1 files changed, 195 insertions, 0 deletions
diff --git a/servo/components/style/queries/feature.rs b/servo/components/style/queries/feature.rs
new file mode 100644
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+++ b/servo/components/style/queries/feature.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/. */
+
+//! Query features.
+
+use super::condition::KleeneValue;
+use crate::parser::ParserContext;
+use crate::values::computed::{self, CSSPixelLength, Ratio, Resolution};
+use crate::Atom;
+use cssparser::Parser;
+use std::fmt;
+use style_traits::ParseError;
+
+/// A generic discriminant for an enum value.
+pub type KeywordDiscriminant = u8;
+
+type QueryFeatureGetter<T> = fn(device: &computed::Context) -> T;
+
+/// Serializes a given discriminant.
+///
+/// FIXME(emilio): we could prevent this allocation if the ToCss code would
+/// generate a method for keywords to get the static string or something.
+pub type KeywordSerializer = fn(KeywordDiscriminant) -> String;
+
+/// Parses a given identifier.
+pub type KeywordParser = for<'a, 'i, 't> fn(
+    context: &'a ParserContext,
+    input: &'a mut Parser<'i, 't>,
+) -> Result<KeywordDiscriminant, ParseError<'i>>;
+
+/// An evaluator for a given feature.
+///
+/// This determines the kind of values that get parsed, too.
+#[allow(missing_docs)]
+pub enum Evaluator {
+    Length(QueryFeatureGetter<CSSPixelLength>),
+    OptionalLength(QueryFeatureGetter<Option<CSSPixelLength>>),
+    Integer(QueryFeatureGetter<i32>),
+    Float(QueryFeatureGetter<f32>),
+    BoolInteger(QueryFeatureGetter<bool>),
+    /// A non-negative number ratio, such as the one from device-pixel-ratio.
+    NumberRatio(QueryFeatureGetter<Ratio>),
+    OptionalNumberRatio(QueryFeatureGetter<Option<Ratio>>),
+    /// A resolution.
+    Resolution(QueryFeatureGetter<Resolution>),
+    /// A keyword value.
+    Enumerated {
+        /// The parser to get a discriminant given a string.
+        parser: KeywordParser,
+        /// The serializer to get a string from a discriminant.
+        ///
+        /// This is guaranteed to be called with a keyword that `parser` has
+        /// produced.
+        serializer: KeywordSerializer,
+        /// The evaluator itself. This is guaranteed to be called with a
+        /// keyword that `parser` has produced.
+        evaluator: fn(&computed::Context, Option<KeywordDiscriminant>) -> KleeneValue,
+    },
+}
+
+/// A simple helper macro to create a keyword evaluator.
+///
+/// This assumes that keyword feature expressions don't accept ranges, and
+/// asserts if that's not true. As of today there's nothing like that (does that
+/// even make sense?).
+macro_rules! keyword_evaluator {
+    ($actual_evaluator:ident, $keyword_type:ty) => {{
+        fn __parse<'i, 't>(
+            context: &$crate::parser::ParserContext,
+            input: &mut $crate::cssparser::Parser<'i, 't>,
+        ) -> Result<$crate::queries::feature::KeywordDiscriminant, ::style_traits::ParseError<'i>>
+        {
+            let kw = <$keyword_type as $crate::parser::Parse>::parse(context, input)?;
+            Ok(kw as $crate::queries::feature::KeywordDiscriminant)
+        }
+
+        fn __serialize(kw: $crate::queries::feature::KeywordDiscriminant) -> String {
+            // This unwrap is ok because the only discriminants that get
+            // back to us is the ones that `parse` produces.
+            let value: $keyword_type = ::num_traits::cast::FromPrimitive::from_u8(kw).unwrap();
+            <$keyword_type as ::style_traits::ToCss>::to_css_string(&value)
+        }
+
+        fn __evaluate(
+            context: &$crate::values::computed::Context,
+            value: Option<$crate::queries::feature::KeywordDiscriminant>,
+        ) -> $crate::queries::condition::KleeneValue {
+            // This unwrap is ok because the only discriminants that get
+            // back to us is the ones that `parse` produces.
