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

Fixes #12766

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

@@ -1,5 +1,5 @@
-tests/cases/compiler/unusedLocalsAndObjectSpread.ts(7,12): error TS6133: 'a' is declared but never used.
-tests/cases/compiler/unusedLocalsAndObjectSpread.ts(14,15): error TS6133: '_' is declared but never used.
+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) ====
@@ -10,8 +10,6 @@ tests/cases/compiler/unusedLocalsAndObjectSpread.ts(14,15): error TS6133: '_' is
         const foo = { a: 1, b: 2 };
         // 'a' is declared but never used
         const {a, ...bar} = foo;
-               ~
-!!! error TS6133: 'a' is declared but never used.
         console.log(bar);
     }
     
@@ -19,9 +17,24 @@ tests/cases/compiler/unusedLocalsAndObjectSpread.ts(14,15): error TS6133: '_' is
         const foo = { a: 1, b: 2 };
         // '_' is declared but never used
         const {a: _, ...bar} = foo;
-                  ~
-!!! error TS6133: '_' is declared but never used.
         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

@@ -16,6 +16,19 @@ function two() {
     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]
@@ -40,3 +53,15 @@ function two() {
     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

@@ -16,3 +16,16 @@ function two() {
     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);
+}