cmd/go: special error message for direct self import

This attempts to sort out confusion about cyclical imports. In addition
to the error message "import cycle not allowed", for direct self
imports we will now instead give a specialized error message, namely:

    self import not allowed

Fixes golang#23295

Change-Id: I14e2418aa4eceb8caf2e6520c686fac7e85cb0dd

src/cmd/go/go_test.go

@@ -1351,6 +1351,22 @@ func TestImportCommentConflict(t *testing.T) {
 	tg.grepStderr("found import comments", "go build did not mention comment conflict")
 }
 
+func TestImportCycle(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.setenv("GOPATH", filepath.Join(tg.pwd(), "testdata/importcycle"))
+	tg.runFail("build", "./testdata/importcycle/cycle.go")
+	tg.grepStderr("import cycle not allowed", "go build did not mention cyclical import")
+}
+
+func TestImportCycleSelfImport(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.setenv("GOPATH", filepath.Join(tg.pwd(), "testdata/importcycle"))
+	tg.runFail("build", "./testdata/importcycle/selfimport.go")
+	tg.grepStderr("self import not allowed", "go build did not mention self import")
+}
+
 // cmd/go: custom import path checking should not apply to Go packages without import comment.
 func TestIssue10952(t *testing.T) {
 	testenv.MustHaveExternalNetwork(t)

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

@@ -598,6 +598,19 @@ func hasGoFiles(dir string) bool {
 	return false
 }
 
+// getImportError returns the error message for cyclical imports. It gives a
+// special message for a packages that directly imports itself. For all other
+// cyclical imports it gives a general message about import cycles.
+func getImportError(p *Package) string {
+	for _, i := range p.Imports {
+		if i == p.ImportPath {
+			return "self import not allowed"
+		}
+	}
+
+	return "import cycle not allowed"
+}
+
 // reusePackage reuses package p to satisfy the import at the top
 // of the import stack stk. If this use causes an import loop,
 // reusePackage updates p's error information to record the loop.
@@ -609,7 +622,7 @@ func reusePackage(p *Package, stk *ImportStack) *Package {
 		if p.Error == nil {
 			p.Error = &PackageError{
 				ImportStack:   stk.Copy(),
-				Err:           "import cycle not allowed",
+				Err:           getImportError(p),
 				IsImportCycle: true,
 			}
 		}

src/cmd/go/testdata/importcycle/cycle.go

@@ -0,0 +1,3 @@
+package p
+
+import "cycle1"

src/cmd/go/testdata/importcycle/selfimport.go

@@ -0,0 +1,3 @@
+package p
+
+import "selfimport"

src/cmd/go/testdata/importcycle/src/cycle1/cycle1.go

@@ -0,0 +1,6 @@
+package cycle1
+
+import (
+	"_noncyclic"
+	"cycle2"
+)

src/cmd/go/testdata/importcycle/src/cycle2/cycle2.go

@@ -0,0 +1,3 @@
+package cycle2
+
+import "cycle1"

src/cmd/go/testdata/importcycle/src/noncyclic/noncyclic.go

@@ -0,0 +1 @@
+package _noncyclic

src/cmd/go/testdata/importcycle/src/selfimport/selfimport.go

@@ -0,0 +1,6 @@
+package selfimport
+
+import (
+	"_noncyclic"
+	"selfimport"
+)