fix(vitest-plugin): explain a missing benchmark function

packages/vitest-plugin/src/__tests__/instrumented.test.ts

@@ -188,4 +188,29 @@ describe("CodSpeedRunner", () => {
       "teardown:run:test bench",
     ]);
   });
+  it("should explain a benchmark function it cannot find", async () => {
+    // What the runner sees when the benches were registered in another
+    // vitest copy than the one this plugin imports.
+    mockedGetBenchFn.mockReturnValue(
+      undefined as unknown as ReturnType<typeof getBenchFn>,
+    );
+
+    const runner = new CodSpeedRunner(fromPartial({}));
+    const suite = fromPartial<RunnerTestSuite>({
+      file: { filepath: __filename },
+      name: "test suite",
+      tasks: [
+        {
+          type: "test",
+          mode: "run",
+          meta: { benchmark: true },
+          name: "test bench",
+        },
+      ],
+    });
+
+    await expect(runner.runSuite(suite)).rejects.toThrow(
+      /no benchmark function found for ".*::test bench"\. The benchmark file and @codspeed\/vitest-plugin most likely resolved different copies of vitest/,
+    );
+  });
 });

packages/vitest-plugin/src/legacy/analysis.ts

@@ -45,6 +45,16 @@ async function runAnalysisBench(
   const uri = `${currentSuiteName}::${benchmark.name}`;
   const fn = getBenchFn(benchmark);
 
+  // The registry `getBenchFn` reads lives in the vitest instance this plugin
+  // imports. When the benchmark file registered its benches in another copy
+  // (pnpm installs one per set of peers, e.g. per vite major), it is empty
+  // here, and calling it fails with a bare "fn is not a function".
+  if (typeof fn !== "function") {
+    throw new Error(
+      `[CodSpeed] no benchmark function found for "${uri}". The benchmark file and @codspeed/vitest-plugin most likely resolved different copies of vitest, so the plugin cannot see the registered benchmarks. Install @codspeed/vitest-plugin in the package that runs the benchmarks and import it from that package's vitest config, so both resolve the same vitest.`,
+    );
+  }
+
   // Constructing a Bench applies tinybench's no-op defaults for the setup and
   // teardown hooks and gives them the Task they expect. The bench itself is
   // never run: this runner drives the benchmark function directly.