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>

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) {

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

@@ -2,10 +2,16 @@ env GO111MODULE=on
 
 # explicit get should report errors about bad names
 ! go get appengine
-stderr 'malformed module path "appengine": missing dot in first path element'
+stderr '^go get appengine: package appengine is not in GOROOT \(.*\)$'
 ! go get x/y.z
 stderr 'malformed module path "x/y.z": missing dot in first path element'
 
+# 'go list -m' should report errors about module names, never GOROOT.
+! go list -m -versions appengine
+stderr 'malformed module path "appengine": missing dot in first path element'
+! go list -m -versions x/y.z
+stderr 'malformed module path "x/y.z": missing dot in first path element'
+
 # build should report all unsatisfied imports,
 # but should be more definitive about non-module import paths
 ! go build ./useappengine

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

@@ -2,7 +2,7 @@
 # Verifies golang.org/issue/34393.
 
 go list -e -deps -f '{{with .Error}}{{.Pos}}: {{.Err}}{{end}}' ./main
-stdout 'bad[/\\]bad.go:3:8: malformed module path "string": missing dot in first path element'
+stdout 'bad[/\\]bad.go:3:8: malformed module path "🐧.example.com/string": invalid char ''🐧'''
 
 -- go.mod --
 module m
@@ -19,4 +19,4 @@ func main() {}
 -- bad/bad.go --
 package bad
 
-import _ "string"
+import _ "🐧.example.com/string"

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

@@ -0,0 +1,53 @@
+env GO111MODULE=on
+
+# Regression test for https://golang.org/issue/34769.
+# Missing standard-library imports should refer to GOROOT rather than
+# complaining about a malformed module path.
+# This is especially important when GOROOT is set incorrectly,
+# since such an error will occur for every package in std.
+
+# Building a nonexistent std package directly should fail usefully.
+
+! go build -mod=readonly nonexist
+! stderr 'import lookup disabled'
+! stderr 'missing dot'
+stderr '^can''t load package: package nonexist is not in GOROOT \('$GOROOT'[/\\]src[/\\]nonexist\)$'
+
+! go build nonexist
+! stderr 'import lookup disabled'
+! stderr 'missing dot'
+stderr '^can''t load package: package nonexist is not in GOROOT \('$GOROOT'[/\\]src[/\\]nonexist\)$'
+
+# Building a nonexistent std package indirectly should also fail usefully.
+
+! go build -mod=readonly ./importnonexist
+! stderr 'import lookup disabled'
+! stderr 'missing dot'
+stderr '^importnonexist[/\\]x.go:2:8: package nonexist is not in GOROOT \('$GOROOT'[/\\]src[/\\]nonexist\)$'
+
+! go build ./importnonexist
+! stderr 'import lookup disabled'
+! stderr 'missing dot'
+stderr '^importnonexist[/\\]x.go:2:8: package nonexist is not in GOROOT \('$GOROOT'[/\\]src[/\\]nonexist\)$'
+
+# Building an *actual* std package should fail if GOROOT is set to something bogus.
+
+[!short] go build ./importjson  # Prove that it works when GOROOT is valid.
+
+env GOROOT=$WORK/not-a-valid-goroot
+! go build ./importjson
+! stderr 'import lookup disabled'
+! stderr 'missing dot'
+stderr 'importjson[/\\]x.go:2:8: package encoding/json is not in GOROOT \('$WORK'[/\\]not-a-valid-goroot[/\\]src[/\\]encoding[/\\]json\)$'
+
+-- go.mod --
+module example.com
+go 1.14
+-- importnonexist/x.go --
+package importnonexist
+import _ "nonexist"
+-- importjson/x.go --
+package importjson
+import _ "encoding/json"
+-- $WORK/not-a-valid-goroot/README --
+This directory is not a valid GOROOT.

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

@@ -4,12 +4,12 @@ env GO111MODULE=on
 # 'go mod tidy' and 'go mod vendor' should not hide loading errors.
 
 ! go mod tidy
-stderr '^issue27063 imports\n\tnonexist: malformed module path "nonexist": missing dot in first path element'
+stderr '^issue27063 imports\n\tnonexist: package nonexist is not in GOROOT \(.*\)'
 stderr '^issue27063 imports\n\tnonexist.example.com: cannot find module providing package nonexist.example.com'
 stderr '^issue27063 imports\n\tissue27063/other imports\n\tother.example.com/nonexist: cannot find module providing package other.example.com/nonexist'
 
 ! go mod vendor
-stderr '^issue27063 imports\n\tnonexist: malformed module path "nonexist": missing dot in first path element'
+stderr '^issue27063 imports\n\tnonexist: package nonexist is not in GOROOT \(.*\)'
 stderr '^issue27063 imports\n\tnonexist.example.com: cannot find module providing package nonexist.example.com'
 stderr '^issue27063 imports\n\tissue27063/other imports\n\tother.example.com/nonexist: cannot find module providing package other.example.com/nonexist'