Ignore null output option (#510)

Co-authored-by: Harry Sarson <[email protected]>
Co-authored-by: Simon Lydell <[email protected]>

lib/elm-test.js

@@ -78,6 +78,17 @@ function getPathToElmBinary(compiler /*: string | void */) /*: string */ {
   }
 }
 
+function outputOptionError() {
+  return `
+Only /dev/null is allowed.
+
+NOTE: Including this option will not have any effect, as \`elm-test\` does not
+produce any output files. It is meant to improve compatibility with editor
+plugins explicitly setting the --output flag to /dev/null when using
+\`elm-test make\` as a drop-in replacement for \`elm make\`.'
+  `.trim();
+}
+
 const examples = `
 elm-test
   Run tests in the tests/ folder
@@ -133,6 +144,17 @@ function main() {
       'Use a custom path to an Elm executable (default: elm)',
       undefined
     )
+    // Ensure compatibility with editor plugins setting --output=/dev/null when
+    // running `make`. This has caused issues with Emacs' flycheck-elm package.
+    .addOption(
+      new Option('--output <output>')
+        .argParser((value) => {
+          if (value !== '/dev/null') {
+            throw new InvalidOptionArgumentError(outputOptionError());
+          }
+        })
+        .hideHelp()
+    )
     .version(packageInfo.version, '--version', 'Print version and exit')
     .helpOption('-h, --help', 'Show help')
     .addHelpCommand('help [command]', 'Show help');

tests/flags.js

@@ -189,6 +189,29 @@ describe('flags', () => {
       assert.ok(Number.isInteger(runResult.status));
       assert.notStrictEqual(runResult.status, 0);
     }).timeout(60000);
+
+    describe('--output', () => {
+      it('should ignore --output flag when set to /dev/null', () => {
+        const runResult = execElmTest([
+          'make',
+          '--output=/dev/null',
+          'tests/Passing/One.elm',
+        ]);
+
+        assert.strictEqual(runResult.status, 0);
+      }).timeout(60000);
+
+      it('should fail if setting --output to anything other than /dev/null', () => {
+        const runResult = execElmTest([
+          'make',
+          '--output=output_file',
+          'tests/Passing/One.elm',
+        ]);
+
+        assert.ok(Number.isInteger(runResult.status));
+        assert.notStrictEqual(runResult.status, 0);
+      }).timeout(60000);
+    });
   });
 
   describe('--help', () => {