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diff --git a/servo/components/style_traits/specified_value_info.rs b/servo/components/style_traits/specified_value_info.rs
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+++ b/servo/components/style_traits/specified_value_info.rs
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+/* 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 https://mozilla.org/MPL/2.0/. */
+
+//! Value information for devtools.
+
+use crate::arc_slice::ArcSlice;
+use crate::owned_slice::OwnedSlice;
+use servo_arc::Arc;
+use std::ops::Range;
+use std::sync::Arc as StdArc;
+
+/// 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 InspectorPropertyType 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;
+}
+
+/// See SpecifiedValueInfo::collect_completion_keywords.
+pub type KeywordsCollectFn<'a> = &'a mut dyn FnMut(&[&'static str]);
+
+/// Information of values of a given specified value type.
+///
+/// This trait is derivable with `#[derive(SpecifiedValueInfo)]`.
+///
+/// The algorithm traverses the type definition. For `SUPPORTED_TYPES`,
+/// it puts an or'ed value of `SUPPORTED_TYPES` of all types it finds.
+/// For `collect_completion_keywords`, it recursively invokes this
+/// method on types found, and lists all keyword values and function
+/// names following the same rule as `ToCss` in that method.
+///
+/// Some attributes of `ToCss` can affect the behavior, specifically:
+/// * If `#[css(function)]` is found, the content inside the annotated
+///   variant (or the whole type) isn't traversed, only the function
+///   name is listed in `collect_completion_keywords`.
+/// * If `#[css(skip)]` is found, the content inside the variant or
+///   field is ignored.
+/// * Values listed in `#[css(if_empty)]`, `#[parse(aliases)]`, and
+///   `#[css(keyword)]` are added into `collect_completion_keywords`.
+///
+/// In addition to `css` attributes, it also has `value_info` helper
+/// attributes, including:
+/// * `#[value_info(ty = "TYPE")]` can be used to specify a constant
+///   from `CssType` to `SUPPORTED_TYPES`.
+/// * `#[value_info(other_values = "value1,value2")]` can be used to
+///   add other values related to a field, variant, or the type itself
+///   into `collect_completion_keywords`.
+/// * `#[value_info(starts_with_keyword)]` can be used on variants to
+///   add the name of a non-unit variant (serialized like `ToCss`) into
+///   `collect_completion_keywords`.
+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;
+
+    /// Collect value starting words for the given specified value type.
+    /// This includes keyword and function names which can appear at the
+    /// beginning of a value of this type.
+    ///
+    /// Caller should pass in a callback function to accept the list of
+    /// values. The callback function can be called multiple times, and
+    /// some values passed to the callback may be duplicate.
+    fn collect_completion_keywords(_f: KeywordsCollectFn) {}
+}
+
+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 usize {}
+impl SpecifiedValueInfo for str {}
+impl SpecifiedValueInfo for String {}
+impl SpecifiedValueInfo for crate::owned_str::OwnedStr {}
+
+#[cfg(feature = "servo")]
+impl SpecifiedValueInfo for ::servo_atoms::Atom {}
+#[cfg(feature = "servo")]
+impl SpecifiedValueInfo for ::servo_url::ServoUrl {}
+
+impl<T: SpecifiedValueInfo + ?Sized> SpecifiedValueInfo for Box<T> {
+    const SUPPORTED_TYPES: u8 = T::SUPPORTED_TYPES;
+    fn collect_completion_keywords(f: KeywordsCollectFn) {
+        T::collect_completion_keywords(f);
+    }
+}
+
+impl<T: SpecifiedValueInfo> SpecifiedValueInfo for [T] {
+    const SUPPORTED_TYPES: u8 = T::SUPPORTED_TYPES;
+    fn collect_completion_keywords(f: KeywordsCollectFn) {
+        T::collect_completion_keywords(f);
+    }
+}
+
+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;
+            fn collect_completion_keywords(f: KeywordsCollectFn) {
+                $param::collect_completion_keywords(f);
+            }
+        }
+    };
+}
+impl_generic_specified_value_info!(Option<T>);
+impl_generic_specified_value_info!(OwnedSlice<T>);
+impl_generic_specified_value_info!(Vec<T>);
+impl_generic_specified_value_info!(Arc<T>);
+impl_generic_specified_value_info!(StdArc<T>);
+impl_generic_specified_value_info!(ArcSlice<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;
+
+    fn collect_completion_keywords(f: KeywordsCollectFn) {
+        T1::collect_completion_keywords(f);
+        T2::collect_completion_keywords(f);
+    }
+}