simple/s1009: improve diagnostic message

We made the message significantly worse in
5de5bd3 by printing something like "len
for []int is defined as zero", which is wrong and not helpful. This
change restores the original message, "len for nil slices is defined as
zero".

simple/s1009/s1009.go

@@ -157,13 +157,17 @@ func run(pass *analysis.Pass) (interface{}, error) {
 		// finally check that xx type is one of array, slice, map or chan
 		// this is to prevent false positive in case if xx is a pointer to an array
 		typ := pass.TypesInfo.TypeOf(xx)
+		var nilType string
 		ok = typeutil.All(typ, func(term *types.Term) bool {
 			switch term.Type().Underlying().(type) {
 			case *types.Slice:
+				nilType = "nil slices"
 				return true
 			case *types.Map:
+				nilType = "nil maps"
 				return true
 			case *types.Chan:
+				nilType = "nil channels"
 				return true
 			case *types.Pointer:
 				return false
@@ -178,7 +182,7 @@ func run(pass *analysis.Pass) (interface{}, error) {
 			return
 		}
 
-		report.Report(pass, expr, fmt.Sprintf("should omit nil check; len() for %s is defined as zero", typ), report.FilterGenerated())
+		report.Report(pass, expr, fmt.Sprintf("should omit nil check; len() for %s is defined as zero", nilType), report.FilterGenerated())
 	}
 	code.Preorder(pass, fn, (*ast.BinaryExpr)(nil))
 	return nil, nil

simple/s1009/testdata/go1.0/CheckRedundantNilCheckWithLen/nil-len.go

@@ -6,11 +6,11 @@ func fn() {
 	var m map[int]int
 	var ch chan int
 
-	if s == nil || len(s) == 0 { //@ diag(`should omit nil check`)
+	if s == nil || len(s) == 0 { //@ diag(re`should omit nil check.+for nil slices`)
 	}
-	if m == nil || len(m) == 0 { //@ diag(`should omit nil check`)
+	if m == nil || len(m) == 0 { //@ diag(re`should omit nil check.+for nil maps`)
 	}
-	if ch == nil || len(ch) == 0 { //@ diag(`should omit nil check`)
+	if ch == nil || len(ch) == 0 { //@ diag(re`should omit nil check.+for nil channels`)
 	}
 
 	if s != nil && len(s) != 0 { //@ diag(`should omit nil check`)