From 52d1ddc2c2b6b474a8602abf18ac56ae9fc7c905 Mon Sep 17 00:00:00 2001
From: Justin Ridgewell <justin@ridgewell.name>
Date: Fri, 23 Feb 2024 01:30:32 -0500
Subject: [PATCH] Implement rest params in abstract closures (#573)

---
 src/lint/rules/variable-use-def.ts | 15 ++++-
 test/lint-variable-use-def.js      | 96 ++++++++++++++++++++++++++++++
 2 files changed, 110 insertions(+), 1 deletion(-)

diff --git a/src/lint/rules/variable-use-def.ts b/src/lint/rules/variable-use-def.ts
index 8058248b..3a9e9361 100644
--- a/src/lint/rules/variable-use-def.ts
+++ b/src/lint/rules/variable-use-def.ts
@@ -265,9 +265,21 @@ function walkAlgorithm(
           continue;
         }
         // TODO this kind of parsing should really be factored out
+        let varName = true;
         for (let i = 0; i < paramList.items.length; ++i) {
           const item = paramList.items[i];
-          if (i % 2 === 0) {
+          if (varName) {
+            if (item.name === 'text' && item.contents === '...') {
+              if (i < paramList.items.length - 2) {
+                report({
+                  ruleId: 'bad-ac',
+                  message: `expected rest param to come last`,
+                  ...offsetToLineAndColumn(algorithmSource, item.location.start.offset),
+                });
+                continue stepLoop;
+              }
+              continue;
+            }
             if (item.name !== 'underscore') {
               report({
                 ruleId: 'bad-ac',
@@ -287,6 +299,7 @@ function walkAlgorithm(
               continue stepLoop;
             }
           }
+          varName = !varName;
         }
       }
 
diff --git a/test/lint-variable-use-def.js b/test/lint-variable-use-def.js
index 7679990a..81f77c3b 100644
--- a/test/lint-variable-use-def.js
+++ b/test/lint-variable-use-def.js
@@ -401,6 +401,102 @@ describe('variables are declared and used appropriately', () => {
       );
     });
 
+    it('abstract closure rest parameters must be last', async () => {
+      await assertLint(
+        positioned`
+          <emu-clause id="sec-object.fromentries">
+          <h1>Object.fromEntries ( _obj_ )</h1>
+          <emu-alg>
+            1. Let _closure_ be a new Abstract Closure with parameters (${M}..._x_, _y_) that captures nothing and performs the following steps when called:
+              1. Do something with _x_.
+              1. Return *undefined*.
+            1. Return _closure_.
+          </emu-alg>
+        </emu-clause>
+        `,
+        {
+          ruleId: 'bad-ac',
+          nodeType: 'emu-alg',
+          message: 'expected rest param to come last',
+        },
+      );
+
+      await assertLint(
+        positioned`
+          <emu-clause id="sec-object.fromentries">
+          <h1>Object.fromEntries ( _obj_ )</h1>
+          <emu-alg>
+            1. Let _closure_ be a new Abstract Closure with parameters (${M}..._x_,) that captures nothing and performs the following steps when called:
+              1. Do something with _x_.
+              1. Return *undefined*.
+            1. Return _closure_.
+          </emu-alg>
+        </emu-clause>
+        `,
+        {
+          ruleId: 'bad-ac',
+          nodeType: 'emu-alg',
+          message: 'expected rest param to come last',
+        },
+      );
+    });
+
+    it('abstract closure rest parameters must be variables', async () => {
+      await assertLint(
+        positioned`
+          <emu-clause id="sec-object.fromentries">
+          <h1>Object.fromEntries ( _obj_ )</h1>
+          <emu-alg>
+            1. Let _closure_ be a new Abstract Closure with parameters (${M}...x) that captures nothing and performs the following steps when called:
+              1. Do something with _x_.
+              1. Return *undefined*.
+            1. Return _closure_.
+          </emu-alg>
+        </emu-clause>
+        `,
+        {
+          ruleId: 'bad-ac',
+          nodeType: 'emu-alg',
+          message: 'expected to find a parameter name here',
+        },
+      );
+
+      await assertLint(
+        positioned`
+          <emu-clause id="sec-object.fromentries">
+          <h1>Object.fromEntries ( _obj_ )</h1>
+          <emu-alg>
+            1. Let _closure_ be a new Abstract Closure with parameters (${M}..._x_,) that captures nothing and performs the following steps when called:
+              1. Do something with _x_.
+              1. Return *undefined*.
+            1. Return _closure_.
+          </emu-alg>
+        </emu-clause>
+        `,
+        {
+          ruleId: 'bad-ac',
+          nodeType: 'emu-alg',
+          message: 'expected rest param to come last',
+        },
+      );
+    });
+
+    it('abstract closure rest parameters are visible', async () => {
+      await assertLintFree(
+        `
+          <emu-clause id="sec-object.fromentries">
+          <h1>Object.fromEntries ( _obj_ )</h1>
+          <emu-alg>
+            1. Let _closure_ be a new Abstract Closure with parameters (..._x_) that captures nothing and performs the following steps when called:
+              1. Do something with _x_.
+              1. Return *undefined*.
+            1. Return _closure_.
+          </emu-alg>
+        </emu-clause>
+        `,
+      );
+    });
+
     it('multiple declarations are visible', async () => {
       await assertLintFree(
         `