S1017: guard against redefnitions of the len builtin

(cherry picked from commit 5bc9f4e)

Author: dominikh

simple/lint.go

@@ -942,7 +942,7 @@ func CheckTrim(pass *analysis.Pass) (interface{}, error) {
 		if !ok {
 			return false
 		}
-		if fn, ok := call.Fun.(*ast.Ident); !ok || fn.Name != "len" {
+		if !code.IsCallTo(pass, call, "len") {
 			return false
 		}
 		if len(call.Args) != 1 {
@@ -1059,7 +1059,7 @@ func CheckTrim(pass *analysis.Pass) (interface{}, error) {
 				if fun != "HasPrefix" {
 					return
 				}
-				if fn, ok := index.Fun.(*ast.Ident); !ok || fn.Name != "len" {
+				if !code.IsCallTo(pass, index, "len") {
 					return
 				}
 				if len(index.Args) != 1 {

simple/testdata/src/trim/trim.go

@@ -139,3 +139,17 @@ func fn() {
 		println(id1)
 	}
 }
+
+func fn3() {
+	const s1 = "a string value"
+
+	var id1 = "a string value"
+	len := func(string) int { return 0 } // don't accept non-builtin definition of len
+	if strings.HasPrefix(id1, s1) {
+		id1 = id1[len(s1):]
+	}
+
+	if strings.HasSuffix(id1, s1) {
+		id1 = id1[:len(id1)-len(s1)]
+	}
+}