#Rust Port: Architecture Guide

Reference for key data structures, patterns, and constraints in the Rust compiler port. See rust-port-research.md for detailed per-pass analysis and rust-port-notes.md for the original design decisions.

#Arenas and ID Types

All shared mutable data is stored in arenas on Environment, referenced by copyable ID types. This replaces JavaScript's shared object references.

Arena	ID Type	Stored On	Replaces
identifiers: Vec<Identifier>	IdentifierId	Environment	Shared Identifier object references across Place values
scopes: Vec<ReactiveScope>	ScopeId	Environment	Shared ReactiveScope references across identifiers
functions: Vec<HIRFunction>	FunctionId	Environment	Inline HIRFunction on FunctionExpression/ObjectMethod
types: Vec<Type>	TypeId	Environment	Inline Type on Identifier

All ID types are Copy + Clone + Hash + Eq + PartialEq newtypes wrapping u32.

#Instructions and EvaluationOrder

• HirFunction.instructions: Vec<Instruction> — flat instruction table
• BasicBlock.instructions: Vec<InstructionId> — indices into the table above
• The old TypeScript InstructionId is renamed to EvaluationOrder — it represents evaluation order and appears on both instructions and terminals
• The new InstructionId is an index into HirFunction.instructions, giving passes a single copyable ID to reference any instruction

#Place is Clone, MutableRange is on Identifier/Scope

Place stores an IdentifierId (not a shared reference), making it small and cheap to clone. Mutation of mutable_range goes through the identifier arena:

env.identifiers[place.identifier].mutable_range.end = new_end;

After InferReactiveScopeVariables, an identifier's effective mutable range is its scope's range. Downstream passes access this through the scope arena:

let range = match env.identifiers[id].scope {
    Some(scope_id) => env.scopes[scope_id].range,
    None => env.identifiers[id].mutable_range,
};

#Function Arena and FunctionId

FunctionExpression and ObjectMethod instruction values store a FunctionId instead of an inline HIRFunction. Inner functions are accessed via the arena:

let inner = &env.functions[function_id];       // read
let inner = &mut env.functions[function_id];    // write

This makes CreateFunction aliasing effects store FunctionId, and function signature caches key by FunctionId.

#AliasingEffect

Effects own cloned Place values (cheap since Place contains IdentifierId). Key variants:

• Apply — clones the args Vec<PlaceOrSpreadOrHole> from the instruction value
• CreateFunction — stores FunctionId (not the FunctionExpression itself), plus cloned captures: Vec<Place>

Effect interning uses content hashing. The interned EffectId serves as both dedup key and allocation-site identity for abstract interpretation in InferMutationAliasingEffects.

#Environment: Separate from HirFunction

HirFunction does not store env. Passes receive env: &mut Environment as a separate parameter. Fields are flat (no sub-structs) to allow precise sliced borrows:

// Simultaneous borrow of different fields is fine:
let id = &env.identifiers[some_id];
let scope = &env.scopes[some_scope_id];

#Ordered Maps

Use IndexMap/IndexSet (from the indexmap crate) wherever the TypeScript uses Map/Set and iteration order matters. The primary case is HIR.blocks: IndexMap<BlockId, BasicBlock> which maintains reverse postorder.

#Side Maps

Side maps fall into four categories:

1. ID-only maps — HashMap<IdType, T> / HashSet<IdType>. No borrow issues. Most passes use this.
2. Reference-identity maps — TypeScript Map<Identifier, T> becomes HashMap<IdentifierId, T>. Similarly DisjointSet<Identifier> becomes DisjointSet<IdentifierId>, DisjointSet<ReactiveScope> becomes DisjointSet<ScopeId>.
3. Instruction/value reference maps — Store InstructionId or FunctionId instead of references. Access the actual data through the instruction table or function arena when needed.
4. Scope reference sets with mutation — Store ScopeId in sets. Mutate through the arena: env.scopes[scope_id].range.start = new_start.

When a pass needs to both iterate over data and mutate the HIR, use two-phase collect/apply: collect IDs or updates into a Vec, then apply mutations in a second loop.

#Error Handling

TypeScript Pattern	Rust Approach
Non-null assertion (!)	.unwrap() (panic)
CompilerError.invariant(), CompilerError.throwTodo(), throw ...	Return Err(CompilerDiagnostic) via Result
env.recordError() or pushDiagnostic() with an invariant error	Return Err(CompilerDiagnostic)
env.recordError() or pushDiagnostic() with a NON invariant error	Keep as-is — accumulate on Environment

Preserve full error details: reason, description, location, suggestions, category.

#JS→Rust Boundary

The JS side serializes the Babel AST and Babel's scope information (scope tree, bindings, reference-to-binding map) to Rust. Keep this serialization thin: only send the core data structures that Babel already computed during parsing. Any derived analysis — identifier source locations, JSX classification, captured variables, etc. — should be computed on the Rust side by walking the AST. See scope.ts.

#Pipeline and Pass Structure

fn compile(ast: Ast, scope: Scope, env: &mut Environment)
    -> Result<CompileResult, CompilerDiagnostic>
{
    let mut hir = lower(ast, scope, env)?;
    some_pass(&mut hir, env)?;
    // ...
    let ast = codegen(...)?;

    if env.has_errors() {
        Ok(CompileResult::Failure(env.take_errors()))
    } else {
        Ok(CompileResult::Success(ast))
    }
}

Pass signatures follow these patterns:

// Most passes: mutable HIR + mutable environment
fn pass(func: &mut HirFunction, env: &mut Environment) -> Result<(), CompilerDiagnostic>;

// Passes that don't need env
fn pass(func: &mut HirFunction);

// Validation passes: read-only HIR, env for error recording
fn validate(func: &HirFunction, env: &mut Environment) -> Result<(), CompilerDiagnostic>;

Use ? to propagate errors that would have thrown or short-circuited in TypeScript. Non-fatal errors are accumulated on env and checked at the end via env.has_errors().

#Structural Similarity to TypeScript

Target ~85-95% structural correspondence. A developer should be able to view TypeScript and Rust side-by-side and trace the logic. The ported code should preserve:

• Same high-level data flow through the code. Only deviate where strictly necessary due to data model differences (arenas, borrow checker workarounds, etc.).
• Same grouping of types, functions, and "classes" (structs with methods) into files. A TypeScript file maps to a Rust file with the same logical contents.
• Similar filenames, type names, and identifier names, adjusted for Rust naming conventions (camelCase -> snake_case for functions/variables, PascalCase preserved for types).
• Crate structure: The monolithic babel-plugin-react-compiler package is split into crates, roughly 1:1 by top-level folder (e.g., src/HIR/ -> a crate, src/Inference/ -> a crate, etc.). We split the lowering logic (BuildHIR and HIRBuilder) into react_compiler_lowering bc of its complexity.

Key mechanical translations:

TypeScript	Rust
switch (value.kind)	match &value (exhaustive)
Map<Identifier, T>	HashMap<IdentifierId, T>
for...of with Set.delete()	`set.retain(\
instr.value = { kind: 'X', ... }	std::mem::replace + reconstruct
{ ...place, effect: Effect.Read }	Place { effect: Effect::Read, ..place.clone() }
array.filter(x => ...)	`vec.retain(\
identifier.mutableRange.end = x	env.identifiers[id].mutable_range.end = x
Builder closures setting outer variables	Return values from closures