cmd/go/internal/modload: don't infer a /v1 suffix module path

src/cmd/go/internal/modload/init.go

@@ -1075,13 +1075,16 @@ func CreateModFile(ctx context.Context, modPath string) {
 		base.Fatalf("go: %s already exists", modFilePath)
 	}
 
+	modPathError := modulePathError{reason: fmt.Sprintf("invalid module path %q", modPath)}
 	if modPath == "" {
-		var err error
-		modPath, err = findModulePath(modRoot)
+		inferredModPath, err := findModulePath(modRoot)
 		if err != nil {
 			base.Fatal(err)
 		}
-	} else if err := module.CheckImportPath(modPath); err != nil {
+		modPath = inferredModPath
+		modPathError.reason = fmt.Sprintf("invalid module path %q inferred from directory in GOPATH", inferredModPath)
+	}
+	if err := module.CheckImportPath(modPath); err != nil {
 		if pathErr, ok := err.(*module.InvalidPathError); ok {
 			pathErr.Kind = "module"
 			// Same as build.IsLocalPath()
@@ -1090,14 +1093,18 @@ func CreateModFile(ctx context.Context, modPath string) {
 				pathErr.Err = errors.New("is a local import path")
 			}
 		}
-		base.Fatal(err)
-	} else if _, _, ok := module.SplitPathVersion(modPath); !ok {
+		modPathError.message = err.Error()
+		base.Fatal(modPathError)
+	}
+	if _, _, ok := module.SplitPathVersion(modPath); !ok {
 		if strings.HasPrefix(modPath, "gopkg.in/") {
-			invalidMajorVersionMsg := fmt.Errorf("module paths beginning with gopkg.in/ must always have a major version suffix in the form of .vN:\n\tgo mod init %s", suggestGopkgIn(modPath))
-			base.Fatalf(`go: invalid module path "%v": %v`, modPath, invalidMajorVersionMsg)
+			modPathError.message = "module paths beginning with gopkg.in/ must always have a major version suffix in the form of .vN"
+			modPathError.suggestions = []string{fmt.Sprintf("go mod init %s", suggestGopkgIn(modPath))}
+		} else {
+			modPathError.message = "major version suffixes must be in the form of /vN and are only allowed for v2 or later"
+			modPathError.suggestions = []string{fmt.Sprintf("go mod init %s", suggestModulePath(modPath))}
 		}
-		invalidMajorVersionMsg := fmt.Errorf("major version suffixes must be in the form of /vN and are only allowed for v2 or later:\n\tgo mod init %s", suggestModulePath(modPath))
-		base.Fatalf(`go: invalid module path "%v": %v`, modPath, invalidMajorVersionMsg)
+		base.Fatal(modPathError)
 	}
 
 	fmt.Fprintf(os.Stderr, "go: creating new go.mod: module %s\n", modPath)
@@ -1140,6 +1147,36 @@ func CreateModFile(ctx context.Context, modPath string) {
 	}
 }
 
+// modulePathError is an error that occurs when a module path is invalid.
+//
+//	 Format:
+//		go: <reason>: <message>
+//
+//		Example usage:
+//			<suggestions>
+//
+//		Run 'go help mod init' for more information.
+type modulePathError struct {
+	reason      string
+	message     string
+	suggestions []string
+}
+
+func (e modulePathError) Error() string {
+	buf := strings.Builder{}
+	buf.WriteString(fmt.Sprintf("%s: %s\n", e.reason, e.message))
+	if len(e.suggestions) > 0 {
+		buf.WriteString("\nExample usage:\n")
+		for _, suggestion := range e.suggestions {
+			buf.WriteString("\t")
+			buf.WriteString(suggestion)
+			buf.WriteString("\n")
+		}
+	}
+	buf.WriteString("\nRun 'go help mod init' for more information.\n")
+	return buf.String()
+}
+
 // fixVersion returns a modfile.VersionFixer implemented using the Query function.
 //
 // It resolves commit hashes and branch names to versions,
@@ -1698,37 +1735,23 @@ func findModulePath(dir string) (string, error) {
 	}
 
 	// Look for path in GOPATH.
-	var badPathErr error
 	for _, gpdir := range filepath.SplitList(cfg.BuildContext.GOPATH) {
 		if gpdir == "" {
 			continue
 		}
 		if rel := search.InDir(dir, filepath.Join(gpdir, "src")); rel != "" && rel != "." {
-			path := filepath.ToSlash(rel)
-			// gorelease will alert users publishing their modules to fix their paths.
-			if err := module.CheckImportPath(path); err != nil {
-				badPathErr = err
-				break
-			}
-			return path, nil
+			return filepath.ToSlash(rel), nil
 		}
 	}
 
