ICE: Expected type for ... but found Lifetime(ReErased) when substituting

[saethlin]

https://github.com/oxidecomputer/cancel-safe-futures on commit 857ddd3a30c5f82e6edb72ac7c8a471860501d9f

RUSTFLAGS="-Zinline-mir -Zcross-crate-inline-threshold=always -Zinline-mir-hint-threshold=10000 -Zinline-mir-threshold=10000" cargo +nightly test --target=x86_64-unknown-linux-gnu

If you don't set a --target you will OOM while optimizing syn with these very silly flags.

error: internal compiler error: compiler/rustc_middle/src/ty/generic_args.rs:885:9: expected type for `T/#0` (T/#0/0) but found Lifetime(ReErased) when substituting

error: internal compiler error: compiler/rustc_middle/src/ty/generic_args.rs:885:9: expected type for `T/#0` (T/#0/0) but found Lifetime(ReErased) when substituting

I also have another example of this that produces a "but found Const` ICE. They look similar, I'll post the other one later if someone wants.

rustc-ice-2023-12-18T05_32_24-344139.txt

[saethlin]

I've been able to minimize to this:

[package]
name = "cancel-safe-futures"
edition = "2021"

[dependencies]
futures-core = { version = "0.3.28" }
futures-util = { version = "0.3.28" }
tokio = { version = "1.28.2", features = ["macros", "sync"] }
futures-lite = "2.1.0"

use futures_core::future::BoxFuture;
use futures_util::FutureExt;
use tokio::sync::MutexGuard;
use futures_lite::future::block_on;

async fn run() {
    let m = tokio::sync::Mutex::new(());
    let _ = async move {
        let permit = ActionPermit { guard: m.lock().await };
        cancel_perform_async_boxed(permit).await;
    }
    .await;
}

fn main() {
    block_on(run())
}

async fn cancel_perform_async_boxed<T>(permit: ActionPermit<'_, T>) {
    let fut = permit.perform_async_boxed(|_| async move {}.boxed());
    tokio::select! {
        _ = fut => {}
    }
}

struct ActionPermit<'a, T> {
    guard: MutexGuard<'a, T>,
}

impl<'a, T> ActionPermit<'a, T> {
    async fn perform_async_boxed<R, F>(mut self, action: F) -> R
    where
        F: for<'lock> FnOnce(&'lock mut T) -> BoxFuture<'lock, R>,
    {
        action(&mut *self.guard).await
    }
}

[kpreid]

Here is a further minimized core-only repro:

pub async fn run(permit: ActionPermit<'_, ()>, ctx: &mut core::task::Context<'_>) {
    run2(permit, ctx);
}

fn run2<T>(permit: ActionPermit<'_, T>, ctx: &mut core::task::Context) {
    _ = || {
        let mut fut = ActionPermit::perform(permit);
        let fut = unsafe { core::pin::Pin::new_unchecked(&mut fut) };
        _ = core::future::Future::poll(fut, ctx);
    };
}

pub struct ActionPermit<'a, T> {
    _guard: core::cell::Ref<'a, T>,
}

impl<'a, T> ActionPermit<'a, T> {
    async fn perform(self) {
        core::future::ready(()).await
    }
}

The poll came out of the innards of tokio::select!. The use of core::cell::Ref is not critical, but the use of a type with both a lifetime and a type generic is; for example, std::borrow::Cow will do too. Tested with the above RUSTFLAGS, under rustc 1.76.0-nightly (d86d65bbc 2023-12-10) x86_64-apple-darwin.

error: internal compiler error: compiler/rustc_middle/src/ty/generic_args.rs:885:9: expected type for `T/#0` (T/#0/0) but found Lifetime(ReErased) when substituting, args=[ReErased, T/#0, std::future::ResumeTy, (), (), CoroutineWitness(DefId(0:16 ~ scratchpad[7815]::{impl#0}::perform::{closure#0}), [ReErased, T/#0]), (ActionPermit<ReErased, T/#0>,)]

