delegation: OOM #155060

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1. matthiaskrgr

   added

   I-compilememIssue: Problems and improvements with respect to memory usage during compilation.Issue: Problems and improvements with respect to memory usage during compilation.

   C-bugCategory: This is a bug.Category: This is a bug.

   F-fn_delegation`#![feature(fn_delegation)]``#![feature(fn_delegation)]`

   on Apr 9, 2026
2. rustbot

   added

   needs-triageThis issue may need triage. Remove when done. See docs forge.rust-lang.org/release/issue-triagingThis issue may need triage. Remove when done. See docs forge.rust-lang.org/release/issue-triaging

   on Apr 9, 2026
3. spqr79 commented on Apr 9, 2026

   

   spqr79

   on Apr 9, 2026 · Hidden as low-quality

   show commentMore actions
4. aerooneqq commented on Apr 10, 2026

   

   aerooneqq

   on Apr 10, 2026

   Contributor

   More actions

   #154389 will fix OOM, the problem that delayed lowering of delegations causes query cycles will be hopefully soon fixed with more infrastructural solution.
5. petrochenkov commented on Apr 15, 2026

   

   petrochenkov

   on Apr 15, 2026

   Contributor

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   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`.
   
6. aerooneqq commented on Apr 22, 2026

   

   aerooneqq

   on Apr 22, 2026

   Contributor

   More actions

   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() {}
7. jieyouxu

   added

   T-compilerRelevant to the compiler team, which will review and decide on the PR/issue.Relevant to the compiler team, which will review and decide on the PR/issue.

   and removed

   needs-triageThis issue may need triage. Remove when done. See docs forge.rust-lang.org/release/issue-triagingThis issue may need triage. Remove when done. See docs forge.rust-lang.org/release/issue-triaging

   on Oct 7, 2026
8. matthiaskrgr

   marked [ICE]: error: internal compiler error: query cycle when printing cycle detected / when getting HIR crate items #165127 as a duplicate of this issue on Oct 29, 2026
9. matthiaskrgr

   added

   I-cycleIssue: A query cycle occurred while none was expectedIssue: A query cycle occurred while none was expected

   on Oct 29, 2026