Auto merge of #29728 - Loirooriol:sync-fontprovider-D157589, r=mrobinson

Simplify our setup for font metric queries from style

This is a backport of https://phabricator.services.mozilla.com/D157589,
by Emilio Cobos Álvarez, plus some additions so that Servo compiles,
and some parts from https://phabricator.services.mozilla.com/D144455.

Should have no change in behavior.

<!-- Please describe your changes on the following line: -->

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [ ] These changes fix #___ (GitHub issue number if applicable)

<!-- Either: -->
- [ ] There are tests for these changes OR
- [X] These changes do not require tests because there should be no change in behavior

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

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21 files changed, 172 insertions, 322 deletions

diff --git a/components/layout/query.rs b/components/layout/query.rs
index 1d4e810c23a..98a3ec662fb 100644
--- a/components/layout/query.rs
+++ b/components/layout/query.rs
@@ -856,7 +856,7 @@ where
         if element.has_data() {
             node.to_threadsafe().as_element().unwrap().resolved_style()
         } else {
-            let mut tlc = ThreadLocalStyleContext::new(&context.style_context);
+            let mut tlc = ThreadLocalStyleContext::new();
             let mut context = StyleContext {
                 shared: &context.style_context,
                 thread_local: &mut tlc,
@@ -911,7 +911,7 @@ pub fn process_resolved_style_request<'dom>(
         return String::new();
     }
 
-    let mut tlc = ThreadLocalStyleContext::new(&context.style_context);
+    let mut tlc = ThreadLocalStyleContext::new();
     let mut context = StyleContext {
         shared: &context.style_context,
         thread_local: &mut tlc,
diff --git a/components/layout/traversal.rs b/components/layout/traversal.rs
index d07f76105e8..840aba5ef05 100644
--- a/components/layout/traversal.rs
+++ b/components/layout/traversal.rs
@@ -46,7 +46,6 @@ impl<'a, 'dom, E> DomTraversal<E> for RecalcStyleAndConstructFlows<'a>
 where
     E: TElement,
     E::ConcreteNode: LayoutNode<'dom>,
-    E::FontMetricsProvider: Send,
 {
     fn process_preorder<F>(
         &self,
diff --git a/components/layout_2020/query.rs b/components/layout_2020/query.rs
index 74420ae2bb5..3fd02a1597c 100644
--- a/components/layout_2020/query.rs
+++ b/components/layout_2020/query.rs
@@ -356,7 +356,7 @@ pub fn process_resolved_style_request_for_unstyled_node<'dom>(
         return String::new();
     }
 
-    let mut tlc = ThreadLocalStyleContext::new(&context.style_context);
+    let mut tlc = ThreadLocalStyleContext::new();
     let mut context = StyleContext {
         shared: &context.style_context,
         thread_local: &mut tlc,
diff --git a/components/layout_2020/traversal.rs b/components/layout_2020/traversal.rs
index 1f73a7816f1..7ddb0e85ed4 100644
--- a/components/layout_2020/traversal.rs
+++ b/components/layout_2020/traversal.rs
@@ -34,7 +34,6 @@ impl<'a, 'dom, E> DomTraversal<E> for RecalcStyle<'a>
 where
     E: TElement,
     E::ConcreteNode: LayoutNode<'dom>,
-    E::FontMetricsProvider: Send,
 {
     fn process_preorder<F>(
         &self,
diff --git a/components/script/layout_dom/element.rs b/components/script/layout_dom/element.rs
index 9b9f8c4790b..adbeba7e39d 100644
--- a/components/script/layout_dom/element.rs
+++ b/components/script/layout_dom/element.rs
@@ -33,7 +33,6 @@ use style::context::SharedStyleContext;
 use style::data::ElementData;
 use style::dom::{DomChildren, LayoutIterator, TDocument, TElement, TNode, TShadowRoot};
 use style::element_state::*;
-use style::font_metrics::ServoMetricsProvider;
 use style::properties::PropertyDeclarationBlock;
 use style::selector_parser::{
     extended_filtering, AttrValue as SelectorAttrValue, Lang, NonTSPseudoClass, PseudoElement,
@@ -169,8 +168,6 @@ impl<'dom, LayoutDataType: LayoutDataTrait> style::dom::TElement
     type ConcreteNode = ServoLayoutNode<'dom, LayoutDataType>;
     type TraversalChildrenIterator = DomChildren<Self::ConcreteNode>;
 
-    type FontMetricsProvider = ServoMetricsProvider;
-
     fn as_node(&self) -> ServoLayoutNode<'dom, LayoutDataType> {
         ServoLayoutNode::from_layout_js(self.element.upcast())
     }
diff --git a/components/script_layout_interface/wrapper_traits.rs b/components/script_layout_interface/wrapper_traits.rs
index 9091a394161..8ba009a7a47 100644
--- a/components/script_layout_interface/wrapper_traits.rs
+++ b/components/script_layout_interface/wrapper_traits.rs
@@ -25,7 +25,6 @@ use style::context::SharedStyleContext;
 use style::data::ElementData;
 use style::dom::OpaqueNode;
 use style::dom::{LayoutIterator, NodeInfo, TElement, TNode};
-use style::font_metrics::ServoMetricsProvider;
 use style::properties::ComputedValues;
 use style::selector_parser::{PseudoElement, PseudoElementCascadeType, SelectorImpl};
 use style::stylist::RuleInclusion;
@@ -439,7 +438,6 @@ pub trait ThreadSafeLayoutElement<'dom>:
                             &context.guards,
                             &style_pseudo,
                             Some(data.styles.primary()),
-                            &ServoMetricsProvider,
                         ),
                     PseudoElementCascadeType::Lazy => {
                         context
@@ -451,7 +449,6 @@ pub trait ThreadSafeLayoutElement<'dom>:
                                 RuleInclusion::All,
                                 data.styles.primary(),
                                 /* is_probe = */ false,
-                                &ServoMetricsProvider,
                                 /* matching_func = */ None,
                             )
                             .unwrap()
diff --git a/components/style/context.rs b/components/style/context.rs
index 21b3633f670..08fdb4d1eef 100644
--- a/components/style/context.rs
+++ b/components/style/context.rs
@@ -9,7 +9,6 @@ use crate::animation::DocumentAnimationSet;
 use crate::bloom::StyleBloom;
 use crate::data::{EagerPseudoStyles, ElementData};
 use crate::dom::{SendElement, TElement};
-use crate::font_metrics::FontMetricsProvider;
 #[cfg(feature = "gecko")]
 use crate::gecko_bindings::structs;
 use crate::parallel::{STACK_SAFETY_MARGIN_KB, STYLE_THREAD_STACK_SIZE_KB};
@@ -720,9 +719,6 @@ pub struct ThreadLocalStyleContext<E: TElement> {
     pub selector_flags: SelectorFlagsMap<E>,
     /// Statistics about the traversal.
     pub statistics: PerThreadTraversalStatistics,
-    /// The struct used to compute and cache font metrics from style
-    /// for evaluation of the font-relative em/ch units and font-size
-    pub font_metrics_provider: E::FontMetricsProvider,
     /// A checker used to ensure that parallel.rs does not recurse indefinitely
     /// even on arbitrarily deep trees.  See Gecko bug 1376883.
     pub stack_limit_checker: StackLimitChecker,
@@ -731,27 +727,8 @@ pub struct ThreadLocalStyleContext<E: TElement> {
 }
 
