go/printer: don't emit unnecessary //line directives before empty lines

1) Split atLineBegin into its two components: writing of // line directives
and writing of indentation (no functionality changes).

2) Don't call writeLineDirective at the beginning of a line if we're
writing white space - it's not necessary. This is the bug fix.

3) Move testing of the SourcePos mode out of writeLineDirective and
into the (single) caller. Clearer and more efficient.

(Instead of these 3 changes one could also have simply called the
original atLineBegin with position p.out rather than p.pos. This
would have caused atLineBegin to not write a line directive.
Factoring the code seemed like a cleaner and more direct approach.)

Fixes #5945.

Change-Id: Ia8710806b6d3d4e5044116b142c036a4ab5a1764
Reviewed-on: https://go-review.googlesource.com/44651
Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/printer/printer.go

@@ -69,7 +69,7 @@ type printer struct {
 	// The out position differs from the pos position when the result
 	// formatting differs from the source formatting (in the amount of
 	// white space). If there's a difference and SourcePos is set in
-	// ConfigMode, //line comments are used in the output to restore
+	// ConfigMode, //line directives are used in the output to restore
 	// original source positions for a reader.
 	pos     token.Position // current position in AST (source) space
 	out     token.Position // current position in output space
@@ -203,19 +203,20 @@ func (p *printer) lineFor(pos token.Pos) int {
 	return p.cachedLine
 }
 
-// atLineBegin emits a //line comment if necessary and prints indentation.
-func (p *printer) atLineBegin(pos token.Position) {
-	// write a //line comment if necessary
-	if p.Config.Mode&SourcePos != 0 && pos.IsValid() && (p.out.Line != pos.Line || p.out.Filename != pos.Filename) {
+// writeLineDirective writes a //line directive if necessary.
+func (p *printer) writeLineDirective(pos token.Position) {
+	if pos.IsValid() && (p.out.Line != pos.Line || p.out.Filename != pos.Filename) {
 		p.output = append(p.output, tabwriter.Escape) // protect '\n' in //line from tabwriter interpretation
 		p.output = append(p.output, fmt.Sprintf("//line %s:%d\n", pos.Filename, pos.Line)...)
 		p.output = append(p.output, tabwriter.Escape)
-		// p.out must match the //line comment
+		// p.out must match the //line directive
 		p.out.Filename = pos.Filename
 		p.out.Line = pos.Line
 	}
+}
 
-	// write indentation
+// writeIndent writes indentation.
+func (p *printer) writeIndent() {
 	// use "hard" htabs - indentation columns
 	// must not be discarded by the tabwriter
 	n := p.Config.Indent + p.indent // include base indentation
@@ -230,9 +231,11 @@ func (p *printer) atLineBegin(pos token.Position) {
 }
 
 // writeByte writes ch n times to p.output and updates p.pos.
+// Only used to write formatting (white space) characters.
 func (p *printer) writeByte(ch byte, n int) {
 	if p.out.Column == 1 {
-		p.atLineBegin(p.pos)
+		// no need to write line directives before white space
+		p.writeIndent()
 	}
 
 	for i := 0; i < n; i++ {
@@ -265,13 +268,16 @@ func (p *printer) writeByte(ch byte, n int) {
 //
 func (p *printer) writeString(pos token.Position, s string, isLit bool) {
 	if p.out.Column == 1 {
-		p.atLineBegin(pos)
+		if p.Config.Mode&SourcePos != 0 {
+			p.writeLineDirective(pos)
+		}
+		p.writeIndent()
 	}
 
 	if pos.IsValid() {
 		// update p.pos (if pos is invalid, continue with existing p.pos)
 		// Note: Must do this after handling line beginnings because
-		// atLineBegin updates p.pos if there's indentation, but p.pos
+		// writeIndent updates p.pos if there's indentation, but p.pos
 		// is the position of s.
 		p.pos = pos
 	}
@@ -1237,7 +1243,7 @@ const (
 	RawFormat Mode = 1 << iota // do not use a tabwriter; if set, UseSpaces is ignored
 	TabIndent                  // use tabs for indentation independent of UseSpaces
 	UseSpaces                  // use spaces instead of tabs for alignment
-	SourcePos                  // emit //line comments to preserve original source positions
+	SourcePos                  // emit //line directives to preserve original source positions
 )
 
 // A Config node controls the output of Fprint.

src/go/printer/printer_test.go

@@ -363,7 +363,7 @@ func identCount(f *ast.File) int {
 	return n
 }
 
-// Verify that the SourcePos mode emits correct //line comments
+// Verify that the SourcePos mode emits correct //line directives
 // by testing that position information for matching identifiers
 // is maintained.
 func TestSourcePos(t *testing.T) {
@@ -394,7 +394,7 @@ func (t *t) foo(a, b, c int) int {
 	}
 
 	// parse pretty printed original
-	// (//line comments must be interpreted even w/o parser.ParseComments set)
+	// (//line directives must be interpreted even w/o parser.ParseComments set)
 	f2, err := parser.ParseFile(fset, "", buf.Bytes(), 0)
 	if err != nil {
 		t.Fatalf("%s\n%s", err, buf.Bytes())
@@ -434,6 +434,53 @@ func (t *t) foo(a, b, c int) int {
 	}
 }
 
+// Verify that the SourcePos mode doesn't emit unnecessary //line directives
+// before empty lines.
+func TestIssue5945(t *testing.T) {
+	const orig = `
+package p   // line 2
+func f() {} // line 3
+
+var x, y, z int
+
+
+func g() { // line 8
+}
+`
+
+	const want = `//line src.go:2
+package p
+
+//line src.go:3
+func f() {}
+
+var x, y, z int
+
+//line src.go:8
+func g() {
+}
+`
+
+	// parse original
+	f1, err := parser.ParseFile(fset, "src.go", orig, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// pretty-print original
+	var buf bytes.Buffer
+	err = (&Config{Mode: UseSpaces | SourcePos, Tabwidth: 8}).Fprint(&buf, fset, f1)
+	if err != nil {
+		t.Fatal(err)
+	}
+	got := buf.String()
+
+	// compare original with desired output
+	if got != want {
+		t.Errorf("got:\n%s\nwant:\n%s\n", got, want)
+	}
+}
+
 var decls = []string{
 	`import "fmt"`,
 	"const pi = 3.1415\nconst e = 2.71828\n\nvar x = pi",