Feat/58561 allow leading underscore to bypass no unused locals

src/compiler/checker.ts

@@ -44409,21 +44409,6 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         return tryCast(getRootDeclaration(node), isParameter);
     }
 
-    function isValidUnusedLocalDeclaration(declaration: Declaration): boolean {
-        if (isBindingElement(declaration)) {
-            if (isObjectBindingPattern(declaration.parent)) {
-                /**
-                 * ignore starts with underscore names _
-                 * const { a: _a } = { a: 1 }
-                 */
-                return !!(declaration.propertyName && isIdentifierThatStartsWithUnderscore(declaration.name));
-            }
-            return isIdentifierThatStartsWithUnderscore(declaration.name);
-        }
-        return isAmbientModule(declaration) ||
-            (isVariableDeclaration(declaration) && isForInOrOfStatement(declaration.parent.parent) || isImportedDeclaration(declaration)) && isIdentifierThatStartsWithUnderscore(declaration.name!);
-    }
-
     function checkUnusedLocalsAndParameters(nodeWithLocals: HasLocals, addDiagnostic: AddUnusedDiagnostic): void {
         // Ideally we could use the ImportClause directly as a key, but must wait until we have full ES6 maps. So must store key along with value.
         const unusedImports = new Map<string, [ImportClause, ImportedDeclaration[]]>();
@@ -44438,7 +44423,8 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
 
             if (local.declarations) {
                 for (const declaration of local.declarations) {
-                    if (isValidUnusedLocalDeclaration(declaration)) {
+                    const name = getNameOfDeclaration(declaration);
+                    if (isAmbientModule(declaration) || name && isIdentifierThatStartsWithUnderscore(name)) {
                         continue;
                     }
 