-	reason := "outside GOPATH, module path must be specified"
-	if badPathErr != nil {
-		// return a different error message if the module was in GOPATH, but
-		// the module path determined above would be an invalid path.
-		reason = fmt.Sprintf("bad module path inferred from directory in GOPATH: %v", badPathErr)
+	return "", modulePathError{
+		reason:  "cannot determine module path for source directory",
+		message: "outside GOPATH, module path must be specified",
+		suggestions: []string{
+			"'go mod init example.com/m' to initialize a v0 or v1 module",
+			"'go mod init example.com/m/v2' to initialize a v2 module",
+		},
 	}
-	msg := `cannot determine module path for source directory %s (%s)
-
-Example usage:
-	'go mod init example.com/m' to initialize a v0 or v1 module
-	'go mod init example.com/m/v2' to initialize a v2 module
-
-Run 'go help mod init' for more information.
-`
-	return "", fmt.Errorf(msg, dir, reason)
 }
 
 var (

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

@@ -8,6 +8,26 @@ stdout '^example.com$'
 go list
 stdout '^example.com$'
 
+# Reset $GOPATH
+env GOPATH=$WORK/gopath
+
+# 'go mod init' should not create a go.mod file in v0 or v1 directory.
+cd $GOPATH/src/example.com/m/v0
+! go mod init
+stderr '(?s)^go: invalid module path "example.com/m/v0" inferred from directory in GOPATH: major version suffixes must be in the form of /vN and are only allowed for v2 or later(.*)go mod init example.com/m/v2(.*)'
+
+cd $GOPATH/src/example.com/m/v1
+! go mod init
+stderr '(?s)^go: invalid module path "example.com/m/v1" inferred from directory in GOPATH: major version suffixes must be in the form of /vN and are only allowed for v2 or later(.*)go mod init example.com/m/v2(.*)'
+
+cd $GOPATH/src/example.com/m/v2
+go mod init
+stderr '^go: creating new go.mod: module example.com/m/v2$'
+
+cd $GOPATH/src/gopkg.in/m
+! go mod init
+stderr '(?s)^go: invalid module path "gopkg.in/m" inferred from directory in GOPATH: module paths beginning with gopkg.in/ must always have a major version suffix in the form of .vN(.*)'
+
 -- go.mod --
 module example.com
 
@@ -19,3 +39,12 @@ func main() {}
 
 -- $WORK/invalid-gopath
 This is a text file, not a directory.
+
+-- example.com/m/v0/main.go --
+package main
+-- example.com/m/v1/main.go --
+package main
+-- example.com/m/v2/main.go --
+package main
+-- gopkg.in/m/main.go --
+package main

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

@@ -2,34 +2,34 @@ env GO111MODULE=on
 env GOFLAGS=-mod=mod
 
 ! go mod init example.com/user/repo/v0
-stderr '(?s)^go: invalid module path "example.com/user/repo/v0": major version suffixes must be in the form of /vN and are only allowed for v2 or later(.*)go mod init example.com/user/repo/v2$'
+stderr '(?s)^go: invalid module path "example.com/user/repo/v0": major version suffixes must be in the form of /vN and are only allowed for v2 or later(.*)go mod init example.com/user/repo/v2(.*)'
 
 ! go mod init example.com/user/repo/v02
-stderr '(?s)^go: invalid module path "example.com/user/repo/v02": major version suffixes must be in the form of /vN and are only allowed for v2 or later(.*)go mod init example.com/user/repo/v2$'
+stderr '(?s)^go: invalid module path "example.com/user/repo/v02": major version suffixes must be in the form of /vN and are only allowed for v2 or later(.*)go mod init example.com/user/repo/v2(.*)'
 
 ! go mod init example.com/user/repo/v023
-stderr '(?s)^go: invalid module path "example.com/user/repo/v023": major version suffixes must be in the form of /vN and are only allowed for v2 or later(.*)go mod init example.com/user/repo/v23$'
+stderr '(?s)^go: invalid module path "example.com/user/repo/v023": major version suffixes must be in the form of /vN and are only allowed for v2 or later(.*)go mod init example.com/user/repo/v23(.*)'
 
 ! go mod init example.com/user/repo/v1
-stderr '(?s)^go: invalid module path "example.com/user/repo/v1": major version suffixes must be in the form of /vN and are only allowed for v2 or later(.*)go mod init example.com/user/repo/v2$'
+stderr '(?s)^go: invalid module path "example.com/user/repo/v1": major version suffixes must be in the form of /vN and are only allowed for v2 or later(.*)go mod init example.com/user/repo/v2(.*)'
 
 ! go mod init example.com/user/repo/v2.0
-stderr '(?s)^go: invalid module path "example.com/user/repo/v2.0": major version suffixes must be in the form of /vN and are only allowed for v2 or later(.*)go mod init example.com/user/repo/v2$'
+stderr '(?s)^go: invalid module path "example.com/user/repo/v2.0": major version suffixes must be in the form of /vN and are only allowed for v2 or later(.*)go mod init example.com/user/repo/v2(.*)'
 
 ! go mod init example.com/user/repo/v2.1.4
-stderr '(?s)^go: invalid module path "example.com/user/repo/v2.1.4": major version suffixes must be in the form of /vN and are only allowed for v2 or later(.*)go mod init example.com/user/repo/v2$'
+stderr '(?s)^go: invalid module path "example.com/user/repo/v2.1.4": major version suffixes must be in the form of /vN and are only allowed for v2 or later(.*)go mod init example.com/user/repo/v2(.*)'
 
