cmd/go: improve error message for missing import starting with cmd/

In modload.Import, confirm that the import path does not start with
"cmd/" before calling QueryPackage, which returns a less helpful
error.

In load.loadPackageData, don't wrap errors with "unknown import path".
The wrapped error should always include the import path, and it's also
repeated in the PackageError wrapper.

Fixes #31031

Change-Id: I071efa22e3842c62831d096f888a8006811fe724
Reviewed-on: https://go-review.googlesource.com/c/go/+/189157
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go.sum

@@ -7,11 +7,13 @@ golang.org/x/arch v0.0.0-20181203225421-5a4828bb7045/go.mod h1:cYlCBUl1MsqxdiKgm
 golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
 golang.org/x/crypto v0.0.0-20190325154230-a5d413f7728c h1:Vj5n4GlwjmQteupaxJ9+0FNOmBrHfq7vN4btdGoDZgI=
 golang.org/x/crypto v0.0.0-20190325154230-a5d413f7728c/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
+golang.org/x/net v0.0.0-20190311183353-d8887717615a h1:oWX7TPOiFAMXLq8o0ikBYfCJVlRHBcsciT5bXOrH628=
 golang.org/x/net v0.0.0-20190311183353-d8887717615a/go.mod h1:t9HGtf8HONx5eT2rtn7q6eTqICYqUVnKs3thJo3Qplg=
 golang.org/x/sync v0.0.0-20190423024810-112230192c58/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
 golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
 golang.org/x/sys v0.0.0-20190502175342-a43fa875dd82 h1:vsphBvatvfbhlb4PO1BYSr9dzugGxJ/SQHoNufZJq1w=
 golang.org/x/sys v0.0.0-20190502175342-a43fa875dd82/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
+golang.org/x/text v0.3.0 h1:g61tztE5qeGQ89tm6NTjjM9VPIm088od1l6aSorWRWg=
 golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
 golang.org/x/tools v0.0.0-20190611154301-25a4f137592f h1:6awn5JC4pwVI5HiBqs7MDtRxnwV9PpO5iSA9v6P09pA=
 golang.org/x/tools v0.0.0-20190611154301-25a4f137592f/go.mod h1:/rFqwRUd4F7ZHNgwSSTFct+R/Kf4OFW1sUzUTQQTgfc=

src/cmd/go/internal/load/pkg.go

@@ -653,7 +653,7 @@ func loadPackageData(path, parentPath, parentDir, parentRoot string, parentIsStd
 			}
 		} else if r.err != nil {
 			data.p = new(build.Package)
-			data.err = fmt.Errorf("unknown import path %q: %v", r.path, r.err)
+			data.err = r.err
 		} else if cfg.ModulesEnabled && path != "unsafe" {
 			data.p = new(build.Package)
 			data.err = fmt.Errorf("unknown import path %q: internal error: module loader did not resolve import", r.path)

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

@@ -22,6 +22,7 @@ import (
 	"cmd/go/internal/par"
 	"cmd/go/internal/search"
 	"cmd/go/internal/semver"
+	"cmd/go/internal/str"
 )
 
 type ImportMissingError struct {
@@ -35,6 +36,9 @@ type ImportMissingError struct {
 
 func (e *ImportMissingError) Error() string {
 	if e.Module.Path == "" {
+		if str.HasPathPrefix(e.ImportPath, "cmd") {
+			return fmt.Sprintf("package %s is not in GOROOT (%s)", e.ImportPath, filepath.Join(cfg.GOROOT, "src", e.ImportPath))
+		}
 		return "cannot find module providing package " + e.ImportPath
 	}
 	return "missing module for import: " + e.Module.Path + "@" + e.Module.Version + " provides " + e.ImportPath
@@ -74,6 +78,9 @@ 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{ImportPath: path}
+	}
 
 	// -mod=vendor is special.
 	// Everything must be in the main module or the main module's vendor directory.

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

+env GO111MODULE=on
+
+# Regression test for golang.org/issue/31031:
+# Importing or loading a non-existent package in cmd/ should print
+# a clear error in module mode.
+
+! go list cmd/unknown
+stderr '^can''t load package: package cmd/unknown: package cmd/unknown is not in GOROOT \('$GOROOT'[/\\]src[/\\]cmd[/\\]unknown\)$'
+
+go list -f '{{range .DepsErrors}}{{.Err}}{{end}}' x.go
+stdout '^package cmd/unknown is not in GOROOT \('$GOROOT'[/\\]src[/\\]cmd[/\\]unknown\)$'
+
+-- x.go --
+package x
+
+import _ "cmd/unknown"