[patch] Make holey array handling compatible with older babel versions

--- 

Copied from comments 

Older versions of babel have a validation bug fixed by 

- https://github.com/babel/babel/pull/10917 

- (code pointer) 
https://github.com/babel/babel/commit/e7b80a2cb93cf28010207fc3cdd19b4568ca35b9#diff-19b555d2f3904c206af406540d9df200b1e16befedb83ff39ebfcbd876f7fa8aL52 

Link to buggy older version (observe that elements must be PatternLikes here) 


https://github.com/babel/babel/blob/v7.7.4/packages/babel-types/src/definitions/es2015.js#L50-L53 

Link to newer versions with correct validation (observe elements can be 
PatternLike | null) 


https://github.com/babel/babel/blob/v7.23.0/packages/babel-types/src/definitions/core.ts#L1306-L1311 

Tested on 3000+ components in fb: 

P898166848 

> Unexpected Errors 

> (count = 0) 

[patch] Make holey array handling compatible with older babel versions 

--- 

Copied from comments 

Older versions of babel have a validation bug fixed by 

- https://github.com/babel/babel/pull/10917 

- (code pointer) 
https://github.com/babel/babel/commit/e7b80a2cb93cf28010207fc3cdd19b4568ca35b9#diff-19b555d2f3904c206af406540d9df200b1e16befedb83ff39ebfcbd876f7fa8aL52 

Link to buggy older version (observe that elements must be PatternLikes here) 


https://github.com/babel/babel/blob/v7.7.4/packages/babel-types/src/definitions/es2015.js#L50-L53 

Link to newer versions with correct validation (observe elements can be 
PatternLike | null) 


https://github.com/babel/babel/blob/v7.23.0/packages/babel-types/src/definitions/core.ts#L1306-L1311 

Tested on 3000+ components in fb: 

P898166848 

> Unexpected Errors 

> (count = 0)

compiler/packages/babel-plugin-react-forget/src/HIR/BuildHIR.ts

@@ -1456,14 +1456,6 @@ function lowerExpression(
           elements.push({
             kind: "Hole",
           });
-          if (builder.environment.config.bailoutOnHoleyArrays) {
-            builder.errors.push({
-              reason: `(BuildHIR::lower) Fix babel holey array backward compatibility.`,
-              severity: ErrorSeverity.Todo,
-              loc: expr.node.loc ?? null,
-              suggestions: null,
-            });
-          }
           continue;
         } else if (element.isExpression()) {
           elements.push(lowerExpressionToTemporary(builder, element));
@@ -3289,15 +3281,6 @@ function lowerAssignment(
           items.push({
             kind: "Hole",
           });
-
-          if (builder.environment.config.bailoutOnHoleyArrays) {
-            builder.errors.push({
-              reason: `(BuildHIR::lower) Fix babel holey array backward compatibility.`,
-              severity: ErrorSeverity.Todo,
-              loc: lvalue.node.loc ?? null,
-              suggestions: null,
-            });
-          }
           continue;
         }
         if (element.isRestElement()) {

compiler/packages/babel-plugin-react-forget/src/HIR/Environment.ts

@@ -256,30 +256,6 @@ const EnvironmentConfigSchema = z.object({
   // Enable validation of mutable ranges
   assertValidMutableRanges: z.boolean().default(false),
 
-  /*
-   *
-   * Instead of handling holey arrays, bail out with a TODO error.
-   *
-   * Older versions of babel seem to have inconsistent handling of holey arrays,
-   * at least when paired with HermesParser. When using these versions, we should
-   * bail out instead of throwing a Babel validation error.
-   *
-   * The babel ast definition for array elements changed from Array<PatternLike>
-   * to Array<PatternLike | null>. Older versions does not expect null in the
-   * ArrayPattern ast and will throw a validation error.
-   *
-   * - HermesParser will parse [, b] into [NodePath<null>, NodePath<Identifier>]
-   * - Forget will try to preserve this holey array when we codegen back to js
-   *   (e.g. we call a babel builder function arrayPattern([null, identifier]))
-   * - Babel will fail with `TypeError: Property elements[0] of ArrayPattern
-   *   expected node to be of a type ["PatternLike"] but instead got null`
-   *
-   * PR that changed the AST definition
-   * https://github.com/babel/babel/pull/10917/files#diff-19b555d2f3904c206af406540d9df200b1e16befedb83ff39ebfcbd876f7fa8aL52-R56
-   *
-   */
-  bailoutOnHoleyArrays: z.boolean().default(false),
-
   /*
    * Enable emitting "change variables" which store the result of whether a particular
    * reactive scope dependency has changed since the scope was last executed.

