rust-lang / rust-lang/rust

ICE: broken MIR: NoSolution on HRTB over GAT in trait object

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A-dyn-trait A-GATs A-higher-ranked A-lifetimes A-trait-system C-bug fixed-by-next-solver I-ICE S-bug-has-test T-compiler
Dominant language
Rust
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Description

Code

MWE below, also tested on nightly (via the playground). Interestingly the error goes away if I manually inline the Node::new calls. Minimised from a filter graph that uses trait objects for type erasure.

pub trait Transform {
    type Output<'a>;
}

pub trait Propagate<Input> {}

type Child<T> = Box<dyn for<'a> Propagate<<T as Transform>::Output<'a>>>;

pub struct Node<T>
where
    T: Transform,
{
    transform: T,
    children: Vec<Child<T>>,
}

impl<T> Node<T>
where
    T: Transform,
{
    pub fn new(transform: T, children: Vec<Child<T>>) -> Self {
        Node {
            transform,
            children,
        }
    }
}

impl<Input, T> Propagate<Input> for Node<T> where T: Transform {}

pub fn main() {
    struct Noop;

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

    let node = Box::new(Node::new(Noop, vec![Box::new(Node::new(Noop, vec![]))]));
}
Meta

rustc --version --verbose:

rustc 1.81.0 (eeb90cda1 2024-09-04)
binary: rustc
commit-hash: eeb90cda1969383f56a2637cbd3037bdf598841c
commit-date: 2024-09-04
host: x86_64-unknown-linux-gnu
release: 1.81.0
LLVM version: 18.1.7
Error output
error: internal compiler error: broken MIR in DefId(0:31 ~ playground[6f8d]::test::run#1) ([move _11]): std::boxed::Box<dyn [Binder { value: Trait(Propagate<()>), bound_vars: [Region(BrNamed(DefId(0:10 ~ playground[6f8d]::Child::'a), 'a))] }] + '?14, std::alloc::Global> is not a subtype of std::boxed::Box<dyn [Binder { value: Trait(Propagate<<test::Noop as Transform>::Output<'a>>), bound_vars: [Region(BrNamed(DefId(0:10 ~ playground[6f8d]::Child::'a), 'a))] }] + '?7, std::alloc::Global>: NoSolution
  --> src/lib.rs:43:45
   |
43 |         let node = Box::new(Node::new(Noop, vec![Box::new(Node::new(Noop, vec![]))]));
   |                                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
note: delayed at compiler/rustc_borrowck/src/type_check/mod.rs:2570:17 - disabled backtrace
  --> src/lib.rs:43:45
   |
43 |         let node = Box::new(Node::new(Noop, vec![Box::new(Node::new(Noop, vec![]))]));
   |                                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   = note: this error: internal compiler error originates in the macro `vec` (in Nightly builds, run with -Z macro-backtrace for more info)

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 1.81.0 (eeb90cda1 2024-09-04) running on x86_64-unknown-linux-gnu

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

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

query stack during panic:
end of query stack
Backtrace 

   Compiling my_test v0.1.0 (/tmp/my_test)
warning: unused variable: `node`
  --> src/main.rs:38:9
   |
38 |     let node = Box::new(Node::new(Noop, vec![Box::new(Node::new(Noop, vec![]))]));
   |         ^^^^ help: if this is intentional, prefix it with an underscore: `_node`
   |
   = note: `#[warn(unused_variables)]` on by default

warning: fields `transform` and `children` are never read
  --> src/main.rs:13:5
   |
9  | pub struct Node<T>
   |            ---- fields in this struct
...
13 |     transform: T,
   |     ^^^^^^^^^
14 |     children: Vec<Child<T>>,
   |     ^^^^^^^^
   |
   = note: `#[warn(dead_code)]` on by default

