feat: Evaluate flags that depend on other flags by emyller · Pull Request #44 · Flagsmith/flagsmith-rust-flag-engine

emyller · 2026-10-30T20:11:27Z

The Rust engine ignores segments that target flags by other flags, so the Rust SDK and edge-proxy-rs serve different flags than Flagsmith.

Closes SDK-11. Tested by engine-test-data v3.12.1.

Changes

Flags can depend on other flags being enabled, or having a given value or variant, including chains of dependencies.
Flags that depend on themselves report a circular dependency error.
Flag results report the variant served, and whether a flag has any targeting.
Environment flags can be evaluated without segments that need an identity.

🤖 Generated with Claude Code

serde_json_path keeps parsed paths private, while jsonpath-rust exposes them, so conditions can tell which flag a path reads. Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Segment conditions can read another flag's `enabled`, `value` or `variant` through `$.flags.<feature>`, as in flagsmith-engine. Flags in a dependency cycle get `ERROR; code=CIRCULAR_DEPENDENCY`. Also, as engine-test-data v3.12.1 expects: - Flag results report the selected multivariate variant. - Untargeted flags report `STATIC`, and targeted ones left at their default `DEFAULT`. - Segments evaluate in context order, which also breaks override priority ties. Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Environment flags can't resolve segments that need an identity, so `remove_identity_from_context` keeps only those reading the environment or other flags, as Flagsmith does for `/api/v1/flags`. Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

linear-code · 2026-10-30T20:11:34Z

SDK-11

khvn26 · 2026-10-30T21:27:50Z

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coderabbitai · 2026-10-30T21:27:58Z

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coderabbitai · 2026-10-30T21:28:05Z

Findings

Mark the FeatureValue.key addition as a breaking change, or keep the struct closed to external lite…
Memoise cyclic flag results, or bound cycle re-traversal.
Keep a segment whose ANY rule matches without relying on a cycle.
Populate the selected mapped variant key.
Treat the FlagResult field addition as a breaking API change.

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Inline comments:
Review comments at @src/engine_eval/context.rs:
- Line 35: **Mark the `FeatureValue.key` addition as a breaking change, or keep
the struct closed to external literals.**

`FeatureValue` is a public struct with all fields public and no
`#[non_exhaustive]`. Downstream crates that build it with a struct literal now
fail to compile, because the new `key` field is not supplied. Serde's `default`
does not apply to Rust struct literals. The in-repo literal in
`src/engine_eval/mappers.rs` already sets `key: None`, so the crate itself
builds. Release tooling derives versions from commit messages (`CHANGELOG.md`
shows release-please entries), so a non-breaking `feat:` commit can ship this
break in a compatible version. Mark the change as breaking (`feat!:` or a
`BREAKING CHANGE:` footer) so the release is versioned as breaking. If source
compatibility must be kept, do not add a public field to this struct.

No safe localized edit is established, so this comment has no diff.

Review comments at @src/engine_eval/mappers.rs:
- Line 95: **Populate the selected mapped variant key.**

When a mapped split selects an option, `src/engine_eval/mappers.rs` sets
`FeatureValue.key` to `None`, and `src/engine.rs` copies that value to
`FlagResult.variant`. The result therefore reports `variant: null`, although
`FlagResult.variant` documents `None` as meaning that no split applies.

The repository does not establish that decimal option IDs are the required
public variant keys. Define the variant-key contract, then map the supported
identifier to `FeatureValue.key`. Do not assume `id.to_string()` without that
contract.

Review comments at @src/engine_eval/result.rs:
- Line 34: **Treat the `FlagResult` field addition as a breaking API change.**

`FlagResult` is public, and downstream struct literals that omit `variant` no
longer compile. `#[serde(default)]` only affects deserialisation. Adding
`#[non_exhaustive]` now would not restore those existing literals; it would
instead prevent external literal construction. Release this change as a breaking
version and document the compatibility change.

