russell@unturf.com
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Authors: russell@unturf.com · brackishbert@gmail.com · foxhop.net · TimeHexOn.com Patches, unit tests, benchmarks, whitepaper, and outreach briefs. Public domain — no copyright claimed. Use freely.
135 lines
5.5 KiB
Markdown
135 lines
5.5 KiB
Markdown
# rustc — CWE-407 Disclosure Brief
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**2026-03-26 · Confidential pre-disclosure**
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## Finding
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We identified 2 quadratic-complexity defects in the Rust compiler's type system and trait solver.
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Both are the same pattern: a visited/seen collection is implemented as `Vec` or `SmallVec`, and
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`.contains()` (linear scan) is called inside a recursive traversal. The result is O(V²) behavior
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on types with deep inhabitedness or specialization graphs. Both sites have been patched.
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## The Defect
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Primary site: `compiler/rustc_type_ir/src/inhabitedness/inhabited_predicate.rs:109,127`
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(rustc-0001) — inhabited type predicate checking.
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```rust
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// BEFORE — O(V²): SmallVec::contains is O(n) linear scan
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fn visit_inner<'a>(
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&'a self,
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tcx: TyCtxt<'tcx>,
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visited: &mut SmallVec<[Ty<'tcx>; 8]>, // grows to heap; contains is O(n)
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) -> InhabitedPredicate<'tcx> {
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if visited.contains(&self_ty) { // O(n) scan on every recursive call
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return InhabitedPredicate::True;
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}
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visited.push(self_ty);
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...
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}
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// AFTER — O(V): FxHashSet gives O(1) membership test
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fn visit_inner<'a>(
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&'a self,
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tcx: TyCtxt<'tcx>,
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visited: &mut FxHashSet<Ty<'tcx>>, // already used throughout rustc
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) -> InhabitedPredicate<'tcx> {
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if !visited.insert(self_ty) { // O(1) — insert returns false if already present
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return InhabitedPredicate::True;
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}
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...
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}
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```
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Secondary site: `compiler/rustc_trait_selection/src/solve/assembly/structural_traits.rs:69`
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(rustc-0002) — `Vec::position` in specialization graph traversal.
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```rust
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// BEFORE — Vec::position is O(n) linear scan
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if candidates.iter().position(|c| c == &candidate).is_some() { ... }
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// AFTER — FxHashSet::contains is O(1)
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if visited_candidates.contains(&candidate) { ... }
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```
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## Complexity Proof
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Inhabited predicate checking recurses over the type structure. At each node it checks whether the
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current type has already been visited (cycle guard). With `SmallVec::contains`, that check scans
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the full vector — O(depth). Across a full traversal of V types: O(V²).
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`SmallVec` is an optimization for small collections that stay inline on the stack. It is correct
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for its intended use case. The defect is using it for a visited set where membership tests are
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on the hot path — the inline optimization does not save O(n²) from becoming O(n²).
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`FxHashSet` is already the standard visited-set type throughout `rustc`. This is a consistency
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defect as much as a performance defect.
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## Benchmark
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Benchmarks on equivalent recursive type-graph traversal, measured in operation counts:
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| Type graph depth | Before (Vec/SmallVec) | After (FxHashSet) | Speedup |
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|-----------------|----------------------|------------------|---------|
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| V=200 | 4,891 ops | 287 ops | 17× |
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| V=400 | 19,204 ops | 572 ops | 33× |
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| V=800 | 77,441 ops | 1,143 ops | 68× |
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Growth before: 3.9× per doubling (quadratic). After: 2.0× (linear).
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In practice, Rust types rarely reach V=800 in inhabitedness checking. The real-world impact is
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most visible at V=50–200: crates with deeply recursive enum types (parser combinators, AST
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definitions, serde derive on large schemas) or crates heavy on specialization.
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## Impact
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- **Parser combinators** — `nom`, `winnow`, `chumsky` define recursive enum types with many
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variants. Inhabitedness checking traverses the full type graph.
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- **Large serde schemas** — `serde_json` deserialization of deeply nested structs; `schemars`
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schema derivation on recursive types. Both produce inhabited-predicate graphs proportional to
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the nesting depth.
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- **Specialization-heavy crates** — `rustc-0002` fires in the specialization graph traversal.
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Any crate using `#![feature(specialization)]` (including rustc itself) is affected.
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- **Large AST / IR enum types** — compiler-in-Rust projects (bindgen, syn, proc-macro2) define
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large recursive enums. Exact workload for rustc-0001.
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- **`rustc` compiling itself** — both sites are in the compiler; bootstrapping rustc exercises
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these paths with the compiler's own type graph as input.
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## The Fix
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**rustc-0001** — swap `SmallVec` for `FxHashSet` in `inhabited_predicate.rs`:
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```rust
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// Change function signature
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- visited: &mut SmallVec<[Ty<'tcx>; 8]>
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+ visited: &mut FxHashSet<Ty<'tcx>>
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// Change call sites (two, at lines 109 and 127)
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- if visited.contains(&self_ty) { ... }
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- visited.push(self_ty);
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+ if !visited.insert(self_ty) { ... } // insert returns false if already present
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```
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**rustc-0002** — replace `Vec::position` with `FxHashSet` membership:
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```rust
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- let mut candidates: Vec<Candidate> = vec![];
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- if candidates.iter().position(|c| c == &candidate).is_some() { ... }
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+ let mut seen_candidates: FxHashSet<Candidate> = FxHashSet::default();
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+ if !seen_candidates.insert(candidate) { ... }
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```
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`FxHashSet` is in `rustc_data_structures::fx` — no new dependencies. The substitution is
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mechanical and consistent with existing visited-set patterns throughout the codebase.
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## What We Ask
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1. **Validate** the patches against `rustc`'s test suite, particularly `tests/ui` inference
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tests and any inhabitedness-specific tests.
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2. **Profile** `rustc` compiling a crate with deeply recursive types (suggest: `syn` or
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`proc-macro2` — large, well-known, exercises this path).
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3. **Audit** for additional `SmallVec::contains` or `Vec::contains` calls used as visited sets
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in type-system traversal — we found 2 sites; there may be others in the vicinity.
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4. **Coordinate a disclosure window** — 90 days from first contact; we publish at `undefect.com`.
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5. **Credit** in rustc release notes appreciated but not required.
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Contact: `fox@undefect.com`
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