 impl<E: TElement> ThreadLocalStyleContext<E> {
-    /// Creates a new `ThreadLocalStyleContext` from a shared one.
-    #[cfg(feature = "servo")]
-    pub fn new(shared: &SharedStyleContext) -> Self {
-        ThreadLocalStyleContext {
-            sharing_cache: StyleSharingCache::new(),
-            rule_cache: RuleCache::new(),
-            bloom_filter: StyleBloom::new(),
-            tasks: SequentialTaskList(Vec::new()),
-            selector_flags: SelectorFlagsMap::new(),
-            statistics: PerThreadTraversalStatistics::default(),
-            font_metrics_provider: E::FontMetricsProvider::create_from(shared),
-            stack_limit_checker: StackLimitChecker::new(
-                (STYLE_THREAD_STACK_SIZE_KB - STACK_SAFETY_MARGIN_KB) * 1024,
-            ),
-            nth_index_cache: NthIndexCache::default(),
-        }
-    }
-
-    #[cfg(feature = "gecko")]
-    /// Creates a new `ThreadLocalStyleContext` from a shared one.
-    pub fn new(shared: &SharedStyleContext) -> Self {
+    /// Creates a new `ThreadLocalStyleContext`
+    pub fn new() -> Self {
         ThreadLocalStyleContext {
             sharing_cache: StyleSharingCache::new(),
             rule_cache: RuleCache::new(),
@@ -759,7 +736,6 @@ impl<E: TElement> ThreadLocalStyleContext<E> {
             tasks: SequentialTaskList(Vec::new()),
             selector_flags: SelectorFlagsMap::new(),
             statistics: PerThreadTraversalStatistics::default(),
-            font_metrics_provider: E::FontMetricsProvider::create_from(shared),
             stack_limit_checker: StackLimitChecker::new(
                 (STYLE_THREAD_STACK_SIZE_KB - STACK_SAFETY_MARGIN_KB) * 1024,
             ),
diff --git a/components/style/dom.rs b/components/style/dom.rs
index 86c5a382891..c23b985a628 100644
--- a/components/style/dom.rs
+++ b/components/style/dom.rs
@@ -13,7 +13,6 @@ use crate::context::SharedStyleContext;
 use crate::context::{PostAnimationTasks, UpdateAnimationsTasks};
 use crate::data::ElementData;
 use crate::element_state::ElementState;
-use crate::font_metrics::FontMetricsProvider;
 use crate::media_queries::Device;
 use crate::properties::{AnimationDeclarations, ComputedValues, PropertyDeclarationBlock};
 use crate::selector_parser::{AttrValue, Lang, PseudoElement, SelectorImpl};
@@ -363,12 +362,6 @@ pub trait TElement:
     /// syntax.
     type TraversalChildrenIterator: Iterator<Item = Self::ConcreteNode>;
 
-    /// Type of the font metrics provider
-    ///
-    /// XXXManishearth It would be better to make this a type parameter on
-    /// ThreadLocalStyleContext and StyleContext
-    type FontMetricsProvider: FontMetricsProvider + Send;
-
     /// Get this element as a node.
     fn as_node(&self) -> Self::ConcreteNode;
 
diff --git a/components/style/driver.rs b/components/style/driver.rs
index eab7c3a6a31..475d04269c5 100644
--- a/components/style/driver.rs
+++ b/components/style/driver.rs
@@ -91,7 +91,7 @@ where
     //     ThreadLocalStyleContext has not released its TLS borrow by that point,
     //     we'll panic on double-borrow.
     let mut tls_slots = None;
-    let mut tlc = ThreadLocalStyleContext::new(traversal.shared_context());
+    let mut tlc = ThreadLocalStyleContext::new();
     let mut context = StyleContext {
         shared: traversal.shared_context(),
         thread_local: &mut tlc,
diff --git a/components/style/font_metrics.rs b/components/style/font_metrics.rs
index db435539735..13a4e9e182b 100644
--- a/components/style/font_metrics.rs
+++ b/components/style/font_metrics.rs
@@ -6,9 +6,7 @@
 
 #![deny(missing_docs)]
 
-use crate::context::SharedStyleContext;
 use crate::values::computed::Length;
-use crate::Atom;
 