Review comments at @src/engine_eval/segment_evaluator.rs:
- Around line 191-193: **Memoise cyclic flag results, or bound cycle
re-traversal.**

Segment conditions that read a flag in a dependency cycle re-run that flag's
resolution on every read. `resolve_feature` returns before caching a cyclic
flag, and segment verdicts that depend on a cycle are not cached. With chained
cyclic flags that each override in several segments, one evaluation can grow
exponentially with the cycle length. The change needs a per-evaluation cache for
cyclic results, or a work budget, before it ships.

Review comments at @src/engine.rs:
- Around line 53-56: **Keep a segment whose ANY rule matches without relying on
a cycle.**

The guard at `src/engine.rs:53-57` discards a segment whenever its evaluation
cuts a dependency cycle. An `ANY` rule can match on a later true condition, so
the verdict does not depend on the cycle. Its override is then lost, and the
feature returns the environment default. Discard a segment only when its verdict
depends on an unresolved flag.

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Commits

Reviewing files that changed from the base of the PR and between f083e82 and 0552139.

Files selected for processing: 11

.gitmodules
Cargo.toml
src/engine.rs
src/engine_eval/context.rs
src/engine_eval/mappers.rs
src/engine_eval/mod.rs
src/engine_eval/result.rs
src/engine_eval/segment_evaluator.rs
tests/engine_eval_test.rs
tests/engine_tests/engine-test-data
tests/mappers_test.rs

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Walkthrough

The evaluation context now preserves segment insertion order and supports removing identity data while filtering segments by their rule conditions. Segment conditions can resolve referenced flags lazily, with cycle tracking and cached results. The engine selects segment overrides by priority, assigns fallback reasons to untargeted or targeted features, and uses resolved dependency flags in evaluation results. Flag results now include an optional selected variant key. The test data submodule reference and evaluation test deserialisation were also updated.

Estimated code review effort: 4 (Complex) | ~45 minutes

Merge Risk: Moderate · previous review 05521

Resolve cyclic-evaluation performance and incorrect targeting results before merging, populate selected variant keys, and declare the public API changes as breaking.

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coderabbitai

Actionable comments posted: 5

Nitpick comments: 1

tests/mappers_test.rs: 1

344-348: Trivial · Maintainability & Code Quality · Quick win

Add an end-to-end assertion for retained flag paths.

The new cases only assert that remove_identity_from_context(&context) retains the segment. They do not evaluate $.flags.banner.enabled or assert a selected banner override. A regression in flag-path resolution or override selection can therefore pass these tests.

Additional comments: 1

src/engine_eval/segment_evaluator.rs-262-272: 1

262-272: Minor · Potential issue · Poor tradeoff

Unverified finding
We couldn't verify this from the code. It is shown for review, not as a verified issue.

get_segment_override evaluates even weaker overrides after finding a winner, so a self-referencing condition in a weaker segment can mark the feature cyclic although an independent, higher-priority override already wins.

A can receive a circular-dependency error, and the other flag cannot target on A, despite the independently determined winning value.

This can occur when: Feature A has a matching priority-1 override and a later priority-2 segment whose condition reads $.flags.A.enabled; another flag reads A during evaluation.

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Commits

Reviewing files that changed from the base of the PR and between f083e82 and 0552139.

Files selected for processing: 11

.gitmodules
Cargo.toml
src/engine.rs
src/engine_eval/context.rs
src/engine_eval/mappers.rs
src/engine_eval/mod.rs
src/engine_eval/result.rs
src/engine_eval/segment_evaluator.rs
tests/engine_eval_test.rs
tests/engine_tests/engine-test-data
tests/mappers_test.rs

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coderabbitai · 2026-10-30T21:37:08Z

 pub struct FeatureValue {
+    /// The key identifying this variant in flag results.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub key: Option<String>,


