cmd/go: eliminate redundancy in import error messages

This change introduces a new interface, load.ImportPathError. An error
may satisfy this by providing an ImportPath method and including the
import path in its error text. modload.ImportMissingError satisfies
this interface. load.ImportErrorf also provides a convenient way to
create an error satisfying this interface with an arbitrary message.

When load.PackageError formats its error text, it may omit the last
path on the import stack if the wrapped error satisfies
ImportPathError and has a matching path.

To make this work, PackageError.Err is now an error instead of a
string. PackageError.MarshalJSON will write Err as a string for
'go list -json' output.

When go/build.Import invokes 'go list' in module mode, it now runs
with '-e' and includes '.Error' in the output format instead of
expecting the error to be in the raw stderr text. If a package error
is printed and a directory was not found, the error will be returned
without extra decoration.

Fixes #34752

Change-Id: I2d81dab7dec19e0ae9f51f6412bc9f30433a8596
Reviewed-on: https://go-review.googlesource.com/c/go/+/199840
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/fmtcmd/fmt.go

@@ -72,7 +72,7 @@ func runFmt(cmd *base.Command, args []string) {
 			continue
 		}
 		if pkg.Error != nil {
-			if strings.HasPrefix(pkg.Error.Err, "build constraints exclude all Go files") {
+			if strings.HasPrefix(pkg.Error.Err.Error(), "build constraints exclude all Go files") {
 				// Skip this error, as we will format
 				// all files regardless.
 			} else {

src/cmd/go/internal/get/get.go

@@ -274,7 +274,7 @@ func download(arg string, parent *load.Package, stk *load.ImportStack, mode int)
 		stk.Push(arg)
 		err := downloadPackage(p)
 		if err != nil {
-			base.Errorf("%s", &load.PackageError{ImportStack: stk.Copy(), Err: err.Error()})
+			base.Errorf("%s", &load.PackageError{ImportStack: stk.Copy(), Err: err})
 			stk.Pop()
 			return
 		}
@@ -355,7 +355,7 @@ func download(arg string, parent *load.Package, stk *load.ImportStack, mode int)
 				stk.Push(path)
 				err := &load.PackageError{
 					ImportStack: stk.Copy(),
-					Err:         "must be imported as " + path[j+len("vendor/"):],
+					Err:         load.ImportErrorf(path, "%s must be imported as %s", path, path[j+len("vendor/"):]),
 				}
 				stk.Pop()
 				base.Errorf("%s", err)

src/cmd/go/internal/load/pkg.go

@@ -7,6 +7,7 @@ package load
 
 import (
 	"bytes"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"go/build"
@@ -304,9 +305,9 @@ func (p *Package) copyBuild(pp *build.Package) {
 type PackageError struct {
 	ImportStack   []string // shortest path from package named on command line to this one
 	Pos           string   // position of error
-	Err           string   // the error itself
-	IsImportCycle bool     `json:"-"` // the error is an import cycle
-	Hard          bool     `json:"-"` // whether the error is soft or hard; soft errors are ignored in some places
+	Err           error    // the error itself
+	IsImportCycle bool     // the error is an import cycle
+	Hard          bool     // whether the error is soft or hard; soft errors are ignored in some places
 }
 
