Permit omission of hi bound in slices.

R=r, rsc
https://golang.org/cl/157082

src/cmd/cgo/ast.go

@@ -178,6 +178,9 @@ func walk(x interface{}, p *Prog, context string) {
 	case *ast.IndexExpr:
 		walk(&n.X, p, "expr");
 		walk(&n.Index, p, "expr");
+	case *ast.SliceExpr:
+		walk(&n.X, p, "expr");
+		walk(&n.Index, p, "expr");
 		if n.End != nil {
 			walk(&n.End, p, "expr")
 		}

src/pkg/exp/eval/expr.go

@@ -581,20 +581,25 @@ func (a *exprCompiler) compile(x ast.Expr, callCtx bool) *expr {
 		return ei.compileIdent(a.block, a.constant, callCtx, x.Value)
 
 	case *ast.IndexExpr:
-		if x.End != nil {
+		l, r := a.compile(x.X, false), a.compile(x.Index, false);
+		if l == nil || r == nil {
+			return nil
+		}
+		return ei.compileIndexExpr(l, r);
+
+	case *ast.SliceExpr:
+		end := x.End;
+		if end == nil {
+			// TODO: set end to len(x.X)
+			panic("unimplemented")
+		}
 		arr := a.compile(x.X, false);
 		lo := a.compile(x.Index, false);
-			hi := a.compile(x.End, false);
+		hi := a.compile(end, false);
 		if arr == nil || lo == nil || hi == nil {
 			return nil
 		}
 		return ei.compileSliceExpr(arr, lo, hi);
-		}
-		l, r := a.compile(x.X, false), a.compile(x.Index, false);
-		if l == nil || r == nil {
-			return nil
-		}
-		return ei.compileIndexExpr(l, r);
 
 	case *ast.KeyValueExpr:
 		goto notimpl

src/pkg/go/ast/ast.go

@@ -172,10 +172,16 @@ type (
 		Sel	*Ident;	// field selector
 	};
 
-	// An IndexExpr node represents an expression followed by an index or slice.
+	// An IndexExpr node represents an expression followed by an index.
 	IndexExpr	struct {
 		X	Expr;	// expression
-		Index	Expr;	// index expression or beginning of slice range
+		Index	Expr;	// index expression
+	};
+
+	// An SliceExpr node represents an expression followed by slice indices.
+	SliceExpr	struct {
+		X	Expr;	// expression
+		Index	Expr;	// beginning of slice range
 		End	Expr;	// end of slice range; or nil
 	};
 
@@ -305,6 +311,7 @@ func (x *FuncLit) Pos() token.Position		{ return x.Type.Pos() }
 func (x *CompositeLit) Pos() token.Position	{ return x.Type.Pos() }
 func (x *SelectorExpr) Pos() token.Position	{ return x.X.Pos() }
 func (x *IndexExpr) Pos() token.Position	{ return x.X.Pos() }
+func (x *SliceExpr) Pos() token.Position	{ return x.X.Pos() }
 func (x *TypeAssertExpr) Pos() token.Position	{ return x.X.Pos() }
 func (x *CallExpr) Pos() token.Position		{ return x.Fun.Pos() }
 func (x *BinaryExpr) Pos() token.Position	{ return x.X.Pos() }
@@ -323,6 +330,7 @@ func (x *CompositeLit) exprNode()	{}
 func (x *ParenExpr) exprNode()		{}
 func (x *SelectorExpr) exprNode()	{}
 func (x *IndexExpr) exprNode()		{}
+func (x *SliceExpr) exprNode()		{}
 func (x *TypeAssertExpr) exprNode()	{}
 func (x *CallExpr) exprNode()		{}
 func (x *StarExpr) exprNode()		{}

src/pkg/go/ast/walk.go

@@ -129,6 +129,10 @@ func Walk(v Visitor, node interface{}) {
 	case *IndexExpr:
 		Walk(v, n.X);
 		Walk(v, n.Index);
+
+	case *SliceExpr:
+		Walk(v, n.X);
+		Walk(v, n.Index);
 		Walk(v, n.End);
 
 	case *TypeAssertExpr:

src/pkg/go/parser/parser.go

@@ -962,23 +962,28 @@ func (p *parser) parseSelectorOrTypeAssertion(x ast.Expr) ast.Expr {
 }
 
