Make --list-different to work with --stdin (#2393)

* Add test for `prettier --stdin --list-different`

See https://github.com/prettier/prettier/issues/2389

* Pass test for `prettier --stdin --list-different`

This fixes https://github.com/prettier/prettier/issues/2389

* Deduplicate --list-different handling

* Exit-early to reduce nesting in listDifferent()

* Simplify condition in listDifferent()

bin/prettier.js

@@ -252,6 +252,10 @@ if (argv["help"] || (!filepatterns.length && !stdin)) {
 
 if (stdin) {
   getStream(process.stdin).then(input => {
+    if (listDifferent(input, options, "(stdin)")) {
+      return;
+    }
+
     try {
       writeOutput(format(input, options));
     } catch (e) {
@@ -279,19 +283,7 @@ if (stdin) {
       return;
     }
 
-    if (argv["list-different"]) {
-      if (
-        !prettier.check(
-          input,
-          Object.assign({}, options, { filepath: filename })
-        )
-      ) {
-        if (!write) {
-          console.log(filename);
-        }
-        process.exitCode = 1;
-      }
-    }
+    listDifferent(input, options, filename);
 
     const start = Date.now();
 
@@ -350,6 +342,23 @@ if (stdin) {
   });
 }
 
+function listDifferent(input, options, filename) {
+  if (!argv["list-different"]) {
+    return;
+  }
+
+  options = Object.assign({}, options, { filepath: filename });
+
+  if (!prettier.check(input, options)) {
+    if (!write) {
+      console.log(filename);
+    }
+    process.exitCode = 1;
+  }
+
+  return true;
+}
+
 function writeOutput(result) {
   // Don't use `console.log` here since it adds an extra newline at the end.
   process.stdout.write(result.formatted);

tests_integration/__tests__/list-different.js

@@ -0,0 +1,13 @@
+"use strict";
+
+const runPrettier = require("../runPrettier");
+
+test("checks stdin with --list-different", () => {
+  const result = runPrettier("cli/with-shebang", ["--list-different"], {
+    input: "0"
+  });
+
+  expect(result.stdout).toEqual("(stdin)\n");
+  expect(result.stderr).toEqual("");
+  expect(result.status).not.toEqual(0);
+});