From 980fcbb56b674c1c78e1483b6d66a2f49e58d80b Mon Sep 17 00:00:00 2001
From: "Alex Lam S.L" <alexlamsl@gmail.com>
Date: Thu, 3 Sep 2020 10:41:33 +0100
Subject: [PATCH] enhance `unused` (#4090)

---
 lib/compress.js              | 53 ++++++++++++++++++++++-----------
 test/compress/drop-unused.js | 57 ++++++++++++++++++++++++++++++++++++
 test/compress/functions.js   |  3 +-
 3 files changed, 94 insertions(+), 19 deletions(-)

diff --git a/lib/compress.js b/lib/compress.js
index 9aa70ef9..273c5a75 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -4312,6 +4312,7 @@ merge(Compressor.prototype, {
             return sym;
         };
         var assign_in_use = Object.create(null);
+        var for_ins = Object.create(null);
         var in_use = [];
         var in_use_ids = Object.create(null); // avoid expensive linear scans of in_use
         var value_read = Object.create(null);
@@ -4602,23 +4603,37 @@ merge(Compressor.prototype, {
                         return !def || fn.name && def === fn.name.definition();
                     }
                 });
-                if (head.length == 0 && tail.length == duplicated) {
-                    [].unshift.apply(side_effects, tail.map(function(def) {
-                        AST_Node.warn("Dropping duplicated definition of variable {name} [{file}:{line},{col}]", template(def.name));
-                        var sym = def.name.definition();
-                        var ref = make_node(AST_SymbolRef, def.name, def.name);
-                        sym.references.push(ref);
-                        var assign = make_node(AST_Assign, def, {
-                            operator: "=",
-                            left: ref,
-                            right: def.value
-                        });
-                        var index = indexOf_assign(sym, def);
-                        if (index >= 0) assign_in_use[sym.id][index] = assign;
-                        sym.eliminated++;
-                        return assign;
-                    }));
-                } else if (head.length > 0 || tail.length > 0) {
+                switch (head.length) {
+                  case 0:
+                    if (tail.length == 0) break;
+                    if (tail.length == duplicated) {
+                        [].unshift.apply(side_effects, tail.map(function(def) {
+                            AST_Node.warn("Dropping duplicated definition of variable {name} [{file}:{line},{col}]", template(def.name));
+                            var sym = def.name.definition();
+                            var ref = make_node(AST_SymbolRef, def.name, def.name);
+                            sym.references.push(ref);
+                            var assign = make_node(AST_Assign, def, {
+                                operator: "=",
+                                left: ref,
+                                right: def.value
+                            });
+                            var index = indexOf_assign(sym, def);
+                            if (index >= 0) assign_in_use[sym.id][index] = assign;
+                            sym.eliminated++;
+                            return assign;
+                        }));
+                        break;
+                    }
+                  case 1:
+                    if (tail.length == 0) {
+                        var id = head[0].name.definition().id;
+                        if (id in for_ins) {
+                            node.definitions = head;
+                            for_ins[id].init = node;
+                            break;
+                        }
+                    }
+                  default:
                     node.definitions = head.concat(tail);
                     body.push(node);
                 }
@@ -4814,6 +4829,10 @@ merge(Compressor.prototype, {
                 return true;
             }
             if (node instanceof AST_ForIn) {
+                if (node.init instanceof AST_SymbolRef && scope === self) {
+                    var id = node.init.definition().id;
+                    if (!(id in for_ins)) for_ins[id] = node;
+                }
                 if (!drop_vars || !compressor.option("loops")) return;
                 if (!is_empty(node.body)) return;
                 if (node.init.has_side_effects(compressor)) return;
diff --git a/test/compress/drop-unused.js b/test/compress/drop-unused.js
index 6ab4fe3b..052a013b 100644
--- a/test/compress/drop-unused.js
+++ b/test/compress/drop-unused.js
@@ -2848,3 +2848,60 @@ issue_4025: {
         "1 1 1",
     ]
 }
+
+forin_var_1: {
+    options = {
+        unused: true,
+    }
+    input: {
+        var k;
+        for (k in [ 1, 2 ])
+            console.log(k);
+        for (k in { PASS: 3 })
+            console.log(k);
+        console.log(k);
+    }
+    expect: {
+        for (var k in [ 1, 2 ])
+            console.log(k);
+        for (k in { PASS: 3 })
+            console.log(k);
+        console.log(k);
+    }
+    expect_stdout: [
+        "0",
+        "1",
+        "PASS",
+        "PASS",
+    ]
+}
+
+forin_var_2: {
+    options = {
+        unused: true,
+    }
+    input: {
+        console.log(function() {
+            switch (0) {
+              case function() {
+                    for (a in 0);
+                }:
+                var b = 0;
+            }
+            for (var c = 0; a;);
+            var a;
+        }());
+    }
+    expect: {
+        console.log(function() {
+            switch (0) {
+              case function() {
+                    for (a in 0);
+                }:
+            }
+            for (; a;);
+            var a;
+        }());
+    }
+    expect_stdout: "undefined"
+}
diff --git a/test/compress/functions.js b/test/compress/functions.js
index 18f4f900..8bfb2eaf 100644
--- a/test/compress/functions.js
+++ b/test/compress/functions.js
@@ -1483,8 +1483,7 @@ issue_2663_2: {
     }
     expect: {
         (function() {
-            var i;
-            for (i in { a: 1, b: 2, c: 3 })
+            for (var i in { a: 1, b: 2, c: 3 })
                 j = i, console.log(j);
             var j;
         })();
-- 
2.34.1