Target-specific native show rules (#6569)

4 files changed, 136 insertions, 18 deletions

diff --git a/crates/typst-library/src/foundations/content/element.rs b/crates/typst-library/src/foundations/content/element.rs
index 49b0b0f9..65c2e28b 100644
--- a/crates/typst-library/src/foundations/content/element.rs
+++ b/crates/typst-library/src/foundations/content/element.rs
@@ -246,12 +246,6 @@ pub trait Synthesize {
         -> SourceResult<()>;
 }
 
-/// Defines a built-in show rule for an element.
-pub trait Show {
-    /// Execute the base recipe for this element.
-    fn show(&self, engine: &mut Engine, styles: StyleChain) -> SourceResult<Content>;
-}
-
 /// Defines built-in show set rules for an element.
 ///
 /// This is a bit more powerful than a user-defined show-set because it can
diff --git a/crates/typst-library/src/foundations/context.rs b/crates/typst-library/src/foundations/context.rs
index bf4bdcd2..56d87775 100644
--- a/crates/typst-library/src/foundations/context.rs
+++ b/crates/typst-library/src/foundations/context.rs
@@ -3,7 +3,7 @@ use comemo::Track;
 use crate::diag::{bail, Hint, HintedStrResult, SourceResult};
 use crate::engine::Engine;
 use crate::foundations::{
-    elem, Args, Construct, Content, Func, Packed, Show, StyleChain, Value,
+    elem, Args, Construct, Content, Func, ShowFn, StyleChain, Value,
 };
 use crate::introspection::{Locatable, Location};
 
@@ -61,7 +61,7 @@ fn require<T>(val: Option<T>) -> HintedStrResult<T> {
 }
 
 /// Executes a `context` block.
-#[elem(Construct, Locatable, Show)]
+#[elem(Construct, Locatable)]
 pub struct ContextElem {
     /// The function to call with the context.
     #[required]
@@ -75,11 +75,8 @@ impl Construct for ContextElem {
     }
 }
 
-impl Show for Packed<ContextElem> {
-    #[typst_macros::time(name = "context", span = self.span())]
-    fn show(&self, engine: &mut Engine, styles: StyleChain) -> SourceResult<Content> {
-        let loc = self.location().unwrap();
-        let context = Context::new(Some(loc), Some(styles));
-        Ok(self.func.call::<[Value; 0]>(engine, context.track(), [])?.display())
-    }
-}
+pub const CONTEXT_RULE: ShowFn<ContextElem> = |elem, engine, styles| {
+    let loc = elem.location().unwrap();
+    let context = Context::new(Some(loc), Some(styles));
+    Ok(elem.func.call::<[Value; 0]>(engine, context.track(), [])?.display())
+};
diff --git a/crates/typst-library/src/foundations/styles.rs b/crates/typst-library/src/foundations/styles.rs
index 978b47d5..0da036f6 100644
--- a/crates/typst-library/src/foundations/styles.rs
+++ b/crates/typst-library/src/foundations/styles.rs
@@ -1,4 +1,5 @@
 use std::any::{Any, TypeId};
+use std::collections::HashMap;
 use std::fmt::{self, Debug, Formatter};
 use std::hash::{Hash, Hasher};
 use std::{mem, ptr};
@@ -13,7 +14,7 @@ use crate::diag::{SourceResult, Trace, Tracepoint};
 use crate::engine::Engine;
 use crate::foundations::{
     cast, ty, Content, Context, Element, Field, Func, NativeElement, OneOrMultiple,
-    RefableProperty, Repr, Selector, SettableProperty,
+    Packed, RefableProperty, Repr, Selector, SettableProperty, Target,
 };
 use crate::text::{FontFamily, FontList, TextElem};
 
