Hang in custom Vec

[ym-xie]

I tried this code:

struct Array<T> {
    data: Vec<T>
}
fn sum_arrays<T>(first: Array<T>) -> Array<T> {
    let mut result = Array {data: first.data.clone()};
    result.data.push(8);
    result
}
struct Vec<T> {data: Vec<T> }
impl<T> Vec<T> {
    pub fn push(mut self: &mut Self, object: &T) -> &mut Self {
        self
    }
}

Command: timeout 30s rustc file.rs

It did not finish within 30 seconds.

Meta

rustc --version --verbose:

rustc 1.93.0-nightly (c90bcb957 2025-11-06)
binary: rustc
commit-hash: c90bcb9571b7aab0d8beaa2ce8a998ffaf079d38
commit-date: 2025-11-06
host: aarch64-apple-darwin
release: 1.93.0-nightly
LLVM version: 21.1.3

Backtrace

<no backtrace>

[ym-xie]

@rustbot label I-hang

[saethlin]

Smaller:

struct Vec<T> {
    data: Vec<T>,
}
impl<T> Vec<T> {
    pub fn push(&mut self) -> &mut Self {
        self
    }
}

And this is fixed by next-solver.

[JonathanBrouwer]

Did a bit of bisection, nightly-2025-04-02 works, nightly-2025-04-03 hangs.
Don't have time to look into it further atm

That leaves these commits:

  commit[0] 2025-04-01: Auto merge of #138928 - ChrisDenton:fix-uwp, r=tgross35
  commit[1] 2025-04-01: Auto merge of #139220 - matthiaskrgr:rollup-v1un5wz, r=matthiaskrgr
  commit[2] 2025-04-02: Auto merge of #138478 - nnethercote:rm-NtExpr-NtLiteral, r=petrochenkov
  commit[3] 2025-04-02: Auto merge of #139229 - Zalathar:rollup-5cs3f4d, r=Zalathar
  commit[4] 2025-04-02: Auto merge of #138848 - clubby789:cargo-update, r=Mark-Simulacrum
  commit[5] 2025-04-02: Auto merge of #139018 - oli-obk:incremental-trait-impls, r=compiler-errors
  commit[6] 2025-04-02: Auto merge of #139257 - TaKO8Ki:rollup-vjzdas7, r=TaKO8Ki
  commit[7] 2025-04-02: Auto merge of #139269 - matthiaskrgr:rollup-pk78gig, r=matthiaskrgr

[estebank]

It gets stuck in rustc_trait_selection::traits::normalize::normalize_with_depth_to depth=2, value=Vec<T/#0>.

Details

error[E0072]: recursive type `Vec` has infinite size
 --> f18.rs:1:1
  |
1 | struct Vec<T> {
  | ^^^^^^^^^^^^^
2 |     data: Vec<T>,
  |           ------ recursive without indirection
  |
help: insert some indirection (e.g., a `Box`, `Rc`, or `&`) to break the cycle
  |
2 |     data: Box<Vec<T>>,
  |           ++++      +