Major · Data Integrity & Integration · Quick win

Supported by static analysis

Script executed:

rg -n 'FeatureValue\s*\{' src tests; sed -n '1,45p' Cargo.toml

Repository: Flagsmith/flagsmith-rust-flag-engine

Length of output: 1179

Script executed:

set -u echo "== lib.rs / re-exports ==" rg -n -e 'pub use' -e 'pub mod' -e 'non_exhaustive' src/lib.rs src/engine_eval/mod.rs 2>/dev/null || true echo "== FeatureValue references across src/tests ==" rg -n -F -e 'FeatureValue' src tests || test "$?" -eq 1 echo "== context.rs FeatureValue definition ==" nl -ba src/engine_eval/context.rs | sed -n '25,45p' echo "== mappers.rs construction site ==" nl -ba src/engine_eval/mappers.rs | sed -n '85,100p' echo "== CHANGELOG / semver / release docs ==" ls -a | rg -i -e 'changelog' -e 'contributing' -e 'readme' || true rg -n -i -e 'semver' -e 'breaking' -e 'backward' -e 'compatib' README.md CONTRIBUTING.md CHANGELOG.md docs 2>/dev/null || true echo "== version history of Cargo.toml version line ==" git log --oneline -n 15 -- Cargo.toml || true git log -p -n 40 -- Cargo.toml | rg -n '^[-+]version' || true echo "== non_exhaustive usage in repo ==" rg -n -F 'non_exhaustive' src tests || test "$?" -eq 1 echo "== base commit presence ==" git cat-file -t f083e820268c4611136b5cca4b2f9d5f0f73b6fe 2>&1 || true git diff --stat f083e820268c4611136b5cca4b2f9d5f0f73b6fe 0552139222db80ea1ec16ec5a6a7e4ae5912bcc5 -- Cargo.toml src/engine_eval/context.rs 2>&1 || true

Repository: Flagsmith/flagsmith-rust-flag-engine

Length of output: 4334

Web query:

Rust Reference struct expressions all fields must be specified missing field error E0063 external crate non_exhaustive

Result:

Sources consulted: - Type system - The Rust Reference — https://doc.rust-lang.org/reference/attributes/type_system.html - Struct expressions - The Rust Reference — https://doc.rust-lang.org/reference/expressions/struct-expr.html - E0063 - Error codes index — https://doc.rust-lang.org/error_codes/E0063.html - codes.rs - source — https://doc.rust-lang.org/stable/nightly-rustc/src/rustc_errors/codes.rs.html - expr.rs - source — https://doc.rust-lang.org/stable/nightly-rustc/src/rustc_hir_typeck/expr.rs.html - …and 7 more

Mark the FeatureValue.key addition as a breaking change, or keep the struct closed to external literals.

FeatureValue is a public struct with all fields public and no #[non_exhaustive]. Downstream crates that build it with a struct literal now fail to compile, because the new key field is not supplied. Serde's default does not apply to Rust struct literals. The in-repo literal in src/engine_eval/mappers.rs already sets key: None, so the crate itself builds. Release tooling derives versions from commit messages (CHANGELOG.md shows release-please entries), so a non-breaking feat: commit can ship this break in a compatible version. Mark the change as breaking (feat!: or a BREAKING CHANGE: footer) so the release is versioned as breaking. If source compatibility must be kept, do not add a public field to this struct.

No safe localized edit is established, so this comment has no diff.

coderabbitai · 2026-10-30T21:37:08Z

    mv_values
        .iter()
        .map(|mv| FeatureValue {
+            key: None,


Minor · Functional Correctness · Quick win

Supported by static analysis

Script executed:

sed -n '45,110p' src/engine_eval/mappers.rs sed -n '140,200p' src/engine_eval/segment_evaluator.rs sed -n '155,210p' src/engine.rs rg -n 'variant|MultivariateFeatureOption' src tests README.md

Repository: Flagsmith/flagsmith-rust-flag-engine

Length of output: 9959

Populate the selected mapped variant key.

