exp/eval: Implement x[lo:]

R=golang-dev, gri
CC=golang-dev
https://golang.org/cl/908044

src/pkg/exp/eval/expr.go

@@ -589,14 +589,16 @@ func (a *exprCompiler) compile(x ast.Expr, callCtx bool) *expr {
 		return ei.compileIndexExpr(l, r)
 
 	case *ast.SliceExpr:
-		end := x.End
-		if end == nil {
-			// TODO: set end to len(x.X)
-			panic("unimplemented")
-		}
+		var hi *expr
 		arr := a.compile(x.X, false)
 		lo := a.compile(x.Index, false)
-		hi := a.compile(end, false)
+		if x.End == nil {
+			// End was omitted, so we need to compute len(x.X)
+			ei := &exprInfo{a.compiler, x.Pos()}
+			hi = ei.compileBuiltinCallExpr(a.block, lenType, []*expr{arr})
+		} else {
+			hi = a.compile(x.End, false)
+		}
 		if arr == nil || lo == nil || hi == nil {
 			return nil
 		}

src/pkg/exp/eval/expr_test.go

@@ -85,6 +85,15 @@ var exprTests = []test{
 	RErr("s[-i]", "negative index"),
 	RErr("s[3]", "index 3 exceeds"),
 
+	Val("ai[0:2]", vslice{varray{1, 2}, 2, 2}),
+	Val("ai[0:1]", vslice{varray{1, 2}, 1, 2}),
+	Val("ai[0:]", vslice{varray{1, 2}, 2, 2}),
+	Val("ai[i:]", vslice{varray{2}, 1, 1}),
+
+	Val("sli[0:2]", vslice{varray{1, 2, 3}, 2, 3}),
+	Val("sli[0:i]", vslice{varray{1, 2, 3}, 1, 3}),
+	Val("sli[1:]", vslice{varray{2, 3}, 1, 2}),
+
 	CErr("1(2)", "cannot call"),
 	CErr("fn(1,2)", "too many"),
 	CErr("fn()", "not enough"),
@@ -112,8 +121,14 @@ var exprTests = []test{
 	Val("len(s)", 3),
 	Val("len(ai)", 2),
 	Val("len(&ai)", 2),
+	Val("len(ai[0:])", 2),
+	Val("len(ai[1:])", 1),
+	Val("len(ai[2:])", 0),
 	Val("len(aai)", 2),
 	Val("len(sli)", 2),
+	Val("len(sli[0:])", 2),
+	Val("len(sli[1:])", 1),
+	Val("len(sli[2:])", 0),
 	// TODO(austin) Test len of map
 	CErr("len(0)", opTypes),
 	CErr("len(i)", opTypes),