Document ternary expression style and the else-after-return prohibition. No bug, rs=luke over IRC, DONTBUILD because this is modifying docs but touching no code

--HG--
extra : rebase_source : 0c11adc6052878710300eea87520345ca7e9143f

mfbt/STYLE

@@ -42,6 +42,23 @@ A corollary: don't mix declaration types by declaring a T and a T* (or a T**,
 
   T* foo, bar; // BAD
 
+== Expressions ==
+
+Ternary expressions (a ? b : c) should use only one line if sufficiently short.
+Longer ternary expressions should use multiple lines.  The condition,
+consequent, and alternative should each be on separate lines (each part
+overflowing to additional lines as necessary), and the ? and : should be aligned
+with the start of the condition:
+
+  size_t
+  BinaryTree::height()
+  {
+    return isLeaf()
+           ? 0
+           : 1 + std::max(left()->height(),
+                          right()->height());
+  }
+
 == Bracing ==
 
 Don't brace single statements.
@@ -347,3 +364,20 @@ interprets that argument.
   DefineProperty(JSObject* obj, const char* name, Value v, Enumerability e);
 
 Use NULL for the null pointer constant.
+
+If a consequent in an if-statement ends with a return, don't specify an else.
+The else would be redundant with the return, and not using it avoids excess
+indentation.  If you feel the if-else alternation is important as a way to
+think about the choice being made, consider a ternary expression instead.
+
+  // BAD
+  if (f())
+    return 2;
+  else
+    return 5;
+  // GOOD
+  if (f())
+    return 2;
+  return 5;
+  // GOOD
+  return f() ? 2 : 5