Simplify altrep.hpp as much as possible

Likely still need to keep it around for compatibility reasons

Author: DavisVaughan

inst/include/cpp11/R.hpp

@@ -27,7 +27,6 @@
 // clang-format on
 
 #include <type_traits>
-#include "cpp11/altrep.hpp"
 
 #if defined(R_VERSION) && R_VERSION >= R_Version(4, 4, 0)
 // Use R's new macro

inst/include/cpp11/altrep.hpp

@@ -1,42 +1,6 @@
 #pragma once
 
-#include "Rversion.h"
-
-#if defined(R_VERSION) && R_VERSION >= R_Version(3, 5, 0)
+// It would be nice to remove this since all supported versions of R have ALTREP, but
+// some groups rely on both this `#define` and `altrep.hpp` itself existing, like arrow:
+// https://github.com/apache/arrow/blob/50f2d6e04e8323119d4dd31506827ee398d6b8e4/r/src/altrep.cpp#L27-L29
 #define HAS_ALTREP
-#endif
-
-#ifndef HAS_ALTREP
-
-#define ALTREP(x) false
-
-#define REAL_ELT(x, i) REAL(x)[i]
-#define INTEGER_ELT(x, i) INTEGER(x)[i]
-#define LOGICAL_ELT(x, i) LOGICAL(x)[i]
-#define RAW_ELT(x, i) RAW(x)[i]
-
-#define SET_REAL_ELT(x, i, val) REAL(x)[i] = val
-#define SET_INTEGER_ELT(x, i, val) INTEGER(x)[i] = val
-#define SET_LOGICAL_ELT(x, i, val) LOGICAL(x)[i] = val
-#define SET_RAW_ELT(x, i, val) RAW(x)[i] = val
-
-#define REAL_GET_REGION(...) \
-  do {                       \
-  } while (false)
-
-#define INTEGER_GET_REGION(...) \
-  do {                          \
-  } while (false)
-#endif
-
-#if !defined HAS_ALTREP || (defined(R_VERSION) && R_VERSION < R_Version(3, 6, 0))
-
-#define LOGICAL_GET_REGION(...) \
-  do {                          \
-  } while (false)
-
-#define RAW_GET_REGION(...) \
-  do {                      \
-  } while (false)
-
-#endif