@@ -938,3 +939,129 @@ fn block_wrong_type(func: Element, id: u8, value: &Block) -> ! {
         value
     )
 }
+
+/// Holds native show rules.
+pub struct NativeRuleMap {
+    rules: HashMap<(Element, Target), NativeShowRule>,
+}
+
+/// The signature of a native show rule.
+pub type ShowFn<T> = fn(
+    elem: &Packed<T>,
+    engine: &mut Engine,
+    styles: StyleChain,
+) -> SourceResult<Content>;
+
+impl NativeRuleMap {
+    /// Creates a new rule map.
+    ///
+    /// Should be populated with rules for all target-element combinations that
+    /// are supported.
+    ///
+    /// Contains built-in rules for a few special elements.
+    pub fn new() -> Self {
+        let mut rules = Self { rules: HashMap::new() };
+
+        // ContextElem is as special as SequenceElem and StyledElem and could,
+        // in theory, also be special cased in realization.
+        rules.register_builtin(crate::foundations::CONTEXT_RULE);
+
+        // CounterDisplayElem only exists because the compiler can't currently
+        // express the equivalent of `context counter(..).display(..)` in native
+        // code (no native closures).
+        rules.register_builtin(crate::introspection::COUNTER_DISPLAY_RULE);
+
+        // These are all only for introspection and empty on all targets.
+        rules.register_empty::<crate::introspection::CounterUpdateElem>();
+        rules.register_empty::<crate::introspection::StateUpdateElem>();
+        rules.register_empty::<crate::introspection::MetadataElem>();
+        rules.register_empty::<crate::model::PrefixInfo>();
+
+        rules
+    }
+
+    /// Registers a rule for all targets.
+    fn register_empty<T: NativeElement>(&mut self) {
+        self.register_builtin::<T>(|_, _, _| Ok(Content::empty()));
+    }
+
+    /// Registers a rule for all targets.
+    fn register_builtin<T: NativeElement>(&mut self, f: ShowFn<T>) {
+        self.register(Target::Paged, f);
+        self.register(Target::Html, f);
+    }
+
+    /// Registers a rule for a target.
+    ///
+    /// Panics if a rule already exists for this target-element combination.
+    pub fn register<T: NativeElement>(&mut self, target: Target, f: ShowFn<T>) {
+        let res = self.rules.insert((T::ELEM, target), NativeShowRule::new(f));
+        if res.is_some() {
+            panic!(
+                "duplicate native show rule for `{}` on {target:?} target",
+                T::ELEM.name()
+            )
+        }
+    }
+
+    /// Retrieves the rule that applies to the `content` on the current
+    /// `target`.
+    pub fn get(&self, target: Target, content: &Content) -> Option<NativeShowRule> {
+        self.rules.get(&(content.func(), target)).copied()
+    }
+}
+
+impl Default for NativeRuleMap {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+pub use rule::NativeShowRule;
+
+mod rule {
+    use super::*;
+
+    /// The show rule for a native element.
+    #[derive(Copy, Clone)]
+    pub struct NativeShowRule {
+        /// The element to which this rule applies.
+        elem: Element,
+        /// Must only be called with content of the appropriate type.
+        f: unsafe fn(
+            elem: &Content,
+            engine: &mut Engine,
+            styles: StyleChain,
+        ) -> SourceResult<Content>,
+    }
+
+    impl NativeShowRule {
+        /// Create a new type-erased show rule.
+        pub fn new<T: NativeElement>(f: ShowFn<T>) -> Self {
+            Self {
+                elem: T::ELEM,
+                // Safety: The two function pointer types only differ in the
+                // first argument, which changes from `&Packed<T>` to
+                // `&Content`. `Packed<T>` is a transparent wrapper around
+                // `Content`. The resulting function is unsafe to call because
+                // content of the correct type must be passed to it.
+                #[allow(clippy::missing_transmute_annotations)]
+                f: unsafe { std::mem::transmute(f) },
+            }
+        }
+
+        /// Applies the rule to content. Panics if the content is of the wrong
+        /// type.
+        pub fn apply(
+            &self,
+            content: &Content,
+            engine: &mut Engine,
+            styles: StyleChain,
+        ) -> SourceResult<Content> {
+            assert_eq!(content.elem(), self.elem);
+
+            // Safety: We just checked that the element is of the correct type.
+            unsafe { (self.f)(content, engine, styles) }
+        }
+    }
+}
diff --git a/crates/typst-library/src/foundations/target.rs b/crates/typst-library/src/foundations/target.rs
index 71e7554e..ff90f1f7 100644
--- a/crates/typst-library/src/foundations/target.rs
+++ b/crates/typst-library/src/foundations/target.rs
@@ -4,7 +4,7 @@ use crate::diag::HintedStrResult;
 use crate::foundations::{elem, func, Cast, Context};
 
 /// The export target.
-#[derive(Debug, Default, Copy, Clone, PartialEq, Hash, Cast)]
+#[derive(Debug, Default, Copy, Clone, Eq, PartialEq, Hash, Cast)]
 pub enum Target {
     /// The target that is used for paged, fully laid-out content.
     #[default]