Implement assumptions defined in the babel/rfcs#5 RFC

- `mutableTemplateObject` and `ignoreToPrimitiveHint` (#12408)
- `setClassMethods` (#12407)
- `setComputedProperties` (#12490)
- `ignoreFunctionLength` (#12491)
- `noDocumentAll` (#12481)
- `iterableIsArray` and `arrayLikeIsIterable` (#12489)
- `pureGetters` (#12504)
- `skipForOfIteratorClosing` (#12496)
- `objectRestNoSymbols`, `setSpreadProperties` and `pureGetters` (#12505)
- `noNewArrows` (#12613, #12793)
- `setPublicClassFields` and `privateFieldsAsProperties` (#12497)
- `constantReexports` and `enumerableModuleMeta` (#12618)
- `constantSuper`, `superIsCallableConstructor` and `noClassCalls` (#12726)

Co-authored-by: Justin Ridgewell <justin@ridgewell.name>
Co-authored-by: Huáng Jùnliàng <JLHwung@users.noreply.github.com>

packages/babel-plugin-transform-parameters/src/index.js

@@ -6,7 +6,10 @@ export { convertFunctionParams };
 export default declare((api, options) => {
   api.assertVersion(7);
 
-  const { loose } = options;
+  const ignoreFunctionLength =
+    api.assumption("ignoreFunctionLength") ?? options.loose;
+  const noNewArrows = api.assumption("noNewArrows");
+
   return {
     name: "transform-parameters",
 
@@ -19,11 +22,14 @@ export default declare((api, options) => {
             .some(param => param.isRestElement() || param.isAssignmentPattern())
         ) {
           // default/rest visitors require access to `arguments`, so it cannot be an arrow
-          path.arrowFunctionToExpression();
+          path.arrowFunctionToExpression({ noNewArrows });
         }
 
         const convertedRest = convertFunctionRest(path);
-        const convertedParams = convertFunctionParams(path, loose);
+        const convertedParams = convertFunctionParams(
+          path,
+          ignoreFunctionLength,
+        );
 
         if (convertedRest || convertedParams) {
           // Manually reprocess this scope to ensure that the moved params are updated.

packages/babel-plugin-transform-parameters/src/params.js

@@ -44,7 +44,7 @@ const iifeVisitor = {
 // last 2 parameters are optional -- they are used by proposal-object-rest-spread/src/index.js
 export default function convertFunctionParams(
   path,
-  loose,
+  ignoreFunctionLength,
   shouldTransformParam,
   replaceRestElement,
 ) {
@@ -123,7 +123,10 @@ export default function convertFunctionParams(
     }
 
     const paramIsAssignmentPattern = param.isAssignmentPattern();
-    if (paramIsAssignmentPattern && (loose || node.kind === "set")) {
+    if (
+      paramIsAssignmentPattern &&
+      (ignoreFunctionLength || node.kind === "set")
+    ) {
       const left = param.get("left");
       const right = param.get("right");
 
@@ -203,6 +206,8 @@ export default function convertFunctionParams(
     // sure that we correctly handle this and arguments.
     const bodyPath = path.get("body.body");
     const arrowPath = bodyPath[bodyPath.length - 1].get("argument.callee");
+
+    // This is an IIFE, so we don't need to worry about the noNewArrows assumption
     arrowPath.arrowFunctionToExpression();
 
     arrowPath.node.generator = path.node.generator;

packages/babel-plugin-transform-parameters/test/fixtures/assumption-ignoreFunctionLength/options.json

@@ -3,12 +3,14 @@
     "proposal-class-properties",
     "external-helpers",
     "syntax-flow",
-    ["transform-parameters", { "loose": true }],
+    "transform-parameters",
     "transform-block-scoping",
     "transform-spread",
     "transform-classes",
     "transform-destructuring",
-    "transform-arrow-functions",
-    "transform-for-of"
-  ]
+    "transform-arrow-functions"
+  ],
+  "assumptions": {
+    "ignoreFunctionLength": true
+  }
 }

packages/babel-plugin-transform-parameters/test/fixtures/loose/ignoreFunctionLength/input.js

@@ -0,0 +1,3 @@
+function fn(a, b = c()) {
+  return b;
+}

packages/babel-plugin-transform-parameters/test/fixtures/loose/ignoreFunctionLength/output.js

@@ -0,0 +1,7 @@
+function fn(a, b) {
+  if (b === void 0) {
+    b = c();
+  }
+
+  return b;
+}

packages/babel-plugin-transform-parameters/test/fixtures/loose/options.json

@@ -0,0 +1,5 @@
+{
+  "plugins": [
+    ["transform-parameters", { "loose": true }]
+  ]
+}