proc: Short circuit evaluation of && and || like go does

Change evaluation of binary operators so that both && and || only
evaluate their second argument conditionally, like go does.

_fixtures/testvariables2.go

@@ -244,6 +244,8 @@ func main() {
 
 	longstr := "very long string 0123456789a0123456789b0123456789c0123456789d0123456789e0123456789f0123456789g012345678h90123456789i0123456789j0123456789"
 
+	var nilstruct *astruct = nil
+
 	var amb1 = 1
 	runtime.Breakpoint()
 	for amb1 := 0; amb1 < 10; amb1++ {
@@ -251,5 +253,5 @@ func main() {
 	}
 
 	runtime.Breakpoint()
-	fmt.Println(i1, i2, i3, p1, amb1, s1, s3, a1, p2, p3, s2, as1, str1, f1, fn1, fn2, nilslice, nilptr, ch1, chnil, m1, mnil, m2, m3, up1, i4, i5, i6, err1, err2, errnil, iface1, iface2, ifacenil, arr1, parr, cpx1, const1, iface3, iface4, recursive1, recursive1.x, iface5, iface2fn1, iface2fn2, bencharr, benchparr, mapinf, mainMenu, b, b2, sd, anonstruct1, anonstruct2, anoniface1, anonfunc, mapanonstruct1, ifacearr, efacearr, ni8, ni16, ni32, pinf, ninf, nan, zsvmap, zsslice, zsvar, tm, errtypednil, emptyslice, emptymap, byteslice, runeslice, longstr)
+	fmt.Println(i1, i2, i3, p1, amb1, s1, s3, a1, p2, p3, s2, as1, str1, f1, fn1, fn2, nilslice, nilptr, ch1, chnil, m1, mnil, m2, m3, up1, i4, i5, i6, err1, err2, errnil, iface1, iface2, ifacenil, arr1, parr, cpx1, const1, iface3, iface4, recursive1, recursive1.x, iface5, iface2fn1, iface2fn2, bencharr, benchparr, mapinf, mainMenu, b, b2, sd, anonstruct1, anonstruct2, anoniface1, anonfunc, mapanonstruct1, ifacearr, efacearr, ni8, ni16, ni32, pinf, ninf, nan, zsvmap, zsslice, zsvar, tm, errtypednil, emptyslice, emptymap, byteslice, runeslice, longstr, nilstruct)
 }

pkg/proc/eval.go

@@ -931,19 +931,28 @@ func (scope *EvalScope) evalBinary(node *ast.BinaryExpr) (*Variable, error) {
 	if err != nil {
 		return nil, err
 	}
+	xv.loadValue(loadFullValue)
+	if xv.Unreadable != nil {
+		return nil, xv.Unreadable
+	}
+
+	// short circuits logical operators
+	switch node.Op {
+	case token.LAND:
+		if !constant.BoolVal(xv.Value) {
+			return newConstant(xv.Value, xv.mem), nil
+		}
+	case token.LOR:
+		if constant.BoolVal(xv.Value) {
+			return newConstant(xv.Value, xv.mem), nil
+		}
+	}
 
 	yv, err := scope.evalAST(node.Y)
 	if err != nil {
 		return nil, err
 	}
-
-	xv.loadValue(loadFullValue)
 	yv.loadValue(loadFullValue)
-
-	if xv.Unreadable != nil {
-		return nil, xv.Unreadable
-	}
-
 	if yv.Unreadable != nil {
 		return nil, yv.Unreadable
 	}

service/test/variables_test.go

@@ -735,6 +735,10 @@ func TestEvalExpression(t *testing.T) {
 		{"ch1.dataqsiz", false, "10", "10", "uint", nil},
 		{"ch1.buf", false, `*[10]int [1,4,3,2,0,0,0,0,0,0]`, `(*[10]int)(…`, "*[10]int", nil},
 		{"ch1.buf[0]", false, "1", "1", "int", nil},
+
+		// shortcircuited logical operators
+		{"nilstruct != nil && nilstruct.A == 1", false, "false", "false", "", nil},
+		{"nilstruct == nil || nilstruct.A == 1", false, "true", "true", "", nil},
 	}
 
 	ver, _ := goversion.Parse(runtime.Version())