feat(eslint-plugin): [prefer-nullish-coalescing] logic and test for strict null checks

packages/eslint-plugin/src/rules/prefer-nullish-coalescing.ts

@@ -1,5 +1,6 @@
 import type { TSESLint, TSESTree } from '@typescript-eslint/utils';
 import { AST_NODE_TYPES, AST_TOKEN_TYPES } from '@typescript-eslint/utils';
+import * as tsutils from 'tsutils';
 import * as ts from 'typescript';
 
 import * as util from '../util';
@@ -9,13 +10,15 @@ export type Options = [
     ignoreConditionalTests?: boolean;
     ignoreTernaryTests?: boolean;
     ignoreMixedLogicalExpressions?: boolean;
+    allowRuleToRunWithoutStrictNullChecksIKnowWhatIAmDoing?: boolean;
   },
 ];
 
 export type MessageIds =
   | 'preferNullishOverOr'
   | 'preferNullishOverTernary'
-  | 'suggestNullish';
+  | 'suggestNullish'
+  | 'noStrictNullCheck';
 
 export default util.createRule<Options, MessageIds>({
   name: 'prefer-nullish-coalescing',
@@ -34,6 +37,8 @@ export default util.createRule<Options, MessageIds>({
       preferNullishOverTernary:
         'Prefer using nullish coalescing operator (`??`) instead of a ternary expression, as it is simpler to read.',
       suggestNullish: 'Fix to nullish coalescing operator (`??`).',
+      noStrictNullCheck:
+        'This rule requires the `strictNullChecks` compiler option to be turned on to function correctly.',
     },
     schema: [
       {
@@ -48,6 +53,9 @@ export default util.createRule<Options, MessageIds>({
           ignoreMixedLogicalExpressions: {
             type: 'boolean',
           },
+          allowRuleToRunWithoutStrictNullChecksIKnowWhatIAmDoing: {
+            type: 'boolean',
+          },
         },
         additionalProperties: false,
       },
@@ -58,6 +66,7 @@ export default util.createRule<Options, MessageIds>({
       ignoreConditionalTests: true,
       ignoreTernaryTests: true,
       ignoreMixedLogicalExpressions: true,
+      allowRuleToRunWithoutStrictNullChecksIKnowWhatIAmDoing: false,
     },
   ],
   create(
@@ -67,12 +76,31 @@ export default util.createRule<Options, MessageIds>({
         ignoreConditionalTests,
         ignoreTernaryTests,
         ignoreMixedLogicalExpressions,
+        allowRuleToRunWithoutStrictNullChecksIKnowWhatIAmDoing,
       },
     ],
   ) {
     const parserServices = util.getParserServices(context);
+    const compilerOptions = parserServices.program.getCompilerOptions();
     const sourceCode = context.getSourceCode();
     const checker = parserServices.program.getTypeChecker();
+    const isStrictNullChecks = tsutils.isStrictCompilerOptionEnabled(
+      compilerOptions,
+      'strictNullChecks',
+    );
+
+    if (
+      !isStrictNullChecks &&
+      allowRuleToRunWithoutStrictNullChecksIKnowWhatIAmDoing !== true
+    ) {
+      context.report({
+        loc: {
+          start: { line: 0, column: 0 },
+          end: { line: 0, column: 0 },
+        },
+        messageId: 'noStrictNullCheck',
+      });
+    }
 
     return {
       ConditionalExpression(node: TSESTree.ConditionalExpression): void {

packages/eslint-plugin/tests/rules/prefer-nullish-coalescing.test.ts

@@ -1,4 +1,5 @@
 import type { TSESLint } from '@typescript-eslint/utils';
+import * as path from 'path';
 
 import type {
   MessageIds,
@@ -385,6 +386,25 @@ x ?? y;
       ],
     })),
 
+    // noStrictNullCheck
+    {
+      code: `
+declare const x: string[] | null;
+if (x) {
+}
+      `,
+      errors: [
+        {
+          messageId: 'noStrictNullCheck',
+          line: 0,
+          column: 1,
+        },
+      ],
+      parserOptions: {
+        tsconfigRootDir: path.join(rootPath, 'unstrict'),
+      },
+    },
+
     // ignoreConditionalTests
     ...nullishTypeInvalidTest((nullish, type) => ({
       code: `