compiler/packages/babel-plugin-react-forget/src/ReactiveScopes/CodegenReactiveFunction.ts

@@ -10,6 +10,7 @@ import { pruneUnusedLValues, pruneUnusedLabels, renameVariables } from ".";
 import { CompilerError, ErrorSeverity } from "../CompilerError";
 import { Environment, EnvironmentConfig, ExternalFunction } from "../HIR";
 import {
+  ArrayPattern,
   BlockId,
   GeneratedSource,
   Identifier,
@@ -1827,20 +1828,51 @@ function codegenObjectPropertyKey(
   }
 }
 
+function codegenArrayPattern(
+  cx: Context,
+  pattern: ArrayPattern
+): t.ArrayPattern {
+  const hasHoles = !pattern.items.every((e) => e.kind !== "Hole");
+  if (hasHoles) {
+    const result = t.arrayPattern([]);
+    /*
+     * Older versions of babel have a validation bug fixed by
+     * https://github.com/babel/babel/pull/10917
+     * https://github.com/babel/babel/commit/e7b80a2cb93cf28010207fc3cdd19b4568ca35b9#diff-19b555d2f3904c206af406540d9df200b1e16befedb83ff39ebfcbd876f7fa8aL52
+     *
+     * Link to buggy older version (observe that elements must be PatternLikes here)
+     * https://github.com/babel/babel/blob/v7.7.4/packages/babel-types/src/definitions/es2015.js#L50-L53
+     *
+     * Link to newer versions with correct validation (observe elements can be PatternLike | null)
+     * https://github.com/babel/babel/blob/v7.23.0/packages/babel-types/src/definitions/core.ts#L1306-L1311
+     */
+    for (const item of pattern.items) {
+      if (item.kind === "Hole") {
+        result.elements.push(null);
+      } else {
+        result.elements.push(codegenLValue(cx, item));
+      }
+    }
+    return result;
+  } else {
+    return t.arrayPattern(
+      pattern.items.map((item) => {
+        if (item.kind === "Hole") {
+          return null;
+        }
+        return codegenLValue(cx, item);
+      })
+    );
+  }
+}
+
 function codegenLValue(
   cx: Context,
   pattern: Pattern | Place | SpreadPattern
 ): t.ArrayPattern | t.ObjectPattern | t.RestElement | t.Identifier {
   switch (pattern.kind) {
     case "ArrayPattern": {
-      return t.arrayPattern(
-        pattern.items.map((item) => {
-          if (item.kind === "Hole") {
-            return null;
-          }
-          return codegenLValue(cx, item);
-        })
-      );
+      return codegenArrayPattern(cx, pattern);
     }
     case "ObjectPattern": {
       return t.objectPattern(

compiler/packages/babel-plugin-react-forget/src/__tests__/fixtures/compiler/error.backwards-compatible-holey-arrays.expect.md

@@ -1,20 +0,0 @@
-
-## Input
-
-```javascript
-// @bailoutOnHoleyArrays
-
-function Component() {
-  return [1, , 3];
-}
-
-```
-
-
-## Error
-
-```
-[ReactForget] Todo: (BuildHIR::lower) Fix babel holey array backward compatibility. (4:4)
-```
-          
-      

compiler/packages/babel-plugin-react-forget/src/__tests__/fixtures/compiler/error.backwards-compatible-holey-arrays.js

@@ -1,5 +0,0 @@
-// @bailoutOnHoleyArrays
-
-function Component() {
-  return [1, , 3];
-}