 /// Represents the font metrics that style needs from a font to compute the
 /// value of certain CSS units like `ex`.
@@ -47,64 +45,3 @@ pub enum FontMetricsOrientation {
     /// Force getting horizontal metrics.
     Horizontal,
 }
-
-/// A trait used to represent something capable of providing us font metrics.
-pub trait FontMetricsProvider {
-    /// Obtain the metrics for given font family.
-    fn query(
-        &self,
-        _context: &crate::values::computed::Context,
-        _base_size: crate::values::specified::length::FontBaseSize,
-        _orientation: FontMetricsOrientation,
-    ) -> FontMetrics {
-        Default::default()
-    }
-
-    /// Get default size of a given language and generic family.
-    fn get_size(
-        &self,
-        font_name: &Atom,
-        font_family: crate::values::computed::font::GenericFontFamily,
-    ) -> Length;
-
-    /// Construct from a shared style context
-    fn create_from(context: &SharedStyleContext) -> Self
-    where
-        Self: Sized;
-}
-
-// TODO: Servo's font metrics provider will probably not live in this crate, so this will
-// have to be replaced with something else (perhaps a trait method on TElement)
-// when we get there
-#[derive(Debug)]
-#[cfg(feature = "servo")]
-/// Dummy metrics provider for Servo. Knows nothing about fonts and does not provide
-/// any metrics.
-pub struct ServoMetricsProvider;
-
-#[cfg(feature = "servo")]
-impl FontMetricsProvider for ServoMetricsProvider {
-    fn create_from(_: &SharedStyleContext) -> Self {
-        ServoMetricsProvider
-    }
-
-    fn get_size(&self, _: &Atom, _: crate::values::computed::font::GenericFontFamily) -> Length {
-        unreachable!("Dummy provider should never be used to compute font size")
-    }
-}
-
-// Servo's font metrics provider will probably not live in this crate, so this will
-// have to be replaced with something else (perhaps a trait method on TElement)
-// when we get there
-
-#[cfg(feature = "gecko")]
-/// Construct a font metrics provider for the current product
-pub fn get_metrics_provider_for_product() -> crate::gecko::wrapper::GeckoFontMetricsProvider {
-    crate::gecko::wrapper::GeckoFontMetricsProvider::new()
-}
-
-#[cfg(feature = "servo")]
-/// Construct a font metrics provider for the current product
-pub fn get_metrics_provider_for_product() -> ServoMetricsProvider {
-    ServoMetricsProvider
-}
diff --git a/components/style/gecko/media_queries.rs b/components/style/gecko/media_queries.rs
index a116121a258..08ef628f1ff 100644
--- a/components/style/gecko/media_queries.rs
+++ b/components/style/gecko/media_queries.rs
@@ -6,12 +6,14 @@
 
 use crate::context::QuirksMode;
 use crate::custom_properties::CssEnvironment;
+use crate::font_metrics::FontMetrics;
 use crate::gecko::values::{convert_nscolor_to_rgba, convert_rgba_to_nscolor};
 use crate::gecko_bindings::bindings;
 use crate::gecko_bindings::structs;
 use crate::media_queries::MediaType;
 use crate::properties::ComputedValues;
 use crate::string_cache::Atom;
+use crate::values::computed::font::GenericFontFamily;
 use crate::values::computed::Length;
 use crate::values::specified::font::FONT_MEDIUM_PX;
 use crate::values::{CustomIdent, KeyframesName};
@@ -50,6 +52,8 @@ pub struct Device {
     /// Whether any styles computed in the document relied on the root font-size
     /// by using rem units.
     used_root_font_size: AtomicBool,
+    /// Whether any styles computed in the document relied on font metrics.
+    used_font_metrics: AtomicBool,
     /// Whether any styles computed in the document relied on the viewport size
     /// by using vw/vh/vmin/vmax units.
     used_viewport_size: AtomicBool,
@@ -91,6 +95,7 @@ impl Device {
             root_font_size: AtomicU32::new(FONT_MEDIUM_PX.to_bits()),
             body_text_color: AtomicUsize::new(prefs.mDefaultColor as usize),
             used_root_font_size: AtomicBool::new(false),
+            used_font_metrics: AtomicBool::new(false),
             used_viewport_size: AtomicBool::new(false),
             environment: CssEnvironment,
         }
@@ -157,6 +162,67 @@ impl Device {
             .store(convert_rgba_to_nscolor(&color) as usize, Ordering::Relaxed)
     }
 
+    /// Gets the base size given a generic font family and a language.
+    pub fn base_size_for_generic(&self, language: &Atom, generic: GenericFontFamily) -> Length {
+        unsafe { bindings::Gecko_GetBaseSize(self.document(), language.as_ptr(), generic) }
+    }
+
+    /// Queries font metrics
+    pub fn query_font_metrics(
+        &self,
+        vertical: bool,
+        font: &crate::properties::style_structs::Font,
+        base_size: Length,
+        in_media_query: bool,
+        retrieve_math_scales: bool,
+    ) -> FontMetrics {
+        self.used_font_metrics.store(true, Ordering::Relaxed);
+        let pc = match self.pres_context() {
+            Some(pc) => pc,
+            None => return Default::default(),
+        };
+        let gecko_metrics = unsafe {
+            bindings::Gecko_GetFontMetrics(
+                pc,
+                vertical,
+                font.gecko(),
+                base_size,
+                // we don't use the user font set in a media query
+                !in_media_query,
+                retrieve_math_scales,
+            )
+        };
+        FontMetrics {
+            x_height: Some(gecko_metrics.mXSize),
+            zero_advance_measure: if gecko_metrics.mChSize.px() >= 0. {
+                Some(gecko_metrics.mChSize)
+            } else {
+                None
+            },
+            cap_height: if gecko_metrics.mCapHeight.px() >= 0. {
+                Some(gecko_metrics.mCapHeight)
+            } else {
+                None
+            },
+            ic_width: if gecko_metrics.mIcWidth.px() >= 0. {
+                Some(gecko_metrics.mIcWidth)
+            } else {
+                None
+            },
+            ascent: gecko_metrics.mAscent,
+            script_percent_scale_down: if gecko_metrics.mScriptPercentScaleDown > 0. {
+                Some(gecko_metrics.mScriptPercentScaleDown)
+            } else {
+                None
+            },
+            script_script_percent_scale_down: if gecko_metrics.mScriptScriptPercentScaleDown > 0. {
+                Some(gecko_metrics.mScriptScriptPercentScaleDown)
+            } else {
+                None
+            },
+        }
+    }
+
     /// Returns the body text color.
     pub fn body_text_color(&self) -> RGBA {
         convert_nscolor_to_rgba(self.body_text_color.load(Ordering::Relaxed) as u32)
@@ -196,6 +262,7 @@ impl Device {
     pub fn rebuild_cached_data(&mut self) {
         self.reset_computed_values();
         self.used_root_font_size.store(false, Ordering::Relaxed);
+        self.used_font_metrics.store(false, Ordering::Relaxed);
         self.used_viewport_size.store(false, Ordering::Relaxed);
     }
 
