go/printer: forbid empty line before first comment in block

To improve readability when exported fields are removed,
forbid the printer from emitting an empty line before the first comment
in a const, var, or type block.
Also, when printing the "Has filtered or unexported fields." message,
add an empty line before it to separate the message from the struct
or interfact contents.

Before the change:
<<<
type NamedArg struct {

        // Name is the name of the parameter placeholder.
        //
        // If empty, the ordinal position in the argument list will be
        // used.
        //
        // Name must omit any symbol prefix.
        Name string

        // Value is the value of the parameter.
        // It may be assigned the same value types as the query
        // arguments.
        Value interface{}
        // contains filtered or unexported fields
}
>>>

After the change:
<<<
type NamedArg struct {
        // Name is the name of the parameter placeholder.
        //
        // If empty, the ordinal position in the argument list will be
        // used.
        //
        // Name must omit any symbol prefix.
        Name string

        // Value is the value of the parameter.
        // It may be assigned the same value types as the query
        // arguments.
        Value interface{}

        // contains filtered or unexported fields
}
>>>

Fixes #18264

Change-Id: I9fe17ca39cf92fcdfea55064bd2eaa784ce48c88
Reviewed-on: https://go-review.googlesource.com/71990
Run-TryBot: Joe Tsai <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>

Author: dsnet

misc/cgo/test/buildid_linux.go

@@ -39,7 +39,6 @@ func testBuildID(t *testing.T) {
 		}
 
 		for len(d) > 0 {
-
 			// ELF standards differ as to the sizes in
 			// note sections.  Both the GNU linker and
 			// gold always generate 32-bit sizes, so that

src/cmd/compile/internal/ssa/gen/rulegen.go

@@ -659,7 +659,6 @@ func extract(val string) (op string, typ string, auxint string, aux string, args
 // It returns the op and unparsed strings for typ, auxint, and aux restrictions and for all args.
 // oparch is the architecture that op is located in, or "" for generic.
 func parseValue(val string, arch arch, loc string) (op opData, oparch string, typ string, auxint string, aux string, args []string) {
-
 	// Resolve the op.
 	var s string
 	s, typ, auxint, aux, args = extract(val)

src/cmd/internal/obj/ppc64/asm9.go

@@ -2468,7 +2468,6 @@ func (c *ctxt9) asmout(p *obj.Prog, o *Optab, out []uint32) {
 		d := c.vregoff(p.GetFrom3())
 		var a int
 		switch p.As {
-
 		// These opcodes expect a mask operand that has to be converted into the
 		// appropriate operand.  The way these were defined, not all valid masks are possible.
 		// Left here for compatibility in case they were used or generated.

src/cmd/internal/obj/x86/asm6.go

@@ -1996,7 +1996,6 @@ func span6(ctxt *obj.Link, s *obj.LSym, newprog obj.ProgAlloc) {
 		c = 0
 		for p := s.Func.Text; p != nil; p = p.Link {
 			if ctxt.Headtype == objabi.Hnacl && p.Isize > 0 {
-
 				// pad everything to avoid crossing 32-byte boundary
 				if c>>5 != (c+int32(p.Isize)-1)>>5 {
 					c = naclpad(ctxt, s, c, -c&31)

src/cmd/link/internal/ld/data.go

@@ -1857,7 +1857,6 @@ func assignAddress(ctxt *Link, sect *sym.Section, n int, s *sym.Symbol, va uint6
 	// Only break at outermost syms.
 
 	if ctxt.Arch.InFamily(sys.PPC64) && s.Outer == nil && ctxt.IsELF && ctxt.LinkMode == LinkExternal && va-sect.Vaddr+funcsize+maxSizeTrampolinesPPC64(s, isTramp) > 0x1c00000 {
-
 		// Set the length for the previous text section
 		sect.Length = va - sect.Vaddr
 

src/cmd/link/internal/ppc64/asm.go

@@ -523,7 +523,6 @@ func archrelocaddr(ctxt *ld.Link, r *sym.Reloc, s *sym.Symbol, val *int64) bool
 
