Prevent sibling 'it'

Author: rattrayalex

src/index.js

@@ -1009,6 +1009,14 @@ export default function (babel) {
     }, null);
   }
 
+  function prependDeclaration(path, id, init, kind = "const") {
+    path.insertBefore(t.variableDeclaration(kind, [
+      t.variableDeclarator(id, init)
+    ]));
+    const constPath = path.getSibling(path.key - 1);
+    path.scope.registerBinding(kind, constPath);
+  }
+
   // TYPE DEFINITIONS
   definePluginType("ForInArrayStatement", {
     visitor: ["idx", "elem", "array", "body"],
@@ -1578,9 +1586,10 @@ export default function (babel) {
           argRef = discriminant;
         } else {
           argRef = alias;
-          path.insertBefore(t.variableDeclaration("const", [
-            t.variableDeclarator(argRef, discriminant)
-          ]));
+          if (path.scope.hasOwnBinding(argRef.name)) {
+            throw path.buildCodeFrameError("Cannot re-use `it` binding in the same scope. Try a new shorthand name (eg; `match foo as x:`).");
+          }
+          prependDeclaration(path, argRef, discriminant);
         }
 
         const matchBody = transformMatchCases(argRef, path.get("cases"));

test/fixtures/match/illegal-sibling-no-as/actual.js

@@ -0,0 +1,5 @@
+match foo():
+  | 1: "ok"
+
+match bar():
+  | 2: "oops"

test/fixtures/match/illegal-sibling-no-as/options.json

@@ -0,0 +1,3 @@
+{
+  "throws": "Cannot re-use `it` binding in the same scope. Try a new shorthand name (eg; `match foo as x:`)."
+}

test/fixtures/match/nested-it/actual.js

@@ -0,0 +1,7 @@
+match foo():
+  | 1:
+    match it:
+      | 2: it
+  | with { x }:
+    match x:
+      | 2: it

test/fixtures/match/nested-it/expected.js

@@ -0,0 +1,18 @@
+function _hasProps(obj) { if (obj == null) return false; if (typeof obj !== "object" && typeof obj !== "function") return false; var i = arguments.length; while (--i > 0) { if (!(arguments[i] in obj)) return false; } return true; }
+
+const it = foo();
+
+if (it === 1) {
+  const it = it;
+
+  if (it === 2) {
+      it;
+    }
+} else if (_hasProps(it, "x")) {
+  const { x } = it;
+  const it = x;
+
+  if (it === 2) {
+      it;
+    }
+}

test/fixtures/match/sibling-as/actual.js

@@ -0,0 +1,5 @@
+match foo():
+  | 1: "ok"
+
+match bar() as x:
+  | 2: "ok 2"

test/fixtures/match/sibling-as/expected.js

@@ -0,0 +1,11 @@
+const it = foo();
+
+if (it === 1) {
+    "ok";
+  }
+
+const x = bar();
+
+if (x === 2) {
+    "ok 2";
+  }