feat(31388): Destructuring array, ability to explicitly tell the variable is unused

src/compiler/checker.ts

@@ -33362,13 +33362,16 @@ namespace ts {
         }
 
         function isValidUnusedLocalDeclaration(declaration: Declaration): boolean {
-            if (isBindingElement(declaration) && isIdentifierThatStartsWithUnderscore(declaration.name)) {
-                return !!findAncestor(declaration.parent, ancestor =>
-                    isArrayBindingPattern(ancestor) || isVariableDeclaration(ancestor) || isVariableDeclarationList(ancestor) ? false :
-                        isForOfStatement(ancestor) ? true : "quit"
-                );
+            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!);
         }

tests/baselines/reference/unusedVariablesWithUnderscoreInBindingElement.errors.txt

@@ -0,0 +1,145 @@
+tests/cases/compiler/unusedVariablesWithUnderscoreInBindingElement.ts(12,17): error TS6133: 'b1' is declared but its value is never read.
+tests/cases/compiler/unusedVariablesWithUnderscoreInBindingElement.ts(13,12): error TS6133: 'a2' is declared but its value is never read.
+tests/cases/compiler/unusedVariablesWithUnderscoreInBindingElement.ts(14,12): error TS6133: 'a3' is declared but its value is never read.
+tests/cases/compiler/unusedVariablesWithUnderscoreInBindingElement.ts(14,16): error TS6133: 'b3' is declared but its value is never read.
+tests/cases/compiler/unusedVariablesWithUnderscoreInBindingElement.ts(23,12): error TS6133: 'a3' is declared but its value is never read.
+tests/cases/compiler/unusedVariablesWithUnderscoreInBindingElement.ts(23,18): error TS6133: 'b3' is declared but its value is never read.
+tests/cases/compiler/unusedVariablesWithUnderscoreInBindingElement.ts(23,22): error TS6133: 'c3' is declared but its value is never read.
+tests/cases/compiler/unusedVariablesWithUnderscoreInBindingElement.ts(23,28): error TS6133: 'd3' is declared but its value is never read.
+tests/cases/compiler/unusedVariablesWithUnderscoreInBindingElement.ts(23,32): error TS6133: 'e3' is declared but its value is never read.
+tests/cases/compiler/unusedVariablesWithUnderscoreInBindingElement.ts(37,22): error TS6133: 'b1' is declared but its value is never read.
+tests/cases/compiler/unusedVariablesWithUnderscoreInBindingElement.ts(38,13): error TS6133: 'a2' is declared but its value is never read.
+tests/cases/compiler/unusedVariablesWithUnderscoreInBindingElement.ts(39,11): error TS6198: All destructured elements are unused.
+tests/cases/compiler/unusedVariablesWithUnderscoreInBindingElement.ts(68,9): error TS6133: 'a3' is declared but its value is never read.
+tests/cases/compiler/unusedVariablesWithUnderscoreInBindingElement.ts(70,18): error TS6198: All destructured elements are unused.
+tests/cases/compiler/unusedVariablesWithUnderscoreInBindingElement.ts(74,9): error TS6133: 'c3' is declared but its value is never read.
+tests/cases/compiler/unusedVariablesWithUnderscoreInBindingElement.ts(75,9): error TS6133: 'd3' is declared but its value is never read.
+tests/cases/compiler/unusedVariablesWithUnderscoreInBindingElement.ts(81,11): error TS6198: All destructured elements are unused.
+
+
+==== tests/cases/compiler/unusedVariablesWithUnderscoreInBindingElement.ts (17 errors) ====
+    function t1() {
+        const [_a1, b1] = [1, 2];
+        console.log(b1);
+
+        const [a2, _b2] = [1, 2];
+        console.log(a2);
+
+        const [_a3, _b3] = [1, 2];
+    }
+
+    function t2() {
