cmd/go: allow -mod=vendor to be set outside of a module

It will behave as if teh command-line-arguments module was vendored,
with zero dependencies, much as -mod=readonly works.

Fixes #56536

Change-Id: Ia02dda2f90d006b5917c3ae002ccb714987f27a3
Reviewed-on: https://go-review.googlesource.com/c/go/+/448019
Reviewed-by: Michael Matloob <matloob@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Michael Matloob <matloob@golang.org>

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

@@ -316,18 +316,22 @@ func importFromModules(ctx context.Context, path string, rs *Requirements, mg *M
 	if cfg.BuildMod == "vendor" {
 		mainModule := MainModules.mustGetSingleMainModule()
 		modRoot := MainModules.ModRoot(mainModule)
-		mainDir, mainOK, mainErr := dirInModule(path, MainModules.PathPrefix(mainModule), modRoot, true)
-		if mainOK {
-			mods = append(mods, mainModule)
-			dirs = append(dirs, mainDir)
-			roots = append(roots, modRoot)
-		}
-		vendorDir, vendorOK, _ := dirInModule(path, "", filepath.Join(modRoot, "vendor"), false)
-		if vendorOK {
-			readVendorList(mainModule)
-			mods = append(mods, vendorPkgModule[path])
-			dirs = append(dirs, vendorDir)
-			roots = append(roots, modRoot)
+		var mainErr error
+		if modRoot != "" {
+			mainDir, mainOK, err := dirInModule(path, MainModules.PathPrefix(mainModule), modRoot, true)
+			mainErr = err
+			if mainOK {
+				mods = append(mods, mainModule)
+				dirs = append(dirs, mainDir)
+				roots = append(roots, modRoot)
+			}
+			vendorDir, vendorOK, _ := dirInModule(path, "", filepath.Join(modRoot, "vendor"), false)
+			if vendorOK {
+				readVendorList(mainModule)
+				mods = append(mods, vendorPkgModule[path])
+				dirs = append(dirs, vendorDir)
+				roots = append(roots, modRoot)
+			}
 		}
 
 		if len(dirs) > 1 {

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

@@ -673,7 +673,7 @@ func LoadModFile(ctx context.Context) *Requirements {
 			modfetch.WorkspaceGoSumFiles = append(modfetch.WorkspaceGoSumFiles, sumFile)
 		}
 		modfetch.GoSumFile = workFilePath + ".sum"
-	} else if modRoots == nil {
+	} else if len(modRoots) == 0 {
 		// We're in module mode, but not inside a module.
 		//
 		// Commands like 'go build', 'go run', 'go list' have no go.mod file to
@@ -707,6 +707,12 @@ func LoadModFile(ctx context.Context) *Requirements {
 			pruning = workspace
 		}
 		requirements = newRequirements(pruning, nil, nil)
+		if cfg.BuildMod == "vendor" {
+			// For issue 56536: Some users may have GOFLAGS=-mod=vendor set.
+			// Make sure it behaves as though the fake module is vendored
+			// with no dependencies.
+			requirements.initVendor(nil)
+		}
 		return requirements
 	}
 

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

@@ -0,0 +1,36 @@
+# baz.go (importing just fmt) works with -mod=mod,  -mod=vendor.
+go build -x -mod=mod my-module/vendor/example.com/another-module/foo/bar/baz.go
+go build -x -mod=readonly my-module/vendor/example.com/another-module/foo/bar/baz.go
+go build -x -mod=vendor my-module/vendor/example.com/another-module/foo/bar/baz.go
+
+# baz_with_outside_dep.go (with a non-std dependency) works with -mod=mod
+# but not with -mod=readonly and -mod=vendor.
+go build -x -mod=mod my-module/vendor/example.com/another-module/foo/bar/baz_with_outside_dep.go
+! go build -x -mod=readonly my-module/vendor/example.com/another-module/foo/bar/baz_with_outside_dep.go
+stderr 'no required module provides package rsc.io/quote'
+! go build -x -mod=vendor my-module/vendor/example.com/another-module/foo/bar/baz_with_outside_dep.go
+stderr 'no required module provides package rsc.io/quote'
+
+-- my-module/go.mod --
+module example.com/my-module
+
+go 1.20
+-- my-module/vendor/example.com/another-module/foo/bar/baz.go --
+package main
+
+import "fmt"
+
+func main() {
+	fmt.Println("hello, world.")
+}
+-- my-module/vendor/example.com/another-module/foo/bar/baz_with_outside_dep.go --
+package main
+
+import (
+    "fmt"
+    "rsc.io/quote"
+)
+
+func main() {
+	fmt.Println(quote.Hello())
+}