build: detect todos in public comments (#4059)

* build: detect todos in public comments

* Creates a naive rule that checks all multi-line comments and reports failures once it detects TODOs inside of it.

* This means that TODOs always need to placed inside of single-line comments.

Fixes #4026

tools/tslint-rules/noExposedTodoRule.js

@@ -0,0 +1,34 @@
+const ts = require('typescript');
+const utils = require('tsutils');
+const Lint = require('tslint');
+
+const ERROR_MESSAGE =
+    'A TODO may only appear in inline (//) style comments. ' +
+    'This is meant to prevent a TODO from being accidentally included in any public API docs.';
+
+/**
+ * Rule that walks through all comments inside of the library and adds failures when it
+ * detects TODO's inside of multi-line comments. TODOs need to be placed inside of single-line
+ * comments.
+ */
+class Rule extends Lint.Rules.AbstractRule {
+
+  apply(sourceFile) {
+    return this.applyWithWalker(new NoExposedTodoWalker(sourceFile, this.getOptions()));
+  }
+}
+
+class NoExposedTodoWalker extends Lint.RuleWalker {
+
+  visitSourceFile(sourceFile) {
+    utils.forEachComment(sourceFile, (fullText, commentRange) => {
+      let isTodoComment = fullText.substring(commentRange.pos, commentRange.end).includes('TODO');
+
+      if (commentRange.kind === ts.SyntaxKind.MultiLineCommentTrivia && isTodoComment) {
+        this.addFailureAt(commentRange.pos, commentRange.end - commentRange.pos, ERROR_MESSAGE);
+      }
+    });
+  }
+}
+
+exports.Rule = Rule;