Rust Port Gap Analysis

Comprehensive comparison of the TypeScript and Rust compiler implementations. Generated 2026-03-30 from systematic review of all major subsystems.

Current test status: Pass 1717/1717, Code 1716/1717, Snap 1717/1718.

Critical Gaps (incorrect compilation possible)

3. Hardcoded useMemoCache identifier name

• TS: ReactiveScopes/CodegenReactiveFunction.ts:166-178

  const useMemoCacheIdentifier = fn.env.programContext.addMemoCacheImport().name;
  

• Rust: react_compiler_reactive_scopes/src/codegen_reactive_function.rs:179

  callee: Box::new(Expression::Identifier(make_identifier("useMemoCache"))),
  

• The TS dynamically resolves the useMemoCache import to _c (from react/compiler-runtime import specifier c). The Rust hardcodes "useMemoCache". The BabelPlugin.ts wrapper handles the rename to _c during AST application, so this works in practice, but it means the codegen output has an incorrect intermediate identifier.

Moderate Gaps (feature gaps or edge cases)

6. Missing optimizeForSSR pass

• TS: Entrypoint/Pipeline.ts:223-226
• Rust: MISSING entirely
• Inlines useState/useReducer, removes effects, strips event handlers from JSX, removes refs. Without it, SSR-mode compilation produces unoptimized output.

7. Missing enableForest codegen path

• TS: ReactiveScopes/CodegenReactiveFunction.ts:1527-1536
• Rust: MISSING
• Skips hook guard wrapping and emits typeArguments on call expressions. Missing in Rust means forest mode has incorrect hook wrapping and dropped type arguments.

8. Function name inference from AssignmentExpression and Property

• TS: Entrypoint/Program.ts:1226-1268 — three cases: parent.isAssignmentExpression(), parent.isProperty(), parent.isAssignmentPattern()
• Rust: program.rs:1483-1488 — only handles VariableDeclarator
• Functions in Foo = () => {}, {useFoo: () => {}}, or {useFoo = () => {}} = {} positions are nameless in Rust, preventing component/hook detection via name heuristics.

9. Missing validations in outlined function pipeline

• TS: Outlined functions go through full compileFn → all validations
• Rust: run_pipeline_passes skips: validateContextVariableLValues, validateUseMemo, validateNoDerivedComputationsInEffects/_exp, validateNoSetStateInEffects, validateNoJSXInTryStatement, validateNoCapitalizedCalls, validateStaticComponents
• Also missing: has_errors() check at end, memo_cache_import registration

10. Reanimated flag injection missing

• TS: Babel/BabelPlugin.ts:48-53 — injectReanimatedFlag(opts) sets enableCustomTypeDefinitionForReanimated = true
• Rust: Detects reanimated but doesn't inject the flag into environment config
• Custom type definitions for reanimated shared values won't activate.

11. Dev-mode enableResetCacheOnSourceFileChanges injection missing

• TS: Babel/BabelPlugin.ts:54-65 — auto-enables in dev mode
• Rust: MISSING
• Fast refresh cache-reset code won't generate in dev mode unless explicitly configured.

12. Outlined functions not re-queued for compilation

• TS: Entrypoint/Program.ts:476-501 — outlined functions with a React function type are pushed back into the compilation queue
• Rust: program.rs:2244-2262 — only does AST insertion
• Outlined functions don't receive full compilation treatment (memoization).

13. Missing addNewReference in RenameVariables

• TS: ReactiveScopes/RenameVariables.ts:163 — this.#programContext.addNewReference(name)
• Rust: MISSING
• Newly created variable names aren't registered with the program context, risking import binding conflicts.

14. known_incompatible not checked for legacy signatures without aliasing config

• TS: Inference/InferMutationAliasingEffects.ts:2351-2370
• Rust: infer_mutation_aliasing_effects.rs:2099-2100 — TODO comment, only checked in the Apply path with aliasing configs
• If any legacy signatures (without aliasing configs) have known_incompatible set, Rust silently continues.

Minor Gaps (cosmetic, defensive, or unlikely to trigger)

15. Missing assertValidMutableRanges pass

• TS: Pipeline.ts:246-249 — gated behind config.assertValidMutableRanges (defaults false)
• Rust: Config flag exists but pass never called
• Debugging-only validation, no production impact.

16. Missing ValidateNoDerivedComputationsInEffects_exp experimental variant

• TS: 842-line experimental validation pass
• Rust: MISSING — only the non-experimental version is ported
• Only affects users who explicitly enable the experimental flag in lint mode.

17. Missing CompileUnexpectedThrow event

• TS: Program.ts:755-769 — logs when a pass incorrectly throws
• Rust: Event type defined but never emitted
• Development-time detection of misbehaving passes.

18. Missing error for sources-specified-without-filename

• TS: Creates a Config error via handleError
• Rust: Silently sets shouldCompile = false

19. Missing codegen_block temporary invariant check

• TS: CodegenReactiveFunction.ts:474-492 — verifies no temporary was overwritten
• Rust: Restores snapshot without checking

20. Extra NullLiteralTypeAnnotation rejection

• Rust rejects Flow null type annotation on first param (over-conservative vs TS)

21. UnsignedShiftRight (>>>) not classified as primitive binary op

• Rust: infer_types.rs:140-159 — missing from is_primitive_binary_op
• Operands won't be constrained to Primitive type, but result is still Primitive.

22. Post-dominator frontier not cached in InferReactivePlaces

• Rust: Recomputes frontier on every call instead of caching per block ID
• Performance issue only, not correctness.

23. Math.random missing restParam

• TS: restParam: Effect.Read
• Rust: rest_param: None (uses default ..Default::default())
• Affects extra-argument fallback effect only.

24. WeakSet.has / WeakMap.has wrong signature shape

• TS: positionalParams: [Effect.Read], restParam: null
• Rust: Uses pure_primitive_fn → positional_params: [], rest_param: Some(Effect::Read)
• Difference in extra-argument fallback behavior only.

25. Missing throwUnknownException__testonly in outlined function pipeline

• Test-only feature.