cmd/go: avoid generating "malformed module path" errors for standard-library paths

If the path looks like it belongs in GOROOT/src and isn't there, we
should mention that in the error message — instead of the fact
that the path is not a valid module path, which the user likely
already knows.

Fixes #34769
Fixes #35734

Change-Id: I3589336d102e420a5ad3bf246816e29f3cbe6d71
Reviewed-on: https://go-review.googlesource.com/c/go/+/210339
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

1 files changed, 20 insertions, 7 deletions

diff --git a/src/cmd/go/internal/modload/import.go b/src/cmd/go/internal/modload/import.go
index dc0fc3c4d0..1899abbd8f 100644
--- a/src/cmd/go/internal/modload/import.go
+++ b/src/cmd/go/internal/modload/import.go
@@ -20,7 +20,6 @@ import (
 	"cmd/go/internal/modfetch"
 	"cmd/go/internal/par"
 	"cmd/go/internal/search"
-	"cmd/go/internal/str"
 
 	"golang.org/x/mod/module"
 	"golang.org/x/mod/semver"
@@ -40,7 +39,7 @@ var _ load.ImportPathError = (*ImportMissingError)(nil)
 
 func (e *ImportMissingError) Error() string {
 	if e.Module.Path == "" {
-		if str.HasPathPrefix(e.Path, "cmd") {
+		if search.IsStandardImportPath(e.Path) {
 			return fmt.Sprintf("package %s is not in GOROOT (%s)", e.Path, filepath.Join(cfg.GOROOT, "src", e.Path))
 		}
 		if i := load.ImportPathError(nil); errors.As(e.QueryErr, &i) {
@@ -121,8 +120,8 @@ func Import(path string) (m module.Version, dir string, err error) {
 	}
 
 	// Is the package in the standard library?
-	if search.IsStandardImportPath(path) &&
-		goroot.IsStandardPackage(cfg.GOROOT, cfg.BuildContext.Compiler, path) {
+	pathIsStd := search.IsStandardImportPath(path)
+	if pathIsStd && goroot.IsStandardPackage(cfg.GOROOT, cfg.BuildContext.Compiler, path) {
 		if targetInGorootSrc {
 			if dir, ok := dirInModule(path, targetPrefix, ModRoot(), true); ok {
 				return Target, dir, nil
@@ -131,9 +130,6 @@ func Import(path string) (m module.Version, dir string, err error) {
 		dir := filepath.Join(cfg.GOROOT, "src", path)
 		return module.Version{}, dir, nil
 	}
-	if str.HasPathPrefix(path, "cmd") {
-		return module.Version{}, "", &ImportMissingError{Path: path}
-	}
 
 	// -mod=vendor is special.
 	// Everything must be in the main module or the main module's vendor directory.
@@ -187,6 +183,12 @@ func Import(path string) (m module.Version, dir string, err error) {
 	// Look up module containing the package, for addition to the build list.
 	// Goal is to determine the module, download it to dir, and return m, dir, ErrMissing.
 	if cfg.BuildMod == "readonly" {
+		if pathIsStd {
+			// 'import lookup disabled' would be confusing for standard-library paths,
+			// since the user probably isn't expecting us to look up a module for
+			// those anyway.
+			return module.Version{}, "", &ImportMissingError{Path: path}
+		}
 		return module.Version{}, "", fmt.Errorf("import lookup disabled by -mod=%s", cfg.BuildMod)
 	}
 	if modRoot == "" && !allowMissingModuleImports {
@@ -253,6 +255,17 @@ func Import(path string) (m module.Version, dir string, err error) {
 		}
 	}
 
+	if pathIsStd {
+		// This package isn't in the standard library, isn't in any module already
+		// in the build list, and isn't in any other module that the user has
+		// shimmed in via a "replace" directive.
+		// Moreover, the import path is reserved for the standard library, so
+		// QueryPackage cannot possibly find a module containing this package.
+		//
+		// Instead of trying QueryPackage, report an ImportMissingError immediately.
+		return module.Version{}, "", &ImportMissingError{Path: path}
+	}
+
 	candidates, err := QueryPackage(path, "latest", Allowed)
 	if err != nil {
 		if errors.Is(err, os.ErrNotExist) {