[dev.cmdgo] cmd/go: replace Target with MainModules, allowing for multiple targets

This change replaces the Target variable that represents the main module
and the pathPrefix and inGorootSrc which provide other information about
the main module with a single MainModules value that represents multiple
main modules and holds their path prefixes, module roots, and whether
they are in GOROOT/src. In cases where the code checks Target or its
previously associated variables, the code now checks or iterates over
MainModules. In some cases, the code still assumes a single main module
by calling MainModules.MustGetSingleMainModule. Some of those cases are
correct: for instance, there is always only one main module for
mod=vendor. Other cases are accompanied with TODOs and will have to be
fixed in future CLs to properly support multiple main modules.

This CL (and other cls on top of it) are planned to be checked into a
branch to allow for those evaluating the workspaces proposal to try it
hands on.

For #45713

Change-Id: I3b699e1d5cad8c76d62dc567b8460de8c73a87ea
Reviewed-on: https://go-review.googlesource.com/c/go/+/334932
Trust: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

1 files changed, 14 insertions, 10 deletions

diff --git a/src/cmd/go/internal/modload/import.go b/src/cmd/go/internal/modload/import.go
index d2bbe5cbe0..7b5305e4bb 100644
--- a/src/cmd/go/internal/modload/import.go
+++ b/src/cmd/go/internal/modload/import.go
@@ -257,11 +257,13 @@ func importFromModules(ctx context.Context, path string, rs *Requirements, mg *M
 	// Is the package in the standard library?
 	pathIsStd := search.IsStandardImportPath(path)
 	if pathIsStd && goroot.IsStandardPackage(cfg.GOROOT, cfg.BuildContext.Compiler, path) {
-		if targetInGorootSrc {
-			if dir, ok, err := dirInModule(path, targetPrefix, ModRoot(), true); err != nil {
-				return module.Version{}, dir, err
-			} else if ok {
-				return Target, dir, nil
+		for _, mainModule := range MainModules.Versions() {
+			if MainModules.InGorootSrc(mainModule) {
+				if dir, ok, err := dirInModule(path, MainModules.PathPrefix(mainModule), MainModules.ModRoot(mainModule), true); err != nil {
+					return module.Version{}, dir, err
+				} else if ok {
+					return mainModule, dir, nil
+				}
 			}
 		}
 		dir := filepath.Join(cfg.GOROOT, "src", path)
@@ -271,8 +273,10 @@ func importFromModules(ctx context.Context, path string, rs *Requirements, mg *M
 	// -mod=vendor is special.
 	// Everything must be in the main module or the main module's vendor directory.
 	if cfg.BuildMod == "vendor" {
-		mainDir, mainOK, mainErr := dirInModule(path, targetPrefix, ModRoot(), true)
-		vendorDir, vendorOK, _ := dirInModule(path, "", filepath.Join(ModRoot(), "vendor"), false)
+		mainModule := MainModules.mustGetSingleMainModule()
+		modRoot := MainModules.ModRoot(mainModule)
+		mainDir, mainOK, mainErr := dirInModule(path, MainModules.PathPrefix(mainModule), modRoot, true)
+		vendorDir, vendorOK, _ := dirInModule(path, "", filepath.Join(modRoot, "vendor"), false)
 		if mainOK && vendorOK {
 			return module.Version{}, "", &AmbiguousImportError{importPath: path, Dirs: []string{mainDir, vendorDir}}
 		}
@@ -280,7 +284,7 @@ func importFromModules(ctx context.Context, path string, rs *Requirements, mg *M
 		// Note that we're not checking that the package exists.
 		// We'll leave that for load.
 		if !vendorOK && mainDir != "" {
-			return Target, mainDir, nil
+			return mainModule, mainDir, nil
 		}
 		if mainErr != nil {
 			return module.Version{}, "", mainErr
@@ -638,8 +642,8 @@ func dirInModule(path, mpath, mdir string, isLocal bool) (dir string, haveGoFile
 // The isLocal return value reports whether the replacement,
 // if any, is local to the filesystem.
 func fetch(ctx context.Context, mod module.Version, needSum bool) (dir string, isLocal bool, err error) {
-	if mod == Target {
-		return ModRoot(), true, nil
+	if modRoot := MainModules.ModRoot(mod); modRoot != "" {
+		return modRoot, true, nil
 	}
 	if r := Replacement(mod); r.Path != "" {
 		if r.Version == "" {