Make prompts forward compatible with fish 3.1.2 by passing locally exported variable

Commit 5d135d555 (prompts: fix pipestatus for jobs prefixed with "not")
introduced a backwards compatibility hack about adding an optional argument
to __fish_print_pipestatus. This hack would break downgrading to fish 3.1.2
if the user copied the new prompt to their config - they would get a backtrace
on every prompt which is arguably worse than the patch's minor improvement.

This does away with the error trace - old fish just won't show the fancy
new pipestatus on `not true`.

Implemented by passing the last $status as the poor man's kwarg, which works
since 3.1.0 (9b86d5dd1 Export all local exported variables in a new scope).

The prompts don't work with fish 3.0.0 or older; downgrading does not seem
too important in general but I think this patch is an okay simplification.

share/functions/__fish_print_pipestatus.fish

@@ -1,8 +1,7 @@
 function __fish_print_pipestatus --description "Print pipestatus for prompt"
-    # take $status as optional argument to maintain compatibility
     set -l last_status
-    if set last_status (string match -r -- '^\d+$' $argv[1])
-        set -e argv[1]
+    if set -q __fish_last_status
+        set last_status $__fish_last_status
     else
         set last_status $argv[-1] # default to $pipestatus[-1]
     end

share/functions/fish_prompt.fish

@@ -3,7 +3,7 @@
 
 function fish_prompt --description 'Write out the prompt'
     set -l last_pipestatus $pipestatus
-    set -l last_status $status
+    set -lx __fish_last_status $status # Export for __fish_print_pipestatus.
     set -l normal (set_color normal)
 
     # Color the prompt differently when we're root
@@ -30,7 +30,7 @@ function fish_prompt --description 'Write out the prompt'
         set bold_flag
     end
     set __fish_prompt_status_generation $status_generation
-    set -l prompt_status (__fish_print_pipestatus $last_status "[" "]" "|" (set_color $fish_color_status) (set_color $bold_flag $fish_color_status) $last_pipestatus)
+    set -l prompt_status (__fish_print_pipestatus "[" "]" "|" (set_color $fish_color_status) (set_color $bold_flag $fish_color_status) $last_pipestatus)
 
     echo -n -s (set_color $fish_color_user) "$USER" $normal @ (set_color $color_host) (prompt_hostname) $normal ' ' (set_color $color_cwd) (prompt_pwd) $normal (fish_vcs_prompt) $normal " "$prompt_status $suffix " "
 end

share/tools/web_config/sample_prompts/classic_status.fish

@@ -4,7 +4,7 @@
 function fish_prompt --description "Write out the prompt"
     # Save our status
     set -l last_pipestatus $pipestatus
-    set -l last_status $status
+    set -lx __fish_last_status $status # Export for __fish_print_pipestatus.
 
     set -l color_cwd
     set -l suffix
@@ -23,6 +23,6 @@ function fish_prompt --description "Write out the prompt"
     end
 
     echo -n -s "$USER" @ (prompt_hostname) ' ' (set_color $color_cwd) (prompt_pwd) \
-        " "(__fish_print_pipestatus $last_status "[" "]" "|" (set_color $fish_color_status) (set_color --bold $fish_color_status) $last_pipestatus) \
+        " "(__fish_print_pipestatus "[" "]" "|" (set_color $fish_color_status) (set_color --bold $fish_color_status) $last_pipestatus) \
         (set_color normal) "$suffix "
 end

share/tools/web_config/sample_prompts/classic_vcs.fish

@@ -3,7 +3,7 @@
 
 function fish_prompt --description 'Write out the prompt'
     set -l last_pipestatus $pipestatus
-    set -l last_status $status
+    set -lx __fish_last_status $status # Export for __fish_print_pipestatus.
     set -l normal (set_color normal)
 
     # Color the prompt differently when we're root
@@ -30,7 +30,7 @@ function fish_prompt --description 'Write out the prompt'
         set bold_flag
     end
     set __fish_prompt_status_generation $status_generation
-    set -l prompt_status (__fish_print_pipestatus $last_status "[" "]" "|" (set_color $fish_color_status) (set_color $bold_flag $fish_color_status) $last_pipestatus)
+    set -l prompt_status (__fish_print_pipestatus "[" "]" "|" (set_color $fish_color_status) (set_color $bold_flag $fish_color_status) $last_pipestatus)
 
     echo -n -s (set_color $fish_color_user) "$USER" $normal @ (set_color $color_host) (prompt_hostname) $normal ' ' (set_color $color_cwd) (prompt_pwd) $normal (fish_vcs_prompt) $normal " "$prompt_status $suffix " "
 end

share/tools/web_config/sample_prompts/informative.fish

@@ -4,7 +4,7 @@
 function fish_prompt --description 'Informative prompt'
     #Save the return status of the previous command
     set -l last_pipestatus $pipestatus
-    set -l last_status $status
+    set -lx __fish_last_status $status # Export for __fish_print_pipestatus.
 
     if functions -q fish_is_root_user; and fish_is_root_user
         printf '%s@%s %s%s%s# ' $USER (prompt_hostname) (set -q fish_color_cwd_root
@@ -12,7 +12,7 @@ function fish_prompt --description 'Informative prompt'
                                                          or set_color $fish_color_cwd) \
             (prompt_pwd) (set_color normal)
     else
-        set -l pipestatus_string (__fish_print_pipestatus $last_status "[" "] " "|" (set_color $fish_color_status) \
+        set -l pipestatus_string (__fish_print_pipestatus "[" "] " "|" (set_color $fish_color_status) \
                                   (set_color --bold $fish_color_status) $last_pipestatus)
 
         printf '[%s] %s%s@%s %s%s %s%s%s \f\r> ' (date "+%H:%M:%S") (set_color brblue) \

share/tools/web_config/sample_prompts/informative_vcs.fish

@@ -3,7 +3,7 @@
 
 function fish_prompt --description 'Write out the prompt'
     set -l last_pipestatus $pipestatus
-    set -l last_status $status
+    set -lx __fish_last_status $status # Export for __fish_print_pipestatus.
 
     if not set -q __fish_git_prompt_show_informative_status
         set -g __fish_git_prompt_show_informative_status 1
@@ -78,7 +78,7 @@ function fish_prompt --description 'Write out the prompt'
 
     printf '%s ' (fish_vcs_prompt)
 
-    set -l pipestatus_string (__fish_print_pipestatus $last_status "[" "] " "|" (set_color $fish_color_status) (set_color --bold $fish_color_status) $last_pipestatus)
+    set -l pipestatus_string (__fish_print_pipestatus "[" "] " "|" (set_color $fish_color_status) (set_color --bold $fish_color_status) $last_pipestatus)
     echo -n $pipestatus_string
     set_color normal