canMergeScopes() — react Function Reference

Source Code

compiler/packages/babel-plugin-react-compiler/src/ReactiveScopes/MergeReactiveScopesThatInvalidateTogether.ts lines 437–502

function canMergeScopes(
  current: ReactiveScopeBlock,
  next: ReactiveScopeBlock,
  temporaries: Map<DeclarationId, DeclarationId>,
): boolean {
  // Don't merge scopes with reassignments
  if (
    current.scope.reassignments.size !== 0 ||
    next.scope.reassignments.size !== 0
  ) {
    log(`  cannot merge, has reassignments`);
    return false;
  }
  // Merge scopes whose dependencies are identical
  if (
    areEqualDependencies(current.scope.dependencies, next.scope.dependencies)
  ) {
    log(`  canMergeScopes: dependencies are equal`);
    return true;
  }
  /*
   * Merge scopes where the outputs of the previous scope are the inputs
   * of the subsequent scope. Note that the output of a scope is not
   * guaranteed to change when its inputs change, for example `foo(x)`
   * may not change when `x` changes, for example `foo(x) { return x < 10}`
   * will not change as x changes from 0 -> 1.
   * Therefore we check that the outputs of the previous scope are of a type
   * that is guaranteed to invalidate with its inputs, and only merge in this case.
   */
  if (
    areEqualDependencies(
      new Set(
        [...current.scope.declarations.values()].map(declaration => ({
          identifier: declaration.identifier,
          reactive: true,
          path: [],
        })),
      ),
      next.scope.dependencies,
    ) ||
    (next.scope.dependencies.size !== 0 &&
      [...next.scope.dependencies].every(
        dep =>
          dep.path.length === 0 &&
          isAlwaysInvalidatingType(dep.identifier.type) &&
          Iterable_some(
            current.scope.declarations.values(),
            decl =>
              decl.identifier.declarationId === dep.identifier.declarationId ||
              decl.identifier.declarationId ===
                temporaries.get(dep.identifier.declarationId),
          ),
      ))
  ) {
    log(`  outputs of prev are input to current`);
    return true;
  }
  log(`  cannot merge scopes:`);
  log(
    `  ${printReactiveScopeSummary(current.scope)} ${[...current.scope.declarations.values()].map(decl => decl.identifier.declarationId)}`,
  );
  log(
    `  ${printReactiveScopeSummary(next.scope)} ${[...next.scope.dependencies].map(dep => `${dep.identifier.declarationId} ${temporaries.get(dep.identifier.declarationId) ?? dep.identifier.declarationId}`)}`,
  );
  return false;
}