Implement import defer parsing support

packages/babel-generator/test/fixtures/import-phases/defer-declaration/input.js

@@ -0,0 +1 @@
+import defer * as x from "y";

packages/babel-generator/test/fixtures/import-phases/defer-declaration/output.js

@@ -0,0 +1 @@
+import defer * as x from "y";

packages/babel-generator/test/fixtures/import-phases/options.json

@@ -1,4 +1,4 @@
 {
-  "plugins": ["sourcePhaseImports"],
+  "plugins": ["sourcePhaseImports", "deferredImportEvaluation"],
   "parserOpts": { "createImportExpressions": true }
 }

packages/babel-parser/data/schema.json

@@ -163,6 +163,7 @@
               "decorators",
               "decorators-legacy",
               "decoratorAutoAccessors",
+              "deferredImportEvaluation",
               "destructuringPrivate",
               "doExpressions",
               "dynamicImport",

packages/babel-parser/src/parse-error/standard-errors.ts

@@ -65,6 +65,8 @@ export default {
     "Decorators must be placed *after* the 'export' keyword. Remove the 'decoratorsBeforeExport: false' option to use the '@decorator export class {}' syntax.",
   DecoratorSemicolon: "Decorators must not be followed by a semicolon.",
   DecoratorStaticBlock: "Decorators can't be used with a static block.",
+  DeferImportRequiresNamespace:
+    'Only `import defer * as x from "./module"` is valid.',
   DeletePrivateField: "Deleting a private field is not allowed.",
   DestructureNamedImport:
     "ES2015 named imports do not destructure. Use another statement for destructuring after the import.",
@@ -74,6 +76,8 @@ export default {
     `\`${exportName}\` has already been exported. Exported identifiers must be unique.`,
   DuplicateProto: "Redefinition of __proto__ property.",
   DuplicateRegExpFlags: "Duplicate regular expression flag.",
+  DynamicImportPhaseRequiresImportExpressions: ({ phase }: { phase: string }) =>
+    `'import.${phase}(...)' can only be parsed when using the 'createImportExpressions' option.`,
   ElementAfterRest: "Rest element must be last element.",
   EscapedCharNotAnIdentifier: "Invalid Unicode escape.",
   ExportBindingIsString: ({
@@ -235,8 +239,6 @@ export default {
     "In non-strict mode code, functions can only be declared at top level, inside a block, or as the body of an if statement.",
   SourcePhaseImportRequiresDefault:
     'Only `import source x from "./module"` is valid.',
-  SourcePhaseDynamicImportRequiresImportExpressions:
-    "'import.source(...)' can only be parsed when using the 'createImportExpressions' option.",
   StaticPrototype: "Classes may not have static property named prototype.",
   SuperNotAllowed:
     "`super()` is only valid inside a class constructor of a subclass. Maybe a typo in the method name ('constructor') or not extending another class?",

packages/babel-parser/src/parser/expression.ts

@@ -1667,16 +1667,27 @@ export default abstract class ExpressionParser extends LValParser {
         this.raise(Errors.ImportMetaOutsideModule, { at: id });
       }
       this.sawUnambiguousESM = true;
-    } else if (this.isContextual(tt._source)) {
-      this.expectPlugin("sourcePhaseImports");
+    } else if (this.isContextual(tt._source) || this.isContextual(tt._defer)) {
+      const isSource = this.isContextual(tt._source);
+
+      // TODO: The proposal doesn't mention import.defer yet because it was
+      // pending on a decision for import.source. Wait to enable it until it's
+      // included in the proposal.
+      if (!isSource) this.unexpected();
+
+      this.expectPlugin(
+        isSource ? "sourcePhaseImports" : "deferredImportEvaluation",
+      );
       if (!this.options.createImportExpressions) {
-        throw this.raise(
-          Errors.SourcePhaseDynamicImportRequiresImportExpressions,
-          { at: this.state.startLoc },
-        );
+        throw this.raise(Errors.DynamicImportPhaseRequiresImportExpressions, {
+          at: this.state.startLoc,
+          phase: this.state.value,
+        });
       }
       this.next();
-      (node as Undone<N.ImportExpression>).phase = "source";
+      (node as Undone<N.ImportExpression>).phase = isSource
+        ? "source"
+        : "defer";
       return this.parseImportCall(node as Undone<N.ImportExpression>);
     }
 

packages/babel-parser/src/parser/statement.ts

@@ -2871,18 +2871,23 @@ export default abstract class StatementParser extends ExpressionParser {
 
