cmd/go: fail if the user tries to modify the GOMOD environment variable

GOMOD was previously silently ignored. now fails if the user tries to modify
the GOMOD environment variable and it is not the same as the module file
that would be used.

Fixes #51217

Author: bstncartwright

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

@@ -6,12 +6,18 @@ package modload
 
 import (
 	"bytes"
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/fsys"
+	"cmd/go/internal/lockedfile"
+	"cmd/go/internal/modconv"
+	"cmd/go/internal/modfetch"
+	"cmd/go/internal/search"
 	"context"
 	"encoding/json"
 	"errors"
 	"fmt"
 	"go/build"
-	"internal/lazyregexp"
 	"io/ioutil"
 	"os"
 	"path"
@@ -20,13 +26,7 @@ import (
 	"strings"
 	"sync"
 
-	"cmd/go/internal/base"
-	"cmd/go/internal/cfg"
-	"cmd/go/internal/fsys"
-	"cmd/go/internal/lockedfile"
-	"cmd/go/internal/modconv"
-	"cmd/go/internal/modfetch"
-	"cmd/go/internal/search"
+	"internal/lazyregexp"
 
 	"golang.org/x/mod/modfile"
 	"golang.org/x/mod/module"
@@ -409,6 +409,10 @@ func Init() {
 			if !mustUseModules {
 				return
 			}
+		} else if os.Getenv("GOMOD") != "" && filepath.Join(modRoot, "go.mod") != filepath.Join(modRoot, os.Getenv("GOMOD")) {
+			// if GOMOD is set and does not point to module file, error as they must use -modfile
+			// see golang.org/issue/51217
+			base.Fatalf("go: GOMOD cannot be used to set the module file. use -modfile")
 		} else {
 			modRoots = []string{modRoot}
 		}
@@ -1420,9 +1424,7 @@ Run 'go help mod init' for more information.
 	return "", fmt.Errorf(msg, dir, reason)
 }
 
-var (
-	importCommentRE = lazyregexp.New(`(?m)^package[ \t]+[^ \t\r\n/]+[ \t]+//[ \t]+import[ \t]+(\"[^"]+\")[ \t]*\r?\n`)
-)
+var importCommentRE = lazyregexp.New(`(?m)^package[ \t]+[^ \t\r\n/]+[ \t]+//[ \t]+import[ \t]+(\"[^"]+\")[ \t]*\r?\n`)
 
 func findImportComment(file string) string {
 	data, err := os.ReadFile(file)