 
-func (p *parser) parseIndex(x ast.Expr) ast.Expr {
+func (p *parser) parseIndexOrSlice(x ast.Expr) ast.Expr {
 	if p.trace {
-		defer un(trace(p, "Index"))
+		defer un(trace(p, "IndexOrSlice"))
 	}
 
 	p.expect(token.LBRACK);
 	p.exprLev++;
-	begin := p.parseExpr();
-	var end ast.Expr;
+	index := p.parseExpr();
 	if p.tok == token.COLON {
 		p.next();
-		end = p.parseExpr();
+		var end ast.Expr;
+		if p.tok != token.RBRACK {
+			end = p.parseExpr()
+		}
+		x = &ast.SliceExpr{x, index, end};
+	} else {
+		x = &ast.IndexExpr{x, index}
 	}
 	p.exprLev--;
 	p.expect(token.RBRACK);
 
-	return &ast.IndexExpr{x, begin, end};
+	return x;
 }
 
 
@@ -1072,6 +1077,7 @@ func (p *parser) checkExpr(x ast.Expr) ast.Expr {
 	case *ast.ParenExpr:
 	case *ast.SelectorExpr:
 	case *ast.IndexExpr:
+	case *ast.SliceExpr:
 	case *ast.TypeAssertExpr:
 		if t.Type == nil {
 			// the form X.(type) is only allowed in type switch expressions
@@ -1168,7 +1174,7 @@ L:	for {
 		case token.PERIOD:
 			x = p.parseSelectorOrTypeAssertion(p.checkExpr(x))
 		case token.LBRACK:
-			x = p.parseIndex(p.checkExpr(x))
+			x = p.parseIndexOrSlice(p.checkExpr(x))
 		case token.LPAREN:
 			x = p.parseCallOrConversion(p.checkExprOrType(x))
 		case token.LBRACE:

src/pkg/go/printer/nodes.go

@@ -665,17 +665,25 @@ func (p *printer) expr1(expr ast.Expr, prec1, depth int, ctxt exprContext, multi
 		p.print(token.RPAREN);
 
 	case *ast.IndexExpr:
+		// 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.End != nil {
-			// blanks around ":" if either side is a binary expression
-			if depth <= 1 && (isBinary(x.Index) || isBinary(x.End)) {
+		p.print(token.RBRACK);
+
+	case *ast.SliceExpr:
+		// 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);
+		// 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)) {
 			p.print(blank, token.COLON, blank)
 		} else {
 			p.print(token.COLON)
 		}
-			p.expr0(x.End, depth+1, multiLine);
+		if x.End != nil {
+			p.expr0(x.End, depth+1, multiLine)
 		}
 		p.print(token.RBRACK);
 

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

@@ -53,6 +53,11 @@ func _() {
 	_ = 1 + 2*3;
 	_ = s[1 : 2*3];
 	_ = s[a : b-c];
+	_ = s[0:];
+	_ = s[a+b];
+	_ = s[a+b:];
+	_ = a[a<<b+1];
+	_ = a[a<<b+1:];
 	_ = s[a+b : len(s)];
 	_ = s[len(s):-a];
 	_ = s[a : len(s)+1];

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

@@ -53,6 +53,11 @@ func _() {
 	_ = 1 + 2*3;
 	_ = s[1 : 2*3];
 	_ = s[a : b-c];
+	_ = s[0:];
+	_ = s[a+b];
+	_ = s[a+b :];
+	_ = a[a<<b+1];
+	_ = a[a<<b+1 :];
 	_ = s[a+b : len(s)];
 	_ = s[len(s) : -a];
 	_ = s[a : len(s)+1];

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

@@ -53,6 +53,11 @@ func _() {
 	_ = 1 + 2*3;
 	_ = s[1 : 2*3];
 	_ = s[a : b-c];
+	_ = s[0:];
+	_ = s[a+b];
+	_ = s[a+b:];
+	_ = a[a<<b+1];
+	_ = a[a<<b+1:];
 	_ = s[a+b : len(s)];
 	_ = s[len(s):-a];
 	_ = s[a : len(s)+1];