tests/baselines/reference/noUnusedLocals_types.errors.txt

@@ -0,0 +1,29 @@
+noUnusedLocals_types.ts(4,6): error TS6196: 'UnusedType1' is declared but never used.
+noUnusedLocals_types.ts(5,11): error TS6196: 'UnusedInterface1' is declared but never used.
+noUnusedLocals_types.ts(6,7): error TS6196: 'UnusedClass1' is declared but never used.
+
+
+==== noUnusedLocals_types.ts (3 errors) ====
+    // Test specifically for type declarations with underscore prefix
+
+    // These should all produce errors (no underscore)
+    type UnusedType1 = string;
+         ~~~~~~~~~~~
+!!! error TS6196: 'UnusedType1' is declared but never used.
+    interface UnusedInterface1 { x: number; }
+              ~~~~~~~~~~~~~~~~
+!!! error TS6196: 'UnusedInterface1' is declared but never used.
+    class UnusedClass1 { }
+          ~~~~~~~~~~~~
+!!! error TS6196: 'UnusedClass1' is declared but never used.
+
+    // These should NOT produce errors (underscore prefix)
+    type _UnusedType2 = string;
+    interface _UnusedInterface2 { x: number; }  
+    class _UnusedClass2 { }
+
+    // Mixed usage - only the one without underscore should error
+    type UsedInOther = number;
+    type _Helper = UsedInOther; // _Helper is not an error, but it uses UsedInOther
+
+    export {};

tests/baselines/reference/noUnusedLocals_types.js

@@ -0,0 +1,35 @@
+//// [tests/cases/compiler/noUnusedLocals_types.ts] ////
+
+//// [noUnusedLocals_types.ts]
+// Test specifically for type declarations with underscore prefix
+
+// These should all produce errors (no underscore)
+type UnusedType1 = string;
+interface UnusedInterface1 { x: number; }
+class UnusedClass1 { }
+
+// These should NOT produce errors (underscore prefix)
+type _UnusedType2 = string;
+interface _UnusedInterface2 { x: number; }  
+class _UnusedClass2 { }
+
+// Mixed usage - only the one without underscore should error
+type UsedInOther = number;
+type _Helper = UsedInOther; // _Helper is not an error, but it uses UsedInOther
+
+export {};
+
+//// [noUnusedLocals_types.js]
+"use strict";
+// Test specifically for type declarations with underscore prefix
+Object.defineProperty(exports, "__esModule", { value: true });
+var UnusedClass1 = /** @class */ (function () {
+    function UnusedClass1() {
+    }
+    return UnusedClass1;
+}());
+var _UnusedClass2 = /** @class */ (function () {
+    function _UnusedClass2() {
+    }
+    return _UnusedClass2;
+}());

tests/baselines/reference/noUnusedLocals_types.symbols

@@ -0,0 +1,36 @@
+//// [tests/cases/compiler/noUnusedLocals_types.ts] ////
+
+=== noUnusedLocals_types.ts ===
+// Test specifically for type declarations with underscore prefix
+
+// These should all produce errors (no underscore)
+type UnusedType1 = string;
+>UnusedType1 : Symbol(UnusedType1, Decl(noUnusedLocals_types.ts, 0, 0))
+
+interface UnusedInterface1 { x: number; }
+>UnusedInterface1 : Symbol(UnusedInterface1, Decl(noUnusedLocals_types.ts, 3, 26))
+>x : Symbol(UnusedInterface1.x, Decl(noUnusedLocals_types.ts, 4, 28))
+
+class UnusedClass1 { }
+>UnusedClass1 : Symbol(UnusedClass1, Decl(noUnusedLocals_types.ts, 4, 41))
+
+// These should NOT produce errors (underscore prefix)
+type _UnusedType2 = string;
+>_UnusedType2 : Symbol(_UnusedType2, Decl(noUnusedLocals_types.ts, 5, 22))
+
+interface _UnusedInterface2 { x: number; }  
+>_UnusedInterface2 : Symbol(_UnusedInterface2, Decl(noUnusedLocals_types.ts, 8, 27))
+>x : Symbol(_UnusedInterface2.x, Decl(noUnusedLocals_types.ts, 9, 29))
+
+class _UnusedClass2 { }
+>_UnusedClass2 : Symbol(_UnusedClass2, Decl(noUnusedLocals_types.ts, 9, 42))
+
+// Mixed usage - only the one without underscore should error
+type UsedInOther = number;
+>UsedInOther : Symbol(UsedInOther, Decl(noUnusedLocals_types.ts, 10, 23))
+
+type _Helper = UsedInOther; // _Helper is not an error, but it uses UsedInOther
+>_Helper : Symbol(_Helper, Decl(noUnusedLocals_types.ts, 13, 26))
+>UsedInOther : Symbol(UsedInOther, Decl(noUnusedLocals_types.ts, 10, 23))
+
+export {};

tests/baselines/reference/noUnusedLocals_types.types

@@ -0,0 +1,41 @@
