delegation: OOM

[matthiaskrgr]

I tried this code:

#![feature(fn_delegation)]
trait Trait<T> {
    fn foo<U>(&self, _: dyn Trait) {}
}

reuse Trait::<_>::foo::<i32> as x;

fn main() {}

I expected to see this happen:
no oom

Instead, this happened:
oom

Meta

rustc --version --verbose:

rustc 1.96.0-nightly (a87c9b960 2026-04-09)
binary: rustc
commit-hash: a87c9b96031d4d8698bb0cd6533e83bc6d77ddaa
commit-date: 2026-04-09
host: x86_64-unknown-linux-gnu
release: 1.96.0-nightly
LLVM version: 22.1.2

Backtrace

   8,68%  librustc_driver-5ecf1647954ff888.so             [.] rustc_query_impl::execution::collect_active_query_jobs
   4,59%  librustc_driver-5ecf1647954ff888.so             [.] rustc_query_impl::execution::collect_active_query_jobs_inner::<rustc_middle::query::caches::DefIdCache<rustc_middle::query::erase::ErasedData<[u8; 16]>>>
   3,57%  librustc_driver-5ecf1647954ff888.so             [.] rustc_query_impl::execution::collect_active_query_jobs_inner::<rustc_data_structures::vec_cache::VecCache<rustc_span::def_id::LocalDefId, rustc_middle::query::erase::ErasedData<[u8; 0]>, rustc_middle::dep_graph::graph::DepNodeIndex>>

[spqr79]

The OOM reproduces on a87c9b9 (1.96.0-nightly, 2026-04-09).
The root cause is two things combining: dyn Trait in the signature of foo is missing its type argument (T), which should be caught as E0107 — but the delegation desugaring path apparently bypasses that well-formedness check. Without the early rejection, the signature-building logic recurses into the self-referential dyn Trait indefinitely and exhausts memory.
Two-part fix in rustc_hir_analysis (delegation lowering):

Before build_delegation_sig runs, validate that any dyn Trait<…> in the source signature has all required type arguments bound. If not, emit E0107 and bail. This is the check that's being skipped in the delegation path.
Even with (1) in place, worth adding an occurs-check before substitution in the signature copy path — if the inferred _ in Trait::<_> resolves back to the same trait, error out rather than expand.

The regression test should go in tests/ui/delegation/ and expect a hard compiler error, not OOM:
rust// tests/ui/delegation/no-oom-self-referential-dyn.rs
#![feature(fn_delegation)]
trait Trait {
fn foo(&self, : dyn Trait) {} //~ ERROR E0107
}
reuse Trait::<>::foo:: as x;
fn main() {}

The fix should be scoped strictly to the delegation lowering path — touching the general trait solver here risks regressions on stable.
No backtrace in the report — if someone wants to pin the exact call frame before sending a PR, adding #![recursion_limit = "10"] to the reproducer should get a stack trace before the process runs out of memory. -Zverbose-internals may also help narrow which query is expanding.

[aerooneqq]

#154389 will fix OOM, the problem that delayed lowering of delegations causes query cycles will be hopefully soon fixed with more infrastructural solution.

[petrochenkov]

Output now:

error: internal compiler error: query cycle when printing cycle detected
  |
  = note: ...when lowering the delayed AST owner `x`
note: ...which requires computing function signature of `Trait::foo`...
 --> src/main.rs:5:5
  |
5 |     fn foo<U>(&self, _: dyn Trait) {}
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  = note: ...which requires calculating trimmed def paths...
  = note: ...which requires looking up span for `x`...
  = note: ...which requires getting HIR ID of `x`...
  = note: ...which again requires lowering the delayed AST owner `x`, completing the cycle
  = note: cycle used when getting HIR crate items
  = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information

error[E0391]: cycle detected when getting HIR crate items
  |
  = note: ...which requires lowering the delayed AST owner `x`...
note: ...which requires computing function signature of `Trait::foo`...
 --> src/main.rs:5:5
  |
5 |     fn foo<U>(&self, _: dyn Trait) {}
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  = note: ...which requires calculating trimmed def paths...
  = note: ...which again requires getting HIR crate items, completing the cycle
  = note: cycle used when running analysis passes on crate `playground`
  = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information

For more information about this error, try `rustc --explain E0391`.

[aerooneqq]

Other examples from #154169 (will not reopen it, let this issue track the problem):

#![feature(fn_delegation)]
#![allow(incomplete_features)]

trait Trait<T> {
    fn foo(&self, x: T) -> S { x }
}
struct F;

struct S<T>(F, T);

impl<T, U> Trait<T> for S<U> {
    reuse to_reuse::foo { &self.0 }
}

fn main() {
    let s = S(F, 42);
    assert_eq!(S::<i32>::foo(F, 1), 1);
}

#![feature(fn_delegation)]
#![allow(incomplete_features)]

trait Trait {
    fn foo() -> Self::Assoc;
    fn bar(&self) -> u8;
}

impl Trait for u8 {
    fn bar(&self) -> u8 { 1 }
}

struct S(u8);

impl Trait for S {
    reuse Trait::* { &self.0 }
    fn bar(&self) -> u8 { 2 }
}

fn main() {}

#![feature(fn_delegation)]
#![allow(incomplete_features)]

trait Trait {
    fn foo(&self) -> Self::Assoc<3> {
        [(); 3]
    }
}

impl () {
    reuse Trait::*;
}

fn main() {}