go/printer, gofmt: fine tuning of line spacing

- no empty lines inside empty structs and interfaces
- top-level declarations are separated by a blank line if
  a) they are of different kind (e.g. const vs type); or
  b) there are documentation comments associated with a
     declaration (this is new)
- applied gofmt -w misc src

The actual changes are in go/printer/nodes.go:397-400 (empty structs/interfaces),
and go/printer/printer.go:307-309 (extra line break). The remaining
changes are cleanups w/o changing the existing functionality.

Fixes issue  2570.

R=rsc
CC=golang-dev
https://golang.org/cl/5493057

src/cmd/gofix/testdata/reflect.type.go.in

@@ -150,6 +150,7 @@ func userType(rt reflect.Type) *userTypeInfo {
 	}
 	return ut
 }
+
 // A typeId represents a gob Type as an integer that can be passed on the wire.
 // Internally, typeIds are used as keys to a map to recover the underlying type info.
 type typeId int32

src/cmd/gofix/testdata/reflect.type.go.out

@@ -150,6 +150,7 @@ func userType(rt reflect.Type) *userTypeInfo {
 	}
 	return ut
 }
+
 // A typeId represents a gob Type as an integer that can be passed on the wire.
 // Internally, typeIds are used as keys to a map to recover the underlying type info.
 type typeId int32

src/pkg/crypto/openpgp/keys.go

@@ -16,6 +16,7 @@ import (
 
 // PublicKeyType is the armor type for a PGP public key.
 var PublicKeyType = "PGP PUBLIC KEY BLOCK"
+
 // PrivateKeyType is the armor type for a PGP private key.
 var PrivateKeyType = "PGP PRIVATE KEY BLOCK"
 

src/pkg/crypto/x509/x509.go

@@ -981,6 +981,7 @@ func CreateCertificate(rand io.Reader, template, parent *Certificate, pub *rsa.P
 // pemCRLPrefix is the magic string that indicates that we have a PEM encoded
 // CRL.
 var pemCRLPrefix = []byte("-----BEGIN X509 CRL")
+
 // pemType is the type of a PEM encoded CRL.
 var pemType = "X509 CRL"
 

src/pkg/encoding/gob/codec_test.go

@@ -102,12 +102,15 @@ func TestIntCodec(t *testing.T) {
 
 // The result of encoding a true boolean with field number 7
 var boolResult = []byte{0x07, 0x01}
+
 // The result of encoding a number 17 with field number 7
 var signedResult = []byte{0x07, 2 * 17}
 var unsignedResult = []byte{0x07, 17}
 var floatResult = []byte{0x07, 0xFE, 0x31, 0x40}
+
 // The result of encoding a number 17+19i with field number 7
 var complexResult = []byte{0x07, 0xFE, 0x31, 0x40, 0xFE, 0x33, 0x40}
+
 // The result of encoding "hello" with field number 7
 var bytesResult = []byte{0x07, 0x05, 'h', 'e', 'l', 'l', 'o'}
 

src/pkg/encoding/gob/type.go

@@ -130,6 +130,7 @@ func userType(rt reflect.Type) *userTypeInfo {
 	}
 	return ut
 }
+
 // A typeId represents a gob Type as an integer that can be passed on the wire.
 // Internally, typeIds are used as keys to a map to recover the underlying type info.
 type typeId int32

src/pkg/exp/ssh/tcpip.go

@@ -10,6 +10,7 @@ import (
 	"io"
 	"net"
 )
+
 // Dial initiates a connection to the addr from the remote host.
 // addr is resolved using net.ResolveTCPAddr before connection. 
 // This could allow an observer to observe the DNS name of the 

src/pkg/go/printer/nodes.go

@@ -364,9 +364,10 @@ func (p *printer) fieldList(fields *ast.FieldList, isStruct, isIncomplete bool)
 	lbrace := fields.Opening
 	list := fields.List
 	rbrace := fields.Closing
+	hasComments := isIncomplete || p.commentBefore(p.fset.Position(rbrace))
 	srcIsOneLine := lbrace.IsValid() && rbrace.IsValid() && p.fset.Position(lbrace).Line == p.fset.Position(rbrace).Line
 
