Auto merge of #12243 - TravisDean:else-brace-check, r=ConnorGBrewster

Add style check and test for else braces

<!-- Please describe your changes on the following line: -->
Added a regex check to enforce project style guidelines. Added unit check. Catches:
```
}
else {
```
when it should be
`} else {`

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [ ] `./mach build -d` does not report any errors (no core files changed)
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #12234 (github issue number if applicable).

<!-- Either: -->
- [X] There are tests for these changes OR
- [ ] These changes do not require tests because _____

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="35" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/12243)
<!-- Reviewable:end -->

components/compositing/compositor.rs

@@ -2261,8 +2261,7 @@ impl<Window: WindowMethods> IOCompositor<Window> {
                             rendergl::render_scene(layer.clone(), context, &self.scene);
                             gl::disable(gl::SCISSOR_TEST);
 
-                        }
+                        } else {
-                        else {
                             rendergl::render_scene(layer.clone(), context, &self.scene);
                         }
                     }

components/gfx/paint_context.rs

@@ -1788,8 +1788,11 @@ impl ScaledFontExtensionMethods for ScaledFont {
         let mut options = struct__AzDrawOptions {
             mAlpha: 1f64 as AzFloat,
             mCompositionOp: CompositionOp::Over as u8,
-            mAntialiasMode: if antialias { AntialiasMode::Subpixel as u8 }
+            mAntialiasMode: if antialias {
-                            else { AntialiasMode::None as u8 }
+                                AntialiasMode::Subpixel as u8
+                            } else {
+                                AntialiasMode::None as u8
+                            }
         };
 
         let mut origin = baseline_origin.clone();

components/net_traits/image/base.rs

@@ -81,12 +81,12 @@ pub fn load_from_memory(buffer: &[u8]) -> Option<Image> {
 
 // https://developer.mozilla.org/en-US/docs/Web/HTML/Element/img
 pub fn detect_image_format(buffer: &[u8]) -> Result<ImageFormat, &str> {
-    if is_gif(buffer)       { Ok(ImageFormat::GIF) }
+    if is_gif(buffer)         { Ok(ImageFormat::GIF)
-    else if is_jpeg(buffer) { Ok(ImageFormat::JPEG) }
+    } else if is_jpeg(buffer) { Ok(ImageFormat::JPEG)
-    else if is_png(buffer)  { Ok(ImageFormat::PNG) }
+    } else if is_png(buffer)  { Ok(ImageFormat::PNG)
-    else if is_bmp(buffer)  { Ok(ImageFormat::BMP) }
+    } else if is_bmp(buffer)  { Ok(ImageFormat::BMP)
-    else if is_ico(buffer)  { Ok(ImageFormat::ICO) }
+    } else if is_ico(buffer)  { Ok(ImageFormat::ICO)
-    else { Err("Image Format Not Supported") }
+    } else { Err("Image Format Not Supported") }
 }
 
 fn is_gif(buffer: &[u8]) -> bool {

components/script/dom/htmlmediaelement.rs

@@ -133,9 +133,8 @@ impl AsyncResponseListener for HTMLMediaElementContext {
 
             // Step 5
             elem.fire_simple_event("error");
-        }
+        } else {
             // => "If the media data cannot be fetched at all..."
-        else {
             elem.queue_dedicated_media_source_failure_steps();
         }
 

components/script/dom/xmlhttprequest.rs

@@ -1128,9 +1128,8 @@ impl XMLHttpRequest {
                 // Step 5
                 if self.response_type.get() == XMLHttpRequestResponseType::_empty {
                     return None;
-                }
+                } else {
                     // Step 6
-                else {
                     temp_doc = self.document_text_html();
                 }
             },
@@ -1143,8 +1142,7 @@ impl XMLHttpRequest {
             Some(Mime(_, mime::SubLevel::Ext(sub), _)) => {
                 if sub.ends_with("+xml") {
                     temp_doc = self.handle_xml();
-                }
+                } else {
-                else {
                     return None;
                 }
             },

components/script/webdriver_handlers.rs

@@ -374,8 +374,7 @@ pub fn handle_is_selected(context: &BrowsingContext,
             }
             else if let Some(_) = node.downcast::<HTMLElement>() {
                 Ok(false) // regular elements are not selectable
-            }
+            } else {
-            else {
                 Err(())
             }
         },

python/tidy/servo_tidy/tidy.py

@@ -415,6 +415,7 @@ def check_rust(file_name, lines):
             (r"^&&", "operators should go at the end of the first line", no_filter),
             (r"\{[A-Za-z0-9_]+\};", "use statement contains braces for single import",
                 lambda match, line: line.startswith('use ')),
+            (r"^\s*else {", "else braces should be on the same line", no_filter),
         ]
 
         for pattern, message, filter_func in regex_rules:

python/tidy/servo_tidy_tests/rust_tidy.rs

@@ -41,6 +41,12 @@ impl test {
         let x = true;
         x
             && x;
+        if x {
+             ;
+        }
+        else {
+             ;
+        }
     }
 
 }

python/tidy/servo_tidy_tests/test_tidy.py

@@ -76,6 +76,7 @@ class CheckTidiness(unittest.TestCase):
         self.assertEqual('use &str instead of &String', errors.next()[2])
         self.assertEqual('use &T instead of &Root<T>', errors.next()[2])
         self.assertEqual('operators should go at the end of the first line', errors.next()[2])
+        self.assertEqual('else braces should be on the same line', errors.next()[2])
         self.assertNoMoreErrors(errors)
 
     def test_spec_link(self):