cgo: add support for C.CString and related functions

testdata/cgo/main.c

@@ -24,6 +24,8 @@ int cflagsConstant = SOME_CONSTANT;
 
 int smallEnumWidth = sizeof(option2_t);
 
+char globalChars[] = {2, 0, 4, 8};
+
 int fortytwo() {
 	return 42;
 }

testdata/cgo/main.go

@@ -135,6 +135,21 @@ func main() {
 	//   void arraydecay(int *buf1, int *buf2[8], int *buf3[7][2]);
 	C.arraydecay((*C.int)(nil), (*[8]C.int)(nil), (*[7][2]C.int)(nil))
 
+	// Test CGo builtins like C.CString.
+	cstr := C.CString("string passed to C")
+	println("cstr length:", C.strlen(cstr))
+	gostr := C.GoString(cstr)
+	println("C.CString:", gostr)
+	charBuf := C.GoBytes(unsafe.Pointer(&C.globalChars[0]), 4)
+	println("C.charBuf:", charBuf[0], charBuf[1], charBuf[2], charBuf[3])
+	binaryString := C.GoStringN(&C.globalChars[0], 4)
+	println("C.CStringN:", len(binaryString), binaryString[0], binaryString[1], binaryString[2], binaryString[3])
+
+	// Test whether those builtins also work with zero length data.
+	println("C.GoString(nil):", C.GoString(nil))
+	println("len(C.GoStringN(nil, 0)):", len(C.GoStringN(nil, 0)))
+	println("len(C.GoBytes(nil, 0)):", len(C.GoBytes(nil, 0)))
+
 	// libc: test whether C functions work at all.
 	buf1 := []byte("foobar\x00")
 	buf2 := make([]byte, len(buf1))

testdata/cgo/main.h

@@ -141,6 +141,8 @@ extern int smallEnumWidth;
 
 extern int cflagsConstant;
 
+extern char globalChars[4];
+
 // test duplicate definitions
 int add(int a, int b);
 extern int global;

testdata/cgo/out.txt

@@ -61,5 +61,12 @@ option 2A: 20
 option 3A: 21
 enum width matches: true
 CFLAGS value: 17
+cstr length: 18
+C.CString: string passed to C
+C.charBuf: 2 0 4 8
+C.CStringN: 4 2 0 4 8
+C.GoString(nil): 
+len(C.GoStringN(nil, 0)): 0
+len(C.GoBytes(nil, 0)): 0
 copied string: foobar
 line written using C puts