When a mapped split selects an option, src/engine_eval/mappers.rs sets FeatureValue.key to None, and src/engine.rs copies that value to FlagResult.variant. The result therefore reports variant: null, although FlagResult.variant documents None as meaning that no split applies.

The repository does not establish that decimal option IDs are the required public variant keys. Define the variant-key contract, then map the supported identifier to FeatureValue.key. Do not assume id.to_string() without that contract.

coderabbitai · 2026-10-30T21:37:08Z


+    /// The key of the selected multivariate variant, "control" when none was selected, or `None` when no split applies.
+    #[serde(default)]
+    pub variant: Option<String>,


Minor · Data Integrity & Integration · Quick win

Supported by static analysis

Script executed:

cat src/engine_eval/result.rs find .github -maxdepth 3 -type f rg -n 'release-please|bump-minor|breaking|versioning|release-type' .github Cargo.toml CHANGELOG.md

Repository: Flagsmith/flagsmith-rust-flag-engine

Length of output: 2315

Script executed:

set -eu printf '%s\n' '--- release workflow ---' nl -ba .github/workflows/release-please.yml printf '%s\n' '--- publish workflow ---' nl -ba .github/workflows/publish-cargo-package.yml printf '%s\n' '--- manifest version and package metadata ---' nl -ba Cargo.toml | sed -n '1,45p' printf '%s\n' '--- FlagResult constructor context ---' nl -ba src/engine.rs | sed -n '185,215p' printf '%s\n' '--- release/version references ---' rg -n -F -e 'release-please' -e 'version' -e 'breaking' -e 'non_exhaustive' --glob '!target/**' --glob '!Cargo.lock' .github Cargo.toml CHANGELOG.md || test "$?" -eq 1

Repository: Flagsmith/flagsmith-rust-flag-engine

Length of output: 4608

Treat the FlagResult field addition as a breaking API change.

FlagResult is public, and downstream struct literals that omit variant no longer compile. #[serde(default)] only affects deserialisation. Adding #[non_exhaustive] now would not restore those existing literals; it would instead prevent external literal construction. Release this change as a breaking version and document the compatibility change.

coderabbitai · 2026-10-30T21:37:08Z

+        if !dependencies.resolved.borrow().contains_key(feature_name) {
+            self.resolve_feature(dependencies, feature_name);
+        }


Major · Performance & Scalability · Heavy lift

Supported by static analysis

Script executed:

sed -n '180,320p' src/engine_eval/segment_evaluator.rs; rg -n 'cyclic|cycle_hits|limit|max_.*segment' src tests | head -80

Repository: Flagsmith/flagsmith-rust-flag-engine

Length of output: 7037

Script executed:

set -e echo '== engine.rs 1-180 ==' nl -ba src/engine.rs | sed -n '1,180p' echo '== segment_evaluator.rs 1-180 ==' nl -ba src/engine_eval/segment_evaluator.rs | sed -n '1,180p' echo '== segment_evaluator.rs 180-245 ==' nl -ba src/engine_eval/segment_evaluator.rs | sed -n '180,245p' echo '== rg cyclic in tests ==' rg -n -F -e 'cyclic' -e 'cycle' tests src || test "$?" -eq 1 echo '== rg is_context_in_segment and condition short-circuit ==' rg -n -F -e 'is_context_in_segment' -e '.all(' -e '.any(' src || test "$?" -eq 1

Repository: Flagsmith/flagsmith-rust-flag-engine

Length of output: 24043

Memoise cyclic flag results, or bound cycle re-traversal.