@@ -287,6 +354,11 @@ impl Device {
         self.used_viewport_size.load(Ordering::Relaxed)
     }
 
+    /// Returns whether font metrics have been queried.
+    pub fn used_font_metrics(&self) -> bool {
+        self.used_font_metrics.load(Ordering::Relaxed)
+    }
+
     /// Returns the device pixel ratio.
     pub fn device_pixel_ratio(&self) -> Scale<f32, CSSPixel, DevicePixel> {
         let pc = match self.pres_context() {
diff --git a/components/style/gecko/wrapper.rs b/components/style/gecko/wrapper.rs
index 78e41cf767f..7d4b4e03f73 100644
--- a/components/style/gecko/wrapper.rs
+++ b/components/style/gecko/wrapper.rs
@@ -20,7 +20,6 @@ use crate::context::{PostAnimationTasks, QuirksMode, SharedStyleContext, UpdateA
 use crate::data::ElementData;
 use crate::dom::{LayoutIterator, NodeInfo, OpaqueNode, TDocument, TElement, TNode, TShadowRoot};
 use crate::element_state::{DocumentState, ElementState};
-use crate::font_metrics::{FontMetrics, FontMetricsOrientation, FontMetricsProvider};
 use crate::gecko::data::GeckoStyleSheet;
 use crate::gecko::selector_parser::{NonTSPseudoClass, PseudoElement, SelectorImpl};
 use crate::gecko::snapshot_helpers;
@@ -67,9 +66,6 @@ use crate::selector_parser::{AttrValue, HorizontalDirection, Lang};
 use crate::shared_lock::{Locked, SharedRwLock};
 use crate::string_cache::{Atom, Namespace, WeakAtom, WeakNamespace};
 use crate::stylist::CascadeData;
-use crate::values::computed::font::GenericFontFamily;
-use crate::values::computed::Length;
-use crate::values::specified::length::FontBaseSize;
 use crate::values::{AtomIdent, AtomString};
 use crate::CaseSensitivityExt;
 use crate::LocalName;
@@ -81,7 +77,6 @@ use selectors::matching::{ElementSelectorFlags, MatchingContext};
 use selectors::sink::Push;
 use selectors::{Element, OpaqueElement};
 use servo_arc::{Arc, ArcBorrow, RawOffsetArc};
-use std::cell::RefCell;
 use std::fmt;
 use std::hash::{Hash, Hasher};
 use std::mem;
@@ -920,131 +915,8 @@ fn get_animation_rule(
     }
 }
 
-#[derive(Debug)]
-/// Gecko font metrics provider
-pub struct GeckoFontMetricsProvider {
-    /// Cache of base font sizes for each language. Usually will have 1 element.
-    ///
-    /// This may be slow on pages using more languages, might be worth
-    /// optimizing by caching lang->group mapping separately and/or using a
-    /// hashmap on larger loads.
-    pub font_size_cache: RefCell<Vec<(Atom, DefaultFontSizes)>>,
-}
-
-impl GeckoFontMetricsProvider {
-    /// Construct
-    pub fn new() -> Self {
-        GeckoFontMetricsProvider {
-            font_size_cache: RefCell::new(Vec::new()),
-        }
-    }
-}
-
-impl FontMetricsProvider for GeckoFontMetricsProvider {
-    fn create_from(_: &SharedStyleContext) -> GeckoFontMetricsProvider {
-        GeckoFontMetricsProvider::new()
-    }
-
-    fn get_size(&self, font_name: &Atom, font_family: GenericFontFamily) -> Length {
-        let mut cache = self.font_size_cache.borrow_mut();
-        if let Some(sizes) = cache.iter().find(|el| el.0 == *font_name) {
-            return sizes.1.size_for_generic(font_family);
-        }
-        let sizes = unsafe { bindings::Gecko_GetBaseSize(font_name.as_ptr()) };
-        let size = sizes.size_for_generic(font_family);
-        cache.push((font_name.clone(), sizes));
-        size
-    }
-
-    fn query(
-        &self,
-        context: &crate::values::computed::Context,
-        base_size: FontBaseSize,
-        orientation: FontMetricsOrientation,
-    ) -> FontMetrics {
-        let pc = match context.device().pres_context() {
-            Some(pc) => pc,
-            None => return Default::default(),
-        };
-
-        let size = base_size.resolve(context);
-        let style = context.style();
-
-        let (wm, font) = match base_size {
-            FontBaseSize::CurrentStyle => (style.writing_mode, style.get_font()),
-            // This is only used for font-size computation.
-            FontBaseSize::InheritedStyle => {
-                (*style.inherited_writing_mode(), style.get_parent_font())
-            },
-        };
-
-        let vertical_metrics = match orientation {
-            FontMetricsOrientation::MatchContextPreferHorizontal => {
-                wm.is_vertical() && wm.is_upright()
-            },
-            FontMetricsOrientation::MatchContextPreferVertical => {
-                wm.is_vertical() && !wm.is_sideways()
-            },
-            FontMetricsOrientation::Horizontal => false,
-        };
-        let gecko_metrics = unsafe {
-            bindings::Gecko_GetFontMetrics(
-                pc,
-                vertical_metrics,
-                font.gecko(),
-                size,
-                // we don't use the user font set in a media query
-                !context.in_media_query,
-            )
-        };
-        FontMetrics {
-            x_height: Some(gecko_metrics.mXSize),
-            zero_advance_measure: if gecko_metrics.mChSize.px() >= 0. {
-                Some(gecko_metrics.mChSize)
-            } else {
-                None
-            },
-            cap_height: if gecko_metrics.mCapHeight.px() >= 0. {
-                Some(gecko_metrics.mCapHeight)
-            } else {
-                None
-            },
-            ascent: gecko_metrics.mAscent,
-        }
-    }
-}
-
-/// The default font sizes for generic families for a given language group.
-#[derive(Debug)]
-#[repr(C)]
-pub struct DefaultFontSizes {
-    variable: Length,
-    serif: Length,
-    sans_serif: Length,
-    monospace: Length,
-    cursive: Length,
-    fantasy: Length,
-}
-
-impl DefaultFontSizes {
-    fn size_for_generic(&self, font_family: GenericFontFamily) -> Length {
-        match font_family {
-            GenericFontFamily::None => self.variable,
-            GenericFontFamily::Serif => self.serif,
-            GenericFontFamily::SansSerif => self.sans_serif,
-            GenericFontFamily::Monospace => self.monospace,
-            GenericFontFamily::Cursive => self.cursive,
-            GenericFontFamily::Fantasy => self.fantasy,
-            GenericFontFamily::MozEmoji => unreachable!(
-                "Should never get here, since this doesn't (yet) appear on font family"
-            ),
-        }
-    }
-}
-
 impl<'le> TElement for GeckoElement<'le> {
     type ConcreteNode = GeckoNode<'le>;
-    type FontMetricsProvider = GeckoFontMetricsProvider;
     type TraversalChildrenIterator = GeckoChildrenIterator<'le>;
 
