Introduce coding-conventions.md

Summary:
Hopefully, the new document makes it clear when one can use QRect::right()
and QRect::bottom().

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: davidedmundson, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D27060

HACKING.md

@@ -91,5 +91,8 @@ KWin uses [KDE's phabricator instance](https://phabricator.kde.org) for code rev
 
 More complete documentation can be found in [KDE's wiki](https://community.kde.org/Infrastructure/Phabricator). Please add "#KWin" as reviewers. Please run KWin's automated test suite prior to uploading a patch to ensure that the change does not break existing code.
 
+# Coding conventions
+KWin's coding conventions are explained in document [coding-conventions.md](doc/coding-conventions.md).
+
 # Coding style
 KWin code follows the [Frameworks coding style](https://techbase.kde.org/Policies/Frameworks_Coding_Style).

doc/coding-conventions.md

@@ -0,0 +1,86 @@
+# Coding Conventions
+
+This document describes some of the recommended coding conventions that should be followed in KWin.
+
+For KWin, it is recommended to follow the KDE Frameworks Coding Style.
+
+
+## `auto` Keyword
+
+Optionally, you can use the `auto` keyword in the following cases. If in doubt, for example if using
+`auto` could make the code less readable, do not use `auto`. Keep in mind that code is read much more
+often than written.
+
+* When it avoids repetition of a type in the same statement.
+
+  ```
+  auto something = new MyCustomType;
+  auto keyEvent = static_cast<QKeyEvent *>(event);
+  auto myList = QStringList({ "FooThing",  "BarThing" });
+  ```
+
+* When assigning iterator types.
+
+  ```
+  auto it = myList.const_iterator();
+  ```
+
+
+## `QRect::right()` and `QRect::bottom()`
+
+For historical reasons, the `QRect::right()` and `QRect::bottom()` functions deviate from the true
+bottom-right corner of the rectangle. Note that this is not the case for the `QRectF` class.
+
+As a general rule, avoid using `QRect::right()` and `QRect::bottom()` as well methods that operate
+on them. There are exceptions, though.
+
+Exception 1: you can use `QRect::moveRight()` and `QRect::moveBottom()` to snap a `QRect` to
+another `QRect` as long as the corresponding borders match, for example
+
+```
+// Ok
+rect.moveRight(anotherRect.right());
+rect.moveBottom(anotherRect.bottom());
+rect.moveBottomRight(anotherRect.bottomRight());
+
+// Bad
+rect.moveRight(anotherRect.left() - 1); // must be rect.moveLeft(anotherRect.left() - rect.width());
+rect.moveBottom(anotherRect.top() - 1); // must be rect.moveTop(anotherRect.top() - rect.height());
+rect.moveBottomRight(anotherRect.topLeft() - QPoint(1, 1));
+```
+
+Exception 2: you can use `QRect::setRight()` and `QRect::setBottom()` to clip a `QRect` by another
+`QRect` as long as the corresponding borders match, for example
+
+```
+// Ok
+rect.setRight(anotherRect.right());
+rect.setBottom(anotherRect.bottom());
+rect.setBottomRight(anotherRect.bottomRight());
+
+// Bad
+rect.setRight(anotherRect.left());
+rect.setBottom(anotherRect.top());
+rect.setBottomRight(anotherRect.topLeft());
+```
+
+Exception 3: you can use `QRect::right()` and `QRect::bottom()` in conditional statements as long
+as the compared borders are the same, for example
+
+```
+// Ok
+if (rect.right() > anotherRect.right()) {
+    return;
+}
+if (rect.bottom() > anotherRect.bottom()) {
+    return;
+}
+
+// Bad
+if (rect.right() > anotherRect.left()) {
+    return;
+}
+if (rect.bottom() > anotherRect.top()) {
+    return;
+}
+```