feat(eslint-plugin): [no-non-null-assert] add suggestion fixer

packages/eslint-plugin/src/rules/no-non-null-assertion.ts

@@ -1,6 +1,12 @@
+import {
+  TSESLint,
+  AST_NODE_TYPES,
+} from '@typescript-eslint/experimental-utils';
 import * as util from '../util';
 
-export default util.createRule({
+type MessageIds = 'noNonNull' | 'suggestOptionalChain';
+
+export default util.createRule<[], MessageIds>({
   name: 'no-non-null-assertion',
   meta: {
     type: 'problem',
@@ -12,16 +18,106 @@ export default util.createRule({
     },
     messages: {
       noNonNull: 'Forbidden non-null assertion.',
+      suggestOptionalChain:
+        'Consider using the optional chain operator `?.` instead. This operator includes runtime checks, so it is safer than the compile-only non-null assertion operator.',
     },
     schema: [],
   },
   defaultOptions: [],
   create(context) {
+    const sourceCode = context.getSourceCode();
     return {
       TSNonNullExpression(node): void {
+        const suggest: TSESLint.ReportSuggestionArray<MessageIds> = [];
+        function convertTokenToOptional(
+          replacement: '?' | '?.',
+        ): TSESLint.ReportFixFunction {
+          return (fixer: TSESLint.RuleFixer): TSESLint.RuleFix | null => {
+            const operator = sourceCode.getTokenAfter(
+              node.expression,
+              util.isNonNullAssertionPunctuator,
+            );
+            if (operator) {
+              return fixer.replaceText(operator, replacement);
+            }
+
+            return null;
+          };
+        }
+        function removeToken(): TSESLint.ReportFixFunction {
+          return (fixer: TSESLint.RuleFixer): TSESLint.RuleFix | null => {
+            const operator = sourceCode.getTokenAfter(
+              node.expression,
+              util.isNonNullAssertionPunctuator,
+            );
+            if (operator) {
+              return fixer.remove(operator);
+            }
+
+            return null;
+          };
+        }
+
+        if (node.parent) {
+          if (
+            (node.parent.type === AST_NODE_TYPES.MemberExpression ||
+              node.parent.type === AST_NODE_TYPES.OptionalMemberExpression) &&
+            node.parent.object === node
+          ) {
+            if (!node.parent.optional) {
+              if (node.parent.computed) {
+                // it is x![y]?.z
+                suggest.push({
+                  messageId: 'suggestOptionalChain',
+                  fix: convertTokenToOptional('?.'),
+                });
+              } else {
+                // it is x!.y?.z
+                suggest.push({
+                  messageId: 'suggestOptionalChain',
+                  fix: convertTokenToOptional('?'),
+                });
+              }
+            } else {
+              if (node.parent.computed) {
+                // it is x!?.[y].z
+                suggest.push({
+                  messageId: 'suggestOptionalChain',
+                  fix: removeToken(),
+                });
+              } else {
+                // it is x!?.y.z
+                suggest.push({
+                  messageId: 'suggestOptionalChain',
+                  fix: removeToken(),
+                });
+              }
+            }
+          } else if (
+            (node.parent.type === AST_NODE_TYPES.CallExpression ||
+              node.parent.type === AST_NODE_TYPES.OptionalCallExpression) &&
+            node.parent.callee === node
+          ) {
+            if (!node.parent.optional) {
+              // it is x.y?.z!()
+              suggest.push({
+                messageId: 'suggestOptionalChain',
+                fix: convertTokenToOptional('?.'),
+              });
+            } else {
+              // it is x.y.z!?.()
+              suggest.push({
+                messageId: 'suggestOptionalChain',
+                fix: removeToken(),
+              });
+            }
+          }
+        }
+
         context.report({
           node,
           messageId: 'noNonNull',
+          suggest,
         });
       },
     };

packages/eslint-plugin/src/util/astUtils.ts

@@ -17,6 +17,17 @@ function isNotOptionalChainPunctuator(
   return !isOptionalChainPunctuator(token);
 }
 
+function isNonNullAssertionPunctuator(
+  token: TSESTree.Token | TSESTree.Comment,
+): boolean {
+  return token.type === AST_TOKEN_TYPES.Punctuator && token.value === '!';
+}
+function isNotNonNullAssertionPunctuator(
+  token: TSESTree.Token | TSESTree.Comment,
+): boolean {
+  return !isNonNullAssertionPunctuator(token);
+}
+
 /**
  * Returns true if and only if the node represents: foo?.() or foo.bar?.()
  */
@@ -32,8 +43,10 @@ function isOptionalOptionalChain(
 }
 
 export {
-  LINEBREAK_MATCHER,
+  isNonNullAssertionPunctuator,
+  isNotNonNullAssertionPunctuator,
   isNotOptionalChainPunctuator,
   isOptionalChainPunctuator,
   isOptionalOptionalChain,
+  LINEBREAK_MATCHER,
 };