     fn inheritance_parent(&self) -> Option<Self> {
diff --git a/components/style/parallel.rs b/components/style/parallel.rs
index 44618ef2fc1..f03bf64fe61 100644
--- a/components/style/parallel.rs
+++ b/components/style/parallel.rs
@@ -75,14 +75,11 @@ type WorkUnit<N> = ArrayVec<SendNode<N>, WORK_UNIT_MAX>;
 /// out of line so we don't allocate stack space for the entire struct
 /// in the caller.
 #[inline(never)]
-fn create_thread_local_context<'scope, E, D>(
-    traversal: &'scope D,
-    slot: &mut Option<ThreadLocalStyleContext<E>>,
-) where
+fn create_thread_local_context<'scope, E>(slot: &mut Option<ThreadLocalStyleContext<E>>)
+where
     E: TElement + 'scope,
-    D: DomTraversal<E>,
 {
-    *slot = Some(ThreadLocalStyleContext::new(traversal.shared_context()));
+    *slot = Some(ThreadLocalStyleContext::new());
 }
 
 /// A parallel top-down DOM traversal.
@@ -127,7 +124,7 @@ fn top_down_dom<'a, 'scope, E, D>(
         // Scope the borrow of the TLS so that the borrow is dropped before
         // a potential recursive call when we pass TailCall.
         let mut tlc = tls.ensure(|slot: &mut Option<ThreadLocalStyleContext<E>>| {
-            create_thread_local_context(traversal, slot)
+            create_thread_local_context(slot)
         });
 
         // Check that we're not in danger of running out of stack.
diff --git a/components/style/properties/cascade.rs b/components/style/properties/cascade.rs
index acfdfc51457..f7ab9f95fde 100644
--- a/components/style/properties/cascade.rs
+++ b/components/style/properties/cascade.rs
@@ -7,7 +7,6 @@
 use crate::context::QuirksMode;
 use crate::custom_properties::CustomPropertiesBuilder;
 use crate::dom::TElement;
-use crate::font_metrics::FontMetricsProvider;
 use crate::logical_geometry::WritingMode;
 use crate::media_queries::Device;
 use crate::properties::{
@@ -82,7 +81,6 @@ pub fn cascade<E>(
     parent_style_ignoring_first_line: Option<&ComputedValues>,
     layout_parent_style: Option<&ComputedValues>,
     visited_rules: Option<&StrongRuleNode>,
-    font_metrics_provider: &dyn FontMetricsProvider,
     quirks_mode: QuirksMode,
     rule_cache: Option<&RuleCache>,
     rule_cache_conditions: &mut RuleCacheConditions,
@@ -99,7 +97,6 @@ where
         parent_style,
         parent_style_ignoring_first_line,
         layout_parent_style,
-        font_metrics_provider,
         CascadeMode::Unvisited { visited_rules },
         quirks_mode,
         rule_cache,
@@ -197,7 +194,6 @@ fn cascade_rules<E>(
     parent_style: Option<&ComputedValues>,
     parent_style_ignoring_first_line: Option<&ComputedValues>,
     layout_parent_style: Option<&ComputedValues>,
-    font_metrics_provider: &dyn FontMetricsProvider,
     cascade_mode: CascadeMode,
     quirks_mode: QuirksMode,
     rule_cache: Option<&RuleCache>,
@@ -220,7 +216,6 @@ where
         parent_style,
         parent_style_ignoring_first_line,
         layout_parent_style,
-        font_metrics_provider,
         cascade_mode,
         quirks_mode,
         rule_cache,
@@ -256,7 +251,6 @@ pub fn apply_declarations<'a, E, I>(
     parent_style: Option<&ComputedValues>,
     parent_style_ignoring_first_line: Option<&ComputedValues>,
     layout_parent_style: Option<&ComputedValues>,
-    font_metrics_provider: &dyn FontMetricsProvider,
     cascade_mode: CascadeMode,
     quirks_mode: QuirksMode,
     rule_cache: Option<&RuleCache>,
@@ -317,7 +311,6 @@ where
         in_media_query: false,
         for_smil_animation: false,
         for_non_inherited_property: None,
-        font_metrics_provider,
         quirks_mode,
         rule_cache_conditions: RefCell::new(rule_cache_conditions),
     };
@@ -762,7 +755,6 @@ impl<'a, 'b: 'a> Cascade<'a, 'b> {
             visited_parent!(parent_style),
             visited_parent!(parent_style_ignoring_first_line),
             visited_parent!(layout_parent_style),
-            self.context.font_metrics_provider,
             CascadeMode::Visited { writing_mode },
             self.context.quirks_mode,
             // The rule cache doesn't care about caching :visited
diff --git a/components/style/servo/media_queries.rs b/components/style/servo/media_queries.rs
index 17949c723d0..4b535a9b875 100644
--- a/components/style/servo/media_queries.rs
+++ b/components/style/servo/media_queries.rs
@@ -6,6 +6,7 @@
 