+        const [_a1, b1] = [1, 2];
+                    ~~
+!!! error TS6133: 'b1' is declared but its value is never read.
+        const [a2, _b2] = [1, 2];
+               ~~
+!!! error TS6133: 'a2' is declared but its value is never read.
+        const [a3, b3] = [1, 2];
+               ~~
+!!! error TS6133: 'a3' is declared but its value is never read.
+                   ~~
+!!! error TS6133: 'b3' is declared but its value is never read.
+    }
+
+    function t3() {
+        const [_a1, [[_b1, c1]], d1, _e1] = [1, [[2, 3]], 4, 5];
+        console.log(c1, d1);
+
+        const [_a2, [[_b2, _c2]], _d2, _e2] = [1, [[2, 3]], 4, 5];
+
+        const [a3, [[b3, c3]], d3, e3] = [1, [[2, 3]], 4, 5];
+               ~~
+!!! error TS6133: 'a3' is declared but its value is never read.
+                     ~~
+!!! error TS6133: 'b3' is declared but its value is never read.
+                         ~~
+!!! error TS6133: 'c3' is declared but its value is never read.
+                               ~~
+!!! error TS6133: 'd3' is declared but its value is never read.
+                                   ~~
+!!! error TS6133: 'e3' is declared but its value is never read.
+    }
+
+    function t4() {
+        const { a1: _a1, b1 } = { a1: 1, b1: 1 };
+        console.log(b1);
+
+        const { a2, b2: _b2 } = { a2: 1, b2: 1 };
+        console.log(a2);
+
+        const { a3: _a3, b3: _b3 } = { a3: 1, b3: 1 };
+    }
+
+    function t5() {
+        const { a1: _a1, b1 } = { a1: 1, b1: 1 };
+                         ~~
+!!! error TS6133: 'b1' is declared but its value is never read.
+        const { a2, b2: _b2 } = { a2: 1, b2: 1 };
+                ~~
+!!! error TS6133: 'a2' is declared but its value is never read.
+        const { a3, b3 } = { a3: 1, b3: 1 };
+              ~~~~~~~~~~
+!!! error TS6198: All destructured elements are unused.
+    }
+
+    function t6() {
+        const {
+            a1: _a1,
+            b1: {
+                b11: {
+                    b111: _b111,
+                    b112
+                }
+            },
+            c1,
+            d1: _d1
+        } = { a1: 1, b1: { b11: { b111: 1, b112: 1 } }, c1: 1, d1: 1 };
+        console.log(b112, c1);
+
+        const {
+            a2: _a2,
+            b2: {
+                b21: {
+                    b211: _b211, b212: _b212
+                }
+            },
+            c2: _c2,
+            d2: _d2
+        } = { a2: 1, b2: { b21: { b211: 1, b212: 1 } }, c2: 1, d2: 1 };
+
+        const {
+            a3,
+            ~~
+!!! error TS6133: 'a3' is declared but its value is never read.
+            b3: {
+                b31: {
+                     ~
+                    b311, b312
+    ~~~~~~~~~~~~~~~~~~~~~~~~~~
+                }
+    ~~~~~~~~~~~~~
+!!! error TS6198: All destructured elements are unused.
+            },
+            c3,
+            ~~
+!!! error TS6133: 'c3' is declared but its value is never read.
+            d3
+            ~~
+!!! error TS6133: 'd3' is declared but its value is never read.
+        } = { a3: 1, b3: { b31: { b311: 1, b312: 1 } }, c3: 1, d3: 1 };
+    }
+
+    function t7() {
+        // error
+        const { _a1, _b1 } = { _a1: 1, _b1: 1 };
+              ~~~~~~~~~~~~
+!!! error TS6198: All destructured elements are unused.
+
+        // ok
+        const { a2: _a2, b2: _b2 } = { a2: 1, b2: 1 };
+
+        // ok
+        const { _a3: _ignoreA3, _b3: _ignoreB3 } = { _a3: 1, _b3: 1 };
+    }
+

