Work around absent `string' in old fishies upgrading.

Improves experience during upgrades, accidentally running
an old fish with a new environment. No errors just from
printing a prompt. Fixes #3057.

Print helpful notice also when launching mismatched fish.

Autoloadable string.fish -- only create function if not builtin.

share/config.fish

@@ -18,8 +18,23 @@ function __fish_default_command_not_found_handler
 end
 
 if status --is-interactive
+	# Existance of string is a good pre-2.3.0 check. Could also check $FISH_VERSION in the future.
+	# This is a "launch", not an issue caused by autoloading during upgrades.
+	if not contains "string" (builtin -n)
+		# the string.fish message to `exec` will probably not help here, so this will that.
+		set -g __is_launched_without_string 1
+
+		set_color --bold
+		echo "You appear to be trying to launch an old fish binary with newer scripts "
+		echo "installed into" (set_color --underline)"$__fish_datadir"
+		set_color normal
+		echo -e "\nThis is an unsupported configuration.\n"
+		set_color yellow
+		echo "You may need to uninstall and reinstall fish!"
+		set_color normal
+	else 
 		# Enable truecolor/24-bit support for select terminals
-	if not set -q NVIM_LISTEN_ADDRESS # Neovim will swallow the 24bit sequences, rendering text white
+		if not set -q NVIM_LISTEN_ADDRESS # (Neovim will swallow the 24bit sequences, rendering text white)
 			and begin
 				set -q KONSOLE_PROFILE_NAME # KDE's konsole
 				or string match -q -- "*:*" $ITERM_SESSION_ID # Supporting versions of iTerm2 will include a colon here
@@ -30,6 +45,7 @@ if status --is-interactive
 			# Only set it if it isn't to allow override by setting to 0
 			set -q fish_term24bit; or set -g fish_term24bit 1
 		end
+	end
 else
 	# Hook up the default as the principal command_not_found handler
 	# in case we are not interactive

share/functions/__fish_urlencode.fish

@@ -1,4 +1,8 @@
 function __fish_urlencode --description "URL-encode stdin"
+	if not type -q string
+		return 1
+	end
+	
 	set -l join ''
 	set -l chars
 	# Set locale to C and IFS to "" in order to split a line into bytes.

share/functions/prompt_pwd.fish

@@ -5,6 +5,12 @@ function prompt_pwd --description "Print the current working directory, shortene
 			return 0
 	end
 
+	# If we don't have a string builtin, we have no hope of maniuplating $PWD - just output it as-is.
+	if not type -q string
+		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; or set -l fish_prompt_pwd_dir_length 1
 

share/functions/string.fish

@@ -0,0 +1,16 @@
+if not contains string (builtin -n)
+	function string
+		if not set -q __is_launched_without_string
+			if status --is-interactive
+				# We've been autoloaded after fish < 2.3.0 upgraded to >= 2.3.1 - no string builtin
+				set_color --bold
+				echo "Fish has been upgraded, and the scripts on your system are not compatible"
+				echo "with this prior instance of fish. You can probably run:"
+				set_color green
+				echo "\n exec fish"
+				set_color normal
+				echo "… to replace this process with a new one in-place."
+			end
+	     end
+	end                                                                  
+end