Use fish -c string ... as the string fallback.

We can again drop the prompt_pwd check.

share/config.fish

@@ -26,8 +26,6 @@ if status --is-interactive
 		# When executed on fish 2.2.0, the `else' block after this would
 		# force on 24-bit mode due to changes to in test behavior.
 		# These "XXX nostring" hacks were added for 2.3.1
-
-		# Let's explain to the user what's going on. 
 		set_color --bold
 		echo "You appear to be trying to launch an old fish binary with newer scripts "
 		echo "installed into" (set_color --underline)"$__fish_datadir" 

share/functions/prompt_pwd.fish

@@ -5,11 +5,6 @@ function prompt_pwd --description "Print the current working directory, shortene
             __fish_print_help prompt_pwd
             return 0
     end
-    if set -q $__is_launched_without_string
-        # There is no `string' builtin - do something sensible. XXX nostring
-        echo $PWD
-        return 0
-    end
 
     # This allows overriding fish_prompt_pwd_dir_length from the outside (global or universal) without leaking it
     set -q fish_prompt_pwd_dir_length

share/functions/string.fish

@@ -1,5 +1,5 @@
+# XXX nostring
 if not contains string (builtin -n)
-	# XXX nostring
 	function string
 		if not set -q __is_launched_without_string
 			if status --is-interactive
@@ -14,6 +14,10 @@ if not contains string (builtin -n)
 				set -g __is_launched_without_string 1
 			end
 	    end
-	    return 127
+	    # We hope that in $__fish_bin_path is a newer fish that can do `string` for us.
+	    
+	    set fish_user_paths $__fish_bin_dir $fish_user_paths
+	    set string_cmd string \'$argv\'
+	    fish -c "$string_cmd"
 	end                                                                  
 end