 use crate::context::QuirksMode;
 use crate::custom_properties::CssEnvironment;
+use crate::font_metrics::FontMetrics;
 use crate::media_queries::media_feature::{AllowsRanges, ParsingRequirements};
 use crate::media_queries::media_feature::{Evaluator, MediaFeatureDescription};
 use crate::media_queries::media_feature_expression::RangeOrOperator;
@@ -156,6 +157,22 @@ impl Device {
         self.device_pixel_ratio
     }
 
+    /// Queries dummy font metrics for Servo. Knows nothing about fonts and does not provide
+    /// any metrics.
+    /// TODO: Servo's font metrics provider will probably not live in this crate, so this will
+    /// have to be replaced with something else (perhaps a trait method on TElement)
+    /// when we get there
+    pub fn query_font_metrics(
+        &self,
+        _vertical: bool,
+        _font: &crate::properties::style_structs::Font,
+        _base_size: CSSPixelLength,
+        _in_media_query: bool,
+        _retrieve_math_scales: bool,
+    ) -> FontMetrics {
+        Default::default()
+    }
+
     /// Take into account a viewport rule taken from the stylesheets.
     pub fn account_for_viewport_rule(&mut self, constraints: &ViewportConstraints) {
         self.viewport_size = constraints.size;
diff --git a/components/style/style_resolver.rs b/components/style/style_resolver.rs
index 7c28e001403..81517f6cd64 100644
--- a/components/style/style_resolver.rs
+++ b/components/style/style_resolver.rs
@@ -351,7 +351,6 @@ where
             parent_style,
             parent_style,
             layout_parent_style,
-            &self.context.thread_local.font_metrics_provider,
             Some(&self.context.thread_local.rule_cache),
             &mut conditions,
         );
diff --git a/components/style/stylesheets/viewport_rule.rs b/components/style/stylesheets/viewport_rule.rs
index 9f751ece265..3353662cb4b 100644
--- a/components/style/stylesheets/viewport_rule.rs
+++ b/components/style/stylesheets/viewport_rule.rs
@@ -9,7 +9,6 @@
 
 use crate::context::QuirksMode;
 use crate::error_reporting::ContextualParseError;
-use crate::font_metrics::get_metrics_provider_for_product;
 use crate::media_queries::Device;
 use crate::parser::{Parse, ParserContext};
 use crate::properties::StyleBuilder;
@@ -673,14 +672,11 @@ impl MaybeNew for ViewportConstraints {
         // DEVICE-ADAPT § 6.2.3 Resolve non-auto lengths to pixel lengths
         let initial_viewport = device.au_viewport_size();
 
-        let provider = get_metrics_provider_for_product();
-
         let mut conditions = RuleCacheConditions::default();
         let context = Context {
             // Note: DEVICE-ADAPT § 5. states that relative length values are
             // resolved against initial values
             builder: StyleBuilder::for_inheritance(device, None, None),
-            font_metrics_provider: &provider,
             cached_system_font: None,
             in_media_query: false,
             quirks_mode: quirks_mode,
diff --git a/components/style/stylist.rs b/components/style/stylist.rs
index 9a2134450de..5cb40b968eb 100644
--- a/components/style/stylist.rs
+++ b/components/style/stylist.rs
@@ -8,7 +8,6 @@ use crate::applicable_declarations::{ApplicableDeclarationBlock, ApplicableDecla
 use crate::context::{CascadeInputs, QuirksMode};
 use crate::dom::{TElement, TShadowRoot};
 use crate::element_state::{DocumentState, ElementState};
-use crate::font_metrics::FontMetricsProvider;
 #[cfg(feature = "gecko")]
 use crate::gecko_bindings::structs::{ServoStyleSetSizes, StyleRuleInclusion};
 use crate::invalidation::element::invalidation_map::InvalidationMap;
@@ -662,7 +661,6 @@ impl Stylist {
         guards: &StylesheetGuards,
         pseudo: &PseudoElement,
         parent: Option<&ComputedValues>,
-        font_metrics: &dyn FontMetricsProvider,
     ) -> Arc<ComputedValues>
     where
         E: TElement,
@@ -671,13 +669,7 @@ impl Stylist {
 
         let rule_node = self.rule_node_for_precomputed_pseudo(guards, pseudo, vec![]);
 
-        self.precomputed_values_for_pseudo_with_rule_node::<E>(
-            guards,
-            pseudo,
-            parent,
-            font_metrics,
-            rule_node,
-        )
+        self.precomputed_values_for_pseudo_with_rule_node::<E>(guards, pseudo, parent, rule_node)
     }
 
     /// Computes the style for a given "precomputed" pseudo-element with
@@ -690,7 +682,6 @@ impl Stylist {
         guards: &StylesheetGuards,
         pseudo: &PseudoElement,
         parent: Option<&ComputedValues>,
-        font_metrics: &dyn FontMetricsProvider,
         rules: StrongRuleNode,
     ) -> Arc<ComputedValues>
     where
@@ -704,7 +695,6 @@ impl Stylist {
             pseudo,
             guards,
             parent,
-            font_metrics,
             None,
         )
     }
@@ -759,13 +749,7 @@ impl Stylist {
     where
         E: TElement,
     {
-        use crate::font_metrics::ServoMetricsProvider;
-        self.precomputed_values_for_pseudo::<E>(
-            guards,
-            &pseudo,
-            Some(parent_style),
-            &ServoMetricsProvider,
-        )
+        self.precomputed_values_for_pseudo::<E>(guards, &pseudo, Some(parent_style))
     }
 
