gofmt: permit omission of first index in slice expression

R=rsc
CC=golang-dev, r
https://golang.org/cl/2053041

src/pkg/go/ast/ast.go

@@ -196,7 +196,7 @@ type (
 	// An SliceExpr node represents an expression followed by slice indices.
 	SliceExpr struct {
 		X     Expr // expression
-		Index Expr // beginning of slice range
+		Index Expr // beginning of slice range; or nil
 		End   Expr // end of slice range; or nil
 	}
 

src/pkg/go/parser/parser.go

@@ -913,7 +913,10 @@ func (p *parser) parseIndexOrSlice(x ast.Expr) ast.Expr {
 
 	p.expect(token.LBRACK)
 	p.exprLev++
-	index := p.parseExpr()
+	var index ast.Expr
+	if p.tok != token.COLON {
+		index = p.parseExpr()
+	}
 	if p.tok == token.COLON {
 		p.next()
 		var end ast.Expr

src/pkg/go/printer/nodes.go

@@ -826,9 +826,11 @@ func (p *printer) expr1(expr ast.Expr, prec1, depth int, ctxt exprContext, multi
 		// TODO(gri): should treat[] like parentheses and undo one level of depth
 		p.expr1(x.X, token.HighestPrec, 1, 0, multiLine)
 		p.print(token.LBRACK)
-		p.expr0(x.Index, depth+1, multiLine)
+		if x.Index != nil {
+			p.expr0(x.Index, depth+1, multiLine)
+		}
 		// blanks around ":" if both sides exist and either side is a binary expression
-		if depth <= 1 && x.End != nil && (isBinary(x.Index) || isBinary(x.End)) {
+		if depth <= 1 && x.Index != nil && x.End != nil && (isBinary(x.Index) || isBinary(x.End)) {
 			p.print(blank, token.COLON, blank)
 		} else {
 			p.print(token.COLON)

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

@@ -31,6 +31,9 @@ func _() {
 	_ = 1 + a
 	_ = a + 1
 	_ = a + b + 1
+	_ = s[a]
+	_ = s[a:]
+	_ = s[:b]
 	_ = s[1:2]
 	_ = s[a:b]
 	_ = s[0:len(s)]
@@ -56,6 +59,7 @@ func _() {
 	_ = s[a : b-c]
 	_ = s[0:]
 	_ = s[a+b]
+	_ = s[:b-c]
 	_ = s[a+b:]
 	_ = a[a<<b+1]
 	_ = a[a<<b+1:]

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

@@ -31,6 +31,9 @@ func _() {
 	_ = 1+a
 	_ = a+1
 	_ = a+b+1
+	_ = s[a]
+	_ = s[a:]
+	_ = s[:b]
 	_ = s[1:2]
 	_ = s[a:b]
 	_ = s[0:len(s)]
@@ -56,6 +59,7 @@ func _() {
 	_ = s[a : b-c]
 	_ = s[0:]
 	_ = s[a+b]
+	_ = s[: b-c]
 	_ = s[a+b :]
 	_ = a[a<<b+1]
 	_ = a[a<<b+1 :]

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

@@ -31,6 +31,9 @@ func _() {
 	_ = 1 + a
 	_ = a + 1
 	_ = a + b + 1
+	_ = s[a]
+	_ = s[a:]
+	_ = s[:b]
 	_ = s[1:2]
 	_ = s[a:b]
 	_ = s[0:len(s)]
@@ -56,6 +59,7 @@ func _() {
 	_ = s[a : b-c]
 	_ = s[0:]
 	_ = s[a+b]
+	_ = s[:b-c]
 	_ = s[a+b:]
 	_ = a[a<<b+1]
 	_ = a[a<<b+1:]