Allow underscore prefix to bypass noUnusedLocals warning

This PR continues [Allow leading underscore for
types...](#58884),
taking into account feedback from https://github.com/microsoft/TypeScript/pull/58884/files#r1643108204.

The initial commit on this PR is original work by @a-tarasyuk. The second
commit enhances the test suite and reverts the change to checker.ts to
get a new baseline on the test suite for easier comparison with the
changes. The third commit changes checker.ts to allow any unused local if
it is prefixed with an underscore. This has the effect of newly allowing
the following unused declarations:

```ts
// Variables were previously allowed only in destructuring and for-loops
const _unusedVar = 2;
let _unusedLet = 4;
var _unusedVar2 = 6;

function _unusedFunc() { }
const _unusedArrow = () => { };
class _UnusedClass { }
interface _UnusedInterface { }
type _UnusedType = string;
enum _UnusedEnum { A }
namespace _UnusedNamespace { ... }
```

As far as I can tell, enum members and properties are not checked in the
checkUnusedLocalsAndParameters function, and we do not need to special-case
them.

Closes #58561

Author: jonshea

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 (name && isIdentifierThatStartsWithUnderscore(name)) {
                         continue;
                     }
 

tests/baselines/reference/noUnusedLocals_types.errors.txt

@@ -1,13 +1,9 @@
 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(9,6): error TS6196: '_UnusedType2' is declared but never used.
-noUnusedLocals_types.ts(10,11): error TS6196: '_UnusedInterface2' is declared but never used.
-noUnusedLocals_types.ts(11,7): error TS6196: '_UnusedClass2' is declared but never used.
-noUnusedLocals_types.ts(15,6): error TS6196: '_Helper' is declared but never used.
 
 
-==== noUnusedLocals_types.ts (7 errors) ====
+==== noUnusedLocals_types.ts (3 errors) ====
     // Test specifically for type declarations with underscore prefix
 
     // These should all produce errors (no underscore)
@@ -23,19 +19,11 @@ noUnusedLocals_types.ts(15,6): error TS6196: '_Helper' is declared but never use
 
     // These should NOT produce errors (underscore prefix)
     type _UnusedType2 = string;
-         ~~~~~~~~~~~~
-!!! error TS6196: '_UnusedType2' is declared but never used.
     interface _UnusedInterface2 { x: number; }  
-              ~~~~~~~~~~~~~~~~~
-!!! error TS6196: '_UnusedInterface2' is declared but never used.
     class _UnusedClass2 { }
-          ~~~~~~~~~~~~~
-!!! error TS6196: '_UnusedClass2' is declared but never used.
 
     // Mixed usage - only the one without underscore should error
     type UsedInOther = number;
     type _Helper = UsedInOther; // _Helper is not an error, but it uses UsedInOther
-         ~~~~~~~
-!!! error TS6196: '_Helper' is declared but never used.
 
     export {};

tests/baselines/reference/unusedLocalsStartingWithUnderscore.errors.txt

@@ -1,25 +1,15 @@
 a.ts(2,7): error TS6133: 'unusedVar' is declared but its value is never read.
-a.ts(3,7): error TS6133: '_unusedVar' is declared but its value is never read.
 a.ts(5,5): error TS6133: 'unusedLet' is declared but its value is never read.
-a.ts(6,5): error TS6133: '_unusedLet' is declared but its value is never read.
 a.ts(8,5): error TS6133: 'unusedVar2' is declared but its value is never read.
-a.ts(9,5): error TS6133: '_unusedVar2' is declared but its value is never read.
 a.ts(12,10): error TS6133: 'unusedFunc' is declared but its value is never read.
-a.ts(13,10): error TS6133: '_unusedFunc' is declared but its value is never read.
 a.ts(15,7): error TS6133: 'unusedArrow' is declared but its value is never read.
-a.ts(16,7): error TS6133: '_unusedArrow' is declared but its value is never read.
 a.ts(19,7): error TS6196: 'UnusedClass' is declared but never used.
-a.ts(20,7): error TS6196: '_UnusedClass' is declared but never used.
 a.ts(23,11): error TS6196: 'UnusedInterface' is declared but never used.
-a.ts(24,11): error TS6196: '_UnusedInterface' is declared but never used.
 a.ts(27,6): error TS6196: 'UnusedType' is declared but never used.
-a.ts(28,6): error TS6196: '_UnusedType' is declared but never used.
 a.ts(31,6): error TS6196: 'UnusedEnum' is declared but never used.
-a.ts(32,6): error TS6196: '_UnusedEnum' is declared but never used.
 a.ts(36,12): error TS6133: 'x' is declared but its value is never read.
 a.ts(38,12): error TS6133: 'x' is declared but its value is never read.
 a.ts(41,11): error TS6133: 'UnusedNamespace' is declared but its value is never read.
-a.ts(44,11): error TS6133: '_UnusedNamespace' is declared but its value is never read.
 a.ts(49,7): error TS6133: 'unusedA' is declared but its value is never read.
 a.ts(51,8): error TS6133: 'unusedC' is declared but its value is never read.
 a.ts(63,7): error TS6196: 'TestClass' is declared but never used.
@@ -36,75 +26,57 @@ a.ts(81,7): error TS6133: 'obj' is declared but its value is never read.
     declare function get(): string;
     export { get };
 
