go/scanner: interpret //line directives sans filename sensibly

A //line directive without a filename now denotes the same
filename as the previous line (as in C).
Previously it denoted the file's directory (!).

Fixes #7765

LGTM=gri
R=gri
CC=golang-codereviews
https://golang.org/cl/86990044

src/pkg/go/scanner/scanner.go

@@ -148,11 +148,17 @@ func (s *Scanner) interpretLineComment(text []byte) {
 		// get filename and line number, if any
 		if i := bytes.LastIndex(text, []byte{':'}); i > 0 {
 			if line, err := strconv.Atoi(string(text[i+1:])); err == nil && line > 0 {
-				// valid //line filename:line comment;
-				filename := filepath.Clean(string(text[len(prefix):i]))
-				if !filepath.IsAbs(filename) {
-					// make filename relative to current directory
-					filename = filepath.Join(s.dir, filename)
+				// valid //line filename:line comment
+				filename := string(bytes.TrimSpace(text[len(prefix):i]))
+				if filename == "" {
+					// assume same file as for previous line
+					filename = s.file.Position(s.file.Pos(s.lineOffset)).Filename
+				} else {
+					filename = filepath.Clean(filename)
+					if !filepath.IsAbs(filename) {
+						// make filename relative to current directory
+						filename = filepath.Join(s.dir, filename)
+					}
 				}
 				// update scanner position
 				s.file.AddLineInfo(s.lineOffset+len(text)+1, filename, line) // +len(text)+1 since comment applies to next line

src/pkg/go/scanner/scanner_test.go

@@ -493,9 +493,10 @@ var segments = []segment{
 	{"\nline3  //line File1.go:100", filepath.Join("dir", "TestLineComments"), 3}, // bad line comment, ignored
 	{"\nline4", filepath.Join("dir", "TestLineComments"), 4},
 	{"\n//line File1.go:100\n  line100", filepath.Join("dir", "File1.go"), 100},
+	{"\n//line :42\n  line1", "dir/File1.go", 42},
 	{"\n//line File2.go:200\n  line200", filepath.Join("dir", "File2.go"), 200},
-	{"\n//line :1\n  line1", "dir", 1},
-	{"\n//line foo:42\n  line42", filepath.Join("dir", "foo"), 42},
+	{"\n//line  \t :123\n  line1", "dir/File2.go", 123},
+	{"\n//line foo\t:42\n  line42", filepath.Join("dir", "foo"), 42},
 	{"\n //line foo:42\n  line44", filepath.Join("dir", "foo"), 44},           // bad line comment, ignored
 	{"\n//line foo 42\n  line46", filepath.Join("dir", "foo"), 46},            // bad line comment, ignored
 	{"\n//line foo:42 extra text\n  line48", filepath.Join("dir", "foo"), 48}, // bad line comment, ignored