feat(fish): Enable left and right transience (#4204)

* feat(fish): Enable left and right transience

* Update README.md

* Mention limitation

2 files changed, 79 insertions, 2 deletions

diff --git a/docs/advanced-config/README.md b/docs/advanced-config/README.md
index e030cadf3..9718e94d9 100644
--- a/docs/advanced-config/README.md
+++ b/docs/advanced-config/README.md
@@ -71,6 +71,41 @@ end
 load(io.popen('starship init cmd'):read("*a"))()
 ```
 
+## TransientPrompt and TransientRightPrompt in Fish
+
+It is possible to replace the previous-printed prompt with a custom string. This
+is useful in cases where all the prompt information is not always needed. To enable
+this, run `enable_transience` in the shell session. To make it permanent, put
+this statement in your `~/.config/fish/config.fish`. Transience can be disabled on-the-fly with
+`disable_transience`.
+
+Note that in case of Fish, the transient prompt is only printed if the commandline is non-empty,
+and syntactically correct.
+
+- By default, the left side of input gets replaced with a bold-green `❯`. To customize this,
+  define a new function called `starship_transient_prompt_func`. For example, to
+  display Starship's `character` module here, you would do
+
+```fish
+function starship_transient_prompt_func
+  starship module character
+end
+starship init fish | source
+enable_transience
+```
+
+- By default, the right side of input is empty. To customize this, define a new
+  function called `starship_transient_rprompt_func`. For example, to display
+  the time at which the last command was started here, you would do
+
+```fish
+function starship_transient_rprompt_func
+  starship module time
+end
+starship init fish | source
+enable_transience
+```
+
 ## Custom pre-prompt and pre-execution Commands in Cmd
 
 Clink provides extremely flexible APIs to run pre-prompt and pre-exec commands
diff --git a/src/init/starship.fish b/src/init/starship.fish
index 569968eba..b427ec243 100644
--- a/src/init/starship.fish
+++ b/src/init/starship.fish
@@ -10,7 +10,18 @@ function fish_prompt
     # Account for changes in variable name between v2.7 and v3.0
     set STARSHIP_DURATION "$CMD_DURATION$cmd_duration"
     set STARSHIP_JOBS (count (jobs -p))
-    ::STARSHIP:: prompt --terminal-width="$COLUMNS" --status=$STARSHIP_CMD_STATUS --pipestatus="$STARSHIP_CMD_PIPESTATUS" --keymap=$STARSHIP_KEYMAP --cmd-duration=$STARSHIP_DURATION --jobs=$STARSHIP_JOBS
+    if test "$TRANSIENT" = "1"
+        # Clear from cursor to end of screen as `commandline -f repaint` does not do this
+        # See https://github.com/fish-shell/fish-shell/issues/8418
+        printf \e\[0J
+        if type -q starship_transient_prompt_func
+            starship_transient_prompt_func
+        else
+            printf "\e[1;32m❯\e[0m "
+        end
+    else
+        ::STARSHIP:: prompt --terminal-width="$COLUMNS" --status=$STARSHIP_CMD_STATUS --pipestatus="$STARSHIP_CMD_PIPESTATUS" --keymap=$STARSHIP_KEYMAP --cmd-duration=$STARSHIP_DURATION --jobs=$STARSHIP_JOBS
+    end
 end
 
 function fish_right_prompt
@@ -25,7 +36,15 @@ function fish_right_prompt
     # Account for changes in variable name between v2.7 and v3.0
     set STARSHIP_DURATION "$CMD_DURATION$cmd_duration"
     set STARSHIP_JOBS (count (jobs -p))
-    ::STARSHIP:: prompt --right --terminal-width="$COLUMNS" --status=$STARSHIP_CMD_STATUS --pipestatus="$STARSHIP_CMD_PIPESTATUS" --keymap=$STARSHIP_KEYMAP --cmd-duration=$STARSHIP_DURATION --jobs=$STARSHIP_JOBS
+    if test "$TRANSIENT" = "1"
+        if type -q starship_transient_rprompt_func
+            starship_transient_rprompt_func
+        else
+            printf ""
+        end
+    else
+        ::STARSHIP:: prompt --right --terminal-width="$COLUMNS" --status=$STARSHIP_CMD_STATUS --pipestatus="$STARSHIP_CMD_PIPESTATUS" --keymap=$STARSHIP_KEYMAP --cmd-duration=$STARSHIP_DURATION --jobs=$STARSHIP_JOBS
+    end
 end
 
 # Disable virtualenv prompt, it breaks starship
@@ -36,6 +55,29 @@ builtin functions -e fish_mode_prompt
 
 set -gx STARSHIP_SHELL "fish"
 
+# Transience related functions
+function reset-transient --on-event fish_postexec
+    set -g TRANSIENT 0
+end
+
+function transient_execute
+    if commandline --is-valid
+        set -g TRANSIENT 1
+        commandline -f repaint
+    else
+        set -g TRANSIENT 0
+    end
+    commandline -f execute
+end
+
+function enable_transience
+    bind \r transient_execute
+end
+
+function disable_transience
+    bind \r execute
+end
+
 # Set up the session key that will be used to store logs
 # We don't use `random [min] [max]` because it is unavailable in older versions of fish shell
 set -gx STARSHIP_SESSION_KEY (string sub -s1 -l16 (random)(random)(random)(random)(random)0000000000000000)