 func (p *PackageError) Error() string {
@@ -317,12 +318,77 @@ func (p *PackageError) Error() string {
 	if p.Pos != "" {
 		// Omit import stack. The full path to the file where the error
 		// is the most important thing.
-		return p.Pos + ": " + p.Err
+		return p.Pos + ": " + p.Err.Error()
 	}
-	if len(p.ImportStack) == 0 {
-		return p.Err
+
+	// If the error is an ImportPathError, and the last path on the stack appears
+	// in the error message, omit that path from the stack to avoid repetition.
+	// If an ImportPathError wraps another ImportPathError that matches the
+	// last path on the stack, we don't omit the path. An error like
+	// "package A imports B: error loading C caused by B" would not be clearer
+	// if "imports B" were omitted.
+	stack := p.ImportStack
+	var ierr ImportPathError
+	if len(stack) > 0 && errors.As(p.Err, &ierr) && ierr.ImportPath() == stack[len(stack)-1] {
+		stack = stack[:len(stack)-1]
+	}
+	if len(stack) == 0 {
+		return p.Err.Error()
+	}
+	return "package " + strings.Join(stack, "\n\timports ") + ": " + p.Err.Error()
+}
+
+// PackageError implements MarshalJSON so that Err is marshaled as a string
+// and non-essential fields are omitted.
+func (p *PackageError) MarshalJSON() ([]byte, error) {
+	perr := struct {
+		ImportStack []string
+		Pos         string
+		Err         string
+	}{p.ImportStack, p.Pos, p.Err.Error()}
+	return json.Marshal(perr)
+}
+
+// ImportPathError is a type of error that prevents a package from being loaded
+// for a given import path. When such a package is loaded, a *Package is
+// returned with Err wrapping an ImportPathError: the error is attached to
+// the imported package, not the importing package.
+//
+// The string returned by ImportPath must appear in the string returned by
+// Error. Errors that wrap ImportPathError (such as PackageError) may omit
+// the import path.
+type ImportPathError interface {
+	error
+	ImportPath() string
+}
+
+type importError struct {
+	importPath string
+	err        error // created with fmt.Errorf
+}
+
+var _ ImportPathError = (*importError)(nil)
+
+func ImportErrorf(path, format string, args ...interface{}) ImportPathError {
+	err := &importError{importPath: path, err: fmt.Errorf(format, args...)}
+	if errStr := err.Error(); !strings.Contains(errStr, path) {
+		panic(fmt.Sprintf("path %q not in error %q", path, errStr))
 	}
-	return "package " + strings.Join(p.ImportStack, "\n\timports ") + ": " + p.Err
+	return err
+}
+
+func (e *importError) Error() string {
+	return e.err.Error()
+}
+
+func (e *importError) Unwrap() error {
+	// Don't return e.err directly, since we're only wrapping an error if %w
+	// was passed to ImportErrorf.
+	return errors.Unwrap(e.err)
+}
+
+func (e *importError) ImportPath() string {
+	return e.importPath
 }
 
 // An ImportStack is a stack of import paths, possibly with the suffix " (test)" appended.
@@ -489,7 +555,7 @@ func loadImport(pre *preload, path, srcDir string, parent *Package, stk *ImportS
 				ImportPath: path,
 				Error: &PackageError{
 					ImportStack: stk.Copy(),
-					Err:         err.Error(),
+					Err:         err,
 				},
 			},
 		}
@@ -516,7 +582,7 @@ func loadImport(pre *preload, path, srcDir string, parent *Package, stk *ImportS
 		if !cfg.ModulesEnabled && path != cleanImport(path) {
 			p.Error = &PackageError{
 				ImportStack: stk.Copy(),
-				Err:         fmt.Sprintf("non-canonical import path: %q should be %q", path, pathpkg.Clean(path)),
+				Err:         fmt.Errorf("non-canonical import path: %q should be %q", path, pathpkg.Clean(path)),
 			}
 			p.Incomplete = true
 		}
@@ -536,20 +602,22 @@ func loadImport(pre *preload, path, srcDir string, parent *Package, stk *ImportS
 		perr := *p
 		perr.Error = &PackageError{
 			ImportStack: stk.Copy(),
-			Err:         fmt.Sprintf("import %q is a program, not an importable package", path),
+			Err:         ImportErrorf(path, "import %q is a program, not an importable package", path),
 		}
 		return setErrorPos(&perr, importPos)
 	}
 
