Opaque type collection: Guard against endlessly recursing free alias types #143793
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fmease
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Jul 11, 2025
| @@ -223,7 +223,10 @@ impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for OpaqueTypeCollector<'tcx> { | |||
| } | |||
| // Skips type aliases, as they are meant to be transparent. | |||
| // FIXME(type_alias_impl_trait): can we require mentioning nested type aliases explicitly? | |||
| ty::Alias(ty::Free, alias_ty) if alias_ty.def_id.is_local() => { | |||
| ty::Alias(ty::Free, alias_ty) if let Some(def_id) = alias_ty.def_id.as_local() => { | |||
| if !self.seen.insert(def_id) { | |||
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Of course, this only guards against cyclic alias types, not against "overly deep" ones. It's within the realm of possibility that this still overflows the stack on inputs like type T${n} = T${n+1}; type T${N} = (); where 0 ≤ n < N where N is large (like >10_000 which is the maximum I tested and which took a while to compile but didn't exhibit a stack overflow).
However, rustc struggles on such inputs anyway today whether eager or lazy (I already found a bunch of boring hangs).
Keeping track of depth instead of visited types would address that but I didn't want to make large modifications to this collector. I thought about utilizing tcx.expand_free_alias_tys in appropriate places which can deal with such inputs. However, that might be incompatible with // FIXME(type_alias_impl_trait): can we require mentioning nested type aliases explicitly? which I guess is referring to constructions like:
#![feature(type_alias_impl_trait)]
// #[define_opaque(Inner)] // <-- (?)
type Outer = Inner;
type Inner = impl Sized;
// ...or sth. like that >.<
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Yea, those are fine. I think we can address generated worst-case input code situations as they occur. Most likely we have enough ensure_sufficient_stack in place already.
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@bors r+ rollup |
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Opaque type collection: Guard against endlessly recursing free alias types See test description for technical details. Fixes rust-lang#131994. r? oli-obk (sry, your queue is large, so no rush & feel free to reassign)
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Opaque type collection: Guard against endlessly recursing free alias types See test description for technical details. Fixes rust-lang#131994. r? oli-obk (sry, your queue is large, so no rush & feel free to reassign)
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Opaque type collection: Guard against endlessly recursing free alias types See test description for technical details. Fixes rust-lang#131994. r? oli-obk (sry, your queue is large, so no rush & feel free to reassign)
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Rollup of 11 pull requests Successful merges: - #143595 (add `const_make_global`; err for `const_allocate` ptrs if didn't call) - #143678 (Added error for invalid char cast) - #143793 (Opaque type collection: Guard against endlessly recursing free alias types) - #143820 (Fixed a core crate compilation failure when enabling the `optimize_for_size` feature on some targets) - #143829 (Trim `BorrowedCursor` API) - #143856 (Linting public reexport of private dependencies) - #143891 (Port `#[coverage]` to the new attribute system) - #143914 (Reword mismatched-lifetime-syntaxes text based on feedback) - #143922 (Improve path segment joining) - #143926 (Remove deprecated fields in bootstrap) - #143975 (type_id_eq: check that the hash fully matches the type) r? `@ghost` `@rustbot` modify labels: rollup
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Rollup of 11 pull requests Successful merges: - #143326 (Remove deprecated `Error::description` impl from `c_str::FromBytesWithNulError`) - #143431 (Use relative visibility when noting sealed trait to reduce false positive) - #143550 (resolve: Use interior mutability for extern module map) - #143631 (update to literal-escaper-0.0.5) - #143793 (Opaque type collection: Guard against endlessly recursing free alias types) - #143880 (tests: Test line debuginfo for linebreaked function parameters) - #143914 (Reword mismatched-lifetime-syntaxes text based on feedback) - #143926 (Remove deprecated fields in bootstrap) - #143955 (Make frame spans appear on a separate trace line) - #143975 (type_id_eq: check that the hash fully matches the type) - #143984 (Fix ice for feature-gated `cfg` attributes applied to the crate) r? `@ghost` `@rustbot` modify labels: rollup
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See test description for technical details.
Fixes #131994.
r? oli-obk (sry, your queue is large, so no rush & feel free to reassign)