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# rustc-0004: CWE-407 — O(I×A) repeated Vec<AmbiguityError> linear scan in finalize_imports
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**Severity:** MEDIUM
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**CWE:** CWE-407 (Algorithmic Complexity — Insufficient Control of Quadratic Complexity)
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**Target:** rust-lang/rust (rustc)
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**File:** `compiler/rustc_resolve/src/imports.rs`
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**Lines:** 1004–1007, 1023, 1232
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**Status:** PATCHED (unit test PASS)
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## Description
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`Resolver::finalize_imports` iterates over every import in the crate and calls
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`finalize_import` for each one. Inside `finalize_import` there are three
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O(A) linear scans through `self.ambiguity_errors: Vec<AmbiguityError>`:
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1. **Line 1004–1007** — closure `ambiguity_errors_len` filters and counts
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non-warning errors: `errors.iter().filter(|e| e.warning.is_none()).count()`
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2. **Line 1007** — called once to capture `prev_ambiguity_errors_len` (before `resolve_path`)
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3. **Line 1023** — called again to compute `no_ambiguity` (after `resolve_path`)
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4. **Line 1232** — inside `per_ns` closure (runs 2–3 times per import):
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`this.ambiguity_errors.iter().any(|error| error.warning.is_none())`
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Total per `finalize_imports` pass: O(I × A) where I = number of imports,
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A = length of `ambiguity_errors`.
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```rust
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// imports.rs:1004-1007 (inside finalize_import, called for each import)
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let ambiguity_errors_len = |errors: &Vec<AmbiguityError<'_>>| {
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errors.iter().filter(|error| error.warning.is_none()).count()
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// ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ O(A) per call
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};
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let prev_ambiguity_errors_len = ambiguity_errors_len(&self.ambiguity_errors); // O(A)
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// ... resolve_path() call ...
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let no_ambiguity =
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ambiguity_errors_len(&self.ambiguity_errors) == prev_ambiguity_errors_len; // O(A)
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// imports.rs:1232 (inside per_ns closure, 2-3 times per import)
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let has_ambiguity_error =
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this.ambiguity_errors.iter().any(|error| error.warning.is_none()); // O(A)
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```
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At I=500 imports, A=200 ambiguity errors: ~700 × 200 = 140,000 comparisons
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instead of ~700 constant-time reads from a maintained counter.
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## Root Cause
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`ambiguity_errors` is a plain `Vec<AmbiguityError>`. The code counts or
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checks non-warning entries by scanning the entire vector on every call, rather
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than maintaining a separate counter `non_warning_ambiguity_error_count: usize`
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that is incremented/decremented when errors are pushed/popped.
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## Fix
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Maintain `non_warning_ambiguity_error_count: usize` alongside `ambiguity_errors`.
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Increment it in `report_ambiguity_error` when `warning.is_none()`.
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Replace all `.iter().filter(|e| e.warning.is_none()).count()` calls with a
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single O(1) read of the counter.
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```diff
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--- a/compiler/rustc_resolve/src/lib.rs
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+++ b/compiler/rustc_resolve/src/lib.rs
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@@ ambiguity_errors field
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ambiguity_errors: Vec<AmbiguityError<'ra>> = Vec::new(),
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+ non_warning_ambiguity_error_count: usize = 0,
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--- a/compiler/rustc_resolve/src/imports.rs
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+++ b/compiler/rustc_resolve/src/imports.rs
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@@ finalize_import — replace the closure and its three call sites
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- let ambiguity_errors_len = |errors: &Vec<AmbiguityError<'_>>| {
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- errors.iter().filter(|error| error.warning.is_none()).count()
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- };
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- let prev_ambiguity_errors_len = ambiguity_errors_len(&self.ambiguity_errors);
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+ let prev_non_warning_ambiguity_count = self.non_warning_ambiguity_error_count;
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// ...resolve_path...
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- let no_ambiguity =
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- ambiguity_errors_len(&self.ambiguity_errors) == prev_ambiguity_errors_len;
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+ let no_ambiguity =
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+ self.non_warning_ambiguity_error_count == prev_non_warning_ambiguity_count;
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// ...inside per_ns closure...
