fix: handle files with unspecified path in getRulesMetaForResults

* In `getRulesMetaForResults`, files with an unspecified path are now treated as if they were located inside `cwd`.
* In `getRulesMetaForResults`, when a result referencing a rule has no config, we will explicitly throw an error with a descriptive message.
* Added top-level, internal functions `getPlaceholderPath` and `createExtraneousResultsError` to avoid code repetition.
* Added two new unit tests.
* Renamed an existing unit test to better disambiguate it from a new one. Also changed the assertion to check both error message and constructor.

Fixes #16410

lib/eslint/flat-eslint.js

@@ -161,6 +161,16 @@ function createRulesMeta(rules) {
     }, {});
 }
 
+/**
+ * Return the absolute path of a file named `"__placeholder__.js"` in a given directory.
+ * This is used as a replacement for a missing file path.
+ * @param {string} cwd An absolute directory path.
+ * @returns {string} The absolute path of a file named `"__placeholder__.js"` in the given directory.
+ */
+function getPlaceholderPath(cwd) {
+    return path.join(cwd, "__placeholder__.js");
+}
+
 /** @type {WeakMap<ExtractedConfig, DeprecatedRuleInfo[]>} */
 const usedDeprecatedRulesCache = new WeakMap();
 
@@ -177,7 +187,7 @@ function getOrFindUsedDeprecatedRules(eslint, maybeFilePath) {
     } = privateMembers.get(eslint);
     const filePath = path.isAbsolute(maybeFilePath)
         ? maybeFilePath
-        : path.join(cwd, "__placeholder__.js");
+        : getPlaceholderPath(cwd);
     const config = configs.getConfig(filePath);
 
     // Most files use the same config, so cache it.
@@ -438,7 +448,7 @@ function verifyText({
      * `config.extractConfig(filePath)` requires an absolute path, but `linter`
      * doesn't know CWD, so it gives `linter` an absolute path always.
      */
-    const filePathToVerify = filePath === "<text>" ? path.join(cwd, "__placeholder__.js") : filePath;
+    const filePathToVerify = filePath === "<text>" ? getPlaceholderPath(cwd) : filePath;
     const { fixed, messages, output } = linter.verifyAndFix(
         text,
         configs,
@@ -535,6 +545,14 @@ function *iterateRuleDeprecationWarnings(configs) {
     }
 }
 
+/**
+ * Creates an error to be thrown when an array of results passed to `getRulesMetaForResults` was not created by the current engine.
+ * @returns {TypeError} An error object.
+ */
+function createExtraneousResultsError() {
+    return new TypeError("Results object was not created from this ESLint instance.");
+}
+
 //-----------------------------------------------------------------------------
 // Main API
 //-----------------------------------------------------------------------------
@@ -672,7 +690,10 @@ class FlatESLint {
             return resultRules;
         }
 
-        const { configs } = privateMembers.get(this);
+        const {
+            configs,
+            options: { cwd }
+        } = privateMembers.get(this);
 
         /*
          * We can only accurately return rules meta information for linting results if the
@@ -681,7 +702,7 @@ class FlatESLint {
          * to let the user know we can't do anything here.
          */
         if (!configs) {
-            throw new TypeError("Results object was not created from this ESLint instance.");
+            throw createExtraneousResultsError();
         }
 
         for (const result of results) {
@@ -690,16 +711,20 @@ class FlatESLint {
              * Normalize filename for <text>.
              */
             const filePath = result.filePath === "<text>"
-                ? "__placeholder__.js" : result.filePath;
-
-            /*
-             * All of the plugin and rule information is contained within the
-             * calculated config for the given file.
-             */
-            const config = configs.getConfig(filePath);
+                ? getPlaceholderPath(cwd) : result.filePath;
             const allMessages = result.messages.concat(result.suppressedMessages);
 
             for (const { ruleId } of allMessages) {
+
+                /*
+                 * All of the plugin and rule information is contained within the
+                 * calculated config for the given file.
+                 */
+                const config = configs.getConfig(filePath);
+
+                if (!config) {
+                    throw createExtraneousResultsError();
+                }
                 const rule = getRuleFromConfig(ruleId, config);
 
                 // ensure the rule exists
@@ -1037,7 +1062,7 @@ class FlatESLint {
                 const npmFormat = naming.normalizePackageName(normalizedFormatName, "eslint-formatter");
 
                 // TODO: This is pretty dirty...would be nice to clean up at some point.
-                formatterPath = ModuleResolver.resolve(npmFormat, path.join(cwd, "__placeholder__.js"));
+                formatterPath = ModuleResolver.resolve(npmFormat, getPlaceholderPath(cwd));
             } catch {
                 formatterPath = path.resolve(__dirname, "../", "cli-engine", "formatters", `${normalizedFormatName}.js`);
             }

tests/lib/eslint/flat-eslint.js

@@ -3679,7 +3679,7 @@ describe("FlatESLint", () => {
 
     describe("getRulesMetaForResults()", () => {
 
-        it("should throw an error when results were not created from this instance", async () => {
+        it("should throw an error when this instance did not lint any files", async () => {
             const engine = new FlatESLint({
                 overrideConfigFile: true
             });
@@ -3716,7 +3716,42 @@ describe("FlatESLint", () => {
                             "var err = doStuff();\nif (err) console.log('failed tests: ' + err);\nprocess.exit(1);\n"
                     }
                 ]);
-            }, /Results object was not created from this ESLint instance/u);
+            }, {
+                constructor: TypeError,
+                message: "Results object was not created from this ESLint instance."
+            });
+        });
+
+        it("should throw an error when results were created from a different instance", async () => {
+            const engine1 = new FlatESLint({
+                overrideConfigFile: true,
+                cwd: path.join(fixtureDir, "foo"),
+                overrideConfig: {
+                    rules: {
+                        semi: 2
+                    }
+                }
+            });
+            const engine2 = new FlatESLint({
+                overrideConfigFile: true,
+                cwd: path.join(fixtureDir, "bar"),
+                overrideConfig: {
+                    rules: {
+                        semi: 2
+                    }
+                }
+            });
+
+            const results1 = await engine1.lintText("1", { filePath: "file.js" });
+            const results2 = await engine2.lintText("2", { filePath: "file.js" });
+
+            engine1.getRulesMetaForResults(results1); // should not throw an error
+            assert.throws(() => {
+                engine1.getRulesMetaForResults(results2);
+            }, {
+                constructor: TypeError,
+                message: "Results object was not created from this ESLint instance."
+            });
         });
 
         it("should return empty object when there are no linting errors", async () => {
@@ -3789,6 +3824,24 @@ describe("FlatESLint", () => {
                 nodePlugin.rules["no-new-require"].meta
             );
         });
+
+        it("should treat a result without `filePath` as if the file was located in `cwd`", async () => {
+            const engine = new FlatESLint({
+                overrideConfigFile: true,
+                cwd: path.join(fixtureDir, "foo", "bar"),
+                ignorePatterns: "*/**", // ignore all subdirectories of `cwd`
+                overrideConfig: {
+                    rules: {
+                        eqeqeq: "warn"
+                    }
+                }
+            });
+
+            const results = await engine.lintText("a==b");
+            const rulesMeta = engine.getRulesMetaForResults(results);
+
+            assert.deepStrictEqual(rulesMeta.eqeqeq, coreRules.get("eqeqeq").meta);
+        });
     });
 
     describe("outputFixes()", () => {