Fix long field accesses not being broken onto separate lines

Fixes #512

src/chains.rs

@@ -194,8 +194,22 @@ fn rewrite_chain_expr(expr: &ast::Expr,
                                 width,
                                 offset)
         }
-        ast::Expr_::ExprField(_, ref field) => Some(format!(".{}", field.node)),
-        ast::Expr_::ExprTupField(_, ref field) => Some(format!(".{}", field.node)),
+        ast::Expr_::ExprField(_, ref field) => {
+            let s = format!(".{}", field.node);
+            if s.len() <= width {
+                Some(s)
+            } else {
+                None
+            }
+        }
+        ast::Expr_::ExprTupField(_, ref field) => {
+            let s = format!(".{}", field.node);
+            if s.len() <= width {
+                Some(s)
+            } else {
+                None
+            }
+        }
         _ => unreachable!(),
     }
 }

tests/source/long_field_access.rs

@@ -0,0 +1,3 @@
+fn f() {
+    block_flow.base.stacking_relative_position_of_display_port = self.base.stacking_relative_position_of_display_port;
+}

tests/target/long_field_access.rs

@@ -0,0 +1,4 @@
+fn f() {
+    block_flow.base.stacking_relative_position_of_display_port =
+        self.base.stacking_relative_position_of_display_port;
+}