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-spread/src/index.js

@@ -5,13 +5,18 @@ import { types as t } from "@babel/core";
 export default declare((api, options) => {
   api.assertVersion(7);
 
-  const { loose, allowArrayLike } = options;
+  const iterableIsArray = api.assumption("iterableIsArray") ?? options.loose;
+  const arrayLikeIsIterable =
+    options.allowArrayLike ?? api.assumption("arrayLikeIsIterable");
 
   function getSpreadLiteral(spread, scope) {
-    if (loose && !t.isIdentifier(spread.argument, { name: "arguments" })) {
+    if (
+      iterableIsArray &&
+      !t.isIdentifier(spread.argument, { name: "arguments" })
+    ) {
       return spread.argument;
     } else {
-      return scope.toArray(spread.argument, true, allowArrayLike);
+      return scope.toArray(spread.argument, true, arrayLikeIsIterable);
     }
   }
 

packages/babel-plugin-transform-spread/test/fixtures/assumption-arrayLikeIsIterable/holes/exec.js

@@ -0,0 +1,6 @@
+var p2 = { 0: "a", 2: "c", length: 3 };
+
+var arr = [...p2, "d"];
+
+expect(arr).toEqual(["a", undefined, "c", "d"]);
+expect(1 in arr).toBe(true); // Not holey

packages/babel-plugin-transform-spread/test/fixtures/assumption-arrayLikeIsIterable/length-cropped/exec.js

@@ -0,0 +1,5 @@
+var p2 = { 0: "b", 1: "c", 2: "d", length: 2 };
+
+var arr = ["a", ...p2, "e"];
+
+expect(arr).toEqual(["a", "b", "c", "e"]);

packages/babel-plugin-transform-spread/test/fixtures/assumption-arrayLikeIsIterable/options.json

@@ -0,0 +1,9 @@
+{
+  "plugins": [
+    ["external-helpers", { "helperVersion": "7.100.0" }],
+    "transform-spread"
+  ],
+  "assumptions": {
+    "arrayLikeIsIterable": true
+  }
+}

packages/babel-plugin-transform-spread/test/fixtures/assumption-arrayLikeIsIterable/simple/exec.js

@@ -0,0 +1,5 @@
+var p2 = { 0: "b", 1: "c", 2: "d", length: 3 };
+
+var arr = ["a", ...p2, "e"];
+
+expect(arr).toEqual(["a", "b", "c", "d", "e"]);

packages/babel-plugin-transform-spread/test/fixtures/assumption-arrayLikeIsIterable/simple/input.js

@@ -0,0 +1 @@
+var arr = ["a", ...p2, "e"];

packages/babel-plugin-transform-spread/test/fixtures/assumption-arrayLikeIsIterable/simple/output.js

@@ -0,0 +1 @@
+var arr = ["a"].concat(babelHelpers.maybeArrayLike(babelHelpers.toConsumableArray, p2), ["e"]);

packages/babel-plugin-transform-spread/test/fixtures/assumption-iterableIsArray/arguments/input.js

@@ -0,0 +1,5 @@
+function foo() {
+  // We know for sure that 'arguments' is _not_ an array, so we
+  // can ignore the assumption in this case.
+  return [...arguments];
+}

packages/babel-plugin-transform-spread/test/fixtures/assumption-iterableIsArray/arguments/output.js

@@ -0,0 +1,5 @@
+function foo() {
+  // We know for sure that 'arguments' is _not_ an array, so we
+  // can ignore the assumption in this case.
+  return Array.prototype.slice.call(arguments);
+}

packages/babel-plugin-transform-spread/test/fixtures/assumption-iterableIsArray/array-literals/input.js

@@ -0,0 +1 @@
+var lyrics = ["head", "and", "toes", ...parts];

packages/babel-plugin-transform-spread/test/fixtures/assumption-iterableIsArray/array-literals/output.js

@@ -0,0 +1 @@
+var lyrics = ["head", "and", "toes"].concat(parts);

packages/babel-plugin-transform-spread/test/fixtures/assumption-iterableIsArray/function-call/input.js

@@ -0,0 +1 @@
+lyrics(["head", "and", "toes", ...parts]);

packages/babel-plugin-transform-spread/test/fixtures/assumption-iterableIsArray/function-call/output.js

@@ -0,0 +1 @@
+lyrics(["head", "and", "toes"].concat(parts));

packages/babel-plugin-transform-spread/test/fixtures/assumption-iterableIsArray/options.json

@@ -0,0 +1,6 @@
+{
+  "plugins": ["external-helpers", "transform-spread"],
+  "assumptions": {
+    "iterableIsArray": true
+  }
+}