Introduce style::values::generics::rect ▭

This defines a single type Rect<T> which allows us to abstract over
CSS values made of four sides top, right, bottom and left.

components/style/values/specified/rect.rs

@@ -0,0 +1,21 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+//! Computed types for CSS borders.
+
+use cssparser::Parser;
+use parser::ParserContext;
+use values::generics::rect::Rect;
+use values::specified::length::LengthOrNumber;
+
+/// A specified rectangle made of four `<length-or-number>` values.
+pub type LengthOrNumberRect = Rect<LengthOrNumber>;
+
+impl LengthOrNumberRect {
+    /// Parses a `LengthOrNumberRect`, rejecting negative values.
+    #[inline]
+    pub fn parse_non_negative(context: &ParserContext, input: &mut Parser) -> Result<Self, ()> {
+        Rect::parse_with(context, input, LengthOrNumber::parse_non_negative)
+    }
+}