Testing patch for `cpp` in Nixpkgs that removes store paths from the __FILE__ macro

cpp-file-macro.patch

diff --git i/libcpp/macro.c w/libcpp/macro.c
index c2515534504..ab593251def 100644
--- i/libcpp/macro.c
+++ w/libcpp/macro.c
@@ -288,8 +288,8 @@ _cpp_builtin_macro_text (cpp_reader *pfile, cpp_hashnode *node,
     case BT_FILE:
     case BT_BASE_FILE:
       {
-	unsigned int len;
-	const char *name;
+	unsigned int len, storelen, i;
+	const char *name, *nix_store;
 	uchar *buf;
 	
 	if (node->value.builtin == BT_FILE)
@@ -302,9 +302,34 @@ _cpp_builtin_macro_text (cpp_reader *pfile, cpp_hashnode *node,
 	      abort ();
 	  }
 	len = strlen (name);
+
 	buf = _cpp_unaligned_alloc (pfile, len * 2 + 3);
 	result = buf;
 	*buf = '"';
+
+        /* If the name starts with the Nix store path, then replace it with a
+           fake path. Nix store paths look like this:
+               $NIX_STORE/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-foobar
+
+           So, if the string is longer than strlen("$NIX_STORE") + 33
+           characters, we replace the middle hash with the fake, all-`e` hash.
+        */
+        nix_store = getenv ("NIX_STORE");
+        if (nix_store)
+          {
+            storelen = strlen (nix_store);
+            if (len > storelen + 33 &&
+                strncmp (nix_store, name, strlen(nix_store)) == 0)
+              {
+                buf = cpp_quote_string (buf + 1, (const unsigned char *) nix_store, storelen);
+                *buf++ = '/';
+                for (i = 0; i < 32; i++)
+                  *buf++ = 'e';
+
+                name += storelen + 33;
+              }
+          }
+
 	buf = cpp_quote_string (buf + 1, (const unsigned char *) name, len);
 	*buf++ = '"';
 	*buf = '\0';