tests/baselines/reference/unusedVariablesWithUnderscoreInBindingElement.js

@@ -0,0 +1,136 @@
+//// [unusedVariablesWithUnderscoreInBindingElement.ts]
+function t1() {
+    const [_a1, b1] = [1, 2];
+    console.log(b1);
+
+    const [a2, _b2] = [1, 2];
+    console.log(a2);
+
+    const [_a3, _b3] = [1, 2];
+}
+
+function t2() {
+    const [_a1, b1] = [1, 2];
+    const [a2, _b2] = [1, 2];
+    const [a3, b3] = [1, 2];
+}
+
+function t3() {
+    const [_a1, [[_b1, c1]], d1, _e1] = [1, [[2, 3]], 4, 5];
+    console.log(c1, d1);
+
+    const [_a2, [[_b2, _c2]], _d2, _e2] = [1, [[2, 3]], 4, 5];
+
+    const [a3, [[b3, c3]], d3, e3] = [1, [[2, 3]], 4, 5];
+}
+
+function t4() {
+    const { a1: _a1, b1 } = { a1: 1, b1: 1 };
+    console.log(b1);
+
+    const { a2, b2: _b2 } = { a2: 1, b2: 1 };
+    console.log(a2);
+
+    const { a3: _a3, b3: _b3 } = { a3: 1, b3: 1 };
+}
+
+function t5() {
+    const { a1: _a1, b1 } = { a1: 1, b1: 1 };
+    const { a2, b2: _b2 } = { a2: 1, b2: 1 };
+    const { a3, b3 } = { a3: 1, b3: 1 };
+}
+
+function t6() {
+    const {
+        a1: _a1,
+        b1: {
+            b11: {
+                b111: _b111,
+                b112
+            }
+        },
+        c1,
+        d1: _d1
+    } = { a1: 1, b1: { b11: { b111: 1, b112: 1 } }, c1: 1, d1: 1 };
+    console.log(b112, c1);
+
+    const {
+        a2: _a2,
+        b2: {
+            b21: {
+                b211: _b211, b212: _b212
+            }
+        },
+        c2: _c2,
+        d2: _d2
+    } = { a2: 1, b2: { b21: { b211: 1, b212: 1 } }, c2: 1, d2: 1 };
+
+    const {
+        a3,
+        b3: {
+            b31: {
+                b311, b312
+            }
+        },
+        c3,
+        d3
+    } = { a3: 1, b3: { b31: { b311: 1, b312: 1 } }, c3: 1, d3: 1 };
+}
+
+function t7() {
+    // error
+    const { _a1, _b1 } = { _a1: 1, _b1: 1 };
+
+    // ok
+    const { a2: _a2, b2: _b2 } = { a2: 1, b2: 1 };
+
+    // ok
+    const { _a3: _ignoreA3, _b3: _ignoreB3 } = { _a3: 1, _b3: 1 };
+}
+
+
+//// [unusedVariablesWithUnderscoreInBindingElement.js]
+function t1() {
+    var _a = [1, 2], _a1 = _a[0], b1 = _a[1];
+    console.log(b1);
+    var _b = [1, 2], a2 = _b[0], _b2 = _b[1];
+    console.log(a2);
+    var _c = [1, 2], _a3 = _c[0], _b3 = _c[1];
+}
+function t2() {
+    var _a = [1, 2], _a1 = _a[0], b1 = _a[1];
+    var _b = [1, 2], a2 = _b[0], _b2 = _b[1];
+    var _c = [1, 2], a3 = _c[0], b3 = _c[1];
+}
+function t3() {
+    var _a = [1, [[2, 3]], 4, 5], _a1 = _a[0], _b = _a[1][0], _b1 = _b[0], c1 = _b[1], d1 = _a[2], _e1 = _a[3];
+    console.log(c1, d1);
+    var _c = [1, [[2, 3]], 4, 5], _a2 = _c[0], _d = _c[1][0], _b2 = _d[0], _c2 = _d[1], _d2 = _c[2], _e2 = _c[3];
+    var _e = [1, [[2, 3]], 4, 5], a3 = _e[0], _f = _e[1][0], b3 = _f[0], c3 = _f[1], d3 = _e[2], e3 = _e[3];
+}
+function t4() {
+    var _a = { a1: 1, b1: 1 }, _a1 = _a.a1, b1 = _a.b1;
+    console.log(b1);
+    var _b = { a2: 1, b2: 1 }, a2 = _b.a2, _b2 = _b.b2;
+    console.log(a2);
+    var _c = { a3: 1, b3: 1 }, _a3 = _c.a3, _b3 = _c.b3;
+}
+function t5() {
+    var _a = { a1: 1, b1: 1 }, _a1 = _a.a1, b1 = _a.b1;
+    var _b = { a2: 1, b2: 1 }, a2 = _b.a2, _b2 = _b.b2;
+    var _c = { a3: 1, b3: 1 }, a3 = _c.a3, b3 = _c.b3;
+}
+function t6() {
+    var _a = { a1: 1, b1: { b11: { b111: 1, b112: 1 } }, c1: 1, d1: 1 }, _a1 = _a.a1, _b = _a.b1.b11, _b111 = _b.b111, b112 = _b.b112, c1 = _a.c1, _d1 = _a.d1;
+    console.log(b112, c1);
+    var _c = { a2: 1, b2: { b21: { b211: 1, b212: 1 } }, c2: 1, d2: 1 }, _a2 = _c.a2, _d = _c.b2.b21, _b211 = _d.b211, _b212 = _d.b212, _c2 = _c.c2, _d2 = _c.d2;
+    var _e = { a3: 1, b3: { b31: { b311: 1, b312: 1 } }, c3: 1, d3: 1 }, a3 = _e.a3, _f = _e.b3.b31, b311 = _f.b311, b312 = _f.b312, c3 = _e.c3, d3 = _e.d3;
+}
+function t7() {
+    // error
+    var _a = { _a1: 1, _b1: 1 }, _a1 = _a._a1, _b1 = _a._b1;
+    // ok
+    var _b = { a2: 1, b2: 1 }, _a2 = _b.a2, _b2 = _b.b2;
+    // ok
+    var _c = { _a3: 1, _b3: 1 }, _ignoreA3 = _c._a3, _ignoreB3 = _c._b3;
+}