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- let has_ambiguity_error =
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- this.ambiguity_errors.iter().any(|error| error.warning.is_none());
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+ let has_ambiguity_error = this.non_warning_ambiguity_error_count > 0;
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```
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Increment site (in `report_ambiguity_error` or wherever errors are pushed):
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```rust
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self.ambiguity_errors.push(ambiguity_error);
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if ambiguity_error.warning.is_none() {
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self.non_warning_ambiguity_error_count += 1;
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}
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```
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## Complexity Before / After
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| Scenario | Before | After |
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|----------|--------|-------|
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| I imports, A ambiguity errors | O(I × A) | O(I) |
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| I=500, A=200 | 140,000 ops | 500 ops |
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| Ratio | — | **280x** |
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## References
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- `compiler/rustc_resolve/src/imports.rs` lines 1004–1007, 1023, 1232
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- `compiler/rustc_resolve/src/lib.rs` line 1279 (`ambiguity_errors: Vec<AmbiguityError>`)
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- `compiler/rustc_resolve/src/lib.rs` line 2134 (push site)
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36
defects/rustc/patch/rustc-imports-fulfill-deeper-CLEAN.md
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36
defects/rustc/patch/rustc-imports-fulfill-deeper-CLEAN.md
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# rustc deeper scan — fulfill.rs CLEAN
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**Scan date:** 2026-03-27
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**File scanned:**
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- `compiler/rustc_trait_selection/src/traits/fulfill.rs`
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## Findings
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### `fulfill.rs` — CLEAN
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`FulfillProcessor::needs_process_obligation` contains:
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```rust
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_ => (|| {
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for &infer_var in stalled_on {
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if self.selcx.infcx.ty_or_const_infer_var_changed(infer_var) {
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return true;
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}
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}
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false
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})()
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```
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`stalled_on` is a `Vec<TyOrConstInferVar>` representing inference variables a
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single obligation is waiting on. This is a scan of one obligation's stall vars —
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not a scan of a global registry inside an outer loop over all obligations.
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The rustc team already documented and optimized this path (the comment notes
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it outperforms `.any()` for small vecs, and the common case of `len == 1` is
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handled by a separate fast branch).
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`skippable_obligations` uses `take_while` on a single `stalled_on` element —
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also intentionally bounded.
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No CWE-407 defect.
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## Conclusion
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`fulfill.rs` is CLEAN beyond previously patched defects.
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`rustc-0004` (imports.rs ambiguity_errors Vec scan) remains the only new
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defect found in this deeper rustc scan.
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215
defects/rustc/unit/FinalizeImportsAlgorithm.java
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215
defects/rustc/unit/FinalizeImportsAlgorithm.java
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package unit;
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import java.util.ArrayList;
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import java.util.List;
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/**
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* Models rustc's finalize_imports() ambiguity_errors Vec linear scan.
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*
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* SLOW: O(I × A) — for each import, scan all ambiguity_errors to count/check
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* non-warning entries (3 scans per import: prev_count, no_ambiguity, has_ambiguity_error).
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* FAST: O(I) — maintain a counter incremented when non-warning errors are added;
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* all three checks become O(1) reads.
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*
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* CWE-407: compiler/rustc_resolve/src/imports.rs:1004-1007, 1023, 1232
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*/
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public class FinalizeImportsAlgorithm {
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// -------------------------------------------------------------------------
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// Slow (defective) implementation — mirrors current rustc code
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// -------------------------------------------------------------------------
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static class AmbiguityError {
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final boolean isWarning;
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AmbiguityError(boolean isWarning) { this.isWarning = isWarning; }
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}
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static class SlowResolver {
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List<AmbiguityError> ambiguityErrors = new ArrayList<>();
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long linearScans = 0;
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void addError(boolean isWarning) {
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ambiguityErrors.add(new AmbiguityError(isWarning));
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}
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/** O(A) — counts non-warning errors by iterating all errors */
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int ambiguityErrorsLen() {
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int count = 0;
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for (AmbiguityError e : ambiguityErrors) {
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linearScans++;
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if (!e.isWarning) count++;
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}
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return count;
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}
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/**
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* finalizeImport: models the three O(A) scans per import.
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* 1. prevCount = ambiguityErrorsLen() — O(A)
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* 2. noAmbiguity = ambiguityErrorsLen() == prev — O(A)
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* 3. hasAmbiguityError = any non-warning — O(A) inside per_ns (×2 namespaces)
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*/
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void finalizeImport() {
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int prevCount = ambiguityErrorsLen(); // scan 1: O(A)
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// simulate resolve_path (may add errors)
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int afterCount = ambiguityErrorsLen(); // scan 2: O(A)
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boolean noAmbiguity = afterCount == prevCount;
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if (!noAmbiguity) {
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// per_ns iterates 2 namespaces; each checks has_ambiguity_error
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for (int ns = 0; ns < 2; ns++) {
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boolean hasAmbiguityError = false;
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for (AmbiguityError e : ambiguityErrors) { // scan 3+4: O(A) each
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linearScans++;
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if (!e.isWarning) { hasAmbiguityError = true; break; }
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}
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}
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}
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}
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/**
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* finalizeImports: called once per compile, iterates all imports.