 	if p.Internal.Local && parent != nil && !parent.Internal.Local {
 		perr := *p
-		errMsg := fmt.Sprintf("local import %q in non-local package", path)
+		var err error
 		if path == "." {
-			errMsg = "cannot import current directory"
+			err = ImportErrorf(path, "%s: cannot import current directory", path)
+		} else {
+			err = ImportErrorf(path, "local import %q in non-local package", path)
 		}
 		perr.Error = &PackageError{
 			ImportStack: stk.Copy(),
-			Err:         errMsg,
+			Err:         err,
 		}
 		return setErrorPos(&perr, importPos)
 	}
@@ -1125,7 +1193,7 @@ func reusePackage(p *Package, stk *ImportStack) *Package {
 		if p.Error == nil {
 			p.Error = &PackageError{
 				ImportStack:   stk.Copy(),
-				Err:           "import cycle not allowed",
+				Err:           errors.New("import cycle not allowed"),
 				IsImportCycle: true,
 			}
 		}
@@ -1228,7 +1296,7 @@ func disallowInternal(srcDir string, importer *Package, importerPath string, p *
 	perr := *p
 	perr.Error = &PackageError{
 		ImportStack: stk.Copy(),
-		Err:         "use of internal package " + p.ImportPath + " not allowed",
+		Err:         ImportErrorf(p.ImportPath, "use of internal package "+p.ImportPath+" not allowed"),
 	}
 	perr.Incomplete = true
 	return &perr
@@ -1275,7 +1343,7 @@ func disallowVendor(srcDir string, importer *Package, importerPath, path string,
 		perr := *p
 		perr.Error = &PackageError{
 			ImportStack: stk.Copy(),
-			Err:         "must be imported as " + path[i+len("vendor/"):],
+			Err:         ImportErrorf(path, "%s must be imported as %s", path, path[i+len("vendor/"):]),
 		}
 		perr.Incomplete = true
 		return &perr
@@ -1329,7 +1397,7 @@ func disallowVendorVisibility(srcDir string, p *Package, stk *ImportStack) *Pack
 	perr := *p
 	perr.Error = &PackageError{
 		ImportStack: stk.Copy(),
-		Err:         "use of vendored package not allowed",
+		Err:         errors.New("use of vendored package not allowed"),
 	}
 	perr.Incomplete = true
 	return &perr
@@ -1455,7 +1523,7 @@ func (p *Package) load(stk *ImportStack, bp *build.Package, err error) {
 		err = base.ExpandScanner(err)
 		p.Error = &PackageError{
 			ImportStack: stk.Copy(),
-			Err:         err.Error(),
+			Err:         err,
 		}
 		return
 	}
@@ -1472,7 +1540,7 @@ func (p *Package) load(stk *ImportStack, bp *build.Package, err error) {
 		// Report an error when the old code.google.com/p/go.tools paths are used.
 		if InstallTargetDir(p) == StalePath {
 			newPath := strings.Replace(p.ImportPath, "code.google.com/p/go.", "golang.org/x/", 1)
-			e := fmt.Sprintf("the %v command has moved; use %v instead.", p.ImportPath, newPath)
+			e := ImportErrorf(p.ImportPath, "the %v command has moved; use %v instead.", p.ImportPath, newPath)
 			p.Error = &PackageError{Err: e}
 			return
 		}
@@ -1585,7 +1653,7 @@ func (p *Package) load(stk *ImportStack, bp *build.Package, err error) {
 	if f1 != "" {
 		p.Error = &PackageError{
 			ImportStack: stk.Copy(),
-			Err:         fmt.Sprintf("case-insensitive file name collision: %q and %q", f1, f2),
+			Err:         fmt.Errorf("case-insensitive file name collision: %q and %q", f1, f2),
 		}
 		return
 	}
@@ -1601,7 +1669,7 @@ func (p *Package) load(stk *ImportStack, bp *build.Package, err error) {
 		if !SafeArg(file) || strings.HasPrefix(file, "_cgo_") {
 			p.Error = &PackageError{
 				ImportStack: stk.Copy(),
-				Err:         fmt.Sprintf("invalid input file name %q", file),
+				Err:         fmt.Errorf("invalid input file name %q", file),
 			}
 			return
 		}
@@ -1609,14 +1677,14 @@ func (p *Package) load(stk *ImportStack, bp *build.Package, err error) {
 	if name := pathpkg.Base(p.ImportPath); !SafeArg(name) {
 		p.Error = &PackageError{
 			ImportStack: stk.Copy(),
-			Err:         fmt.Sprintf("invalid input directory name %q", name),
+			Err:         fmt.Errorf("invalid input directory name %q", name),
 		}
 		return
 	}
 	if !SafeArg(p.ImportPath) {
 		p.Error = &PackageError{
 			ImportStack: stk.Copy(),
-			Err:         fmt.Sprintf("invalid import path %q", p.ImportPath),
+			Err:         ImportErrorf(p.ImportPath, "invalid import path %q", p.ImportPath),
 		}
 		return
 	}
@@ -1662,31 +1730,31 @@ func (p *Package) load(stk *ImportStack, bp *build.Package, err error) {
 		// code; see issue #16050).
 	}
 
-	setError := func(msg string) {
+	setError := func(err error) {
 		p.Error = &PackageError{
 			ImportStack: stk.Copy(),
-			Err:         msg,
+			Err:         err,
 		}
 	}
 
