gofmt: stability improvement

There are a variety of token pairs that if printed
without separating blank may combine into a different
token sequence. Most of these (except for INT + .)
don't happen at the moment due to the spacing
introduced between expression operands. However, this
will prevent errors should the expression spacing
change.

R=rsc
CC=golang-dev
https://golang.org/cl/2207044

src/pkg/go/printer/nodes.go

@@ -543,7 +543,7 @@ func walkBinary(e *ast.BinaryExpr) (has5, has6 bool, maxProblem int) {
 
 	case *ast.UnaryExpr:
 		switch e.Op.String() + r.Op.String() {
-		case "/*":
+		case "/*", "&&", "&^":
 			maxProblem = 6
 		case "++", "--":
 			if maxProblem < 5 {
@@ -612,11 +612,14 @@ func reduceDepth(depth int) int {
 //	1) If there is a binary operator with a right side unary operand
 //	   that would clash without a space, the cutoff must be (in order):
 //
-//		&^	7
 //		/*	7
+//		&&	7
+//		&^	7
 //		++	6
 //		--	6
 //
+//         (Comparison operators always have spaces around them.)
+//
 //	2) If there is a mix of level 6 and level 5 operators, then the cutoff
 //	   is 6 (use spaces to distinguish precedence) in Normal mode
 //	   and 5 (never use spaces) in Compact mode.

src/pkg/go/printer/printer.go

@@ -746,6 +746,26 @@ func (p *printer) writeWhitespace(n int) {
 // ----------------------------------------------------------------------------
 // Printing interface
 
+
+func mayCombine(prev token.Token, next byte) (b bool) {
+	switch prev {
+	case token.INT:
+		b = next == '.' // 1.
+	case token.ADD:
+		b = next == '+' // ++
+	case token.SUB:
+		b = next == '-' // --
+	case token.QUO:
+		b = next == '*' // /*
+	case token.LSS:
+		b = next == '-' || next == '<' // <- or <<
+	case token.AND:
+		b = next == '&' || next == '^' // && or &^
+	}
+	return
+}
+
+
 // print prints a list of "items" (roughly corresponding to syntactic
 // tokens, but also including whitespace and formatting information).
 // It is the only print function that should be called directly from
@@ -803,8 +823,13 @@ func (p *printer) print(args ...interface{}) {
 			tok = x.Kind
 		case token.Token:
 			s := x.String()
-			if p.lastTok == token.INT && s[0] == '.' {
-				// separate int with blank from '.' so it doesn't become a float
+			if mayCombine(p.lastTok, s[0]) {
+				// the previous and the current token must be
+				// separated by a blank otherwise they combine
+				// into a different incorrect token sequence
+				// (except for token.INT followed by a '.' this
+				// should never happen because it is taken care
+				// of via binary expression formatting)
 				if len(p.buffer) != 0 {
 					p.internalError("whitespace buffer not empty")
 				}

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

@@ -194,6 +194,17 @@ func f(x int, args ...int) {
 	_ = .42.x
 	_ = 42e0.x
 	_ = 42e0.x
+
+	// a blank must remain between the binary operator and the 2nd operand
+	_ = x / *y
+	_ = x < -1
+	_ = x < <-1
+	_ = x + +1
+	_ = x - -1
+	_ = x & &x
+	_ = x & ^x
+
+	_ = f(x / *y, x < -1, x < <-1, x + +1, x - -1, x & &x, x & ^x)
 }
 
 

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

@@ -194,6 +194,17 @@ func f(x int, args ...int) {
 	_ = .42.x
 	_ = 42e0 .x
 	_ = 42e0.x
+
+	// a blank must remain between the binary operator and the 2nd operand
+	_ = x/ *y
+	_ = x< -1
+	_ = x< <-1
+	_ = x+ +1
+	_ = x- -1
+	_ = x& &x
+	_ = x& ^x
+
+	_ = f(x/ *y, x< -1, x< <-1, x+ +1, x- -1, x& &x, x& ^x)
 }
 
 

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

@@ -194,6 +194,17 @@ func f(x int, args ...int) {
 	_ = .42.x
 	_ = 42e0.x
 	_ = 42e0.x
+
+	// a blank must remain between the binary operator and the 2nd operand
+	_ = x / *y
+	_ = x < -1
+	_ = x < <-1
+	_ = x + +1
+	_ = x - -1
+	_ = x & &x
+	_ = x & ^x
+
+	_ = f(x / *y, x < -1, x < <-1, x + +1, x - -1, x & &x, x & ^x)
 }