 // resolve direct jump relocation r in s, and add trampoline if necessary
 func trampoline(ctxt *ld.Link, r *sym.Reloc, s *sym.Symbol) {
-
 	// Trampolines are created if the branch offset is too large and the linker cannot insert a call stub to handle it.
 	// For internal linking, trampolines are always created for long calls.
 	// For external linking, the linker can insert a call stub to handle a long call, but depends on having the TOC address in
@@ -542,7 +541,6 @@ func trampoline(ctxt *ld.Link, r *sym.Reloc, s *sym.Symbol) {
 		if (ctxt.LinkMode == ld.LinkExternal && s.Sect != r.Sym.Sect) || (ctxt.LinkMode == ld.LinkInternal && int64(int32(t<<6)>>6) != t) || (*ld.FlagDebugTramp > 1 && s.File != r.Sym.File) {
 			var tramp *sym.Symbol
 			for i := 0; ; i++ {
-
 				// Using r.Add as part of the name is significant in functions like duffzero where the call
 				// target is at some offset within the function.  Calls to duff+8 and duff+256 must appear as
 				// distinct trampolines.

src/encoding/base32/base32.go

@@ -290,7 +290,6 @@ func (enc *Encoding) decode(dst, src []byte) (n int, end bool, err error) {
 		dlen := 8
 
 		for j := 0; j < 8; {
-
 			// We have reached the end and are missing padding
 			if len(src) == 0 && enc.padChar != NoPadding {
 				return n, false, CorruptInputError(olen - len(src) - j)

src/go/doc/testdata/blank.0.golden

@@ -48,6 +48,7 @@ TYPES
 		H	uint32
 
 		A	uint8
+	
 		// contains filtered or unexported fields
 	}
 

src/go/doc/testdata/blank.2.golden

@@ -48,6 +48,7 @@ TYPES
 		H	uint32
 
 		A	uint8
+	
 		// contains filtered or unexported fields
 	}
 

src/go/doc/testdata/testing.0.golden

@@ -42,6 +42,7 @@ TYPES
 	// B is a type passed to Benchmark functions to manage benchmark ...
 	type B struct {
 		N int
+	
 		// contains filtered or unexported fields
 	}
 

src/go/doc/testdata/testing.2.golden

@@ -42,6 +42,7 @@ TYPES
 	// B is a type passed to Benchmark functions to manage benchmark ...
 	type B struct {
 		N int
+	
 		// contains filtered or unexported fields
 	}
 

src/go/printer/nodes.go

@@ -479,6 +479,7 @@ func (p *printer) fieldList(fields *ast.FieldList, isStruct, isIncomplete bool)
 		if isIncomplete {
 			if len(list) > 0 {
 				p.print(formfeed)
+				p.print(newline)
 			}
 			p.flush(p.posFor(rbrace), token.RBRACE) // make sure we don't lose the last line comment
 			p.setLineComment("// contains filtered or unexported fields")
@@ -506,6 +507,7 @@ func (p *printer) fieldList(fields *ast.FieldList, isStruct, isIncomplete bool)
 		if isIncomplete {
 			if len(list) > 0 {
 				p.print(formfeed)
+				p.print(newline)
 			}
 			p.flush(p.posFor(rbrace), token.RBRACE) // make sure we don't lose the last line comment
 			p.setLineComment("// contains filtered or unexported methods")

src/go/printer/printer.go

@@ -62,7 +62,7 @@ type printer struct {
 	mode        pmode        // current printer mode
 	impliedSemi bool         // if set, a linebreak implies a semicolon
 	lastTok     token.Token  // last token printed (token.ILLEGAL if it's whitespace)
-	prevOpen    token.Token  // previous non-brace "open" token (, [, or token.ILLEGAL
+	prevOpen    token.Token  // previous "open" token: (, [, {, or token.ILLEGAL
 	wsbuf       []whiteSpace // delayed white space
 