   checkImportReflection(node: Undone<N.ImportDeclaration>) {
     const { specifiers } = node;
-    const isSingleDefaultBinding =
-      specifiers.length === 1 &&
-      specifiers[0].type === "ImportDefaultSpecifier";
+    const singleBindingType =
+      specifiers.length === 1 ? specifiers[0].type : null;
 
     if (node.phase === "source") {
-      if (!isSingleDefaultBinding) {
+      if (singleBindingType !== "ImportDefaultSpecifier") {
         this.raise(Errors.SourcePhaseImportRequiresDefault, {
           at: specifiers[0].loc.start,
         });
       }
+    } else if (node.phase === "defer") {
+      if (singleBindingType !== "ImportNamespaceSpecifier") {
+        this.raise(Errors.DeferImportRequiresNamespace, {
+          at: specifiers[0].loc.start,
+        });
+      }
     } else if (node.module) {
-      if (!isSingleDefaultBinding) {
+      if (singleBindingType !== "ImportDefaultSpecifier") {
         this.raise(Errors.ImportReflectionNotBinding, {
           at: specifiers[0].loc.start,
         });
@@ -2930,7 +2935,11 @@ export default abstract class StatementParser extends ExpressionParser {
 
   isPotentialImportPhase(isExport: boolean): boolean {
     if (isExport) return false;
-    return this.isContextual(tt._source) || this.isContextual(tt._module);
+    return (
+      this.isContextual(tt._source) ||
+      this.isContextual(tt._defer) ||
+      this.isContextual(tt._module)
+    );
   }
 
   applyImportPhase(
@@ -2960,6 +2969,9 @@ export default abstract class StatementParser extends ExpressionParser {
     if (phase === "source") {
       this.expectPlugin("sourcePhaseImports", loc);
       (node as N.ImportDeclaration).phase = "source";
+    } else if (phase === "defer") {
+      this.expectPlugin("deferredImportEvaluation", loc);
+      (node as N.ImportDeclaration).phase = "defer";
     } else if (this.hasPlugin("sourcePhaseImports")) {
       (node as N.ImportDeclaration).phase = null;
     }

packages/babel-parser/src/tokenizer/types.ts

@@ -284,6 +284,7 @@ export const tt = {
   _assert: createKeywordLike("assert", { startsExpr }),
   _async: createKeywordLike("async", { startsExpr }),
   _await: createKeywordLike("await", { startsExpr }),
+  _defer: createKeywordLike("defer", { startsExpr }),
   _from: createKeywordLike("from", { startsExpr }),
   _get: createKeywordLike("get", { startsExpr }),
   _let: createKeywordLike("let", { startsExpr }),

packages/babel-parser/src/types.d.ts

@@ -627,7 +627,7 @@ export interface NewExpression extends CallOrNewBase {
 export interface ImportExpression extends NodeBase {
   type: "ImportExpression";
   source: Expression;
-  phase?: null | "source";
+  phase?: null | "source" | "defer";
   options: Expression | null;
 }
 
@@ -928,7 +928,7 @@ export interface ImportDeclaration extends NodeBase {
   >;
   source: Literal;
   importKind?: "type" | "typeof" | "value"; // TODO: Not in spec,
-  phase?: null | "source";
+  phase?: null | "source" | "defer";
   attributes?: ImportAttribute[];
   // @deprecated
   assertions?: ImportAttribute[];

packages/babel-parser/src/typings.d.ts

@@ -8,6 +8,7 @@ export type Plugin =
   | "classStaticBlock" // Enabled by default
   | "decimal"
   | "decorators-legacy"
+  | "deferredImportEvaluation"
   | "decoratorAutoAccessors"
   | "destructuringPrivate"
   | "doExpressions"

packages/babel-parser/test/fixtures/experimental/deferred-import-evaluation/.attributes-expression/input.js

@@ -0,0 +1 @@
+import.defer("x", { with: { attr: "val" } });

packages/babel-parser/test/fixtures/experimental/deferred-import-evaluation/.attributes-expression/options.json

@@ -0,0 +1,4 @@
+{
+  "plugins": ["deferredImportEvaluation", "importAttributes"],
+  "createImportExpressions": true
+}

packages/babel-parser/test/fixtures/experimental/deferred-import-evaluation/.dynamic-import-no-createImportExpressions/input.js

@@ -0,0 +1 @@
+import.defer("foo");

packages/babel-parser/test/fixtures/experimental/deferred-import-evaluation/.dynamic-import-no-createImportExpressions/options.json