Backtrace

thread 'rustc' panicked at /rustc/d86d65bbc19b928387f68427fcc3a0da498d8a19/compiler/rustc_errors/src/lib.rs:1113:75:
Box<dyn Any>
stack backtrace:
   0:        0x1039210eb - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::h5e659123d19f3605
   1:        0x10396d733 - core::fmt::write::h3553f749a30e8cce
   2:        0x10391757e - std::io::Write::write_fmt::ha0a0cbd22c8ebc52
   3:        0x103920ec9 - std::sys_common::backtrace::print::ha58ecd228ca176e9
   4:        0x103924095 - std::panicking::default_hook::{{closure}}::h17c3184394eb8856
   5:        0x103923e14 - std::panicking::default_hook::h2fbacdca4604565a
   6:        0x11137a41e - <alloc[67ea17a77aaf9eaf]::boxed::Box<rustc_driver_impl[36417fddfe0df6e]::install_ice_hook::{closure#0}> as core[2238ef0cf162900b]::ops::function::Fn<(&dyn for<'a, 'b> core[2238ef0cf162900b]::ops::function::Fn<(&'a core[2238ef0cf162900b]::panic::panic_info::PanicInfo<'b>,), Output = ()> + core[2238ef0cf162900b]::marker::Sync + core[2238ef0cf162900b]::marker::Send, &core[2238ef0cf162900b]::panic::panic_info::PanicInfo)>>::call
   7:        0x103924735 - std::panicking::rust_panic_with_hook::h563bb071cac0df33
   8:        0x11141140a - std[d8ec8f7e4f9a1335]::panicking::begin_panic::<rustc_errors[42dabb35cd23131d]::ExplicitBug>::{closure#0}
   9:        0x11140f3d9 - std[d8ec8f7e4f9a1335]::sys_common::backtrace::__rust_end_short_backtrace::<std[d8ec8f7e4f9a1335]::panicking::begin_panic<rustc_errors[42dabb35cd23131d]::ExplicitBug>::{closure#0}, !>
  10:        0x115b41e19 - std[d8ec8f7e4f9a1335]::panicking::begin_panic::<rustc_errors[42dabb35cd23131d]::ExplicitBug>
  11:        0x1113e265d - <rustc_errors[42dabb35cd23131d]::diagnostic_builder::Bug as rustc_errors[42dabb35cd23131d]::diagnostic_builder::EmissionGuarantee>::diagnostic_builder_emit_producing_guarantee
  12:        0x111fd02fb - <rustc_errors[42dabb35cd23131d]::Handler>::bug::<alloc[67ea17a77aaf9eaf]::string::String>
  13:        0x111ff115b - rustc_middle[99776e18600d2c31]::util::bug::opt_span_bug_fmt::<rustc_span[b6253c3433877e36]::span_encoding::Span>::{closure#0}
  14:        0x111fed147 - rustc_middle[99776e18600d2c31]::ty::context::tls::with_opt::<rustc_middle[99776e18600d2c31]::util::bug::opt_span_bug_fmt<rustc_span[b6253c3433877e36]::span_encoding::Span>::{closure#0}, !>::{closure#0}
  15:        0x111fed102 - rustc_middle[99776e18600d2c31]::ty::context::tls::with_context_opt::<rustc_middle[99776e18600d2c31]::ty::context::tls::with_opt<rustc_middle[99776e18600d2c31]::util::bug::opt_span_bug_fmt<rustc_span[b6253c3433877e36]::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
  16:        0x115bf058b - rustc_middle[99776e18600d2c31]::util::bug::bug_fmt
  17:        0x115be6df6 - <rustc_middle[99776e18600d2c31]::ty::generic_args::ArgFolder>::type_param_expected
  18:        0x111ef297b - <rustc_middle[99776e18600d2c31]::ty::generic_args::ArgFolder as rustc_type_ir[2a68048d6213fca9]::fold::TypeFolder<rustc_middle[99776e18600d2c31]::ty::context::TyCtxt>>::fold_ty
  19:        0x111ed3075 - <&rustc_middle[99776e18600d2c31]::ty::list::List<rustc_middle[99776e18600d2c31]::ty::generic_args::GenericArg> as rustc_type_ir[2a68048d6213fca9]::fold::TypeFoldable<rustc_middle[99776e18600d2c31]::ty::context::TyCtxt>>::try_fold_with::<rustc_middle[99776e18600d2c31]::ty::generic_args::ArgFolder>
