style: Centralize construction of computed::Context

For controlled construction and access of upcoming, lazily-evaluated container query size.

Differential Revision: https://phabricator.services.mozilla.com/D158055

components/style/properties/cascade.rs

@@ -26,7 +26,6 @@ use fxhash::FxHashMap;
 use servo_arc::Arc;
 use smallvec::SmallVec;
 use std::borrow::Cow;
-use std::cell::RefCell;
 #[cfg(feature = "gecko")]
 use std::mem;
 
@@ -277,11 +276,11 @@ where
 
     let is_root_element = pseudo.is_none() && element.map_or(false, |e| e.is_root());
 
-    let mut context = computed::Context {
+    let mut context = computed::Context::new(
         // We'd really like to own the rules here to avoid refcount traffic, but
         // animation's usage of `apply_declarations` make this tricky. See bug
         // 1375525.
-        builder: StyleBuilder::new(
+        StyleBuilder::new(
             device,
             parent_style,
             parent_style_ignoring_first_line,
@@ -290,14 +289,9 @@ where
             custom_properties,
             is_root_element,
         ),
-        cached_system_font: None,
-        in_media_query: false,
-        for_smil_animation: false,
-        for_non_inherited_property: None,
-        container_info: None,
         quirks_mode,
-        rule_cache_conditions: RefCell::new(rule_cache_conditions),
-    };
+        rule_cache_conditions,
+    );
 
     let using_cached_reset_properties;
     let mut cascade = Cascade::new(&mut context, cascade_mode, &referenced_properties);

components/style/stylesheets/viewport_rule.rs

@@ -28,7 +28,6 @@ use cssparser::{parse_important, AtRuleParser, DeclarationListParser, Declaratio
 use euclid::Size2D;
 use selectors::parser::SelectorParseErrorKind;
 use std::borrow::Cow;
-use std::cell::RefCell;
 use std::fmt::{self, Write};
 use std::iter::Enumerate;
 use std::str::Chars;
@@ -665,18 +664,13 @@ impl MaybeNew for ViewportConstraints {
         let initial_viewport = device.au_viewport_size();
 
         let mut conditions = RuleCacheConditions::default();
-        let context = Context {
+        let context = Context::new(
             // Note: DEVICE-ADAPT § 5. states that relative length values are
             // resolved against initial values
-            builder: StyleBuilder::for_inheritance(device, None, None),
-            cached_system_font: None,
-            in_media_query: false,
+            StyleBuilder::for_inheritance(device, None, None),
             quirks_mode,
-            container_info: None,
-            for_smil_animation: false,
-            for_non_inherited_property: None,
-            rule_cache_conditions: RefCell::new(&mut conditions),
-        };
+            &mut conditions,
+        );
 
         // DEVICE-ADAPT § 9.3 Resolving 'extend-to-zoom'
         let extend_width;

components/style/values/computed/mod.rs

@@ -246,6 +246,43 @@ impl<'a> Context<'a> {
         f(&context)
     }
 
+    /// Creates a context suitable for more general cases.
+    pub fn new(
+        builder: StyleBuilder<'a>,
+        quirks_mode: QuirksMode,
+        rule_cache_conditions: &'a mut RuleCacheConditions,
+    ) -> Self {
+        Self {
+            builder,
+            cached_system_font: None,
+            in_media_query: false,
+            quirks_mode,
+            container_info: None,
+            for_smil_animation: false,
+            for_non_inherited_property: None,
+            rule_cache_conditions: RefCell::new(rule_cache_conditions),
+        }
+    }
+
+    /// Creates a context suitable for computing animations.
+    pub fn new_for_animation(
+        builder: StyleBuilder<'a>,
+        for_smil_animation: bool,
+        quirks_mode: QuirksMode,
+        rule_cache_conditions: &'a mut RuleCacheConditions,
+    ) -> Self {
+        Self {
+            builder,
+            cached_system_font: None,
+            in_media_query: false,
+            quirks_mode,
+            container_info: None,
+            for_smil_animation,
+            for_non_inherited_property: None,
+            rule_cache_conditions: RefCell::new(rule_cache_conditions),
+        }
+    }
+
     /// The current device.
     pub fn device(&self) -> &Device {
         self.builder.device