improved comment formatting:

- print comments line by line, strip common prefix but do not
  modify comment contents otherwise
- align comments with subsequent keyword if indicated (e.g. case labels)
- terminate "column section" after multi-line expressions for better alignment

R=rsc
http://go/go-review/1017002

src/pkg/go/printer/testdata/comments.go

@@ -124,7 +124,94 @@ func typeswitch(x interface{}) {
 	default:
 		// this comment should be indented
 	}
-	// this comment should be indented
+	// this comment should not be indented
+}
+
+func _() {
+	/* freestanding comment
+	   aligned		line
+	   aligned line
+	*/
+}
+
+func _() {
+	/* freestanding comment
+	   aligned		line
+	   aligned line
+	   */
+}
+
+func _() {
+	/* freestanding comment
+	   aligned		line
+	   aligned line */
+}
+
+func _() {
+	/*	freestanding comment
+		aligned		line
+		aligned line
+	*/
+}
+
+func _() {
+	/*	freestanding comment
+		aligned		line
+		aligned line
+		*/
+}
+
+func _() {
+	/*	freestanding comment
+		aligned		line
+		aligned line */
+}
+
+
+/*
+ * line
+ * of
+ * stars
+ */
+
+/* another line
+ * of
+ * stars */
+
+/*	and another line
+ *	of
+ *	stars */
+
+/*
+aligned in middle
+here
+        not here
+*/
+
+/*
+blank line in middle:
+
+with no leading spaces on blank line.
+*/
+
+func _() {
+	/*
+	 * line
+	 * of
+	 * stars
+	 */
+
+	/*
+	aligned in middle
+	here
+		not here
+	*/
+
+	/*
+	blank line in middle:
+
+	with no leading spaces on blank line.
+*/
 }
 
 

src/pkg/go/printer/testdata/comments.golden

@@ -122,7 +122,94 @@ func typeswitch(x interface{}) {
 	default:
 		// this comment should be indented
 	}
-	// this comment should be indented
+	// this comment should not be indented
+}
+
+func _() {
+	/* freestanding comment
+	   aligned		line
+	   aligned line
+	*/
+}
+
+func _() {
+	/* freestanding comment
+	   aligned		line
+	   aligned line
+	*/
+}
+
+func _() {
+	/* freestanding comment
+	   aligned		line
+	   aligned line */
+}
+
+func _() {
+	/*	freestanding comment
+		aligned		line
+		aligned line
+	*/
+}
+
+func _() {
+	/*	freestanding comment
+		aligned		line
+		aligned line
+	*/
+}
+
+func _() {
+	/*	freestanding comment
+		aligned		line
+		aligned line */
+}
+
+
+/*
+ * line
+ * of
+ * stars
+ */
+
+/* another line
+ * of
+ * stars */
+
+/*	and another line
+ *	of
+ *	stars */
+
+/*
+aligned in middle
+here
+        not here
+*/
+
+/*
+blank line in middle:
+
+with no leading spaces on blank line.
+*/
+
+func _() {
+	/*
+	 * line
+	 * of
+	 * stars
+	 */
+
+	/*
+	aligned in middle
+	here
+		not here
+	*/
+
+	/*
+	blank line in middle:
+
+	with no leading spaces on blank line.
+	*/
 }
 
 

src/pkg/go/printer/testdata/expressions.go

@@ -159,12 +159,24 @@ func _() {
 	c;
 	_ = a < b ||
 		b < a;
-	_ = "1234567890"
-		"1234567890";
-	// this comment should be indented
+	_ = "933262154439441526816992388562667004907159682643816214685929"
+	"638952175999932299156089414639761565182862536979208272237582"
+	"51185210916864000000000000000000000000";  // 100!
+	_ = "170141183460469231731687303715884105727";  // prime
 }
 
