#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.

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#Critical Gaps (incorrect compilation possible)

#3. Hardcoded useMemoCache identifier name

• TS: ReactiveScopes/CodegenReactiveFunction.ts:166-178 typescript const useMemoCacheIdentifier = fn.env.programContext.addMemoCacheImport().name;
• Rust: react_compiler_reactive_scopes/src/codegen_reactive_function.rs:179 rust 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.

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#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.

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#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.