 	// Positions
@@ -426,6 +426,11 @@ func (p *printer) writeCommentPrefix(pos, next token.Position, prev *ast.Comment
 		n := 0
 		if pos.IsValid() && p.last.IsValid() {
 			n = pos.Line - p.last.Line
+			if n > 1 && p.prevOpen != token.ILLEGAL && prev == nil {
+				// Forbid multiple empty lines from appearing immediately
+				// following some type of open paren, bracket, or brace.
+				n = 1
+			}
 			if n < 0 { // should never happen
 				n = 0
 			}
@@ -879,7 +884,7 @@ func (p *printer) print(args ...interface{}) {
 		switch p.lastTok {
 		case token.ILLEGAL:
 			// ignore (white space)
-		case token.LPAREN, token.LBRACK:
+		case token.LPAREN, token.LBRACK, token.LBRACE:
 			p.prevOpen = p.lastTok
 		default:
 			// other tokens followed any opening token

src/go/printer/testdata/comments.golden

@@ -97,6 +97,109 @@ type S3 struct {
 	f3	int	// f3 is not exported
 }
 
+type T struct {	// foo
+
+	// bar
+	x	int
+	X	int
+}
+
+type T struct {	/* foo */
+
+	// bar
+	x int
+}
+
+type T struct {	// foo
+
+	x	int
+
+	X	int
+}
+
+type T struct {	/* foo */
+
+	x int
+}
+
+type T struct {
+	x int
+}
+
+type T struct {
+	x int
+}
+
+type T struct{ x int }
+
+type T struct {
+	x int
+}
+
+type T struct {
+	x	int
+	X	int
+	x	int
+}
+
+type T struct {
+	// Comment
+	x	int
+	// X.
+	X	int
+	// Comment
+	x	int
+}
+
+const (	// foo
+
+	// bar
+	x	int	= 0
+	X	int	= 0
+)
+
+const (	/* foo */
+
+	// bar
+	x int = 0
+)
+
+const (	// foo
+
+	x	int	= 0
+
+	X	int	= 0
+)
+
+const (	/* foo */
+
+	x int = 0
+)
+
+const (
+	x int = 0
+)
+
+const (
+	x	int	= 0
+	X	int	= 0
+)
+
+const (
+	x	int	= 0
+	X	int	= 0
+	x	int	= 0
+)
+
+const (
+	// Comment
+	x	int	= 0
+	// X.
+	X	int	= 0
+	// Comment
+	x	int	= 0
+)
+
 // This comment group should be separated
 // with a newline from the next comment
 // group.

src/go/printer/testdata/comments.input

@@ -97,6 +97,113 @@ type S3 struct {
 	f3 int // f3 is not exported
 }
 
+type T struct { // foo
+
+   // bar
+   x int
+   X int
+}
+
+type T struct { /* foo */
+
+  			 // bar
+   x int
+}
+
+type T struct { // foo
+
+  				 x int
+
+   X int
+}
+
+type T struct { /* foo */
+
+
+
+x int
+}
+
+type T struct {x int
+}
+
+type T struct {
+					x int}
+
+type T struct {x int}
+
+type T struct {
+
+   x int
+}
+
+type T struct {
+   x int
+   X int
+   x int
+}
+
+type T struct {
+   // Comment
+   x int
+   // X.
+   X int
+   // Comment
+   x int
+}
+
+const ( // foo
+
+   // bar
+   x int = 0
+   X int = 0
+)
+
+const ( /* foo */
+
+   // bar
+   x int = 0
+)
+
+const ( // foo
+
+   x int = 0
+
+   X int = 0
+)
+
+const ( /* foo */
+
+   x int = 0
+)
+
+const (
+
+   x int = 0
+)
+
+const (
+
+   x int = 0
+   X int = 0
+
+)
+
+const (
+   x int = 0
+   X int = 0
+   x int = 0
+)
+
+const (
+   // Comment
+   x int = 0
+   // X.
+   X int = 0
+   // Comment
+   x int = 0
+)
+
 // This comment group should be separated
 // with a newline from the next comment
 // group.