Segment conditions that read a flag in a dependency cycle re-run that flag's resolution on every read. resolve_feature returns before caching a cyclic flag, and segment verdicts that depend on a cycle are not cached. With chained cyclic flags that each override in several segments, one evaluation can grow exponentially with the cycle length. The change needs a per-evaluation cache for cyclic results, or a work budget, before it ships.

coderabbitai · 2026-10-30T21:37:08Z

+        let cycle_hits = evaluator.cycle_hits();
+        if !evaluator.is_context_in_segment(segment_context) || evaluator.cycle_hits() != cycle_hits
+        {
+            continue; // Unmatched, or the verdict rests on a flag left unresolved by a cycle


Minor · Functional Correctness · Heavy lift

Supported by static analysis

Script executed:

sed -n '385,460p' src/engine_eval/segment_evaluator.rs; rg -n '"type": "ANY"|CIRCULAR_DEPENDENCY' tests | head -80

Repository: Flagsmith/flagsmith-rust-flag-engine

Length of output: 2409

Script executed:

set -e echo "=== segment_evaluator.rs outline ===" ast-grep outline src/engine_eval/segment_evaluator.rs --view expanded echo "=== segment_evaluator.rs 1-140 ===" nl -ba src/engine_eval/segment_evaluator.rs | sed -n '1,140p' echo "=== segment_evaluator.rs 140-260 ===" nl -ba src/engine_eval/segment_evaluator.rs | sed -n '140,260p' echo "=== segment_evaluator.rs 460-520 ===" nl -ba src/engine_eval/segment_evaluator.rs | sed -n '460,520p' echo "=== engine.rs 1-140 ===" nl -ba src/engine.rs | sed -n '1,140p' echo "=== engine.rs 140-260 ===" nl -ba src/engine.rs | sed -n '140,260p' echo "=== CIRCULAR_DEPENDENCY / cycle hits references ===" rg -n -F -e 'CIRCULAR_DEPENDENCY' -e 'cycle_hits' -e 'CIRCULAR_DEPENDENCY_REASON' src tests || test "$?" -eq 1

Repository: Flagsmith/flagsmith-rust-flag-engine

Length of output: 29417

Script executed:

set -e echo "=== segment_evaluator.rs 250-330 ===" nl -ba src/engine_eval/segment_evaluator.rs | sed -n '250,330p' echo "=== cycle-related references in tests and docs ===" rg -n -i -e 'cycl' -e 'circular' tests docs README.md CHANGELOG.md 2>/dev/null || test "$?" -eq 1 echo "=== submodule fixture directory ===" ls -la tests/engine_tests/engine-test-data 2>/dev/null | head -20 || true echo "=== recent commit subjects touching cycle logic ===" git log --oneline -n 15 -- src/engine.rs src/engine_eval/segment_evaluator.rs || true

Repository: Flagsmith/flagsmith-rust-flag-engine

Length of output: 3914

Keep a segment whose ANY rule matches without relying on a cycle.

The guard at src/engine.rs:53-57 discards a segment whenever its evaluation cuts a dependency cycle. An ANY rule can match on a later true condition, so the verdict does not depend on the cycle. Its override is then lost, and the feature returns the environment default. Discard a segment only when its verdict depends on an unresolved flag.

Read JSONPaths made of member names only, such as `$.identity.identifier` or `$.flags.<feature>.enabled`, straight from the context, without serialising it or evaluating JSONPath. Other paths are still evaluated by jsonpath-rust. Reuse the segment matches memoised while resolving flag dependencies when evaluating segments, so segments read through a dependency are evaluated once. Memoise only once a condition has read a flag. Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

emyller and others added 3 commits October 30, 2026 15:57

refactor: Query JSONPath conditions with jsonpath-rust

0131309

serde_json_path keeps parsed paths private, while jsonpath-rust exposes them, so conditions can tell which flag a path reads. Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

emyller requested a review from a team as a code owner October 30, 2026 20:11

emyller requested review from khvn26 and removed request for a team October 30, 2026 20:11

This was referenced Oct 30, 2026

feat(Flag Dependency): Evaluate flags that depend on other flags Flagsmith/flagsmith-rust-client#74

Open

feat(Flag Dependency): Serve flags that depend on other flags Flagsmith/edge-proxy-rs#30

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coderabbitai Bot reviewed Oct 30, 2026

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feat: Evaluate flags that depend on other flags - #44

feat: Evaluate flags that depend on other flags#44
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