Fix nested attachments when the base in math.attach has attachments (#4832)

crates/typst/src/math/attach.rs

@@ -52,20 +52,22 @@ pub struct AttachElem {
 impl LayoutMath for Packed<AttachElem> {
     #[typst_macros::time(name = "math.attach", span = self.span())]
     fn layout_math(&self, ctx: &mut MathContext, styles: StyleChain) -> SourceResult<()> {
-        let base = ctx.layout_into_fragment(self.base(), styles)?;
+        let new_elem = merge_base(self);
+        let elem = new_elem.as_ref().unwrap_or(self);
 
+        let base = ctx.layout_into_fragment(elem.base(), styles)?;
         let sup_style = style_for_superscript(styles);
         let sup_style_chain = styles.chain(&sup_style);
-        let tl = self.tl(sup_style_chain);
-        let tr = self.tr(sup_style_chain);
+        let tl = elem.tl(sup_style_chain);
+        let tr = elem.tr(sup_style_chain);
         let primed = tr.as_ref().is_some_and(|content| content.is::<PrimesElem>());
-        let t = self.t(sup_style_chain);
+        let t = elem.t(sup_style_chain);
 
         let sub_style = style_for_subscript(styles);
         let sub_style_chain = styles.chain(&sub_style);
-        let bl = self.bl(sub_style_chain);
-        let br = self.br(sub_style_chain);
-        let b = self.b(sub_style_chain);
+        let bl = elem.bl(sub_style_chain);
+        let br = elem.br(sub_style_chain);
+        let b = elem.b(sub_style_chain);
 
         let limits = base.limits().active(styles);
         let (t, tr) = match (t, tr) {
@@ -248,6 +250,43 @@ impl Limits {
     }
 }
 
+/// If an AttachElem's base is also an AttachElem, merge attachments into the
+/// base AttachElem where possible.
+fn merge_base(elem: &Packed<AttachElem>) -> Option<Packed<AttachElem>> {
+    // Extract from an EquationElem.
+    let mut base = elem.base();
+    if let Some(equation) = base.to_packed::<EquationElem>() {
+        base = equation.body();
+    }
+
+    // Move attachments from elem into base where possible.
+    if let Some(base) = base.to_packed::<AttachElem>() {
+        let mut elem = elem.clone();
+        let mut base = base.clone();
+
+        macro_rules! merge {
+            ($content:ident) => {
+                if base.$content.is_none() && elem.$content.is_some() {
+                    base.$content = elem.$content.clone();
+                    elem.$content = None;
+                }
+            };
+        }
+
+        merge!(t);
+        merge!(b);
+        merge!(tl);
+        merge!(tr);
+        merge!(bl);
+        merge!(br);
+
+        elem.base = base.pack();
+        return Some(elem);
+    }
+
+    None
+}
+
 macro_rules! measure {
     ($e: ident, $attr: ident) => {
         $e.as_ref().map(|e| e.$attr()).unwrap_or_default()

tests/suite/math/attach.typ

@@ -69,7 +69,7 @@ $ sqrt(a_(1/2)^zeta), sqrt(a_alpha^(1/2)), sqrt(a_(1/2)^(3/4)) \
   sqrt(attach(a, tl: 1/2, bl: 3/4, tr: 1/2, br: 3/4)) $
 
 --- math-attach-descender-collision ---
-// Test for no collisions between descenders/ascenders and attachments
+// Test for no collisions between descenders/ascenders and attachments.
 
 $ sup_(x in P_i) quad inf_(x in P_i) $
 $ op("fff",limits: #true)^(y) quad op("yyy", limits:#true)_(f) $
@@ -112,7 +112,7 @@ $ attach(A, t: #context oops) $
 $iota_a^b$
 
 --- math-attach-default-placement ---
-// Test default of limit attachments on relations at all sizes
+// Test default of limit attachments on relations at all sizes.
 #set page(width: auto)
 $ a =^"def" b quad a lt.eq_"really" b quad  a arrow.r.long.squiggly^"slowly" b $
 $a =^"def" b quad a lt.eq_"really" b quad a arrow.r.long.squiggly^"slowly" b$
@@ -120,11 +120,22 @@ $a =^"def" b quad a lt.eq_"really" b quad a arrow.r.long.squiggly^"slowly" b$
 $a scripts(=)^"def" b quad a scripts(lt.eq)_"really" b quad a scripts(arrow.r.long.squiggly)^"slowly" b$
 
 --- math-attach-integral ---
-// Test default of scripts attachments on integrals at display size
+// Test default of scripts attachments on integrals at display size.
 $ integral.sect_a^b  quad \u{2a1b}_a^b quad limits(\u{2a1b})_a^b $
 $integral.sect_a^b quad \u{2a1b}_a^b quad limits(\u{2a1b})_a^b$
 
 --- math-attach-large-operator ---
-// Test default of limit attachments on large operators at display size only
+// Test default of limit attachments on large operators at display size only.
 $ tack.t.big_0^1 quad \u{02A0A}_0^1 quad join_0^1 $
 $tack.t.big_0^1 quad \u{02A0A}_0^1 quad join_0^1$
+
+--- math-attach-nested-base ---
+// Test attachments when the base has attachments.
+$ attach(a^b, b: c) quad
+  attach(attach(attach(attach(attach(attach(sum, tl: 1), t: 2), tr: 3), br: 4), b: 5), bl: 6) $
+
+#let a0 = math.attach(math.alpha, b: [0])
+#let a1 = $alpha^1$
+
+$ a0 + a1 + a0_2 \
+  a1_2 + a0^2 + a1^2 $