style: Add a ValueInfo trait for exposing types needed by devtools.

Most of types just derive it using proc_macro directly. Some of value
types need manual impl.

In my current plan, this new trait will be used in bug 1434130 to expose
values as well.

Bug: 1455576
Reviewed-by: emilio
MozReview-Commit-ID: LI7fy45VkRw

components/style_traits/specified_value_info.rs

@@ -0,0 +1,71 @@
+/* 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/. */
+
+//! Value information for devtools.
+
+use servo_arc::Arc;
+use std::ops::Range;
+
+/// Type of value that a property supports. This is used by Gecko's
+/// devtools to make sense about value it parses, and types listed
+/// here should match TYPE_* constants in InspectorUtils.webidl.
+///
+/// XXX This should really be a bitflags rather than a namespace mod,
+/// but currently we cannot use bitflags in const.
+#[allow(non_snake_case)]
+pub mod CssType {
+    /// <color>
+    pub const COLOR: u8 = 1 << 0;
+    /// <gradient>
+    pub const GRADIENT: u8 = 1 << 1;
+    /// <timing-function>
+    pub const TIMING_FUNCTION: u8 = 1 << 2;
+}
+
+/// Information of values of a given specified value type.
+pub trait SpecifiedValueInfo {
+    /// Supported CssTypes by the given value type.
+    ///
+    /// XXX This should be typed CssType when that becomes a bitflags.
+    /// Currently we cannot do so since bitflags cannot be used in constant.
+    const SUPPORTED_TYPES: u8 = 0;
+}
+
+impl SpecifiedValueInfo for bool {}
+impl SpecifiedValueInfo for f32 {}
+impl SpecifiedValueInfo for i8 {}
+impl SpecifiedValueInfo for i32 {}
+impl SpecifiedValueInfo for u8 {}
+impl SpecifiedValueInfo for u16 {}
+impl SpecifiedValueInfo for u32 {}
+impl SpecifiedValueInfo for str {}
+impl SpecifiedValueInfo for String {}
+
+impl<T: SpecifiedValueInfo + ?Sized> SpecifiedValueInfo for Box<T> {
+    const SUPPORTED_TYPES: u8 = T::SUPPORTED_TYPES;
+}
+
+impl<T: SpecifiedValueInfo> SpecifiedValueInfo for [T] {
+    const SUPPORTED_TYPES: u8 = T::SUPPORTED_TYPES;
+}
+
+macro_rules! impl_generic_specified_value_info {
+    ($ty:ident<$param:ident>) => {
+        impl<$param: SpecifiedValueInfo> SpecifiedValueInfo for $ty<$param> {
+            const SUPPORTED_TYPES: u8 = $param::SUPPORTED_TYPES;
+        }
+    }
+}
+impl_generic_specified_value_info!(Option<T>);
+impl_generic_specified_value_info!(Vec<T>);
+impl_generic_specified_value_info!(Arc<T>);
+impl_generic_specified_value_info!(Range<Idx>);
+
+impl<T1, T2> SpecifiedValueInfo for (T1, T2)
+where
+    T1: SpecifiedValueInfo,
+    T2: SpecifiedValueInfo,
+{
+    const SUPPORTED_TYPES: u8 = T1::SUPPORTED_TYPES | T2::SUPPORTED_TYPES;
+}