cmd/go: note when some Go files were ignored on no-Go-files errors

It is pretty confusing when there are Go files ignored for mismatching
build tags and similar and we output "no buildable Go files" without
giving any other information about some Go files have been ignored.

Fixes golang#17008.

Change-Id: I1766ee86a9a7a72f6694deae3f73b47bfc9d0be5
Reviewed-on: https://go-review.googlesource.com/29113
Run-TryBot: Jaana Burcu Dogan <jbd@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/go/build/build.go

@@ -410,11 +410,16 @@ func (ctxt *Context) ImportDir(dir string, mode ImportMode) (*Package, error) {
 // containing no buildable Go source files. (It may still contain
 // test files, files hidden by build tags, and so on.)
 type NoGoError struct {
-	Dir string
+	Dir     string
+	Ignored bool // whether any Go files were ignored due to build tags
 }
 
 func (e *NoGoError) Error() string {
-	return "no buildable Go source files in " + e.Dir
+	msg := "no buildable Go source files in " + e.Dir
+	if e.Ignored {
+		msg += " (.go files ignored due to build tags)"
+	}
+	return msg
 }
 
 // MultiplePackageError describes a directory containing
@@ -846,7 +851,7 @@ Found:
 		return p, badGoError
 	}
 	if len(p.GoFiles)+len(p.CgoFiles)+len(p.TestGoFiles)+len(p.XTestGoFiles) == 0 {
-		return p, &NoGoError{p.Dir}
+		return p, &NoGoError{Dir: p.Dir, Ignored: len(p.IgnoredGoFiles) > 0}
 	}
 
 	for tag := range allTags {

src/go/build/build_test.go

@@ -93,6 +93,17 @@ func TestEmptyFolderImport(t *testing.T) {
 	}
 }
 
+func TestIgnoredGoFilesImport(t *testing.T) {
+	_, err := Import(".", "testdata/ignored", 0)
+	e, ok := err.(*NoGoError)
+	if !ok {
+		t.Fatal(`Import("testdata/ignored") did not return NoGoError.`)
+	}
+	if !e.Ignored {
+		t.Fatal(`Import("testdata/ignored") should have ignored Go files.`)
+	}
+}
+
 func TestMultiplePackageImport(t *testing.T) {
 	_, err := Import(".", "testdata/multi", 0)
 	mpe, ok := err.(*MultiplePackageError)

src/go/build/testdata/ignored/ignored.go

@@ -0,0 +1,3 @@
+// +build alwaysignore
+
+package ignored