Add JSX Fragment syntax support (#6552)

* Add JSX Fragments to babel-types

* Support JSX fragments in the transform-react-jsx plugin

* Add tests JSX fragments

* Update helper-builder and transform plugin documentations for jsx fragment

* Add generator for jsx fragments

* Add test for jsx fragment generator

* Split jsx transform example into normal and fragment examples

* Remove unnecessary fields from ElementState in babel-helper-builder-react-jsx

* inline [skip ci]

packages/babel-helper-builder-react-jsx/src/index.js

@@ -3,11 +3,9 @@ import * as t from "@babel/types";
 
 type ElementState = {
   tagExpr: Object, // tag node
-  tagName: string, // raw string tag name
+  tagName: ?string, // raw string tag name
   args: Array<Object>, // array of call arguments
   call?: Object, // optional call property that can be set to override the call expression returned
-  pre?: Function, // function called with (state: ElementState) before building attribs
-  post?: Function, // function called with (state: ElementState) after building attribs
 };
 
 export default function(opts) {
@@ -30,6 +28,20 @@ You can turn on the 'throwIfNamespace' flag to bypass this warning.`,
     },
   };
 
+  visitor.JSXFragment = {
+    exit(path, file) {
+      if (opts.compat) {
+        throw path.buildCodeFrameError(
+          "Fragment tags are only supported in React 16 and up.",
+        );
+      }
+      const callExpr = buildFragmentCall(path, file);
+      if (callExpr) {
+        path.replaceWith(t.inherits(callExpr, path.node));
+      }
+    },
+  };
+
   return visitor;
 
   function convertJSXIdentifier(node, parent) {
@@ -188,4 +200,35 @@ You can turn on the 'throwIfNamespace' flag to bypass this warning.`,
 
     return attribs;
   }
+
+  function buildFragmentCall(path, file) {
+    if (opts.filter && !opts.filter(path.node, file)) return;
+
+    const openingPath = path.get("openingElement");
+    openingPath.parent.children = t.react.buildChildren(openingPath.parent);
+
+    const args = [];
+    const tagName = null;
+    const tagExpr = file.get("jsxFragIdentifier")();
+
+    const state: ElementState = {
+      tagExpr: tagExpr,
+      tagName: tagName,
+      args: args,
+    };
+
+    if (opts.pre) {
+      opts.pre(state, file);
+    }
+
+    // no attributes are allowed with <> syntax
+    args.push(t.nullLiteral(), ...path.node.children);
+
+    if (opts.post) {
+      opts.post(state, file);
+    }
+
+    file.set("usedFragment", true);
+    return state.call || t.callExpression(state.callee, args);
+  }
 }