 rustc_trait_selection::traits::normalize::normalize_with_depth_to depth=0, value=Vec<T/#0>
 rustc_trait_selection::traits::normalize::normalize_with_depth_to depth=0, value=Binder { value: TraitPredicate(<T as std::marker::Sized>, polarity:Positive), bound_vars: [] }
 rustc_trait_selection::traits::wf::clause_obligations param_env=ParamEnv { caller_bounds: [Binder { value: TraitPredicate(<T as std::marker::Sized>, polarity:Positive), bound_vars: [] }] }, body_id=DefId(0:3 ~ f18[3802]::Vec), clause=Binder { value: TraitPredicate(<T as std::marker::Sized>, polarity:Positive), bound_vars: [] }, span=f18.rs:1:12: 1:13 (#0)
   rustc_trait_selection::traits::normalize::normalize_with_depth_to depth=0, value=Binder { value: WellFormed(Term::Ty(T/#0)), bound_vars: [] }
   0ms  INFO rustc_trait_selection::traits::wf return=[Obligation(predicate=Binder { value: WellFormed(Term::Ty(T/#0)), bound_vars: [] }, depth=0)]
 rustc_trait_selection::traits::normalize::normalize_with_depth_to depth=1, value=Binder { value: TraitPredicate(<T as std::marker::Sized>, polarity:Positive), bound_vars: [] }
 rustc_infer::infer::lexical_region_resolve::collect_errors 
 rustc_trait_selection::traits::normalize::normalize_with_depth_to depth=0, value=Vec<T/#0>
 rustc_trait_selection::traits::normalize::normalize_with_depth_to depth=0, value=Binder { value: TraitPredicate(<T as std::marker::Sized>, polarity:Positive), bound_vars: [] }
 rustc_trait_selection::traits::wf::clause_obligations param_env=ParamEnv { caller_bounds: [Binder { value: TraitPredicate(<T as std::marker::Sized>, polarity:Positive), bound_vars: [] }] }, body_id=DefId(0:6 ~ f18[3802]::{impl#0}), clause=Binder { value: TraitPredicate(<T as std::marker::Sized>, polarity:Positive), bound_vars: [] }, span=f18.rs:4:6: 4:7 (#0)
   rustc_trait_selection::traits::normalize::normalize_with_depth_to depth=0, value=Binder { value: WellFormed(Term::Ty(T/#0)), bound_vars: [] }
   0ms  INFO rustc_trait_selection::traits::wf return=[Obligation(predicate=Binder { value: WellFormed(Term::Ty(T/#0)), bound_vars: [] }, depth=0)]
 rustc_trait_selection::traits::normalize::normalize_with_depth_to depth=1, value=Binder { value: TraitPredicate(<T as std::marker::Sized>, polarity:Positive), bound_vars: [] }
 rustc_trait_selection::traits::normalize::normalize_with_depth_to depth=0, value=Vec<T/#0>
 rustc_trait_selection::traits::normalize::normalize_with_depth_to depth=0, value=Vec<T/#0>
 rustc_trait_selection::traits::normalize::normalize_with_depth_to depth=0, value=Binder { value: TraitPredicate(<T as std::marker::Sized>, polarity:Positive), bound_vars: [] }
 rustc_infer::infer::lexical_region_resolve::collect_errors 
 rustc_trait_selection::traits::normalize::normalize_with_depth_to depth=0, value=&ReLateParam(DefId(0:8 ~ f18[3802]::{impl#0}::push), LateNamed(DefId(0:9 ~ f18[3802]::{impl#0}::push::'_))) mut Vec<T/#0>
 rustc_trait_selection::traits::normalize::normalize_with_depth_to depth=0, value=&ReLateParam(DefId(0:8 ~ f18[3802]::{impl#0}::push), LateNamed(DefId(0:9 ~ f18[3802]::{impl#0}::push::'_))) mut Vec<T/#0>
 rustc_trait_selection::traits::normalize::normalize_with_depth_to depth=0, value=Binder { value: TraitPredicate(<T as std::marker::Sized>, polarity:Positive), bound_vars: [] }
 rustc_trait_selection::traits::wf::clause_obligations param_env=ParamEnv { caller_bounds: [Binder { value: TraitPredicate(<T as std::marker::Sized>, polarity:Positive), bound_vars: [] }] }, body_id=DefId(0:8 ~ f18[3802]::{impl#0}::push), clause=Binder { value: TraitPredicate(<T as std::marker::Sized>, polarity:Positive), bound_vars: [] }, span=f18.rs:4:6: 4:7 (#0)
   rustc_trait_selection::traits::normalize::normalize_with_depth_to depth=0, value=Binder { value: WellFormed(Term::Ty(T/#0)), bound_vars: [] }
   0ms  INFO rustc_trait_selection::traits::wf return=[Obligation(predicate=Binder { value: WellFormed(Term::Ty(T/#0)), bound_vars: [] }, depth=0)]
 rustc_trait_selection::traits::normalize::normalize_with_depth_to depth=0, value=fn(&ReLateParam(DefId(0:8 ~ f18[3802]::{impl#0}::push), LateNamed(DefId(0:9 ~ f18[3802]::{impl#0}::push::'_))) mut Vec<T/#0>) -> &ReLateParam(DefId(0:8 ~ f18[3802]::{impl#0}::push), LateNamed(DefId(0:9 ~ f18[3802]::{impl#0}::push::'_))) mut Vec<T/#0>
 rustc_trait_selection::traits::normalize::normalize_with_depth_to depth=0, value=Vec<T/#0>
 rustc_trait_selection::traits::normalize::normalize_with_depth_to depth=0, value=&ReLateParam(DefId(0:8 ~ f18[3802]::{impl#0}::push), LateNamed(DefId(0:9 ~ f18[3802]::{impl#0}::push::'_))) mut Vec<T/#0>
