go: record location of failed imports for error reporting.

Fixes #2664.

R=rsc
CC=golang-dev, remy
https://golang.org/cl/5593047

src/cmd/go/get.go

@@ -119,7 +119,7 @@ func download(arg string, stk *importStack) {
 		stk.push(p.ImportPath)
 		defer stk.pop()
 		if err := downloadPackage(p); err != nil {
-			errorf("%s", &PackageError{stk.copy(), err.Error()})
+			errorf("%s", &PackageError{ImportStack: stk.copy(), Err: err.Error()})
 			return
 		}
 

src/cmd/go/pkg.go

@@ -59,10 +59,14 @@ type Package struct {
 // A PackageError describes an error loading information about a package.
 type PackageError struct {
 	ImportStack []string // shortest path from package named on command line to this one
+	Pos         string   // position of error
 	Err         string   // the error itself
 }
 
 func (p *PackageError) Error() string {
+	if p.Pos != "" {
+		return strings.Join(p.ImportStack, "\n\timports ") + ": " + p.Pos + ": " + p.Err
+	}
 	return strings.Join(p.ImportStack, "\n\timports ") + ": " + p.Err
 }
 
@@ -360,6 +364,12 @@ Stale:
 		}
 		deps[path] = true
 		p1 := loadPackage(path, stk)
+		if p1.Error != nil {
+			if info.ImportPos != nil && len(info.ImportPos[path]) > 0 {
+				pos := info.ImportPos[path][0]
+				p1.Error.Pos = pos.String()
+			}
+		}
 		imports = append(imports, p1)
 		for _, dep := range p1.Deps {
 			deps[dep] = true

src/pkg/go/build/build_test.go

@@ -71,6 +71,8 @@ func TestBuild(t *testing.T) {
 			t.Errorf("ScanDir(%#q): %v", tt.dir, err)
 			continue
 		}
+		// Don't bother testing import positions.
+		tt.info.ImportPos, tt.info.TestImportPos = info.ImportPos, info.TestImportPos
 		if !reflect.DeepEqual(info, tt.info) {
 			t.Errorf("ScanDir(%#q) = %#v, want %#v\n", tt.dir, info, tt.info)
 			continue

src/pkg/go/build/dir.go

@@ -116,10 +116,11 @@ func envOr(name, def string) string {
 }
 
 type DirInfo struct {
-	Package        string            // Name of package in dir
-	PackageComment *ast.CommentGroup // Package comments from GoFiles
-	ImportPath     string            // Import path of package in dir
-	Imports        []string          // All packages imported by GoFiles
+	Package        string                      // Name of package in dir
+	PackageComment *ast.CommentGroup           // Package comments from GoFiles
+	ImportPath     string                      // Import path of package in dir
+	Imports        []string                    // All packages imported by GoFiles
+	ImportPos      map[string][]token.Position // Source code location of imports
 
 	// Source files
 	GoFiles  []string // .go files in dir (excluding CgoFiles, TestGoFiles, XTestGoFiles)
@@ -134,9 +135,10 @@ type DirInfo struct {
 	CgoLDFLAGS   []string // Cgo LDFLAGS directives
 
 	// Test information
-	TestGoFiles  []string // _test.go files in package
-	XTestGoFiles []string // _test.go files outside package
-	TestImports  []string // All packages imported by (X)TestGoFiles
+	TestGoFiles   []string // _test.go files in package
+	XTestGoFiles  []string // _test.go files outside package
+	TestImports   []string // All packages imported by (X)TestGoFiles
+	TestImportPos map[string][]token.Position
 }
 
 func (d *DirInfo) IsCommand() bool {
@@ -223,8 +225,8 @@ func (ctxt *Context) ScanDir(dir string) (info *DirInfo, err error) {
 
 	var Sfiles []string // files with ".S" (capital S)
 	var di DirInfo
-	imported := make(map[string]bool)
-	testImported := make(map[string]bool)
+	imported := make(map[string][]token.Position)
+	testImported := make(map[string][]token.Position)
 	fset := token.NewFileSet()
 	for _, d := range dirs {
 		if d.IsDir() {
@@ -327,9 +329,9 @@ func (ctxt *Context) ScanDir(dir string) (info *DirInfo, err error) {
 					log.Panicf("%s: parser returned invalid quoted string: <%s>", filename, quoted)
 				}
 				if isTest {
-					testImported[path] = true
+					testImported[path] = append(testImported[path], fset.Position(spec.Pos()))
 				} else {
-					imported[path] = true
+					imported[path] = append(imported[path], fset.Position(spec.Pos()))
 				}
 				if path == "C" {
 					if isTest {
@@ -366,12 +368,14 @@ func (ctxt *Context) ScanDir(dir string) (info *DirInfo, err error) {
 		return nil, fmt.Errorf("%s: no Go source files", dir)
 	}
 	di.Imports = make([]string, len(imported))
+	di.ImportPos = imported
 	i := 0
 	for p := range imported {
 		di.Imports[i] = p
 		i++
 	}
 	di.TestImports = make([]string, len(testImported))
+	di.TestImportPos = testImported
 	i = 0
 	for p := range testImported {
 		di.TestImports[i] = p