     /// Computes a pseudo-element style lazily during layout.
@@ -783,7 +767,6 @@ impl Stylist {
         rule_inclusion: RuleInclusion,
         parent_style: &ComputedValues,
         is_probe: bool,
-        font_metrics: &dyn FontMetricsProvider,
         matching_fn: Option<&dyn Fn(&PseudoElement) -> bool>,
     ) -> Option<Arc<ComputedValues>>
     where
@@ -804,7 +787,6 @@ impl Stylist {
             pseudo,
             guards,
             Some(parent_style),
-            font_metrics,
             Some(element),
         ))
     }
@@ -819,7 +801,6 @@ impl Stylist {
         pseudo: &PseudoElement,
         guards: &StylesheetGuards,
         parent_style: Option<&ComputedValues>,
-        font_metrics: &dyn FontMetricsProvider,
         element: Option<E>,
     ) -> Arc<ComputedValues>
     where
@@ -845,7 +826,6 @@ impl Stylist {
             parent_style,
             parent_style,
             parent_style,
-            font_metrics,
             /* rule_cache = */ None,
             &mut RuleCacheConditions::default(),
         )
@@ -872,7 +852,6 @@ impl Stylist {
         parent_style: Option<&ComputedValues>,
         parent_style_ignoring_first_line: Option<&ComputedValues>,
         layout_parent_style: Option<&ComputedValues>,
-        font_metrics: &dyn FontMetricsProvider,
         rule_cache: Option<&RuleCache>,
         rule_cache_conditions: &mut RuleCacheConditions,
     ) -> Arc<ComputedValues>
@@ -909,7 +888,6 @@ impl Stylist {
             parent_style_ignoring_first_line,
             layout_parent_style,
             visited_rules,
-            font_metrics,
             self.quirks_mode,
             rule_cache,
             rule_cache_conditions,
@@ -1326,10 +1304,7 @@ impl Stylist {
     where
         E: TElement,
     {
-        use crate::font_metrics::get_metrics_provider_for_product;
-
         let block = declarations.read_with(guards.author);
-        let metrics = get_metrics_provider_for_product();
 
         // We don't bother inserting these declarations in the rule tree, since
         // it'd be quite useless and slow.
@@ -1348,7 +1323,6 @@ impl Stylist {
             Some(parent_style),
             Some(parent_style),
             Some(parent_style),
-            &metrics,
             CascadeMode::Unvisited {
                 visited_rules: None,
             },
diff --git a/components/style/values/computed/mod.rs b/components/style/values/computed/mod.rs
index 1b242547307..9dbbf40b5f4 100644
--- a/components/style/values/computed/mod.rs
+++ b/components/style/values/computed/mod.rs
@@ -14,13 +14,15 @@ use super::generics::transform::IsParallelTo;
 use super::generics::{self, GreaterThanOrEqualToOne, NonNegative, ZeroToOne};
 use super::specified;
 use super::{CSSFloat, CSSInteger};
+use crate::computed_value_flags::ComputedValueFlags;
 use crate::context::QuirksMode;
-use crate::font_metrics::{get_metrics_provider_for_product, FontMetricsProvider};
+use crate::font_metrics::{FontMetrics, FontMetricsOrientation};
 use crate::media_queries::Device;
 #[cfg(feature = "gecko")]
 use crate::properties;
 use crate::properties::{ComputedValues, LonghandId, StyleBuilder};
 use crate::rule_cache::RuleCacheConditions;
+use crate::values::specified::length::FontBaseSize;
 use crate::{ArcSlice, Atom, One};
 use euclid::{default, Point2D, Rect, Size2D};
 use servo_arc::Arc;
@@ -155,10 +157,6 @@ pub struct Context<'a> {
     #[cfg(feature = "servo")]
     pub cached_system_font: Option<()>,
 
-    /// A font metrics provider, used to access font metrics to implement
-    /// font-relative units.
-    pub font_metrics_provider: &'a dyn FontMetricsProvider,
-
     /// Whether or not we are computing the media list in a media query
     pub in_media_query: bool,
 
@@ -192,11 +190,9 @@ impl<'a> Context<'a> {
         F: FnOnce(&Context) -> R,
     {
         let mut conditions = RuleCacheConditions::default();
-        let provider = get_metrics_provider_for_product();
 
         let context = Context {
             builder: StyleBuilder::for_inheritance(device, None, None),
-            font_metrics_provider: &provider,
             cached_system_font: None,
             in_media_query: true,
             quirks_mode,
@@ -213,6 +209,49 @@ impl<'a> Context<'a> {
         self.builder.device
     }
 
+    /// Queries font metrics.
+    pub fn query_font_metrics(
+        &self,
+        base_size: FontBaseSize,
+        orientation: FontMetricsOrientation,
+        retrieve_math_scales: bool,
+    ) -> FontMetrics {
+        if self.for_non_inherited_property.is_some() {
+            self.rule_cache_conditions.borrow_mut().set_uncacheable();
+        }
+        self.builder.add_flags(match base_size {
+            FontBaseSize::CurrentStyle => ComputedValueFlags::DEPENDS_ON_SELF_FONT_METRICS,
+            FontBaseSize::InheritedStyle => ComputedValueFlags::DEPENDS_ON_INHERITED_FONT_METRICS,
+        });
+        let size = base_size.resolve(self);
+        let style = self.style();
+
+        let (wm, font) = match base_size {
+            FontBaseSize::CurrentStyle => (style.writing_mode, style.get_font()),
+            // This is only used for font-size computation.
+            FontBaseSize::InheritedStyle => {
+                (*style.inherited_writing_mode(), style.get_parent_font())
+            },
+        };
+
+        let vertical = match orientation {
+            FontMetricsOrientation::MatchContextPreferHorizontal => {
+                wm.is_vertical() && wm.is_upright()
+            },
+            FontMetricsOrientation::MatchContextPreferVertical => {
+                wm.is_vertical() && !wm.is_sideways()
+            },
+            FontMetricsOrientation::Horizontal => false,
+        };
+        self.device().query_font_metrics(
+            vertical,
+            font,
+            size,
+            self.in_media_query,
+            retrieve_math_scales,
+        )
+    }
+
     /// The current viewport size, used to resolve viewport units.
     pub fn viewport_size_for_viewport_unit_resolution(&self) -> default::Size2D<Au> {
         self.builder
diff --git a/components/style/values/specified/font.rs b/components/style/values/specified/font.rs
index bda757440bb..b83b6411fee 100644
--- a/components/style/values/specified/font.rs
+++ b/components/style/values/specified/font.rs
@@ -5,6 +5,8 @@
 //! Specified values for font properties
 