 	// The gc toolchain only permits C source files with cgo or SWIG.
 	if len(p.CFiles) > 0 && !p.UsesCgo() && !p.UsesSwig() && cfg.BuildContext.Compiler == "gc" {
-		setError(fmt.Sprintf("C source files not allowed when not using cgo or SWIG: %s", strings.Join(p.CFiles, " ")))
+		setError(fmt.Errorf("C source files not allowed when not using cgo or SWIG: %s", strings.Join(p.CFiles, " ")))
 		return
 	}
 
 	// C++, Objective-C, and Fortran source files are permitted only with cgo or SWIG,
 	// regardless of toolchain.
 	if len(p.CXXFiles) > 0 && !p.UsesCgo() && !p.UsesSwig() {
-		setError(fmt.Sprintf("C++ source files not allowed when not using cgo or SWIG: %s", strings.Join(p.CXXFiles, " ")))
+		setError(fmt.Errorf("C++ source files not allowed when not using cgo or SWIG: %s", strings.Join(p.CXXFiles, " ")))
 		return
 	}
 	if len(p.MFiles) > 0 && !p.UsesCgo() && !p.UsesSwig() {
-		setError(fmt.Sprintf("Objective-C source files not allowed when not using cgo or SWIG: %s", strings.Join(p.MFiles, " ")))
+		setError(fmt.Errorf("Objective-C source files not allowed when not using cgo or SWIG: %s", strings.Join(p.MFiles, " ")))
 		return
 	}
 	if len(p.FFiles) > 0 && !p.UsesCgo() && !p.UsesSwig() {
-		setError(fmt.Sprintf("Fortran source files not allowed when not using cgo or SWIG: %s", strings.Join(p.FFiles, " ")))
+		setError(fmt.Errorf("Fortran source files not allowed when not using cgo or SWIG: %s", strings.Join(p.FFiles, " ")))
 		return
 	}
 
@@ -1695,7 +1763,7 @@ func (p *Package) load(stk *ImportStack, bp *build.Package, err error) {
 	if other := foldPath[fold]; other == "" {
 		foldPath[fold] = p.ImportPath
 	} else if other != p.ImportPath {
-		setError(fmt.Sprintf("case-insensitive import collision: %q and %q", p.ImportPath, other))
+		setError(ImportErrorf(p.ImportPath, "case-insensitive import collision: %q and %q", p.ImportPath, other))
 		return
 	}
 
@@ -2102,7 +2170,7 @@ func GoFilesPackage(gofiles []string) *Package {
 			pkg.Internal.CmdlineFiles = true
 			pkg.Name = f
 			pkg.Error = &PackageError{
-				Err: fmt.Sprintf("named files must be .go files: %s", pkg.Name),
+				Err: fmt.Errorf("named files must be .go files: %s", pkg.Name),
 			}
 			return pkg
 		}

src/cmd/go/internal/load/test.go

@@ -110,7 +110,7 @@ func TestPackagesAndErrors(p *Package, cover *TestCover) (pmain, ptest, pxtest *
 			// non-test copy of a package.
 			ptestErr = &PackageError{
 				ImportStack:   testImportStack(stk[0], p1, p.ImportPath),
-				Err:           "import cycle not allowed in test",
+				Err:           errors.New("import cycle not allowed in test"),
 				IsImportCycle: true,
 			}
 		}
@@ -271,7 +271,7 @@ func TestPackagesAndErrors(p *Package, cover *TestCover) (pmain, ptest, pxtest *
 	// afterward that gathers t.Cover information.
 	t, err := loadTestFuncs(ptest)
 	if err != nil && pmain.Error == nil {
-		pmain.Error = &PackageError{Err: err.Error()}
+		pmain.Error = &PackageError{Err: err}
 	}
 	t.Cover = cover
 	if len(ptest.GoFiles)+len(ptest.CgoFiles) > 0 {
@@ -322,7 +322,7 @@ func TestPackagesAndErrors(p *Package, cover *TestCover) (pmain, ptest, pxtest *
 
 	data, err := formatTestmain(t)
 	if err != nil && pmain.Error == nil {
-		pmain.Error = &PackageError{Err: err.Error()}
+		pmain.Error = &PackageError{Err: err}
 	}
 	if data != nil {
 		pmain.Internal.TestmainGo = &data

src/cmd/go/internal/modload/import.go

@@ -16,6 +16,7 @@ import (
 	"time"
 