  20:        0x111f4373a - <rustc_middle[99776e18600d2c31]::ty::Ty as rustc_type_ir[2a68048d6213fca9]::fold::TypeSuperFoldable<rustc_middle[99776e18600d2c31]::ty::context::TyCtxt>>::try_super_fold_with::<rustc_middle[99776e18600d2c31]::ty::generic_args::ArgFolder>
  21:        0x11122a936 - <rustc_const_eval[732a1bba9fb60b50]::transform::validate::TypeChecker as rustc_middle[99776e18600d2c31]::mir::visit::Visitor>::visit_projection_elem
  22:        0x11122b82b - <rustc_const_eval[732a1bba9fb60b50]::transform::validate::TypeChecker as rustc_middle[99776e18600d2c31]::mir::visit::Visitor>::visit_place
  23:        0x11122ea58 - <rustc_const_eval[732a1bba9fb60b50]::transform::validate::TypeChecker as rustc_middle[99776e18600d2c31]::mir::visit::Visitor>::visit_statement
  24:        0x111229871 - rustc_const_eval[732a1bba9fb60b50]::transform::validate::validate_types
  25:        0x112323c21 - <rustc_mir_transform[23b75684e486e9bc]::inline::Inliner>::try_inlining
  26:        0x11232294f - <rustc_mir_transform[23b75684e486e9bc]::inline::Inliner>::process_blocks
  27:        0x112322418 - <rustc_mir_transform[23b75684e486e9bc]::inline::Inline as rustc_middle[99776e18600d2c31]::mir::MirPass>::run_pass
  28:        0x1123c3f98 - rustc_mir_transform[23b75684e486e9bc]::pass_manager::run_passes_inner
  29:        0x112331d73 - rustc_mir_transform[23b75684e486e9bc]::optimized_mir
  30:        0x112880081 - rustc_query_impl[52be1802f6aa83c9]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[52be1802f6aa83c9]::query_impl::optimized_mir::dynamic_query::{closure#2}::{closure#0}, rustc_middle[99776e18600d2c31]::query::erase::Erased<[u8; 8usize]>>
  31:        0x1129b2eff - <rustc_query_impl[52be1802f6aa83c9]::query_impl::optimized_mir::dynamic_query::{closure#2} as core[2238ef0cf162900b]::ops::function::FnOnce<(rustc_middle[99776e18600d2c31]::ty::context::TyCtxt, rustc_span[b6253c3433877e36]::def_id::DefId)>>::call_once
  32:        0x1127dc137 - rustc_query_system[48ab03958a5795b4]::query::plumbing::try_execute_query::<rustc_query_impl[52be1802f6aa83c9]::DynamicConfig<rustc_query_system[48ab03958a5795b4]::query::caches::DefaultCache<rustc_span[b6253c3433877e36]::def_id::DefId, rustc_middle[99776e18600d2c31]::query::erase::Erased<[u8; 8usize]>>, false, false, false>, rustc_query_impl[52be1802f6aa83c9]::plumbing::QueryCtxt, true>
  33:        0x112a62db5 - rustc_query_impl[52be1802f6aa83c9]::query_impl::optimized_mir::get_query_incr::__rust_end_short_backtrace
  34:        0x111d96497 - rustc_middle[99776e18600d2c31]::query::plumbing::query_get_at::<rustc_query_system[48ab03958a5795b4]::query::caches::DefaultCache<rustc_span[b6253c3433877e36]::def_id::DefId, rustc_middle[99776e18600d2c31]::query::erase::Erased<[u8; 8usize]>>>
  35:        0x111d9b8b0 - <rustc_metadata[ad3b2a361d891836]::rmeta::encoder::EncodeContext>::encode_crate_root
  36:        0x111da8dc3 - rustc_metadata[ad3b2a361d891836]::rmeta::encoder::encode_metadata
  37:        0x111e412ef - rustc_metadata[ad3b2a361d891836]::fs::encode_and_write_metadata
  38:        0x111c1fa9b - rustc_interface[35319ced0081c4aa]::passes::start_codegen
  39:        0x111bd6bc3 - <rustc_middle[99776e18600d2c31]::ty::context::GlobalCtxt>::enter::<<rustc_interface[35319ced0081c4aa]::queries::Queries>::codegen_and_build_linker::{closure#0}, core[2238ef0cf162900b]::result::Result<rustc_interface[35319ced0081c4aa]::queries::Linker, rustc_span[b6253c3433877e36]::ErrorGuaranteed>>