 
+// Alignment after overlong lines
+const (
+	_ = "991";
+	_ = "2432902008176640000";  // 20!
+	_ = "933262154439441526816992388562667004907159682643816214685929"
+	"638952175999932299156089414639761565182862536979208272237582"
+	"51185210916864000000000000000000000000";  // 100!
+	_ = "170141183460469231731687303715884105727";  // prime
+)
+
+
 func same(t, u *Time) bool {
 	// respect source lines in multi-line expressions
 	return t.Year == u.Year

src/pkg/go/printer/testdata/expressions.golden

@@ -151,12 +151,24 @@ func _() {
 		c;
 	_ = a < b ||
 		b < a;
-	_ = "1234567890"
-	"1234567890";
-	// this comment should be indented
+	_ = "933262154439441526816992388562667004907159682643816214685929"
+	"638952175999932299156089414639761565182862536979208272237582"
+	"51185210916864000000000000000000000000";	// 100!
+	_ = "170141183460469231731687303715884105727";	// prime
 }
 
 
+// Alignment after overlong lines
+const (
+	_	= "991";
+	_	= "2432902008176640000";	// 20!
+	_	= "933262154439441526816992388562667004907159682643816214685929"
+	"638952175999932299156089414639761565182862536979208272237582"
+	"51185210916864000000000000000000000000";	// 100!
+	_	= "170141183460469231731687303715884105727";	// prime
+)
+
+
 func same(t, u *Time) bool {
 	// respect source lines in multi-line expressions
 	return t.Year == u.Year &&

src/pkg/go/printer/testdata/expressions.raw

@@ -151,12 +151,24 @@ func _() {
 		c;
 	_ = a < b ||
 		b < a;
-	_ = "1234567890"
-	"1234567890";
-	// this comment should be indented
+	_ = "933262154439441526816992388562667004907159682643816214685929"
+	"638952175999932299156089414639761565182862536979208272237582"
+	"51185210916864000000000000000000000000";	// 100!
+	_ = "170141183460469231731687303715884105727";	// prime
 }
 
 
+// Alignment after overlong lines
+const (
+	_		= "991";
+	_		= "2432902008176640000";	// 20!
+	_		= "933262154439441526816992388562667004907159682643816214685929"
+	"638952175999932299156089414639761565182862536979208272237582"
+	"51185210916864000000000000000000000000";	// 100!
+	_		= "170141183460469231731687303715884105727";	// prime
+)
+
+
 func same(t, u *Time) bool {
 	// respect source lines in multi-line expressions
 	return t.Year == u.Year &&

src/pkg/go/printer/testdata/statements.go

@@ -62,6 +62,32 @@ func _() {
 	case 1:  // this comment should have no effect on the previous or next line
 		use(x);
 	}
+
+	switch x := 0; x {
+	case 1:
+		x = 0;
+		// this comment should be indented
+	case 2:
+		x = 0;
+	// this comment should not be indented, it is aligned with the next case
+	case 3:
+		x = 0;
+		/* indented comment
+		   aligned
+		   aligned
+		*/
+		// bla
+		/* and more */
+	case 4:
+		x = 0;
+	/* not indented comment
+	   aligned
+	   aligned
+	*/
+	// bla
+	/* and more */
+	case 5:
+	}
 }
 
 

src/pkg/go/printer/testdata/statements.golden

@@ -69,6 +69,32 @@ func _() {
 	case 1:	// this comment should have no effect on the previous or next line
 		use(x);
 	}
+
+	switch x := 0; x {
+	case 1:
+		x = 0;
+		// this comment should be indented
+	case 2:
+		x = 0;
+	// this comment should not be indented, it is aligned with the next case
+	case 3:
+		x = 0;
+		/* indented comment
+		   aligned
+		   aligned
+		*/
+		// bla
+		/* and more */
+	case 4:
+		x = 0;
+	/* not indented comment
+	   aligned
+	   aligned
+	*/
+	// bla
+	/* and more */
+	case 5:
+	}
 }