Do not report unused local error on locals that are intended for removing properties with object spread

src/compiler/checker.ts

@@ -16702,6 +16702,14 @@ namespace ts {
             }
         }
 
+        function isRemovedPropertyFromObjectSpread(node: Node) {
+            if (isBindingElement(node) && isObjectBindingPattern(node.parent)) {
+                const lastElement = lastOrUndefined(node.parent.elements);
+                return lastElement !== node && !!lastElement.dotDotDotToken;
+            }
+            return false;
+        }
+
         function errorUnusedLocal(node: Node, name: string) {
             if (isIdentifierThatStartsWithUnderScore(node)) {
                 const declaration = getRootDeclaration(node.parent);
@@ -16711,7 +16719,10 @@ namespace ts {
                     return;
                 }
             }
-            error(node, Diagnostics._0_is_declared_but_never_used, name);
+
+            if (!isRemovedPropertyFromObjectSpread(node.kind === SyntaxKind.Identifier ? node.parent : node)) {
+                error(node, Diagnostics._0_is_declared_but_never_used, name);
+            }
         }
 
         function parameterNameStartsWithUnderscore(parameterName: DeclarationName) {

tests/baselines/reference/unusedLocalsAndObjectSpread.errors.txt

@@ -0,0 +1,40 @@
+tests/cases/compiler/unusedLocalsAndObjectSpread.ts(21,18): error TS6133: 'bar' is declared but never used.
+tests/cases/compiler/unusedLocalsAndObjectSpread.ts(28,21): error TS6133: 'bar' is declared but never used.
+
+
+==== tests/cases/compiler/unusedLocalsAndObjectSpread.ts (2 errors) ====
+
+    declare var console: { log(a: any): void };
+
+    function one() {
+        const foo = { a: 1, b: 2 };
+        // 'a' is declared but never used
+        const {a, ...bar} = foo;
+        console.log(bar);
+    }
+
+    function two() {
+        const foo = { a: 1, b: 2 };
+        // '_' is declared but never used
+        const {a: _, ...bar} = foo;
+        console.log(bar);
+    }
+
+    function three() {
+        const foo = { a: 1, b: 2 };
+        // 'a' is declared but never used
+        const {a, ...bar} = foo; // bar should be unused
+                     ~~~
+!!! error TS6133: 'bar' is declared but never used.
+        //console.log(bar);
+    }
+
+    function four() {
+        const foo = { a: 1, b: 2 };
+        // '_' is declared but never used
+        const {a: _, ...bar} = foo; // bar should be unused
+                        ~~~
+!!! error TS6133: 'bar' is declared but never used.
+        //console.log(bar);
+    }
+

tests/baselines/reference/unusedLocalsAndObjectSpread.js

@@ -0,0 +1,67 @@
+//// [unusedLocalsAndObjectSpread.ts]
+
+declare var console: { log(a: any): void };
+
+function one() {
+    const foo = { a: 1, b: 2 };
+    // 'a' is declared but never used
+    const {a, ...bar} = foo;
+    console.log(bar);
+}
+
+function two() {
+    const foo = { a: 1, b: 2 };
+    // '_' is declared but never used
+    const {a: _, ...bar} = foo;
+    console.log(bar);
+}
+
+function three() {
+    const foo = { a: 1, b: 2 };
+    // 'a' is declared but never used
+    const {a, ...bar} = foo; // bar should be unused
+    //console.log(bar);
+}
+
+function four() {
+    const foo = { a: 1, b: 2 };
+    // '_' is declared but never used
+    const {a: _, ...bar} = foo; // bar should be unused
+    //console.log(bar);
+}
+
+
+//// [unusedLocalsAndObjectSpread.js]
+var __rest = (this && this.__rest) || function (s, e) {
+    var t = {};
+    for (var p in s) if (Object.prototype.hasOwnProperty.call(s, p) && e.indexOf(p) < 0)
+        t[p] = s[p];
+    if (s != null && typeof Object.getOwnPropertySymbols === "function")
+        for (var i = 0, p = Object.getOwnPropertySymbols(s); i < p.length; i++) if (e.indexOf(p[i]) < 0)
+            t[p[i]] = s[p[i]];
+    return t;
+};
+function one() {
+    var foo = { a: 1, b: 2 };
+    // 'a' is declared but never used
+    var a = foo.a, bar = __rest(foo, ["a"]);
+    console.log(bar);
+}
+function two() {
+    var foo = { a: 1, b: 2 };
+    // '_' is declared but never used
+    var _ = foo.a, bar = __rest(foo, ["a"]);
+    console.log(bar);
+}
+function three() {
+    var foo = { a: 1, b: 2 };
+    // 'a' is declared but never used
+    var a = foo.a, bar = __rest(foo, ["a"]); // bar should be unused
+    //console.log(bar);
+}
+function four() {
+    var foo = { a: 1, b: 2 };
+    // '_' is declared but never used
+    var _ = foo.a, bar = __rest(foo, ["a"]); // bar should be unused
+    //console.log(bar);
+}

tests/cases/compiler/unusedLocalsAndObjectSpread.ts

@@ -0,0 +1,31 @@
+//@noUnusedLocals:true
+
+declare var console: { log(a: any): void };
+
+function one() {
+    const foo = { a: 1, b: 2 };
+    // 'a' is declared but never used
+    const {a, ...bar} = foo;
+    console.log(bar);
+}
+
+function two() {
+    const foo = { a: 1, b: 2 };
+    // '_' is declared but never used
+    const {a: _, ...bar} = foo;
+    console.log(bar);
+}
+
+function three() {
+    const foo = { a: 1, b: 2 };
+    // 'a' is declared but never used
+    const {a, ...bar} = foo; // bar should be unused
+    //console.log(bar);
+}
+
+function four() {
+    const foo = { a: 1, b: 2 };
+    // '_' is declared but never used
+    const {a: _, ...bar} = foo; // bar should be unused
+    //console.log(bar);
+}