+            let value: Option<$keyword_type> =
+                value.map(|kw| ::num_traits::cast::FromPrimitive::from_u8(kw).unwrap());
+            $crate::queries::condition::KleeneValue::from($actual_evaluator(context, value))
+        }
+
+        $crate::queries::feature::Evaluator::Enumerated {
+            parser: __parse,
+            serializer: __serialize,
+            evaluator: __evaluate,
+        }
+    }};
+}
+
+bitflags! {
+    /// Different flags or toggles that change how a expression is parsed or
+    /// evaluated.
+    #[derive(Clone, Copy, ToShmem)]
+    pub struct FeatureFlags : u8 {
+        /// The feature should only be parsed in chrome and ua sheets.
+        const CHROME_AND_UA_ONLY = 1 << 0;
+        /// The feature requires a -webkit- prefix.
+        const WEBKIT_PREFIX = 1 << 1;
+        /// The feature requires the inline-axis containment.
+        const CONTAINER_REQUIRES_INLINE_AXIS = 1 << 2;
+        /// The feature requires the block-axis containment.
+        const CONTAINER_REQUIRES_BLOCK_AXIS = 1 << 3;
+        /// The feature requires containment in the physical width axis.
+        const CONTAINER_REQUIRES_WIDTH_AXIS = 1 << 4;
+        /// The feature requires containment in the physical height axis.
+        const CONTAINER_REQUIRES_HEIGHT_AXIS = 1 << 5;
+        /// The feature evaluation depends on the viewport size.
+        const VIEWPORT_DEPENDENT = 1 << 6;
+    }
+}
+
+impl FeatureFlags {
+    /// Returns parsing requirement flags.
+    pub fn parsing_requirements(self) -> Self {
+        self.intersection(Self::CHROME_AND_UA_ONLY | Self::WEBKIT_PREFIX)
+    }
+
+    /// Returns all the container axis flags.
+    pub fn all_container_axes() -> Self {
+        Self::CONTAINER_REQUIRES_INLINE_AXIS |
+            Self::CONTAINER_REQUIRES_BLOCK_AXIS |
+            Self::CONTAINER_REQUIRES_WIDTH_AXIS |
+            Self::CONTAINER_REQUIRES_HEIGHT_AXIS
+    }
+
+    /// Returns our subset of container axis flags.
+    pub fn container_axes(self) -> Self {
+        self.intersection(Self::all_container_axes())
+    }
+}
+
+/// Whether a feature allows ranges or not.
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+#[allow(missing_docs)]
+pub enum AllowsRanges {
+    Yes,
+    No,
+}
+
+/// A description of a feature.
+pub struct QueryFeatureDescription {
+    /// The feature name, in ascii lowercase.
+    pub name: Atom,
+    /// Whether min- / max- prefixes are allowed or not.
+    pub allows_ranges: AllowsRanges,
+    /// The evaluator, which we also use to determine which kind of value to
+    /// parse.
+    pub evaluator: Evaluator,
+    /// Different feature-specific flags.
+    pub flags: FeatureFlags,
+}
+
+impl QueryFeatureDescription {
+    /// Whether this feature allows ranges.
+    #[inline]
+    pub fn allows_ranges(&self) -> bool {
+        self.allows_ranges == AllowsRanges::Yes
+    }
+}
+
+/// A simple helper to construct a `QueryFeatureDescription`.
+macro_rules! feature {
+    ($name:expr, $allows_ranges:expr, $evaluator:expr, $flags:expr,) => {
+        $crate::queries::feature::QueryFeatureDescription {
+            name: $name,
+            allows_ranges: $allows_ranges,
+            evaluator: $evaluator,
+            flags: $flags,
+        }
+    };
+}
+
+impl fmt::Debug for QueryFeatureDescription {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.debug_struct("QueryFeatureDescription")
+            .field("name", &self.name)
+            .field("allows_ranges", &self.allows_ranges)
+            .field("flags", &self.flags)
+            .finish()
+    }
+}