Fix arbitrarily nested equations in the base of math.attach (#5613)

crates/typst-layout/src/math/attach.rs

@@ -161,7 +161,7 @@ fn stretch_size(
 ) -> Option<Smart<Rel<Length>>> {
     // Extract from an EquationElem.
     let mut base = elem.base();
-    if let Some(equation) = base.to_packed::<EquationElem>() {
+    while let Some(equation) = base.to_packed::<EquationElem>() {
         base = equation.body();
     }
 

crates/typst-library/src/math/attach.rs

@@ -48,7 +48,7 @@ impl Packed<AttachElem> {
     pub fn merge_base(&self) -> Option<Self> {
         // Extract from an EquationElem.
         let mut base = self.base();
-        if let Some(equation) = base.to_packed::<EquationElem>() {
+        while let Some(equation) = base.to_packed::<EquationElem>() {
             base = equation.body();
         }
 

tests/suite/math/attach.typ

@@ -167,9 +167,22 @@ $ attach(a^b, b: c) quad
 
 #let a0 = math.attach(math.alpha, b: [0])
 #let a1 = $alpha^1$
+#let a2 = $attach(a1, bl: 3)$
 
 $ a0 + a1 + a0_2 \
-  a1_2 + a0^2 + a1^2 $
+  a1_2 + a0^2 + a1^2 \
+  a2 + a2_2 + a2^2 $
+
+--- math-attach-nested-deep-base ---
+// Test attachments when the base has attachments and is nested arbitrarily
+// deep.
+#{
+  let var = $x^1$
+  for i in range(24) {
+    var = $var$    
+  }
+  $var_2$
+}
 
 --- math-attach-scripts-extended-shapes ---
 // Test script attachments positioning if the base is an extended shape (or a

tests/suite/math/stretch.typ

@@ -83,3 +83,11 @@ $ stretch(base, size: #50%) $
 // Test stretching with attachments when nested in an equation.
 #let body = $stretch(=)$
 $ body^"text" $
+
+#{
+  let body = $stretch(=)$
+  for i in range(24) {
+    body = $body$    
+  }
+  $body^"long text"$
+}