Add [fence-effects=""] to stop effect propagation

src/Meta.ts

@@ -47,6 +47,9 @@ export default class Meta extends Builder {
         span.appendChild(node.firstChild);
       }
       node.replaceWith(span);
+    } else {
+      // Nothing to render, strip it.
+      node.replaceWith(...node.childNodes);
     }
   }
 }

src/Xref.ts

@@ -91,21 +91,34 @@ export default class Xref extends Builder {
   shouldPropagateEffect(effectName: string) {
     if (!this.isInvocation) return false;
     if (this.clause) {
-      // Xrefs nested inside Abstract Closures should not propagate the
-      // user-code effect, since those are effectively nested functions.
+      // Xrefs should not propagate past explicit fences in parent steps. Fences
+      // must be at the beginning of steps.
+      //
+      // Abstract Closures are considered automatic fences for the user-code
+      // effect, since those are effectively nested functions.
       //
       // Calls to Abstract Closures that can call user code must be explicitly
       // marked as such with <emu-meta effects="user-code">...</emu-meta>.
-      if (effectName === 'user-code') {
-        for (let node = this.node; node.parentElement; node = node.parentElement) {
-          const parent = node.parentElement;
-          // This is super hacky. It's checking the output of ecmarkdown.
-          if (
-            parent.tagName === 'LI' &&
-            parent.textContent?.includes('be a new Abstract Closure')
-          ) {
-            return false;
-          }
+      for (let node = this.node; node.parentElement; node = node.parentElement) {
+        const parent = node.parentElement;
+        // This is super hacky. It's checking the output of ecmarkdown.
+        if (parent.tagName !== 'LI') continue;
+
+        if (
+          effectName === 'user-code' &&
+          parent.textContent?.includes('be a new Abstract Closure')
+        ) {
+          return false;
+        }
+
+        if (
+          parent
+            .getAttribute('fence-effects')
+            ?.split(',')
+            .map(s => s.trim())
+            .includes(effectName)
+        ) {
+          return false;
         }
       }
       if (!this.clause.canHaveEffect(effectName)) {

test/baselines/generated-reference/effect-user-code.html

@@ -121,4 +121,16 @@ <h1><span class="secnum">20</span> ResultOfEvaluating()</h1>
   <p>The abstract operation ResultOfEvaluating takes no arguments. The phrase "the result of evaluating Foo" is automatically considered as can call user code. It performs the following steps when called:</p>
   <emu-alg><ol><li>Let <var>res</var> be the result of <span class="e-user-code">evaluating <emu-nt>Foo</emu-nt></span>.</li></ol></emu-alg>
 </emu-clause>
+
+<emu-clause id="sec-fenced-effects" type="abstract operation" aoid="FencedEffects">
+  <h1><span class="secnum">21</span> FencedEffects()</h1>
+  <p>The abstract operation FencedEffects takes no arguments. Effects don't propagate past fences in parent steps. A fence must be at the beginning of a step. It performs the following steps when called:</p>
+  <emu-alg><ol><li fence-effects="user-code">Fence.<ol><li><emu-xref aoid="UserCode" id="_ref_18"><a href="#sec-user-code" class="e-user-code">UserCode</a></emu-xref>().</li></ol></li></ol></emu-alg>
+</emu-clause>
+
+<emu-clause id="sec-call-fenced-effects" type="abstract operation" aoid="CallFencedEffects">
+  <h1><span class="secnum">22</span> CallFencedEffects()</h1>
+  <p>The abstract operation CallFencedEffects takes no arguments. Effects don't propagate past fences in parent steps. It performs the following steps when called:</p>
+  <emu-alg><ol><li><emu-xref aoid="FencedEffects" id="_ref_19"><a href="#sec-fenced-effects">FencedEffects</a></emu-xref>().</li></ol></emu-alg>
+</emu-clause>
 </div></body>

test/baselines/sources/effect-user-code.html

@@ -221,3 +221,26 @@ <h1>ResultOfEvaluating()</h1>
     1. Let _res_ be the result of evaluating |Foo|.
   </emu-alg>
 </emu-clause>
+
+<emu-clause id="sec-fenced-effects" type="abstract operation">
+  <h1>FencedEffects()</h1>
+  <dl class="header">
+    <dt>description</dt>
+    <dd>Effects don't propagate past fences in parent steps. A fence must be at the beginning of a step.</dd>
+  </dl>
+  <emu-alg>
+    1. [fence-effects="user-code"] Fence.
+      1. UserCode().
+  </emu-alg>
+</emu-clause>
+
+<emu-clause id="sec-call-fenced-effects" type="abstract operation">
+  <h1>CallFencedEffects()</h1>
+  <dl class="header">
+    <dt>description</dt>
+    <dd>Effects don't propagate past fences in parent steps.</dd>
+  </dl>
+  <emu-alg>
+    1. FencedEffects().
+  </emu-alg>
+</emu-clause>