fix corner cases in rests & unused (#5063)

lib/compress.js

@@ -5507,7 +5507,7 @@ merge(Compressor.prototype, {
         if (!(fn.rest instanceof AST_DestructuredArray)) return;
         if (!compressor.drop_fargs(fn, compressor.parent())) return;
         fn.argnames = fn.argnames.concat(fn.rest.elements);
-        fn.rest = null;
+        fn.rest = fn.rest.rest;
     }
 
     OPT(AST_Lambda, function(self, compressor) {
@@ -6428,12 +6428,16 @@ merge(Compressor.prototype, {
                 }
                 if (!(node instanceof AST_Accessor)) {
                     if (node.rest) {
-                        node.rest = node.rest.transform(trimmer);
-                        if (!(node.uses_arguments && !tt.has_directive("use strict"))
-                            && (node.rest instanceof AST_DestructuredArray && node.rest.elements.length == 0
-                                || node.rest instanceof AST_DestructuredObject && node.rest.properties.length == 0)) {
-                            node.rest = null;
+                        var rest = node.rest.transform(trimmer);
+                        if (rest instanceof AST_Destructured && !rest.rest
+                            && (!node.uses_arguments || tt.has_directive("use strict"))) {
+                            if (rest instanceof AST_DestructuredArray) {
+                                if (rest.elements.length == 0) rest = null;
+                            } else if (rest.properties.length == 0) {
+                                rest = null;
+                            }
                         }
+                        node.rest = rest;
                     }
                     var argnames = node.argnames;
                     var trim = compressor.drop_fargs(node, parent) && !node.rest;
@@ -12125,7 +12129,7 @@ merge(Compressor.prototype, {
     OPT(AST_DestructuredArray, function(self, compressor) {
         if (compressor.option("rests") && self.rest instanceof AST_DestructuredArray) {
             self.elements = self.elements.concat(self.rest.elements);
-            self.rest = null;
+            self.rest = self.rest.rest;
         }
         return self;
     });

test/compress/rests.js

@@ -491,7 +491,7 @@ drop_rest_array: {
         rests: true,
     }
     input: {
-        var [ ...[ a ]] = [ "PASS" ];
+        var [ ...[ a ] ] = [ "PASS" ];
         console.log(a);
     }
     expect: {
@@ -542,6 +542,82 @@ drop_rest_lambda: {
     node_version: ">=6"
 }
 
+keep_rest_array: {
+    options = {
+        rests: true,
+    }
+    input: {
+        var [ ...[ ...a ] ] = "PASS";
+        console.log(a.join(""));
+    }
+    expect: {
+        var [ ...a ] = "PASS";
+        console.log(a.join(""));
+    }
+    expect_stdout: "PASS"
+    node_version: ">=6"
+}
+
+keep_rest_arrow: {
+    options = {
+        arrows: true,
+        keep_fargs: false,
+        reduce_vars: true,
+        rests: true,
+    }
+    input: {
+        console.log(((...[ ...a ]) => a.join(""))("PASS"));
+    }
+    expect: {
+        console.log(((...a) => a.join(""))("PASS"));
+    }
+    expect_stdout: "PASS"
+    node_version: ">=6"
+}
+
+keep_rest_lambda_1: {
+    options = {
+        keep_fargs: false,
+        reduce_vars: true,
+        rests: true,
+        toplevel: true,
+    }
+    input: {
+        function f(...[ ...a ]) {
+            return a.join("");
+        }
+        console.log(f("PASS"), f([ 42 ]));
+    }
+    expect: {
+        function f(...a) {
+            return a.join("");
+        }
+        console.log(f("PASS"), f([ 42 ]));
+    }
+    expect_stdout: "PASS 42"
+    node_version: ">=6"
+}
+
+keep_rest_lambda_2: {
+    options = {
+        unused: true,
+    }
+    input: {
+        function f(...[ ...a ]) {
+            return a.join("");
+        }
+        console.log(f("PASS"), f([ 42 ]));
+    }
+    expect: {
+        function f(...[ ...a ]) {
+            return a.join("");
+        }
+        console.log(f("PASS"), f([ 42 ]));
+    }
+    expect_stdout: "PASS 42"
+    node_version: ">=6"
+}
+
 issue_4525_1: {
     options = {
         arguments: true,