  40:        0x111bd29a4 - <rustc_interface[35319ced0081c4aa]::queries::Queries>::codegen_and_build_linker
  41:        0x111303f7d - <rustc_interface[35319ced0081c4aa]::interface::Compiler>::enter::<rustc_driver_impl[36417fddfe0df6e]::run_compiler::{closure#0}::{closure#0}, core[2238ef0cf162900b]::result::Result<core[2238ef0cf162900b]::option::Option<rustc_interface[35319ced0081c4aa]::queries::Linker>, rustc_span[b6253c3433877e36]::ErrorGuaranteed>>
  42:        0x11136cfa6 - rustc_span[b6253c3433877e36]::set_source_map::<core[2238ef0cf162900b]::result::Result<(), rustc_span[b6253c3433877e36]::ErrorGuaranteed>, rustc_interface[35319ced0081c4aa]::interface::run_compiler<core[2238ef0cf162900b]::result::Result<(), rustc_span[b6253c3433877e36]::ErrorGuaranteed>, rustc_driver_impl[36417fddfe0df6e]::run_compiler::{closure#0}>::{closure#0}::{closure#0}>
  43:        0x111380e41 - <scoped_tls[2656f72bfa0842ac]::ScopedKey<rustc_span[b6253c3433877e36]::SessionGlobals>>::set::<rustc_interface[35319ced0081c4aa]::util::run_in_thread_pool_with_globals<rustc_interface[35319ced0081c4aa]::interface::run_compiler<core[2238ef0cf162900b]::result::Result<(), rustc_span[b6253c3433877e36]::ErrorGuaranteed>, rustc_driver_impl[36417fddfe0df6e]::run_compiler::{closure#0}>::{closure#0}, core[2238ef0cf162900b]::result::Result<(), rustc_span[b6253c3433877e36]::ErrorGuaranteed>>::{closure#0}, core[2238ef0cf162900b]::result::Result<(), rustc_span[b6253c3433877e36]::ErrorGuaranteed>>
  44:        0x111378d13 - std[d8ec8f7e4f9a1335]::sys_common::backtrace::__rust_begin_short_backtrace::<rustc_interface[35319ced0081c4aa]::util::run_in_thread_with_globals<rustc_interface[35319ced0081c4aa]::util::run_in_thread_pool_with_globals<rustc_interface[35319ced0081c4aa]::interface::run_compiler<core[2238ef0cf162900b]::result::Result<(), rustc_span[b6253c3433877e36]::ErrorGuaranteed>, rustc_driver_impl[36417fddfe0df6e]::run_compiler::{closure#0}>::{closure#0}, core[2238ef0cf162900b]::result::Result<(), rustc_span[b6253c3433877e36]::ErrorGuaranteed>>::{closure#0}, core[2238ef0cf162900b]::result::Result<(), rustc_span[b6253c3433877e36]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[2238ef0cf162900b]::result::Result<(), rustc_span[b6253c3433877e36]::ErrorGuaranteed>>
  45:        0x111387639 - <<std[d8ec8f7e4f9a1335]::thread::Builder>::spawn_unchecked_<rustc_interface[35319ced0081c4aa]::util::run_in_thread_with_globals<rustc_interface[35319ced0081c4aa]::util::run_in_thread_pool_with_globals<rustc_interface[35319ced0081c4aa]::interface::run_compiler<core[2238ef0cf162900b]::result::Result<(), rustc_span[b6253c3433877e36]::ErrorGuaranteed>, rustc_driver_impl[36417fddfe0df6e]::run_compiler::{closure#0}>::{closure#0}, core[2238ef0cf162900b]::result::Result<(), rustc_span[b6253c3433877e36]::ErrorGuaranteed>>::{closure#0}, core[2238ef0cf162900b]::result::Result<(), rustc_span[b6253c3433877e36]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[2238ef0cf162900b]::result::Result<(), rustc_span[b6253c3433877e36]::ErrorGuaranteed>>::{closure#1} as core[2238ef0cf162900b]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  46:        0x10392da19 - std::sys::unix::thread::Thread::new::thread_start::h04013d08f8688bf1
  47:     0x7ff801e4f4e1 - __pthread_start