+//// [tests/cases/compiler/noUnusedLocals_types.ts] ////
+
+=== noUnusedLocals_types.ts ===
+// Test specifically for type declarations with underscore prefix
+
+// These should all produce errors (no underscore)
+type UnusedType1 = string;
+>UnusedType1 : string
+>            : ^^^^^^
+
+interface UnusedInterface1 { x: number; }
+>x : number
+>  : ^^^^^^
+
+class UnusedClass1 { }
+>UnusedClass1 : UnusedClass1
+>             : ^^^^^^^^^^^^
+
+// These should NOT produce errors (underscore prefix)
+type _UnusedType2 = string;
+>_UnusedType2 : string
+>             : ^^^^^^
+
+interface _UnusedInterface2 { x: number; }  
+>x : number
+>  : ^^^^^^
+
+class _UnusedClass2 { }
+>_UnusedClass2 : _UnusedClass2
+>              : ^^^^^^^^^^^^^
+
+// Mixed usage - only the one without underscore should error
+type UsedInOther = number;
+>UsedInOther : number
+>            : ^^^^^^
+
+type _Helper = UsedInOther; // _Helper is not an error, but it uses UsedInOther
+>_Helper : number
+>        : ^^^^^^
+
+export {};

tests/baselines/reference/unusedLocalsStartingWithUnderscore.errors.txt

@@ -1,22 +1,156 @@
-/a.ts(7,11): error TS6133: '_ns' is declared but its value is never read.
-/a.ts(8,9): error TS6133: '_' is declared but its value is never read.
+unusedLocalsStartingWithUnderscore.ts(2,7): error TS6133: 'unusedVar' is declared but its value is never read.
+unusedLocalsStartingWithUnderscore.ts(5,5): error TS6133: 'unusedLet' is declared but its value is never read.
+unusedLocalsStartingWithUnderscore.ts(8,5): error TS6133: 'unusedVar2' is declared but its value is never read.
+unusedLocalsStartingWithUnderscore.ts(11,9): error TS6133: 'a1' is declared but its value is never read.
+unusedLocalsStartingWithUnderscore.ts(15,10): error TS6133: 'unusedFunc' is declared but its value is never read.
+unusedLocalsStartingWithUnderscore.ts(18,7): error TS6133: 'unusedArrow' is declared but its value is never read.
+unusedLocalsStartingWithUnderscore.ts(22,7): error TS6196: 'UnusedClass' is declared but never used.
+unusedLocalsStartingWithUnderscore.ts(26,11): error TS6196: 'UnusedInterface' is declared but never used.
+unusedLocalsStartingWithUnderscore.ts(30,6): error TS6196: 'UnusedType' is declared but never used.
+unusedLocalsStartingWithUnderscore.ts(34,6): error TS6196: 'UnusedEnum' is declared but never used.
+unusedLocalsStartingWithUnderscore.ts(39,12): error TS6133: 'x' is declared but its value is never read.
+unusedLocalsStartingWithUnderscore.ts(41,12): error TS6133: 'x' is declared but its value is never read.
+unusedLocalsStartingWithUnderscore.ts(44,11): error TS6133: 'UnusedNamespace' is declared but its value is never read.
+unusedLocalsStartingWithUnderscore.ts(52,7): error TS6133: 'unusedA' is declared but its value is never read.
+unusedLocalsStartingWithUnderscore.ts(54,8): error TS6133: 'unusedC' is declared but its value is never read.
+unusedLocalsStartingWithUnderscore.ts(61,7): error TS6196: 'TestClass' is declared but never used.
+unusedLocalsStartingWithUnderscore.ts(62,13): error TS6133: 'unusedMember' is declared but its value is never read.
+unusedLocalsStartingWithUnderscore.ts(63,13): error TS6133: '_unusedMember' is declared but its value is never read.
+unusedLocalsStartingWithUnderscore.ts(65,13): error TS6133: 'unusedMethod' is declared but its value is never read.
+unusedLocalsStartingWithUnderscore.ts(66,13): error TS6133: '_unusedMethod' is declared but its value is never read.
+unusedLocalsStartingWithUnderscore.ts(69,6): error TS6196: 'TestEnum' is declared but never used.
+unusedLocalsStartingWithUnderscore.ts(74,11): error TS6196: 'TestInterface' is declared but never used.
+unusedLocalsStartingWithUnderscore.ts(79,7): error TS6133: 'obj' is declared but its value is never read.
 
 
-==== /a.ts (2 errors) ====
-    import * as _ from "./a";
+==== unusedLocalsStartingWithUnderscore.ts (23 errors) ====
+    // Variables
+    const unusedVar = 1; // error
+          ~~~~~~~~~
+!!! error TS6133: 'unusedVar' is declared but its value is never read.
+    const _unusedVar = 2; // ok
 