@@ -0,0 +1,4 @@
+{
+  "plugins": ["deferredImportEvaluation"],
+  "throws": "TODO (disabled test)"
+}

packages/babel-parser/test/fixtures/experimental/deferred-import-evaluation/.dynamic-import/input.js

@@ -0,0 +1 @@
+import.defer("foo");

packages/babel-parser/test/fixtures/experimental/deferred-import-evaluation/.dynamic-import/options.json

@@ -0,0 +1,4 @@
+{
+  "plugins": ["deferredImportEvaluation"],
+  "createImportExpressions": true
+}

packages/babel-parser/test/fixtures/experimental/deferred-import-evaluation/attributes-declaration/input.js

@@ -0,0 +1 @@
+import defer * as ns from "x" with { attr: "val" };

packages/babel-parser/test/fixtures/experimental/deferred-import-evaluation/attributes-declaration/options.json

@@ -0,0 +1,3 @@
+{
+  "throws": "This experimental syntax requires enabling the parser plugin: \"importAttributes\". (1:35)"
+}

packages/babel-parser/test/fixtures/experimental/deferred-import-evaluation/import-defer/input.js

@@ -0,0 +1 @@
+import defer * as ns from "x";

packages/babel-parser/test/fixtures/experimental/deferred-import-evaluation/import-defer/output.json

@@ -0,0 +1,38 @@
+{
+  "type": "File",
+  "start":0,"end":30,"loc":{"start":{"line":1,"column":0,"index":0},"end":{"line":1,"column":30,"index":30}},
+  "program": {
+    "type": "Program",
+    "start":0,"end":30,"loc":{"start":{"line":1,"column":0,"index":0},"end":{"line":1,"column":30,"index":30}},
+    "sourceType": "module",
+    "interpreter": null,
+    "body": [
+      {
+        "type": "ImportDeclaration",
+        "start":0,"end":30,"loc":{"start":{"line":1,"column":0,"index":0},"end":{"line":1,"column":30,"index":30}},
+        "phase": "defer",
+        "specifiers": [
+          {
+            "type": "ImportNamespaceSpecifier",
+            "start":13,"end":20,"loc":{"start":{"line":1,"column":13,"index":13},"end":{"line":1,"column":20,"index":20}},
+            "local": {
+              "type": "Identifier",
+              "start":18,"end":20,"loc":{"start":{"line":1,"column":18,"index":18},"end":{"line":1,"column":20,"index":20},"identifierName":"ns"},
+              "name": "ns"
+            }
+          }
+        ],
+        "source": {
+          "type": "StringLiteral",
+          "start":26,"end":29,"loc":{"start":{"line":1,"column":26,"index":26},"end":{"line":1,"column":29,"index":29}},
+          "extra": {
+            "rawValue": "x",
+            "raw": "\"x\""
+          },
+          "value": "x"
+        }
+      }
+    ],
+    "directives": []
+  }
+}

packages/babel-parser/test/fixtures/experimental/deferred-import-evaluation/no-default/input.js

@@ -0,0 +1 @@
+import defer x from "x";

packages/babel-parser/test/fixtures/experimental/deferred-import-evaluation/no-default/output.json

@@ -0,0 +1,41 @@
+{
+  "type": "File",
+  "start":0,"end":24,"loc":{"start":{"line":1,"column":0,"index":0},"end":{"line":1,"column":24,"index":24}},
+  "errors": [
+    "SyntaxError: Only `import defer * as x from \"./module\"` is valid. (1:13)"
+  ],
+  "program": {
+    "type": "Program",
+    "start":0,"end":24,"loc":{"start":{"line":1,"column":0,"index":0},"end":{"line":1,"column":24,"index":24}},
+    "sourceType": "module",
+    "interpreter": null,
+    "body": [
+      {
+        "type": "ImportDeclaration",
+        "start":0,"end":24,"loc":{"start":{"line":1,"column":0,"index":0},"end":{"line":1,"column":24,"index":24}},
+        "phase": "defer",
+        "specifiers": [
+          {
+            "type": "ImportDefaultSpecifier",
+            "start":13,"end":14,"loc":{"start":{"line":1,"column":13,"index":13},"end":{"line":1,"column":14,"index":14}},
+            "local": {
+              "type": "Identifier",
+              "start":13,"end":14,"loc":{"start":{"line":1,"column":13,"index":13},"end":{"line":1,"column":14,"index":14},"identifierName":"x"},
+              "name": "x"
+            }
+          }
+        ],
+        "source": {
+          "type": "StringLiteral",
+          "start":20,"end":23,"loc":{"start":{"line":1,"column":20,"index":20},"end":{"line":1,"column":23,"index":23}},
+          "extra": {
+            "rawValue": "x",
+            "raw": "\"x\""
+          },
+          "value": "x"
+        }
+      }
+    ],
+    "directives": []
+  }
+}

packages/babel-parser/test/fixtures/experimental/deferred-import-evaluation/no-named/input.js