 ! go mod init example.com/user/repo/v3.5
-stderr '(?s)^go: invalid module path "example.com/user/repo/v3.5": major version suffixes must be in the form of /vN and are only allowed for v2 or later(.*)go mod init example.com/user/repo/v3$'
+stderr '(?s)^go: invalid module path "example.com/user/repo/v3.5": major version suffixes must be in the form of /vN and are only allowed for v2 or later(.*)go mod init example.com/user/repo/v3(.*)'
 
 ! go mod init example.com/user/repo/v4.1.4
-stderr '(?s)^go: invalid module path "example.com/user/repo/v4.1.4": major version suffixes must be in the form of /vN and are only allowed for v2 or later(.*)go mod init example.com/user/repo/v4$'
+stderr '(?s)^go: invalid module path "example.com/user/repo/v4.1.4": major version suffixes must be in the form of /vN and are only allowed for v2 or later(.*)go mod init example.com/user/repo/v4(.*)'
 
 ! go mod init example.com/user/repo/v.2.3
-stderr '(?s)^go: invalid module path "example.com/user/repo/v.2.3": major version suffixes must be in the form of /vN and are only allowed for v2 or later(.*)go mod init example.com/user/repo/v2$'
+stderr '(?s)^go: invalid module path "example.com/user/repo/v.2.3": major version suffixes must be in the form of /vN and are only allowed for v2 or later(.*)go mod init example.com/user/repo/v2(.*)'
 
 ! go mod init example.com/user/repo/v.5.3
-stderr '(?s)^go: invalid module path "example.com/user/repo/v.5.3": major version suffixes must be in the form of /vN and are only allowed for v2 or later(.*)go mod init example.com/user/repo/v5$'
+stderr '(?s)^go: invalid module path "example.com/user/repo/v.5.3": major version suffixes must be in the form of /vN and are only allowed for v2 or later(.*)go mod init example.com/user/repo/v5(.*)'
 
 ! go mod init gopkg.in/pkg
 stderr '(?s)^go: invalid module path "gopkg.in/pkg": module paths beginning with gopkg.in/ must always have a major version suffix in the form of .vN(.*)go mod init gopkg.in/pkg.v1$'
@@ -63,20 +63,20 @@ stderr '(?s)^go: invalid module path "gopkg.in/user/pkg.v.2.3": module paths beg
 
 # module paths with a trailing dot are rejected as invalid import paths
 ! go mod init example.com/user/repo/v2.
-stderr '(?s)^go: malformed module path "example.com/user/repo/v2.": trailing dot in path element$'
+stderr '(?s)^go: invalid module path "example.com/user/repo/v2.": malformed module path "example.com/user/repo/v2.": trailing dot in path element(.*)'
 
 ! go mod init example.com/user/repo/v2..
-stderr '(?s)^go: malformed module path "example.com/user/repo/v2..": trailing dot in path element$'
+stderr '(?s)^go: invalid module path "example.com/user/repo/v2..": malformed module path "example.com/user/repo/v2..": trailing dot in path element(.*)'
 
 ! go mod init gopkg.in/user/pkg.v.2.
-stderr '(?s)^go: malformed module path "gopkg.in/user/pkg.v.2.": trailing dot in path element$'
+stderr '(?s)^go: invalid module path "gopkg.in/user/pkg.v.2.": malformed module path "gopkg.in/user/pkg.v.2.": trailing dot in path element(.*)'
 
 ! go mod init gopkg.in/user/pkg.v.2..
-stderr '(?s)^go: malformed module path "gopkg.in/user/pkg.v.2..": trailing dot in path element$'
+stderr '(?s)^go: invalid module path "gopkg.in/user/pkg.v.2..": malformed module path "gopkg.in/user/pkg.v.2..": trailing dot in path element(.*)'
 
 # module paths with spaces are also rejected
 ! go mod init 'foo bar'
-stderr '(?s)^go: malformed module path "foo bar": invalid char '' ''$'
+stderr '(?s)^go: invalid module path "foo bar": malformed module path "foo bar": invalid char '' ''(.*)'
 
 ! go mod init 'foo  bar baz'
-stderr '(?s)^go: malformed module path "foo  bar baz": invalid char '' ''$'
+stderr '(?s)^go: invalid module path "foo  bar baz": malformed module path "foo  bar baz": invalid char '' ''(.*)'

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

@@ -1,7 +1,7 @@
 env GO111MODULE=on
 
 ! go mod init .
-stderr '^go: malformed module path ".": is a local import path$'
+stderr '^go: invalid module path ".": malformed module path ".": is a local import path(.*)'
 
 cd x
 go mod init example.com/x

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

@@ -12,7 +12,7 @@ stderr '^go: error reading go.mod: missing module declaration. To specify the mo
 # but are a valid Windows file name.
 cd $WORK/'gopath/src/m''d'
 ! go mod init
-stderr 'cannot determine module path'
+stderr '(?s)^go: invalid module path "m''d" inferred from directory in GOPATH: malformed module path "m''d": invalid char ''\\''''(.*)'
 
 # Test that a go.mod file is rejected when its module declaration has a path that can't
 # possibly be a module path, because it isn't even a valid import path