 rustc_trait_selection::traits::normalize::normalize_with_depth_to depth=1, value=<&mut _ as std::ops::LegacyReceiver>
 rustc_trait_selection::traits::normalize::normalize_with_depth_to depth=1, value=<&mut _ as std::ops::LegacyReceiver>
 rustc_trait_selection::traits::normalize::normalize_with_depth_to depth=1, value=Binder { value: OutlivesPredicate(Vec<T/#0>, ReLateParam(DefId(0:8 ~ f18[3802]::{impl#0}::push), LateNamed(DefId(0:9 ~ f18[3802]::{impl#0}::push::'_)))), bound_vars: [] }
 rustc_trait_selection::traits::normalize::normalize_with_depth_to depth=1, value=Binder { value: TraitPredicate(<T as std::marker::Sized>, polarity:Positive), bound_vars: [] }
 rustc_trait_selection::traits::normalize::normalize_with_depth_to depth=1, value=<&mut _ as std::ops::LegacyReceiver>
 rustc_trait_selection::traits::normalize::normalize_with_depth_to depth=0, value=Vec<T/#0>
 rustc_trait_selection::traits::normalize::normalize_with_depth_to depth=0, value=&ReLateParam(DefId(0:8 ~ f18[3802]::{impl#0}::push), LateNamed(DefId(0:9 ~ f18[3802]::{impl#0}::push::'_))) mut Vec<T/#0>
 rustc_trait_selection::traits::normalize::normalize_with_depth_to depth=0, value=&ReLateParam(DefId(0:8 ~ f18[3802]::{impl#0}::push), LateNamed(DefId(0:9 ~ f18[3802]::{impl#0}::push::'_))) mut Vec<T/#0>
 rustc_trait_selection::traits::normalize::normalize_with_depth_to depth=0, value=&'?0 mut Vec<T/#0>
 rustc_trait_selection::traits::normalize::normalize_with_depth_to depth=0, value=Binder { value: OutlivesPredicate(Vec<T/#0>, '?0), bound_vars: [] }
 rustc_trait_selection::traits::normalize::normalize_with_depth_to depth=0, value=Binder { value: TraitPredicate(<T as std::marker::Sized>, polarity:Positive), bound_vars: [] }
 rustc_infer::infer::lexical_region_resolve::collect_errors 
 rustc_trait_selection::traits::normalize::normalize_with_depth_to depth=0, value=&ReLateParam(DefId(0:8 ~ f18[3802]::{impl#0}::push), LateNamed(DefId(0:9 ~ f18[3802]::{impl#0}::push::'_))) mut Vec<T/#0>
 rustc_trait_selection::traits::normalize::normalize_with_depth_to depth=0, value=&ReLateParam(DefId(0:8 ~ f18[3802]::{impl#0}::push), LateNamed(DefId(0:9 ~ f18[3802]::{impl#0}::push::'_))) mut Vec<T/#0>
 rustc_trait_selection::traits::normalize::normalize_with_depth_to depth=0, value=?0t
 rustc_hir_typeck::coercion::coerce a=&ReLateParam(DefId(0:8 ~ f18[3802]::{impl#0}::push), LateNamed(DefId(0:9 ~ f18[3802]::{impl#0}::push::'_))) mut Vec<T/#0>, b=&ReLateParam(DefId(0:8 ~ f18[3802]::{impl#0}::push), LateNamed(DefId(0:9 ~ f18[3802]::{impl#0}::push::'_))) mut Vec<T/#0>
   rustc_trait_selection::traits::normalize::normalize_with_depth_to depth=1, value=<&mut _ as std::ops::CoerceUnsized<&mut _>>
   rustc_trait_selection::traits::normalize::normalize_with_depth_to depth=1, value=<&mut _ as std::ops::CoerceUnsized<&mut _>>
   rustc_trait_selection::traits::normalize::normalize_with_depth_to depth=1, value=Binder { value: TraitPredicate(<_ as std::marker::Unsize<_>>, polarity:Positive), bound_vars: [] }
   rustc_trait_selection::traits::normalize::normalize_with_depth_to depth=2, value=Vec<T/#0>
   rustc_trait_selection::traits::normalize::normalize_with_depth_to depth=2, value=Vec<T/#0>
   rustc_trait_selection::traits::normalize::normalize_with_depth_to depth=2, value=Vec<T/#0>
   rustc_trait_selection::traits::normalize::normalize_with_depth_to depth=2, value=Vec<T/#0>
   rustc_trait_selection::traits::normalize::normalize_with_depth_to depth=2, value=Vec<T/#0>
   rustc_trait_selection::traits::normalize::normalize_with_depth_to depth=2, value=Vec<T/#0>
   rustc_trait_selection::traits::normalize::normalize_with_depth_to depth=2, value=Vec<T/#0>

Edit: I'm 99% sure that all we need to do is modify coerce_unsized_old_solver to add a check that the nested obligations do not match the current obligation.

rust/compiler/rustc_hir_typeck/src/coercion.rs

Line 795 in 843f8ce

Ok(Some(impl_source)) => queue.extend(impl_source.nested_obligations()),

Edit 2: #148677

[lcnr]

I feel like we probably should properly increment the recursion_depth of the obligations and have a recursion limit check here instead

[theemathas]

Code that causes this issue without having any errors first:

trait Trait {
    type Assoc;
}
impl<T> Trait for T {
    type Assoc = Thing<T>;
}

struct Thing<T>(<T as Trait>::Assoc);

fn foo<T>(x: &Thing<T>) -> &Thing<T> {
    x
}