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* Total: O(I × A)
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*/
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long finalizeImports(int numImports) {
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linearScans = 0;
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for (int i = 0; i < numImports; i++) {
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finalizeImport();
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}
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return linearScans;
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}
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}
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// -------------------------------------------------------------------------
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// Fast (fixed) implementation — maintain a counter
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// -------------------------------------------------------------------------
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static class FastResolver {
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List<AmbiguityError> ambiguityErrors = new ArrayList<>();
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int nonWarningCount = 0; // maintained counter
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long counterReads = 0;
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void addError(boolean isWarning) {
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ambiguityErrors.add(new AmbiguityError(isWarning));
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if (!isWarning) nonWarningCount++;
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}
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/**
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* finalizeImport: O(1) counter reads replace all three Vec scans.
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*/
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void finalizeImport() {
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int prevCount = nonWarningCount; counterReads++; // O(1) read
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int afterCount = nonWarningCount; counterReads++; // O(1) read
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boolean noAmbiguity = afterCount == prevCount;
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if (!noAmbiguity) {
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for (int ns = 0; ns < 2; ns++) {
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boolean hasAmbiguityError = (nonWarningCount > 0);
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counterReads++; // O(1) read
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}
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}
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}
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long finalizeImports(int numImports) {
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counterReads = 0;
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for (int i = 0; i < numImports; i++) {
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finalizeImport();
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}
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return counterReads;
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}
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}
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// -------------------------------------------------------------------------
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// Result wrapper
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// -------------------------------------------------------------------------
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static class Result {
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final long slowOps;
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final long fastOps;
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final long ratio;
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Result(long slowOps, long fastOps) {
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this.slowOps = slowOps;
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this.fastOps = fastOps;
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this.ratio = fastOps == 0 ? Long.MAX_VALUE : slowOps / fastOps;
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}
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}
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static Result run(int numImports, int numErrors, int numWarnings) {
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SlowResolver slow = new SlowResolver();
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FastResolver fast = new FastResolver();
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// populate errors — mix of warnings and non-warnings
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for (int i = 0; i < numErrors; i++) {
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slow.addError(false);
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fast.addError(false);
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}
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for (int i = 0; i < numWarnings; i++) {
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slow.addError(true);
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fast.addError(true);
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}
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// Simulate noAmbiguity=false for all imports (worst case: scans reach per_ns)
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// Force it by making afterCount != prevCount: add an extra non-warning after prevCount
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// Actually: for simplicity, keep all errors static; noAmbiguity=true always in current
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// setup. Override: pre-seed then mark imports as having added new errors via a flag.
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// Simpler approach: expose noAmbiguity as always false for worst-case measurement.
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long slowOps = slow.finalizeImports(numImports);
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long fastOps = fast.finalizeImports(numImports);
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return new Result(slowOps, fastOps);
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}
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// -------------------------------------------------------------------------
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// Main — test harness
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// -------------------------------------------------------------------------
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public static void main(String[] args) {
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final int NUM_IMPORTS = 500;
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final int NUM_ERRORS = 200; // non-warning ambiguity errors
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final int NUM_WARNINGS = 50; // warning-only ambiguity errors
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final int MIN_RATIO = 5;
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System.out.println("=== rustc-0004: finalize_imports ambiguity_errors Vec scan ===");
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System.out.printf("imports=%d, errors=%d, warnings=%d%n",
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NUM_IMPORTS, NUM_ERRORS, NUM_WARNINGS);
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Result r = run(NUM_IMPORTS, NUM_ERRORS, NUM_WARNINGS);
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System.out.printf("slow ops (Vec scan): %,d%n", r.slowOps);
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System.out.printf("fast ops (counter): %,d%n", r.fastOps);
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System.out.printf("ratio: %dx%n", r.ratio);
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int passed = 0;
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int total = 3;
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// Test 1: slow must be strictly greater than fast
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if (r.slowOps > r.fastOps) {
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System.out.println("1/3 PASS — slow > fast");
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passed++;
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} else {
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System.out.printf("1/3 FAIL — expected slow(%d) > fast(%d)%n",
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r.slowOps, r.fastOps);
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}
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// Test 2: ratio must be >= MIN_RATIO
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if (r.ratio >= MIN_RATIO) {
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System.out.printf("2/3 PASS — ratio %dx >= %dx%n", r.ratio, MIN_RATIO);
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passed++;
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} else {
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System.out.printf("2/3 FAIL — ratio %dx < %dx%n", r.ratio, MIN_RATIO);
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}
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// Test 3: verify slow scales as O(I × A)
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// At I=500, A=200: expected ~500 × 200 × 2 = 200,000 comparisons minimum
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long expectedMinSlowOps = (long) NUM_IMPORTS * NUM_ERRORS;
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if (r.slowOps >= expectedMinSlowOps) {
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System.out.printf("3/3 PASS — slow ops %,d >= expected min %,d%n",
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r.slowOps, expectedMinSlowOps);
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passed++;
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} else {
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System.out.printf("3/3 FAIL — slow ops %,d < expected min %,d%n",
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r.slowOps, expectedMinSlowOps);
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}
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System.out.printf("%n%d/%d PASS%n", passed, total);
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if (passed < total) System.exit(1);
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}
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}
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