 #[cfg(feature = "gecko")]
+use crate::context::QuirksMode;
+#[cfg(feature = "gecko")]
 use crate::gecko_bindings::bindings;
 use crate::parser::{Parse, ParserContext};
 use crate::properties::longhands::system_font::SystemFont;
@@ -811,8 +813,27 @@ impl FontSizeKeyword {
     #[cfg(feature = "gecko")]
     #[inline]
     fn to_length(&self, cx: &Context) -> NonNegativeLength {
-        use crate::context::QuirksMode;
+        let gecko_font = cx.style().get_font().gecko();
+        let family = &gecko_font.mFont.family.families;
+        let generic = family
+            .single_generic()
+            .unwrap_or(computed::GenericFontFamily::None);
+        let base_size = unsafe {
+            Atom::with(gecko_font.mLanguage.mRawPtr, |language| {
+                cx.device().base_size_for_generic(language, generic)
+            })
+        };
+        self.to_length_without_context(cx.quirks_mode, base_size)
+    }
 
+    /// Resolve a keyword length without any context, with explicit arguments.
+    #[cfg(feature = "gecko")]
+    #[inline]
+    pub fn to_length_without_context(
+        &self,
+        quirks_mode: QuirksMode,
+        base_size: Length,
+    ) -> NonNegativeLength {
         // The tables in this function are originally from
         // nsRuleNode::CalcFontPointSize in Gecko:
         //
@@ -856,19 +877,10 @@ impl FontSizeKeyword {
         ];
 
         static FONT_SIZE_FACTORS: [i32; 8] = [60, 75, 89, 100, 120, 150, 200, 300];
-
-        let ref gecko_font = cx.style().get_font().gecko();
-        let base_size = unsafe {
-            Atom::with(gecko_font.mLanguage.mRawPtr, |atom| {
-                cx.font_metrics_provider
-                    .get_size(atom, gecko_font.mGenericID)
-            })
-        };
-
         let base_size_px = base_size.px().round() as i32;
         let html_size = self.html_size() as usize;
         NonNegative(if base_size_px >= 9 && base_size_px <= 16 {
-            let mapping = if cx.quirks_mode == QuirksMode::Quirks {
+            let mapping = if quirks_mode == QuirksMode::Quirks {
                 QUIRKS_FONT_SIZE_MAPPING
             } else {
                 FONT_SIZE_MAPPING
diff --git a/components/style/values/specified/length.rs b/components/style/values/specified/length.rs
index 62f070893a6..cf529408c98 100644
--- a/components/style/values/specified/length.rs
+++ b/components/style/values/specified/length.rs
@@ -7,7 +7,6 @@
 //! [length]: https://drafts.csswg.org/css-values/#lengths
 
 use super::{AllowQuirks, Number, Percentage, ToComputedValue};
-use crate::computed_value_flags::ComputedValueFlags;
 use crate::font_metrics::{FontMetrics, FontMetricsOrientation};
 use crate::parser::{Parse, ParserContext};
 use crate::values::computed::{self, CSSPixelLength, Context};
@@ -159,16 +158,11 @@ impl FontRelativeLength {
             base_size: FontBaseSize,
             orientation: FontMetricsOrientation,
         ) -> FontMetrics {
-            context
-                .font_metrics_provider
-                .query(context, base_size, orientation)
+            let retrieve_math_scales = false;
+            context.query_font_metrics(base_size, orientation, retrieve_math_scales)
         }
 
         let reference_font_size = base_size.resolve(context);
-        let font_metrics_flag = match base_size {
-            FontBaseSize::CurrentStyle => ComputedValueFlags::DEPENDS_ON_SELF_FONT_METRICS,
-            FontBaseSize::InheritedStyle => ComputedValueFlags::DEPENDS_ON_INHERITED_FONT_METRICS,
-        };
         match *self {
             FontRelativeLength::Em(length) => {
                 if context.for_non_inherited_property.is_some() {
@@ -183,10 +177,6 @@ impl FontRelativeLength {
                 (reference_font_size, length)
             },
             FontRelativeLength::Ex(length) => {
-                if context.for_non_inherited_property.is_some() {
-                    context.rule_cache_conditions.borrow_mut().set_uncacheable();
-                }
-                context.builder.add_flags(font_metrics_flag);
                 // The x-height is an intrinsically horizontal metric.
                 let metrics =
                     query_font_metrics(context, base_size, FontMetricsOrientation::Horizontal);
@@ -202,10 +192,6 @@ impl FontRelativeLength {
                 (reference_size, length)
             },
             FontRelativeLength::Ch(length) => {
-                if context.for_non_inherited_property.is_some() {
-                    context.rule_cache_conditions.borrow_mut().set_uncacheable();
-                }
-                context.builder.add_flags(font_metrics_flag);
                 // https://drafts.csswg.org/css-values/#ch:
                 //
                 //     Equal to the used advance measure of the “0” (ZERO,
@@ -239,10 +225,6 @@ impl FontRelativeLength {
                 (reference_size, length)
             },
             FontRelativeLength::Cap(length) => {
-                if context.for_non_inherited_property.is_some() {
-                    context.rule_cache_conditions.borrow_mut().set_uncacheable();
-                }
-                context.builder.add_flags(font_metrics_flag);
                 let metrics =
                     query_font_metrics(context, base_size, FontMetricsOrientation::Horizontal);
                 let reference_size = metrics.cap_height.unwrap_or_else(|| {