Make math.class affect the limit configuration (#3231)

crates/typst/src/math/attach.rs

@@ -212,6 +212,15 @@ impl Limits {
         }
     }
 
+    /// The default limit configuration for a math class.
+    pub fn for_class(class: MathClass) -> Self {
+        match class {
+            MathClass::Large => Self::Display,
+            MathClass::Relation => Self::Always,
+            _ => Self::Never,
+        }
+    }
+
     /// Whether limits should be displayed in this context
     pub fn active(&self, ctx: &MathContext) -> bool {
         match self {

crates/typst/src/math/class.rs

@@ -2,12 +2,15 @@ use unicode_math_class::MathClass;
 
 use crate::diag::SourceResult;
 use crate::foundations::{elem, Content, Packed};
-use crate::math::{LayoutMath, MathContext};
+use crate::math::{LayoutMath, Limits, MathContext};
 
 /// Forced use of a certain math class.
 ///
 /// This is useful to treat certain symbols as if they were of a different
-/// class, e.g. to make a symbol behave like a relation.
+/// class, e.g. to make a symbol behave like a relation. The class of a symbol
+/// defines the way it is laid out, including spacing around it, and how its
+/// scripts are attached by default. Note that the latter can always be
+/// overridden using [`{limits}`](math.limits) and [`{scripts}`](math.scripts).
 ///
 /// # Example
 /// ```example
@@ -37,6 +40,7 @@ impl LayoutMath for Packed<ClassElem> {
         ctx.unstyle();
 
         fragment.set_class(*self.class());
+        fragment.set_limits(Limits::for_class(*self.class()));
         ctx.push(fragment);
         Ok(())
     }

tests/typ/math/class.typ

@@ -34,4 +34,14 @@ $ a pluseq 5 $
 
 ---
 // Test exceptions.
-$ sqrt(3)\/2 quad d_0.d_1d_2 dots $
+$ sqrt(3)\/2 quad d_0.d_1d_2 dots $
+
+---
+// Test if the math class changes the limit configuration.
+$ class("normal", ->)_a $
+$class("relation", x)_a$
+$ class("large", x)_a $
+$class("large", ->)_a$
+
+$limits(class("normal", ->))_a$
+$ scripts(class("relation", x))_a $