-==== a.ts (32 errors) ====
+==== a.ts (22 errors) ====
     // Variables
     const unusedVar = 1; // error
           ~~~~~~~~~
 !!! error TS6133: 'unusedVar' is declared but its value is never read.
     const _unusedVar = 2; // ok
-          ~~~~~~~~~~
-!!! error TS6133: '_unusedVar' is declared but its value is never read.
 
     let unusedLet = 3; // error  
         ~~~~~~~~~
 !!! error TS6133: 'unusedLet' is declared but its value is never read.
     let _unusedLet = 4; // ok
-        ~~~~~~~~~~
-!!! error TS6133: '_unusedLet' is declared but its value is never read.
 
     var unusedVar2 = 5; // error
         ~~~~~~~~~~
 !!! error TS6133: 'unusedVar2' is declared but its value is never read.
     var _unusedVar2 = 6; // ok
-        ~~~~~~~~~~~
-!!! error TS6133: '_unusedVar2' is declared but its value is never read.
 
     // Functions
     function unusedFunc() { } // error
              ~~~~~~~~~~
 !!! error TS6133: 'unusedFunc' is declared but its value is never read.
     function _unusedFunc() { } // ok
-             ~~~~~~~~~~~
-!!! error TS6133: '_unusedFunc' is declared but its value is never read.
 
     const unusedArrow = () => { }; // error
           ~~~~~~~~~~~
 !!! error TS6133: 'unusedArrow' is declared but its value is never read.
     const _unusedArrow = () => { }; // ok
-          ~~~~~~~~~~~~
-!!! error TS6133: '_unusedArrow' is declared but its value is never read.
 
     // Classes
     class UnusedClass { } // error
           ~~~~~~~~~~~
 !!! error TS6196: 'UnusedClass' is declared but never used.
     class _UnusedClass { } // ok
-          ~~~~~~~~~~~~
-!!! error TS6196: '_UnusedClass' is declared but never used.
 
     // Interfaces
     interface UnusedInterface { } // error
               ~~~~~~~~~~~~~~~
 !!! error TS6196: 'UnusedInterface' is declared but never used.
     interface _UnusedInterface { } // ok
-              ~~~~~~~~~~~~~~~~
-!!! error TS6196: '_UnusedInterface' is declared but never used.
 
     // Type aliases
     type UnusedType = string; // error
          ~~~~~~~~~~
 !!! error TS6196: 'UnusedType' is declared but never used.
     type _UnusedType = string; // ok
-         ~~~~~~~~~~~
-!!! error TS6196: '_UnusedType' is declared but never used.
 
     // Enums
     enum UnusedEnum { A } // error
          ~~~~~~~~~~
 !!! error TS6196: 'UnusedEnum' is declared but never used.
     enum _UnusedEnum { A } // ok
-         ~~~~~~~~~~~
-!!! error TS6196: '_UnusedEnum' is declared but never used.
 
     // Declarations in for loops
     for (const _x of []) { } // ok
@@ -123,8 +95,6 @@ a.ts(81,7): error TS6133: 'obj' is declared but its value is never read.
         export const x = 1;
     }
     namespace _UnusedNamespace { // ok
-              ~~~~~~~~~~~~~~~~
-!!! error TS6133: '_UnusedNamespace' is declared but its value is never read.
         export const x = 1;
     }