Don't warn on #pragma once in header files

If the file being linted has an extension of .h, .hpp, or .hh enable the
compiler option to ignore #pragma once in the "main file", since it is
perfectly valid here and this is considered the main file.

Author: Arcanemagus

lib/main.js

@@ -2,7 +2,7 @@
 
 // eslint-disable-next-line import/extensions, import/no-extraneous-dependencies
 import { CompositeDisposable } from 'atom';
-import { dirname } from 'path';
+import { dirname, extname } from 'path';
 
 let helpers = null;
 let clangFlags = null;
@@ -73,6 +73,7 @@ export default {
         }
 
         const filePath = editor.getPath();
+        const fileExt = extname(filePath);
         const fileText = editor.getText();
 
         const args = [
@@ -87,6 +88,10 @@ export default {
         const grammar = editor.getGrammar().name;
 
         switch (grammar) {
+          case 'Objective-C':
+            args.push('-xobjective-c');
+            args.push(...this.clangDefaultObjCFlags.split(/\s+/));
+            break;
           case 'Objective-C++':
             args.push('-xobjective-c++');
             args.push(...this.clangDefaultObjCppFlags.split(/\s+/));
@@ -95,10 +100,6 @@ export default {
             args.push('-xc');
             args.push(...this.clangDefaultCFlags.split(/\s+/));
             break;
-          case 'Objective-C':
-            args.push('-xobjective-c');
-            args.push(...this.clangDefaultObjCFlags.split(/\s+/));
-            break;
           default:
           case 'C++':
           case 'C++14':
@@ -107,6 +108,11 @@ export default {
             break;
         }
 
+        if (fileExt === '.hpp' || fileExt === '.hh' || fileExt === '.h') {
+          // Don't warn about #pragma once when linting header files
+          args.push('-Wno-pragma-once-outside-header');
+        }
+
         if (this.clangSuppressWarnings) {
           args.push('-w');
         }

spec/files/pragma/pragma_once.c

@@ -0,0 +1,3 @@
+#pragma once
+
+int main() {}

spec/files/pragma/pragma_once.cpp

@@ -0,0 +1,3 @@
+#pragma once
+
+int main() {}

spec/files/pragma/pragma_once.h

@@ -0,0 +1,3 @@
+#pragma once
+
+int main() {}

spec/files/pragma/pragma_once.hpp

@@ -0,0 +1,3 @@
+#pragma once
+
+int main() {}

spec/linter-clang-spec.js

@@ -7,6 +7,7 @@ import { join } from 'path';
 const lint = require('../lib/main').provideLinter().lint;
 
 const miPath = join(__dirname, 'files', 'missing_import');
+const poPath = join(__dirname, 'files', 'pragma', 'pragma_once');
 const validPath = join(__dirname, 'files', 'valid.c');
 
 describe('The Clang provider for AtomLinter', () => {
@@ -62,4 +63,38 @@ describe('The Clang provider for AtomLinter', () => {
       expect(messages[0].location.position).toEqual([[1, 9], [1, 20]]);
     });
   });
+
+  describe('handles pragma once properly', () => {
+    it('finds a pragma once warning in pragma_once.c', async () => {
+      const editor = await atom.workspace.open(`${poPath}.c`);
+      const messages = await lint(editor);
+      expect(messages.length).toEqual(1);
+      expect(messages[0].severity).toEqual('warning');
+      expect(messages[0].excerpt).toEqual('#pragma once in main file [-Wpragma-once-outside-header]');
+      expect(messages[0].location.file).toBe(`${poPath}.c`);
+      expect(messages[0].location.position).toEqual([[0, 8], [0, 12]]);
+    });
+
+    it('finds a pragma once warning in pragma_once.cpp', async () => {
+      const editor = await atom.workspace.open(`${poPath}.cpp`);
+      const messages = await lint(editor);
+      expect(messages.length).toEqual(1);
+      expect(messages[0].severity).toEqual('warning');
+      expect(messages[0].excerpt).toEqual('#pragma once in main file [-Wpragma-once-outside-header]');
+      expect(messages[0].location.file).toBe(`${poPath}.cpp`);
+      expect(messages[0].location.position).toEqual([[0, 8], [0, 12]]);
+    });
+
+    it("doesn't find a pragma once warning in pragma_once.h", async () => {
+      const editor = await atom.workspace.open(`${poPath}.h`);
+      const messages = await lint(editor);
+      expect(messages.length).toEqual(0);
+    });
+
+    it("doesn't find a pragma once warning in pragma_once.hpp", async () => {
+      const editor = await atom.workspace.open(`${poPath}.hpp`);
+      const messages = await lint(editor);
+      expect(messages.length).toEqual(0);
+    });
+  });
 });