-	if !isIncomplete && !p.commentBefore(p.fset.Position(rbrace)) && srcIsOneLine {
+	if !hasComments && srcIsOneLine {
 		// possibly a one-line struct/interface
 		if len(list) == 0 {
 			// no blank between keyword and {} in this case
@@ -391,9 +392,13 @@ func (p *printer) fieldList(fields *ast.FieldList, isStruct, isIncomplete bool)
 			return
 		}
 	}
+	// hasComments || !srcIsOneLine
+
+	p.print(blank, lbrace, token.LBRACE, indent)
+	if hasComments || len(list) > 0 {
+		p.print(formfeed)
+	}
 
-	// at least one entry or incomplete
-	p.print(blank, lbrace, token.LBRACE, indent, formfeed)
 	if isStruct {
 
 		sep := vtab
@@ -1512,9 +1517,14 @@ func (p *printer) file(src *ast.File) {
 			prev := tok
 			tok = declToken(d)
 			// if the declaration token changed (e.g., from CONST to TYPE)
+			// or the next declaration has documentation associated with it,
 			// print an empty line between top-level declarations
+			// (because p.linebreak is called with the position of d, which
+			// is past any documentation, the minimum requirement is satisfied
+			// even w/o the extra getDoc(d) nil-check - leave it in case the
+			// linebreak logic improves - there's already a TODO).
 			min := 1
-			if prev != tok {
+			if prev != tok || getDoc(d) != nil {
 				min = 2
 			}
 			p.linebreak(p.fset.Position(d.Pos()).Line, min, ignore, false)

src/pkg/go/printer/printer.go

@@ -257,6 +257,7 @@ func (p *printer) writeCommentPrefix(pos, next token.Position, prev, comment *as
 	} else {
 		// comment on a different line:
 		// separate with at least one line break
+		droppedLinebreak := false
 		if prev == nil {
 			// first comment of a comment group
 			j := 0
@@ -282,6 +283,7 @@ func (p *printer) writeCommentPrefix(pos, next token.Position, prev, comment *as
 				case newline, formfeed:
 					// TODO(gri): may want to keep formfeed info in some cases
 					p.wsbuf[i] = ignore
+					droppedLinebreak = true
 				}
 				j = i
 				break
@@ -289,25 +291,41 @@ func (p *printer) writeCommentPrefix(pos, next token.Position, prev, comment *as
 			p.writeWhitespace(j)
 		}
 
-		// turn off indent if we're about to print a line directive.
-		indent := p.indent
-		if strings.HasPrefix(comment.Text, linePrefix) {
-			p.indent = 0
+		// determine number of linebreaks before the comment
+		n := 0
+		if pos.IsValid() && p.last.IsValid() {
+			n = pos.Line - p.last.Line
+			if n < 0 { // should never happen
+				n = 0
+			}
 		}
 
-		// use formfeeds to break columns before a comment;
-		// this is analogous to using formfeeds to separate
-		// individual lines of /*-style comments - but make
-		// sure there is at least one line break if the previous
-		// comment was a line comment
-		n := pos.Line - p.last.Line // if !pos.IsValid(), pos.Line == 0, and n will be 0
-		if n <= 0 && prev != nil && prev.Text[1] == '/' {
+		// at the package scope level only (p.indent == 0),
+		// add an extra newline if we dropped one before:
+		// this preserves a blank line before documentation
+		// comments at the package scope level (issue 2570)
+		if p.indent == 0 && droppedLinebreak {
+			n++
+		}
+
+		// make sure there is at least one line break
+		// if the previous comment was a line comment
+		if n == 0 && prev != nil && prev.Text[1] == '/' {
 			n = 1
 		}
+
 		if n > 0 {
+			// turn off indent if we're about to print a line directive
+			indent := p.indent
+			if strings.HasPrefix(comment.Text, linePrefix) {
+				p.indent = 0
+			}
+			// use formfeeds to break columns before a comment;
+			// this is analogous to using formfeeds to separate
+			// individual lines of /*-style comments
 			p.writeByteN('\f', nlimit(n))
+			p.indent = indent // restore indent
 		}
-		p.indent = indent
 	}
 }
 