@@ -0,0 +1 @@
+import defer { x } from "x";

packages/babel-parser/test/fixtures/experimental/deferred-import-evaluation/no-named/output.json

@@ -0,0 +1,46 @@
+{
+  "type": "File",
+  "start":0,"end":28,"loc":{"start":{"line":1,"column":0,"index":0},"end":{"line":1,"column":28,"index":28}},
+  "errors": [
+    "SyntaxError: Only `import defer * as x from \"./module\"` is valid. (1:15)"
+  ],
+  "program": {
+    "type": "Program",
+    "start":0,"end":28,"loc":{"start":{"line":1,"column":0,"index":0},"end":{"line":1,"column":28,"index":28}},
+    "sourceType": "module",
+    "interpreter": null,
+    "body": [
+      {
+        "type": "ImportDeclaration",
+        "start":0,"end":28,"loc":{"start":{"line":1,"column":0,"index":0},"end":{"line":1,"column":28,"index":28}},
+        "phase": "defer",
+        "specifiers": [
+          {
+            "type": "ImportSpecifier",
+            "start":15,"end":16,"loc":{"start":{"line":1,"column":15,"index":15},"end":{"line":1,"column":16,"index":16}},
+            "imported": {
+              "type": "Identifier",
+              "start":15,"end":16,"loc":{"start":{"line":1,"column":15,"index":15},"end":{"line":1,"column":16,"index":16},"identifierName":"x"},
+              "name": "x"
+            },
+            "local": {
+              "type": "Identifier",
+              "start":15,"end":16,"loc":{"start":{"line":1,"column":15,"index":15},"end":{"line":1,"column":16,"index":16},"identifierName":"x"},
+              "name": "x"
+            }
+          }
+        ],
+        "source": {
+          "type": "StringLiteral",
+          "start":24,"end":27,"loc":{"start":{"line":1,"column":24,"index":24},"end":{"line":1,"column":27,"index":27}},
+          "extra": {
+            "rawValue": "x",
+            "raw": "\"x\""
+          },
+          "value": "x"
+        }
+      }
+    ],
+    "directives": []
+  }
+}

packages/babel-parser/test/fixtures/experimental/deferred-import-evaluation/options.json

@@ -0,0 +1,4 @@
+{
+  "sourceType": "module",
+  "plugins": ["deferredImportEvaluation"]
+}

packages/babel-parser/typings/babel-parser.d.ts

@@ -12,6 +12,7 @@ type Plugin =
   | "classStaticBlock" // Enabled by default
   | "decimal"
   | "decorators-legacy"
+  | "deferredImportEvaluation"
   | "decoratorAutoAccessors"
   | "destructuringPrivate"
   | "doExpressions"

packages/babel-types/src/ast-types/generated/index.ts

@@ -864,7 +864,7 @@ export interface ImportDeclaration extends BaseNode {
   attributes?: Array<ImportAttribute> | null;
   importKind?: "type" | "typeof" | "value" | null;
   module?: boolean | null;
-  phase?: "source" | null;
+  phase?: "source" | "defer" | null;
 }
 
 export interface ImportDefaultSpecifier extends BaseNode {
@@ -888,7 +888,7 @@ export interface ImportExpression extends BaseNode {
   type: "ImportExpression";
   source: Expression;
   options?: Expression | null;
-  phase?: "source" | null;
+  phase?: "source" | "defer" | null;
 }
 
 export interface MetaProperty extends BaseNode {

packages/babel-types/src/definitions/core.ts

@@ -1745,7 +1745,7 @@ defineType("ImportDeclaration", {
     },
     phase: {
       default: null,
-      validate: assertOneOf("source"),
+      validate: assertOneOf("source", "defer"),
     },
     specifiers: {
       validate: chain(
@@ -1816,7 +1816,7 @@ defineType("ImportExpression", {
   fields: {
     phase: {
       default: null,
-      validate: assertOneOf("source"),
+      validate: assertOneOf("source", "defer"),
     },
     source: {
       validate: assertNodeType("Expression"),