feat(eslint-plugin): [prefer-null-coal] opt for suggestion fixer (#1272)

packages/eslint-plugin/docs/rules/prefer-nullish-coalescing.md

@@ -46,13 +46,15 @@ type Options = [
   {
     ignoreConditionalTests?: boolean;
     ignoreMixedLogicalExpressions?: boolean;
+    forceSuggestionFixer?: boolean;
   },
 ];
 
 const defaultOptions = [
   {
     ignoreConditionalTests: true,
-    ignoreMixedLogicalExpressions: true;
+    ignoreMixedLogicalExpressions: true,
+    forceSuggestionFixer: false,
   },
 ];
 ```
@@ -129,7 +131,13 @@ a ?? (b && c) ?? d;
 a ?? (b && c && d);
 ```
 
-**_NOTE:_** Errors for this specific case will be presented as suggestions, instead of fixes. This is because it is not always safe to automatically convert `||` to `??` within a mixed logical expression, as we cannot tell the intended precedence of the operator. Note that by design, `??` requires parentheses when used with `&&` or `||` in the same expression.
+**_NOTE:_** Errors for this specific case will be presented as suggestions (see below), instead of fixes. This is because it is not always safe to automatically convert `||` to `??` within a mixed logical expression, as we cannot tell the intended precedence of the operator. Note that by design, `??` requires parentheses when used with `&&` or `||` in the same expression.
+
+### forceSuggestionFixer
+
+Setting this option to `true` will cause the rule to use ESLint's "suggested fix" mode for all fixes. _This option is provided as to aid in transitioning your codebase onto this rule_.
+
+Suggestion fixes cannot be automatically applied via the `--fix` CLI command, but can be _manually_ chosen to be applied one at a time via an IDE or similar. This makes it safe to run autofixers on an existing codebase without worrying about potential runtime behaviour changes from this rule's fixer.
 
 ## When Not To Use It
 

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

@@ -11,6 +11,7 @@ export type Options = [
   {
     ignoreConditionalTests?: boolean;
     ignoreMixedLogicalExpressions?: boolean;
+    forceSuggestionFixer?: boolean;
   },
 ];
 export type MessageIds = 'preferNullish';
@@ -41,6 +42,9 @@ export default util.createRule<Options, MessageIds>({
           ignoreMixedLogicalExpressions: {
             type: 'boolean',
           },
+          forceSuggestionFixer: {
+            type: 'boolean',
+          },
         },
         additionalProperties: false,
       },
@@ -50,9 +54,19 @@ export default util.createRule<Options, MessageIds>({
     {
       ignoreConditionalTests: true,
       ignoreMixedLogicalExpressions: true,
+      forceSuggestionFixer: false,
     },
   ],
-  create(context, [{ ignoreConditionalTests, ignoreMixedLogicalExpressions }]) {
+  create(
+    context,
+    [
+      {
+        ignoreConditionalTests,
+        ignoreMixedLogicalExpressions,
+        forceSuggestionFixer,
+      },
+    ],
+  ) {
     const parserServices = util.getParserServices(context);
     const sourceCode = context.getSourceCode();
     const checker = parserServices.program.getTypeChecker();
@@ -79,30 +93,34 @@ export default util.createRule<Options, MessageIds>({
           return;
         }
 
-        const barBarOperator = sourceCode.getTokenAfter(
-          node.left,
-          token =>
-            token.type === AST_TOKEN_TYPES.Punctuator &&
-            token.value === node.operator,
-        )!; // there _must_ be an operator
-
-        const fixer = isMixedLogical
-          ? // suggestion instead for cases where we aren't sure if the fixer is completely safe
-            ({
-              suggest: [
-                {
-                  messageId: 'preferNullish',
-                  fix(fixer: TSESLint.RuleFixer): TSESLint.RuleFix {
-                    return fixer.replaceText(barBarOperator, '??');
+        const barBarOperator = util.nullThrows(
+          sourceCode.getTokenAfter(
+            node.left,
+            token =>
+              token.type === AST_TOKEN_TYPES.Punctuator &&
+              token.value === node.operator,
+          ),
+          util.NullThrowsReasons.MissingToken('operator', node.type),
+        );
+
+        const fixer =
+          isMixedLogical || forceSuggestionFixer
+            ? // suggestion instead for cases where we aren't sure if the fixer is completely safe
+              ({
+                suggest: [
+                  {
+                    messageId: 'preferNullish',
+                    fix(fixer: TSESLint.RuleFixer): TSESLint.RuleFix {
+                      return fixer.replaceText(barBarOperator, '??');
+                    },
                   },
+                ],
+              } as const)
+            : {
+                fix(fixer: TSESLint.RuleFixer): TSESLint.RuleFix {
+                  return fixer.replaceText(barBarOperator, '??');
                 },
-              ],
-            } as const)
-          : {
-              fix(fixer: TSESLint.RuleFixer): TSESLint.RuleFix {
-                return fixer.replaceText(barBarOperator, '??');
-              },
-            };
+              };
 
         context.report({
           node: barBarOperator,

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

@@ -435,5 +435,33 @@ if (function werid() { return x ?? 'foo' }) {}
         },
       ],
     })),
+
+    // testing the suggestion fixer option
+    {
+      code: `
+declare const x: string | null;
+x || 'foo';
+      `.trimRight(),
+      output: null,
+      options: [{ forceSuggestionFixer: true }],
+      errors: [
+        {
+          messageId: 'preferNullish',
+          line: 3,
+          column: 3,
+          endLine: 3,
+          endColumn: 5,
+          suggestions: [
+            {
+              messageId: 'preferNullish',
+              output: `
+declare const x: string | null;
+x ?? 'foo';
+              `.trimRight(),
+            },
+          ],
+        },
+      ],
+    },
   ],
 });