add notes and error helper for vendor+reflect error

README.md

@@ -30,17 +30,6 @@ go install github.com/golang/mock/mockgen@v1.5.0
 If you use `mockgen` in your CI pipeline, it may be more appropriate to fixate
 on a specific mockgen version.
 
-## Documentation
-
-After installing, you can use `go doc` to get documentation:
-
-```bash
-go doc github.com/golang/mock/gomock
-```
-
-Alternatively, there is an online reference for the package hosted on GoPkgDoc
-[here][gomock-reference].
-
 ## Running mockgen
 
 `mockgen` has two modes of operation: source and reflect.
@@ -265,3 +254,23 @@ If the received value is `3`, then it will be printed as `03`.
 [ci-runs]:             https://github.com/golang/mock/actions
 [reference-badge]:     https://pkg.go.dev/badge/github.com/golang/mock.svg
 [reference]:           https://pkg.go.dev/github.com/golang/mock
+
+## Debugging Errors
+
+### reflect vendoring error
+
+```text
+cannot find package "."
+... github.com/golang/mock/mockgen/model
+```
+
+If you come across this error while using reflect mode and vendoring
+dependencies there are three workarounds you can choose from:
+
+1. Use source mode.
+2. Include an empty import `import _ "github.com/golang/mock/mockgen/model"`.
+3. Add `--build_flags=--mod=mod` to your mockgen command.
+
+This error is do to changes in default behavior of the go command in more recent
+versions. More details can be found in
+[#494](https://github.com/golang/mock/issues/494).

mockgen/reflect.go

@@ -20,7 +20,9 @@ import (
 	"bytes"
 	"encoding/gob"
 	"flag"
+	"fmt"
 	"go/build"
+	"io"
 	"io/ioutil"
 	"log"
 	"os"
@@ -159,11 +161,18 @@ func runInDir(program []byte, dir string) (*model.Package, error) {
 	cmdArgs = append(cmdArgs, "-o", progBinary, progSource)
 
 	// Build the program.
+	buf := bytes.NewBuffer(nil)
 	cmd := exec.Command("go", cmdArgs...)
 	cmd.Dir = tmpDir
 	cmd.Stdout = os.Stdout
-	cmd.Stderr = os.Stderr
+	cmd.Stderr = io.MultiWriter(os.Stderr, buf)
 	if err := cmd.Run(); err != nil {
+		sErr := buf.String()
+		if strings.Contains(sErr, `cannot find package "."`) &&
+			strings.Contains(sErr, "github.com/golang/mock/mockgen/model") {
+			fmt.Fprint(os.Stderr, "Please reference the steps in the README to fix this error:\n\thttps://github.com/golang/mock#reflect-vendoring-error.")
+			return nil, err
+		}
 		return nil, err
 	}