@@ -812,7 +830,8 @@ func (p *printer) flush(next token.Position, tok token.Token) (wroteNewline, dro
 // getNode returns the ast.CommentGroup associated with n, if any.
 func getDoc(n ast.Node) *ast.CommentGroup {
 	switch n := n.(type) {
-	// *ast.Fields cannot be printed separately - ignore for now
+	case *ast.Field:
+		return n.Doc
 	case *ast.ImportSpec:
 		return n.Doc
 	case *ast.ValueSpec:

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

@@ -106,7 +106,7 @@ type S3 struct {
 var x int	// x
 var ()
 
-// This comment SHOULD be associated with the next declaration.
+// This comment SHOULD be associated with f0.
 func f0() {
 	const pi = 3.14	// pi
 	var s1 struct{}	/* an empty struct */	/* foo */
@@ -115,8 +115,9 @@ func f0() {
 	var s2 struct{} = struct{}{}
 	x := pi
 }
+
 //
-// NO SPACE HERE
+// This comment should be associated with f1, with one blank line before the comment.
 //
 func f1() {
 	f0()

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

@@ -107,7 +107,7 @@ var x int  // x
 var ()
 
 
-// This comment SHOULD be associated with the next declaration.
+// This comment SHOULD be associated with f0.
 func f0() {
 	const pi = 3.14  // pi
 	var s1 struct {}  /* an empty struct */ /* foo */
@@ -117,7 +117,7 @@ func f0() {
 	x := pi
 }
 //
-// NO SPACE HERE
+// This comment should be associated with f1, with one blank line before the comment.
 //
 func f1() {
 	f0()
@@ -130,7 +130,7 @@ func f1() {
 
 
 func _() {
-	// this comment should be properly indented
+// this comment should be properly indented
 }
 
 

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

@@ -115,6 +115,18 @@ import _ "io"
 
 var _ int
 
+// at least one empty line between declarations of the same kind
+// if there is associated documentation (was issue 2570)
+type T1 struct{}
+
+// T2 comment
+type T2 struct {
+}	// should be a two-line struct
+
+// T3 comment
+type T2 struct {
+}	// should be a two-line struct
+
 // printing of constant literals
 const (
 	_	= "foobar"
@@ -286,6 +298,15 @@ type _ struct {
 	}
 }
 
+// no blank lines in empty structs and interfaces, but leave 1- or 2-line layout alone
+type _ struct{}
+type _ struct {
+}
+
+type _ interface{}
+type _ interface {
+}
+
 // no tabs for single or ungrouped decls
 func _() {
 	const xxxxxx = 0

src/pkg/go/printer/testdata/declarations.input

@@ -115,6 +115,20 @@ import (
 import _ "io"
 var _ int
 
+// at least one empty line between declarations of the same kind
+// if there is associated documentation (was issue 2570)
+type T1 struct{}
+// T2 comment
+type T2 struct {
+} // should be a two-line struct
+
+
+// T3 comment
+type T2 struct {
+
+
+} // should be a two-line struct
+
 
 // printing of constant literals
 const (
@@ -293,6 +307,18 @@ type _ struct {
 }
 