 	"cmd/go/internal/cfg"
+	"cmd/go/internal/load"
 	"cmd/go/internal/modfetch"
 	"cmd/go/internal/module"
 	"cmd/go/internal/par"
@@ -25,32 +26,38 @@ import (
 )
 
 type ImportMissingError struct {
-	ImportPath string
-	Module     module.Version
-	QueryErr   error
+	Path     string
+	Module   module.Version
+	QueryErr error
 
 	// newMissingVersion is set to a newer version of Module if one is present
 	// in the build list. When set, we can't automatically upgrade.
 	newMissingVersion string
 }
 
+var _ load.ImportPathError = (*ImportMissingError)(nil)
+
 func (e *ImportMissingError) Error() string {
 	if e.Module.Path == "" {
-		if str.HasPathPrefix(e.ImportPath, "cmd") {
-			return fmt.Sprintf("package %s is not in GOROOT (%s)", e.ImportPath, filepath.Join(cfg.GOROOT, "src", e.ImportPath))
+		if str.HasPathPrefix(e.Path, "cmd") {
+			return fmt.Sprintf("package %s is not in GOROOT (%s)", e.Path, filepath.Join(cfg.GOROOT, "src", e.Path))
 		}
 		if e.QueryErr != nil {
-			return fmt.Sprintf("cannot find module providing package %s: %v", e.ImportPath, e.QueryErr)
+			return fmt.Sprintf("cannot find module providing package %s: %v", e.Path, e.QueryErr)
 		}
-		return "cannot find module providing package " + e.ImportPath
+		return "cannot find module providing package " + e.Path
 	}
-	return fmt.Sprintf("missing module for import: %s@%s provides %s", e.Module.Path, e.Module.Version, e.ImportPath)
+	return fmt.Sprintf("missing module for import: %s@%s provides %s", e.Module.Path, e.Module.Version, e.Path)
 }
 
 func (e *ImportMissingError) Unwrap() error {
 	return e.QueryErr
 }
 
+func (e *ImportMissingError) ImportPath() string {
+	return e.Path
+}
+
 // An AmbiguousImportError indicates an import of a package found in multiple
 // modules in the build list, or found in both the main module and its vendor
 // directory.
@@ -121,7 +128,7 @@ func Import(path string) (m module.Version, dir string, err error) {
 		return module.Version{}, dir, nil
 	}
 	if str.HasPathPrefix(path, "cmd") {
-		return module.Version{}, "", &ImportMissingError{ImportPath: path}
+		return module.Version{}, "", &ImportMissingError{Path: path}
 	}
 
 	// -mod=vendor is special.
@@ -220,7 +227,7 @@ func Import(path string) (m module.Version, dir string, err error) {
 			}
 			_, ok := dirInModule(path, m.Path, root, isLocal)
 			if ok {
-				return m, "", &ImportMissingError{ImportPath: path, Module: m}
+				return m, "", &ImportMissingError{Path: path, Module: m}
 			}
 		}
 	}
@@ -230,7 +237,7 @@ func Import(path string) (m module.Version, dir string, err error) {
 		if errors.Is(err, os.ErrNotExist) {
 			// Return "cannot find module providing package […]" instead of whatever
 			// low-level error QueryPackage produced.
-			return module.Version{}, "", &ImportMissingError{ImportPath: path, QueryErr: err}
+			return module.Version{}, "", &ImportMissingError{Path: path, QueryErr: err}
 		} else {
 			return module.Version{}, "", err
 		}
@@ -255,7 +262,7 @@ func Import(path string) (m module.Version, dir string, err error) {
 			}
 		}
 	}
-	return m, "", &ImportMissingError{ImportPath: path, Module: m, newMissingVersion: newMissingVersion}
+	return m, "", &ImportMissingError{Path: path, Module: m, newMissingVersion: newMissingVersion}
 }
 
 // maybeInModule reports whether, syntactically,

src/cmd/go/internal/work/exec.go

@@ -433,7 +433,7 @@ func (b *Builder) build(a *Action) (err error) {
 			err = fmt.Errorf("go build %s: %v", a.Package.ImportPath, err)
 		}
 		if err != nil && b.IsCmdList && b.NeedError && p.Error == nil {
-			p.Error = &load.PackageError{Err: err.Error()}
+			p.Error = &load.PackageError{Err: err}
 		}
 	}()
 	if cfg.BuildN {

src/cmd/go/testdata/script/cmd_import_error.txt

@@ -5,7 +5,7 @@ env GO111MODULE=on
 # a clear error in module mode.
 
