[clang] Adding model for NSString length to avoid false positive npes.

Reviewed By: akotulski

Differential Revision: D4231656

fbshipit-source-id: 4201e57

infer/models/objc/src/NSString.m

@@ -12,6 +12,8 @@
 
 void __get_array_length(const UInt8);
 
+void __infer_assume(bool cond);
+
 @implementation NSString
 
 + (instancetype)stringWithUTF8String:(const char*)bytes {
@@ -57,4 +59,15 @@ void __get_array_length(const UInt8);
   self->value = format->value;
   return self;
 }
+
+- (int)length {
+  if (self == nil) {
+    return 0;
+  } else {
+    int res;
+    __infer_assume(res >= 0);
+    return res;
+  }
+}
+
 @end

infer/tests/codetoanalyze/objc/errors/Makefile

@@ -33,6 +33,7 @@ SOURCES_DEFAULT = \
   npe/Npe_with_equal_names.m \
   npe/block.m \
   npe/skip_method_with_nil_object.m \
+	npe/Nsstring_length_no_npe.m \
   procdescs/MethodCall.m \
   property/main.c \
   resource_leaks/ResourceLeakExample.m \

infer/tests/codetoanalyze/objc/errors/npe/Nsstring_length_no_npe.m

@@ -0,0 +1,13 @@
+#include <Foundation/Foundation.h>
+@interface Nsstring_length_no_npe : NSObject
+@end
+
+@implementation Nsstring_length_no_npe
+
+- (NSDictionary*)logMessage:(NSString* __nullable)message {
+  if (message.length > 0) {
+    return @{ @"key" : message }; // No NPE because of model of NSString length.
+  } else
+    return nil;
+}
+@end