Code review snippets to address from @davidben

Also:
* sus_if_msvc for attributes should be sus_if_msvc_not_cl
  * "language is msvc variant" vs "compiler is msvc"
  * sus_if_gnuc_language
* test wrapping for each integer size, esp < 32bit
* not destroy nevervalue? since it's in a union can that be a
  requirement on NeverValueTypes, and is C++ okay with constructing
  an object in a place where one already exists and was not destroyed?

sus/CMakeLists.txt

@@ -131,7 +131,6 @@ target_sources(subspace PUBLIC
     "iter/zip.h"
     "macros/__private/compiler_bugs.h"
     "macros/assume.h"
-    "macros/arch.h"
     "macros/builtin.h"
     "macros/compiler.h"
     "macros/eval_macro.h"

sus/macros/pure.h

@@ -21,7 +21,7 @@
 ///
 /// A pure function is allowed to dereference pointers, and access global
 /// memory, but it may not change them. To do so can cause Undefined Behaviour.
-#define sus_pure sus_if_msvc_else([[nodiscard]], __attribute__((pure)))
+#define sus_pure [[nodiscard]] sus_if_not_msvc_compiler(, __attribute__((pure)))
 
 /// Used to mark a function as "const", meaning it does not change any values
 /// outside of its own scope, and does not read global memory.
@@ -31,4 +31,4 @@
 /// A const function is allowed to only read from its inputs and determine an
 /// output from them, without going through pointers or accessing global memory.
 /// To do so anyway can cause Undefined Behaviour.
-#define sus_pure_const sus_if_msvc_else([[nodiscard]], __attribute__((const)))
+#define sus_pure_const [[nodiscard]] sus_if_msvc_else(, __attribute__((const)))