[clang-tidy] Add a little checker for Twine locals in LLVM.

Those often cause use after free bugs and should be generally avoided.
Technically it is safe to have a Twine with >=2 components in a variable
but I don't think it is a good pattern to follow. The almost trivial checker
comes with elaborated fix-it hints that turn the Twine into a std::string
if necessary and otherwise fall back to the original type if the Twine
is created from a single value.

llvm-svn: 212535

1 files changed, 35 insertions, 0 deletions

diff --git a/clang-tools-extra/test/clang-tidy/llvm-twine-local.cpp b/clang-tools-extra/test/clang-tidy/llvm-twine-local.cpp
new file mode 100644
index 00000000000..0f134a5500a
--- /dev/null
+++ b/clang-tools-extra/test/clang-tidy/llvm-twine-local.cpp
@@ -0,0 +1,35 @@
+// RUN: grep -Ev "// *[A-Z-]+:" %s > %t.cpp
+// RUN: clang-tidy %t.cpp -checks='-*,llvm-twine-local' -fix -- > %t.msg 2>&1
+// RUN: FileCheck -input-file=%t.cpp %s
+// RUN: FileCheck -input-file=%t.msg -check-prefix=CHECK-MESSAGES %s
+
+namespace llvm {
+class Twine {
+public:
+  Twine(const char *);
+  Twine(int);
+  Twine &operator+(const Twine &);
+};
+}
+
+using namespace llvm;
+
+void foo(const Twine &x);
+
+static Twine Moo = Twine("bark") + "bah";
+// CHECK-MASSAGES: twine variables are prone to use after free bugs
+// CHECK-MESSAGES: note: FIX-IT applied suggested code changes
+// CHECK: static std::string Moo = (Twine("bark") + "bah").str();
+
+int main() {
+  const Twine t = Twine("a") + "b" + Twine(42);
+// CHECK-MASSAGES: twine variables are prone to use after free bugs
+// CHECK-MESSAGES: note: FIX-IT applied suggested code changes
+// CHECK: std::string t = (Twine("a") + "b" + Twine(42)).str();
+  foo(Twine("a") + "b");
+
+  Twine Prefix = false ? "__INT_FAST" : "__UINT_FAST";
+// CHECK-MASSAGES: twine variables are prone to use after free bugs
+// CHECK-MESSAGES: note: FIX-IT applied suggested code changes
+// CHECK: const char * Prefix = false ? "__INT_FAST" : "__UINT_FAST";
+}