 ! go list cmd/unknown
-stderr '^can''t load package: package cmd/unknown: package cmd/unknown is not in GOROOT \('$GOROOT'[/\\]src[/\\]cmd[/\\]unknown\)$'
+stderr '^can''t load package: package cmd/unknown is not in GOROOT \('$GOROOT'[/\\]src[/\\]cmd[/\\]unknown\)$'
 
 go list -f '{{range .DepsErrors}}{{.Err}}{{end}}' x.go
 stdout '^package cmd/unknown is not in GOROOT \('$GOROOT'[/\\]src[/\\]cmd[/\\]unknown\)$'

src/go/build/build.go

@@ -1046,7 +1046,7 @@ func (ctxt *Context) importGo(p *Package, path, srcDir string, mode ImportMode,
 		parent = d
 	}
 
-	cmd := exec.Command("go", "list", "-compiler="+ctxt.Compiler, "-tags="+strings.Join(ctxt.BuildTags, ","), "-installsuffix="+ctxt.InstallSuffix, "-f={{.Dir}}\n{{.ImportPath}}\n{{.Root}}\n{{.Goroot}}\n", path)
+	cmd := exec.Command("go", "list", "-e", "-compiler="+ctxt.Compiler, "-tags="+strings.Join(ctxt.BuildTags, ","), "-installsuffix="+ctxt.InstallSuffix, "-f={{.Dir}}\n{{.ImportPath}}\n{{.Root}}\n{{.Goroot}}\n{{if .Error}}{{.Error}}{{end}}\n", "--", path)
 
 	// TODO(bcmills): This is wrong if srcDir is in a vendor directory, or if
 	// srcDir is in some module dependency of the main module. The main module
@@ -1073,12 +1073,22 @@ func (ctxt *Context) importGo(p *Package, path, srcDir string, mode ImportMode,
 		return fmt.Errorf("go/build: importGo %s: %v\n%s\n", path, err, stderr.String())
 	}
 
-	f := strings.Split(stdout.String(), "\n")
-	if len(f) != 5 || f[4] != "" {
+	f := strings.SplitN(stdout.String(), "\n", 5)
+	if len(f) != 5 {
 		return fmt.Errorf("go/build: importGo %s: unexpected output:\n%s\n", path, stdout.String())
 	}
+	dir := f[0]
+	errStr := strings.TrimSpace(f[4])
+	if errStr != "" && p.Dir == "" {
+		// If 'go list' could not locate the package, return the same error that
+		// 'go list' reported.
+		// If 'go list' did locate the package (p.Dir is not empty), ignore the
+		// error. It was probably related to loading source files, and we'll
+		// encounter it ourselves shortly.
+		return errors.New(errStr)
+	}
 
-	p.Dir = f[0]
+	p.Dir = dir
 	p.ImportPath = f[1]
 	p.Root = f[2]
 	p.Goroot = f[3] == "true"

src/go/build/build_test.go

@@ -7,6 +7,7 @@ package build
 import (
 	"internal/testenv"
 	"io"
+	"io/ioutil"
 	"os"
 	"path/filepath"
 	"reflect"
@@ -447,3 +448,29 @@ func TestIssue23594(t *testing.T) {
 		t.Fatalf("incorrectly set .Doc to %q", p.Doc)
 	}
 }
+
+// TestMissingImportErrorRepetition checks that when an unknown package is
+// imported, the package path is only shown once in the error.
+// Verifies golang.org/issue/34752.
+func TestMissingImportErrorRepetition(t *testing.T) {
+	testenv.MustHaveGoBuild(t) // need 'go list' internally
+	tmp, err := ioutil.TempDir("", "")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tmp)
+	if err := ioutil.WriteFile(filepath.Join(tmp, "go.mod"), []byte("module m"), 0666); err != nil {
+		t.Fatal(err)
+	}
+	defer os.Setenv("GO111MODULE", os.Getenv("GO111MODULE"))
+	os.Setenv("GO111MODULE", "on")
+	defer os.Setenv("GOPROXY", os.Getenv("GOPROXY"))
+	os.Setenv("GOPROXY", "off")
+
+	pkgPath := "example.com/hello"
+	if _, err = Import(pkgPath, tmp, FindOnly); err == nil {
+		t.Fatal("unexpected success")
+	} else if n := strings.Count(err.Error(), pkgPath); n != 1 {
+		t.Fatalf("package path %q appears in error %d times; should appear once\nerror: %v", pkgPath, n, err)
+	}
+}