implement the fish_breakpoint_prompt function

This is another step to resolving issue #1310. It makes
`fish_breakpoint_prompt` a replacement for `fish_prompt` if it is defined
and we're presenting a prompt in the context of a `breakpoint` command.

doc_src/fish_breakpoint_prompt.txt

@@ -0,0 +1,30 @@
+\section fish_breakpoint_prompt fish_breakpoint_prompt - define the appearance of the command line prompt when in the context of a `breakpoint` command
+
+\subsection fish_breakpoint_prompt-synopsis Synopsis
+\fish{synopsis}
+function fish_breakpoint_prompt
+    ...
+end
+\endfish
+
+\subsection fish_breakpoint_prompt-description Description
+
+By defining the `fish_breakpoint_prompt` function, the user can choose a custom prompt when asking for input in response to a `breakpoint` command. The `fish_breakpoint_prompt` function is executed when the prompt is to be shown, and the output is used as a prompt.
+
+The exit status of commands within `fish_breakpoint_prompt` will not modify the value of <a href="index.html#variables-status">$status</a> outside of the `fish_breakpoint_prompt` function.
+
+`fish` ships with a default version of this function that displays the function name and line number of the current execution context.
+
+
+\subsection fish_breakpoint_prompt-example Example
+
+A simple prompt that is a simplified version of the default debugging prompt:
+
+\fish
+function fish_breakpoint_prompt -d "Write out the debug prompt"
+    set -l function (status current-function)
+    set -l line (status current-line-number)
+    set -l prompt "$function:$line >"
+    echo -ns (set_color $fish_color_status) "BP $prompt" (set_color normal) ' '
+end
+\endfish

share/functions/fish_breakpoint_prompt.fish

@@ -0,0 +1,23 @@
+# Define the default debugging prompt command.
+function fish_breakpoint_prompt --description "A right prompt to be used when `breakpoint` is executed"
+    set -l saved_status $status
+    set -l function (status current-function)
+    set -l line (status current-line-number)
+    # At the moment we don't include the filename because, even if we truncate it, it makes the
+    # prompt too long.
+    #set -l filename (status current-filename)
+    #set -l prompt "$filename:$function:$line >"
+    set -l prompt "$function:$line"
+    if test $saved_status -ne 0
+        set prompt "$prompt [!$saved_status]"
+    end
+    set prompt "$prompt > "
+
+    # Make sure the prompt doesn't consume more than half the terminal width.
+    set -l max_len (math "$COLUMNS / 2")
+    if test (string length -- $prompt) -gt $max_len
+        set prompt ...(string sub -s -(math $max_len - 3) -- $prompt)
+    end
+
+    echo -ns (set_color $fish_color_status) "BP $prompt" (set_color normal) ' '
+end

src/reader.cpp

@@ -92,6 +92,9 @@
 /// The name of the function that prints the fish right prompt (RPROMPT).
 #define RIGHT_PROMPT_FUNCTION_NAME L"fish_right_prompt"
 
+/// The name of the function to use in place of the left prompt if we're in the debugger context.
+#define DEBUG_PROMPT_FUNCTION_NAME L"fish_breakpoint_prompt"
+
 /// The name of the function for getting the input mode indicator.
 #define MODE_PROMPT_FUNCTION_NAME L"fish_mode_prompt"
 
@@ -2251,15 +2254,20 @@ static int read_i(void) {
 
     while ((!data->end_loop) && (!sanity_check())) {
         event_fire_generic(L"fish_prompt");
-        if (function_exists(LEFT_PROMPT_FUNCTION_NAME))
-            reader_set_left_prompt(LEFT_PROMPT_FUNCTION_NAME);
-        else
-            reader_set_left_prompt(DEFAULT_PROMPT);
 
-        if (function_exists(RIGHT_PROMPT_FUNCTION_NAME))
+        if (is_breakpoint && function_exists(DEBUG_PROMPT_FUNCTION_NAME)) {
+            reader_set_left_prompt(DEBUG_PROMPT_FUNCTION_NAME);
+        } else if (function_exists(LEFT_PROMPT_FUNCTION_NAME)) {
+            reader_set_left_prompt(LEFT_PROMPT_FUNCTION_NAME);
+        } else {
+            reader_set_left_prompt(DEFAULT_PROMPT);
+        }
+
+        if (function_exists(RIGHT_PROMPT_FUNCTION_NAME)) {
             reader_set_right_prompt(RIGHT_PROMPT_FUNCTION_NAME);
-        else
+        } else {
             reader_set_right_prompt(L"");
+        }
 
         // Put buff in temporary string and clear buff, so that we can handle a call to
         // reader_set_buffer during evaluation.
@@ -2287,6 +2295,7 @@ static int read_i(void) {
             }
         }
     }
+
     reader_pop();
     return 0;
 }