 
+// no blank lines in empty structs and interfaces, but leave 1- or 2-line layout alone
+type _ struct{            }
+type _ struct {
+
+}
+
+type _ interface{            }
+type _ interface {
+
+}
+
+
 // no tabs for single or ungrouped decls
 func _() {
 	const xxxxxx = 0

src/pkg/log/syslog/syslog.go

@@ -92,11 +92,13 @@ func (w *Writer) Emerg(m string) (err error) {
 	_, err = w.writeString(LOG_EMERG, m)
 	return err
 }
+
 // Crit logs a message using the LOG_CRIT priority.
 func (w *Writer) Crit(m string) (err error) {
 	_, err = w.writeString(LOG_CRIT, m)
 	return err
 }
+
 // ERR logs a message using the LOG_ERR priority.
 func (w *Writer) Err(m string) (err error) {
 	_, err = w.writeString(LOG_ERR, m)
@@ -114,11 +116,13 @@ func (w *Writer) Notice(m string) (err error) {
 	_, err = w.writeString(LOG_NOTICE, m)
 	return err
 }
+
 // Info logs a message using the LOG_INFO priority.
 func (w *Writer) Info(m string) (err error) {
 	_, err = w.writeString(LOG_INFO, m)
 	return err
 }
+
 // Debug logs a message using the LOG_DEBUG priority.
 func (w *Writer) Debug(m string) (err error) {
 	_, err = w.writeString(LOG_DEBUG, m)

src/pkg/math/all_test.go

@@ -22,6 +22,7 @@ var vf = []float64{
 	1.8253080916808550e+00,
 	-8.6859247685756013e+00,
 }
+
 // The expected results below were computed by the high precision calculators
 // at http://keisan.casio.com/.  More exact input values (array vf[], above)
 // were obtained by printing them with "%.26f".  The answers were calculated
@@ -159,6 +160,7 @@ var cos = []float64{
 	-2.517729313893103197176091e-01,
 	-7.39241351595676573201918e-01,
 }
+
 // Results for 100000 * Pi + vf[i]
 var cosLarge = []float64{
 	2.634752141185559426744e-01,
@@ -514,6 +516,7 @@ var sin = []float64{
 	9.6778633541687993721617774e-01,
 	-6.734405869050344734943028e-01,
 }
+
 // Results for 100000 * Pi + vf[i]
 var sinLarge = []float64{
 	-9.646661658548936063912e-01,
@@ -563,6 +566,7 @@ var tan = []float64{
 	-3.843885560201130679995041e+00,
 	9.10988793377685105753416e-01,
 }
+
 // Results for 100000 * Pi + vf[i]
 var tanLarge = []float64{
 	-3.66131656475596512705e+00,

src/pkg/math/sin.go

@@ -98,6 +98,7 @@ var _sin = [...]float64{
 	8.33333333332211858878E-3,  // 0x3f8111111110f7d0
 	-1.66666666666666307295E-1, // 0xbfc5555555555548
 }
+
 // cos coefficients
 var _cos = [...]float64{
 	-1.13585365213876817300E-11, // 0xbda8fa49a0861a9b

src/pkg/net/ipraw_test.go

@@ -59,6 +59,7 @@ func parsePingReply(p []byte) (id, seq int) {
 }
 
 var srchost = flag.String("srchost", "", "Source of the ICMP ECHO request")
+
 // 127.0.0.1 because this is an IPv4-specific test.
 var dsthost = flag.String("dsthost", "127.0.0.1", "Destination for the ICMP ECHO request")
 

src/pkg/sort/sort.go

@@ -240,14 +240,18 @@ func (p StringSlice) Sort() { Sort(p) }
 
 // Ints sorts a slice of ints in increasing order.
 func Ints(a []int) { Sort(IntSlice(a)) }
+
 // Float64s sorts a slice of float64s in increasing order.
 func Float64s(a []float64) { Sort(Float64Slice(a)) }
+
 // Strings sorts a slice of strings in increasing order.
 func Strings(a []string) { Sort(StringSlice(a)) }
 
 // IntsAreSorted tests whether a slice of ints is sorted in increasing order.
 func IntsAreSorted(a []int) bool { return IsSorted(IntSlice(a)) }
+
 // Float64sAreSorted tests whether a slice of float64s is sorted in increasing order.
 func Float64sAreSorted(a []float64) bool { return IsSorted(Float64Slice(a)) }
+
 // StringsAreSorted tests whether a slice of strings is sorted in increasing order.
 func StringsAreSorted(a []string) bool { return IsSorted(StringSlice(a)) }