-    for (const _ of []) { }
+    let unusedLet = 3; // error  
+        ~~~~~~~~~
+!!! error TS6133: 'unusedLet' is declared but its value is never read.
+    let _unusedLet = 4; // ok
 
-    for (const _ in []) { }
+    var unusedVar2 = 5; // error
+        ~~~~~~~~~~
+!!! error TS6133: 'unusedVar2' is declared but its value is never read.
+    var _unusedVar2 = 6; // ok
 
-    namespace _ns {
-              ~~~
-!!! error TS6133: '_ns' is declared but its value is never read.
-        let _;
-            ~
-!!! error TS6133: '_' is declared but its value is never read.
-        for (const _ of []) { }
+    const { a1, _b1 } = { a1: 1, _b1: 2 }; // error on a1
+            ~~
+!!! error TS6133: 'a1' is declared but its value is never read.
+    const { _a2, _b2 } = { _a2: 1, _b2: 2 }; // ok
 
-        for (const _ in []) { }
+    // Functions
+    function unusedFunc() { } // error
+             ~~~~~~~~~~
+!!! error TS6133: 'unusedFunc' is declared but its value is never read.
+    function _unusedFunc() { } // ok
+
+    const unusedArrow = () => { }; // error
+          ~~~~~~~~~~~
+!!! error TS6133: 'unusedArrow' is declared but its value is never read.
+    const _unusedArrow = () => { }; // ok
+
+    // Classes
+    class UnusedClass { } // error
+          ~~~~~~~~~~~
+!!! error TS6196: 'UnusedClass' is declared but never used.
+    class _UnusedClass { } // ok
+
+    // Interfaces
+    interface UnusedInterface { } // error
+              ~~~~~~~~~~~~~~~
+!!! error TS6196: 'UnusedInterface' is declared but never used.
+    interface _UnusedInterface { } // ok
+
+    // Type aliases
+    type UnusedType = string; // error
+         ~~~~~~~~~~
+!!! error TS6196: 'UnusedType' is declared but never used.
+    type _UnusedType = string; // ok
+
+    // Enums
+    enum UnusedEnum { A } // error
+         ~~~~~~~~~~
+!!! error TS6196: 'UnusedEnum' is declared but never used.
+    enum _UnusedEnum { A } // ok
+
+    // Declarations in for loops
+    for (const _x of []) { } // ok
+    for (const x of []) { } // error
+               ~
+!!! error TS6133: 'x' is declared but its value is never read.
+    for (const _x in []) { } // ok
+    for (const x in []) { } // error
+               ~
+!!! error TS6133: 'x' is declared but its value is never read.
+
+    // Namespaces
+    namespace UnusedNamespace { // error
+              ~~~~~~~~~~~~~~~
+!!! error TS6133: 'UnusedNamespace' is declared but its value is never read.
+        export const x = 1;
+    }
+    namespace _UnusedNamespace { // ok
+        export const x = 1;
     }
-
+
+    // Destructuring
+    const { a: unusedA } = { a: 1 }; // error
+          ~~~~~~~~~~~~~~
+!!! error TS6133: 'unusedA' is declared but its value is never read.
+    const { b: _unusedB } = { b: 2 }; // ok
+    const [unusedC] = [3]; // error
+           ~~~~~~~
+!!! error TS6133: 'unusedC' is declared but its value is never read.
+    const [_unusedD] = [4]; // ok
+
+    //
+    // The following declarations may _not_ use an underscore to bypass @noUnusedLocals
+    //
+
+    class TestClass {
+          ~~~~~~~~~
+!!! error TS6196: 'TestClass' is declared but never used.
+        private unusedMember = 1; // error
+                ~~~~~~~~~~~~
+!!! error TS6133: 'unusedMember' is declared but its value is never read.
+        private _unusedMember = 2; // still error
+                ~~~~~~~~~~~~~
+!!! error TS6133: '_unusedMember' is declared but its value is never read.
+
+        private unusedMethod() { } // error
+                ~~~~~~~~~~~~
+!!! error TS6133: 'unusedMethod' is declared but its value is never read.
+        private _unusedMethod() { } // still error
+                ~~~~~~~~~~~~~
+!!! error TS6133: '_unusedMethod' is declared but its value is never read.
+    }
+
+    enum TestEnum {
+         ~~~~~~~~
+!!! error TS6196: 'TestEnum' is declared but never used.
+        UnusedMember = 1, // error
+        _UnusedMember = 2, // still error
+    }
+
+    interface TestInterface {
+              ~~~~~~~~~~~~~
+!!! error TS6196: 'TestInterface' is declared but never used.
+        unusedProp: number; // error
+        _unusedProp: number; // still error
+    }
+
+    const obj = {
+          ~~~
+!!! error TS6133: 'obj' is declared but its value is never read.
+        unusedProp: 1, // error
+        _unusedProp: 2, // still error
+    };
+
+    export { };