index.ts

import { declare } from "@babel/helper-plugin-utils";
import syntacExplicitResourceManagement from "@babel/plugin-syntax-explicit-resource-management";
import { types as t, template, traverse, type PluginPass } from "@babel/core";
import type { NodePath, Visitor } from "@babel/traverse";

export default declare(api => {
  // TOOD: assert version 7.22.0
  api.assertVersion(7);

  const TOP_LEVEL_USING = new WeakSet();
  const TOP_LEVEL_AWAIT_USING = new WeakSet();

  function isUsingDeclaration(node: t.Node): node is t.VariableDeclaration {
    if (!t.isVariableDeclaration(node)) return false;
    return (
      node.kind === "using" ||
      node.kind === "await using" ||
      TOP_LEVEL_USING.has(node) ||
      TOP_LEVEL_AWAIT_USING.has(node)
    );
  }

  const transformUsingDeclarationsVisitor: Visitor<PluginPass> = {
    ForOfStatement(path: NodePath<t.ForOfStatement>) {
      const { left } = path.node;
      if (!isUsingDeclaration(left)) return;

      const { id } = left.declarations[0];
      const tmpId = path.scope.generateUidIdentifierBasedOnNode(id);
      left.declarations[0].id = tmpId;
      left.kind = "const";

      path.ensureBlock();
      path.node.body.body.unshift(
        t.variableDeclaration("using", [
          t.variableDeclarator(id, t.cloneNode(tmpId)),
        ]),
      );
    },
    BlockStatement(path, state) {
      let stackId: t.Identifier | null = null;
      let needsAwait = false;

      for (const node of path.node.body) {
        if (!isUsingDeclaration(node)) continue;
        stackId ??= path.scope.generateUidIdentifier("stack");
        const isAwaitUsing =
          node.kind === "await using" || TOP_LEVEL_AWAIT_USING.has(node);
        needsAwait ||= isAwaitUsing;

        if (
          !TOP_LEVEL_AWAIT_USING.delete(node) &&
          !TOP_LEVEL_USING.delete(node)
        ) {
          node.kind = "const";
        }
        node.declarations.forEach(decl => {
          const args = [t.cloneNode(stackId), decl.init];
          if (isAwaitUsing) args.push(t.booleanLiteral(true));
          decl.init = t.callExpression(state.addHelper("using"), args);
        });
      }
      if (!stackId) return;

      const errorId = path.scope.generateUidIdentifier("error");
      const hasErrorId = path.scope.generateUidIdentifier("hasError");

      let disposeCall: t.Expression = t.callExpression(
        state.addHelper("dispose"),
        [
          t.cloneNode(stackId),
          t.cloneNode(errorId),
          t.cloneNode(hasErrorId),
          // Pass SuppressedError so that it can be used with "pure"
          // polyfills that do not compile the contents of runtime
          // helpers.
          template.expression.ast`
            typeof SuppressedError !== "undefined" && SuppressedError
          `,
        ],
      );
      if (needsAwait) disposeCall = t.awaitExpression(disposeCall);

      const replacement = template.statement.ast`
        try {
          var ${stackId} = [];
          ${path.node.body}
        } catch (_) {
          var ${errorId} = _;
          var ${hasErrorId} = true;
        } finally {
          ${disposeCall}
        }
      `;

      const { parentPath } = path;
      if (
        parentPath.isFunction() ||
        parentPath.isTryStatement() ||
        parentPath.isCatchClause() ||
        parentPath.isStaticBlock()
      ) {
        path.replaceWith(t.blockStatement([replacement]));
      } else {
        path.replaceWith(replacement);
      }
    },
  };

  const transformUsingDeclarationsVisitorSkipFn: Visitor<PluginPass> =
    traverse.visitors.merge([
      transformUsingDeclarationsVisitor,
      {
        Function(path) {
          path.skip();
        },
      },
    ]);

  return {
    name: "proposal-explicit-resource-management",
    inherits: syntacExplicitResourceManagement,

    visitor: traverse.visitors.merge([
      transformUsingDeclarationsVisitor,
      {
        // To transform top-level using declarations, we must wrap the
        // module body in a block after hoisting all the exports and imports.
        // This might cause some variables to be `undefined` rather than TDZ.
        Program(path) {
          if (path.node.sourceType !== "module") return;
          if (!path.node.body.some(isUsingDeclaration)) return;

          const innerBlockBody = [];
          for (const stmt of path.get("body")) {
            if (stmt.isFunctionDeclaration() || stmt.isImportDeclaration()) {
              continue;
            }

            let { node } = stmt;
            let shouldRemove = true;

            if (stmt.isExportDefaultDeclaration()) {
              let { declaration } = stmt.node;
              let varId;
              if (t.isClassDeclaration(declaration)) {
                varId = declaration.id;
                declaration.id = null;
                declaration = t.toExpression(declaration);
              } else if (!t.isExpression(declaration)) {
                continue;
              }

              varId ??= path.scope.generateUidIdentifier("_default");
              innerBlockBody.push(
                t.variableDeclaration("var", [
                  t.variableDeclarator(varId, declaration),
                ]),
              );
              stmt.replaceWith(
                t.exportNamedDeclaration(null, [
                  t.exportSpecifier(
                    t.cloneNode(varId),
                    t.identifier("default"),
                  ),
                ]),
              );
              continue;
            }

            if (stmt.isExportNamedDeclaration()) {
              node = stmt.node.declaration;
              if (!node || t.isFunction(node)) continue;

              stmt.replaceWith(
                t.exportNamedDeclaration(
                  null,
                  Object.keys(t.getOuterBindingIdentifiers(node, false)).map(
                    id => t.exportSpecifier(t.identifier(id), t.identifier(id)),
                  ),
                ),
              );
              shouldRemove = false;
            } else if (stmt.isExportDeclaration()) {
              continue;
            }

            if (t.isClassDeclaration(node)) {
              const { id } = node;
              node.id = null;
              innerBlockBody.push(
                t.variableDeclaration("var", [
                  t.variableDeclarator(id, t.toExpression(node)),
                ]),
              );
            } else if (t.isVariableDeclaration(node)) {
              if (node.kind === "using") {
                TOP_LEVEL_USING.add(stmt.node);
              } else if (node.kind === "await using") {
                TOP_LEVEL_AWAIT_USING.add(stmt.node);
              }
              node.kind = "var";
              innerBlockBody.push(node);
            } else {
              innerBlockBody.push(stmt.node);
            }

            if (shouldRemove) stmt.remove();
          }

          path.pushContainer("body", t.blockStatement(innerBlockBody));
        },
        // We must transform `await using` in async functions before that
        // async-to-generator will transform `await` expressions into `yield`
        Function(path, state) {
          if (path.node.async) {
            path.traverse(transformUsingDeclarationsVisitorSkipFn, state);
          }
        },
      },
    ]),
  };
});