:210:50 9: 0x7f711a89eb15 - std::panicking::default_hook::h341c1030c6a1161b at /rustc/f1edd0429582dd29cccacaf50fd134b05593bd9c/library/std/src/panicking.rs:227:9 10: 0x7f711b11b2b1 - rustc_driver::DEFAULT_HOOK::{{closure}}::{{closure}}::h932547f60770f26a 11: 0x7f711a89f779 - std::panicking::rust_panic_with_hook::h50680ff4b44510c6 at /rustc/f1edd0429582dd29cccacaf50fd134b05593bd9c/library/std/src/panicking.rs:628:17 12: 0x7f711a89f202 - std::panicking::begin_panic_handler::{{closure}}::h9371c0fbb1e8465a at /rustc/f1edd0429582dd29cccacaf50fd134b05593bd9c/library/std/src/panicking.rs:519:13 13: 0x7f711a89bf44 - std::sys_common::backtrace::__rust_end_short_backtrace::h9b3efa22a5768c0f at /rustc/f1edd0429582dd29cccacaf50fd134b05593bd9c/library/std/src/sys_common/backtrace.rs:139:18 14: 0x7f711a89f199 - rust_begin_unwind at /rustc/f1edd0429582dd29cccacaf50fd134b05593bd9c/library/std/src/panicking.rs:517:5 15: 0x7f711a863441 - core::panicking::panic_fmt::h23b9203e89cc61cf at /rustc/f1edd0429582dd29cccacaf50fd134b05593bd9c/library/core/src/panicking.rs:100:14 16: 0x7f711a86338d - core::panicking::panic::h0ba7146865b2f9d6 at /rustc/f1edd0429582dd29cccacaf50fd134b05593bd9c/library/core/src/panicking.rs:50:5 17: 0x7f711c3655a2 - rustc_codegen_llvm::mono_item::<impl rustc_codegen_ssa::traits::declare::PreDefineMethods for rustc_codegen_llvm::context::CodegenCx>::predefine_fn::h1c521c4ac07b4fbd 18: 0x7f711c36cf31 - rustc_codegen_llvm::base::compile_codegen_unit::module_codegen::h16153c4046d114f1 19: 0x7f711cd826a5 - rustc_query_system::dep_graph::graph::DepGraph<K>::with_task::h789e443a348261c1 20: 0x7f711cdc67f3 - rustc_codegen_llvm::base::compile_codegen_unit::hab1bf03723596068 21: 0x7f711cd8dccb - rustc_codegen_ssa::base::codegen_crate::h7a10ba1a5bbbde6f 22: 0x7f711cdbae7a - <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_ssa::traits::backend::CodegenBackend>::codegen_crate::ha56c5fc04fb5674a 23: 0x7f711cd2bf7d - rustc_interface::queries::Queries::ongoing_codegen::h11dabf965b22130b 24: 0x7f711cd009aa - rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter::hf84cd18c24bd5171 25: 0x7f711ccee0ee - rustc_span::with_source_map::h6ab8a240e103b5b9 26: 0x7f711cd002ac - scoped_tls::ScopedKey<T>::set::hd1fbd64c6f645895 27: 0x7f711cceeef5 - std::sys_common::backtrace::__rust_begin_short_backtrace::h0a1328c9fa7f7448 28: 0x7f711cd1a962 - core::ops::function::FnOnce::call_once{{vtable.shim}}::h4ea1ced06d6b3e97 29: 0x7f711a8aa933 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h7bd677a5dc988be6 at /rustc/f1edd0429582dd29cccacaf50fd134b05593bd9c/library/alloc/src/boxed.rs:1691:9 30: 0x7f711a8aa933 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h7b1c1ba11c4db785 at /rustc/f1edd0429582dd29cccacaf50fd134b05593bd9c/library/alloc/src/boxed.rs:1691:9 31: 0x7f711a8aa933 - std::sys::unix::thread::Thread::new::thread_start::h9c58c0d12d84e854 at /rustc/f1edd0429582dd29cccacaf50fd134b05593bd9c/library/std/src/sys/unix/thread.rs:106:17 32: 0x7f711a6835c2 - start_thread 33: 0x7f711a708584 - __clone 34: 0x0 - <unknown> error: internal compiler error: unexpected panic 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 1.57.0 (f1edd0429 2021-11-29) running on x86_64-unknown-linux-gnu note: compiler flags: --crate-type lib query stack during panic: end of query stackYes, I saw that error in one of my repros above. This is smaller though.
Code
Compiled with the command
rustc --edition 2021 --crate-type lib repro.rs. Surprisingly, It is not reproducible with--crate-type bin!With any of the following changes, the code compiles fine:
impl<'a, E, A: Object<'a, Error = E>> Object<'a> for (A,) { type Error = (); - type Future = CustomFut<'a, E, A>; + type Future = CustomFut<'a, A::Error, A>; fn create() -> Self::Future { unimplemented!() }or
pub struct CustomFut<'f, E, A: Object<'f, Error = E>> { - ph: PhantomData<(A::Future,)>, + ph: PhantomData<(E, A::Future,)>, }Meta
The code compiles fine on v1.55.0. It was reproduced on v1.56.0 up to the current nightly with a similar error output.
Last good nightly:
1.56.0-nightly (b03ccace5 2021-08-24)Regressed nightly:
1.56.0-nightly (0afc20860 2021-08-25)rustc --version --verbose:Error output
Output with backtrace
@rustbot label A-async-await A-generators regression-from-stable-to-stable