note: no errors encountered even though delayed bugs were created

note: those delayed bugs will now be shown as internal compiler errors

error: internal compiler error: broken MIR in DefId(0:21 ~ my_test[db07]::main) ([move _11]): std::boxed::Box<dyn [Binder { value: Trait(Propagate<()>), bound_vars: [Region(BrNamed(DefId(0:10 ~ my_test[db07]::Child::'a), 'a))] }] + '?14, std::alloc::Global> is not a subtype of std::boxed::Box<dyn [Binder { value: Trait(Propagate<<main::Noop as Transform>::Output<'a>>), bound_vars: [Region(BrNamed(DefId(0:10 ~ my_test[db07]::Child::'a), 'a))] }] + '?7, std::alloc::Global>: NoSolution
  --> src/main.rs:38:41
   |
38 |     let node = Box::new(Node::new(Noop, vec![Box::new(Node::new(Noop, vec![]))]));
   |                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
note: delayed at compiler/rustc_borrowck/src/type_check/mod.rs:2570:17
         0: <rustc_errors::DiagCtxtInner>::emit_diagnostic
         1: <rustc_errors::DiagCtxtHandle>::emit_diagnostic
         2: <rustc_span::ErrorGuaranteed as rustc_errors::diagnostic::EmissionGuarantee>::emit_producing_guarantee
         3: <rustc_errors::DiagCtxtHandle>::span_delayed_bug::<rustc_span::span_encoding::Span, alloc::string::String>
         4: <rustc_borrowck::type_check::TypeChecker>::typeck_mir
         5: rustc_borrowck::type_check::type_check
         6: rustc_borrowck::nll::compute_regions
         7: rustc_borrowck::do_mir_borrowck
         8: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::mir_borrowck::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 8]>>
         9: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::DynamicConfig<rustc_query_system::query::caches::VecCache<rustc_span::def_id::LocalDefId, rustc_middle::query::erase::Erased<[u8; 8]>>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, true>
        10: rustc_query_impl::query_impl::mir_borrowck::get_query_incr::__rust_end_short_backtrace
        11: rustc_interface::passes::analysis
        12: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 1]>>
        13: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::DynamicConfig<rustc_query_system::query::caches::SingleCache<rustc_middle::query::erase::Erased<[u8; 1]>>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, true>
        14: rustc_query_impl::query_impl::analysis::get_query_incr::__rust_end_short_backtrace
        15: rustc_interface::interface::run_compiler::<core::result::Result<(), rustc_span::ErrorGuaranteed>, rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}
        16: std::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface::util::run_in_thread_with_globals<rustc_interface::interface::run_compiler<core::result::Result<(), rustc_span::ErrorGuaranteed>, rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}, core::result::Result<(), rustc_span::ErrorGuaranteed>>::{closure#0}::{closure#0}, core::result::Result<(), rustc_span::ErrorGuaranteed>>
        17: <<std::thread::Builder>::spawn_unchecked_<rustc_interface::util::run_in_thread_with_globals<rustc_interface::interface::run_compiler<core::result::Result<(), rustc_span::ErrorGuaranteed>, rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}, core::result::Result<(), rustc_span::ErrorGuaranteed>>::{closure#0}::{closure#0}, core::result::Result<(), rustc_span::ErrorGuaranteed>>::{closure#1} as core::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
        18: <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once
                   at /rustc/eeb90cda1969383f56a2637cbd3037bdf598841c/library/alloc/src/boxed.rs:2070:9
        19: <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once
                   at /rustc/eeb90cda1969383f56a2637cbd3037bdf598841c/library/alloc/src/boxed.rs:2070:9
        20: std::sys::pal::unix::thread::Thread::new::thread_start
                   at /rustc/eeb90cda1969383f56a2637cbd3037bdf598841c/library/std/src/sys/pal/unix/thread.rs:108:17
        21: <unknown>
        22: <unknown>
  --> src/main.rs:38:41
   |
38 |     let node = Box::new(Node::new(Noop, vec![Box::new(Node::new(Noop, vec![]))]));
   |                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   = note: this error: internal compiler error originates in the macro `vec` (in Nightly builds, run with -Z macro-backtrace for more info)

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 1.81.0 (eeb90cda1 2024-09-04) running on x86_64-unknown-linux-gnu

note: compiler flags: --crate-type bin -C embed-bitcode=no -C debuginfo=2 -C incremental=[REDACTED]

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

query stack during panic:
end of query stack
warning: `my_test` (bin "my_test") generated 2 warnings
error: could not compile `my_test` (bin "my_test"); 2 warnings emitted

Contributor guide

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First steps

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  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Reproduce the minimal example from the issue in src/lib.rs or src/main.rs, then start at rustc_borrowck/src/type_check/mod.rs:2570 and follow the MIR borrow-checking path shown in the query stack. The issue is resolved when this HRTB-over-GAT trait-object example no longer produces a broken-MIR NoSolution internal compiler error.

Written by the indexing model from the issue text.

Assessment

Tech stack
rust
Domain
compilers
Issue type
Bug
Difficulty
5/5
Estimated time
Over a week
Activity status
Stale
Clarity
Needs clarification
Newbie friendliness
25/100

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