Add self-insert-notfirst readline command

This adds a new readline command self-insert-notfirst, which is
analogous to self-insert, except that it does nothing if the cursor
is at the beginning. This will serve as a higher-performance implementation
for stripping leading spaces on paste.

doc_src/cmds/bind.rst

@@ -162,6 +162,10 @@ The following special input functions are available:
 
 - ``repaint-mode`` reexecutes the fish_mode_prompt function and redraws the prompt. This is useful for vi-mode. If no fish_mode_prompt exists, it acts like a normal repaint.
 
+- ``self-insert``, inserts the matching sequence into the command line
+
+- ``self-insert-notfirst``, inserts the matching sequence into the command line, unless the cursor is at the beginning
+
 - ``suppress-autosuggestion``, remove the current autosuggestion
 
 - ``swap-selection-start-stop``, go to the other end of the highlighted text without changing the selection

src/input.cpp

@@ -117,6 +117,7 @@ static const input_function_metadata_t input_function_metadata[] = {
     {readline_cmd_t::history_token_search_backward, L"history-token-search-backward"},
     {readline_cmd_t::history_token_search_forward, L"history-token-search-forward"},
     {readline_cmd_t::self_insert, L"self-insert"},
+    {readline_cmd_t::self_insert_notfirst, L"self-insert-notfirst"},
     {readline_cmd_t::transpose_chars, L"transpose-chars"},
     {readline_cmd_t::transpose_words, L"transpose-words"},
     {readline_cmd_t::upcase_word, L"upcase-word"},
@@ -497,7 +498,8 @@ char_event_t inputter_t::readch(bool allow_commands) {
 
         if (evt.is_readline()) {
             switch (evt.get_readline()) {
-                case readline_cmd_t::self_insert: {
+                case readline_cmd_t::self_insert:
+                case readline_cmd_t::self_insert_notfirst: {
                     // Typically self-insert is generated by the generic (empty) binding.
                     // However if it is generated by a real sequence, then insert that sequence.
                     for (auto iter = evt.seq.crbegin(); iter != evt.seq.crend(); ++iter) {
@@ -505,7 +507,13 @@ char_event_t inputter_t::readch(bool allow_commands) {
                     }
                     // Issue #1595: ensure we only insert characters, not readline functions. The
                     // common case is that this will be empty.
-                    return read_characters_no_readline();
+                    char_event_t res = read_characters_no_readline();
+
+                    // Hackish: mark the input style.
+                    res.input_style = evt.get_readline() == readline_cmd_t::self_insert_notfirst
+                                          ? char_event_t::style_notfirst
+                                          : char_event_t::style_normal;
+                    return res;
                 }
                 case readline_cmd_t::func_and: {
                     if (function_status_) {

src/input_common.h

@@ -42,6 +42,7 @@ enum class readline_cmd_t {
     history_token_search_backward,
     history_token_search_forward,
     self_insert,
+    self_insert_notfirst,
     transpose_chars,
     transpose_words,
     upcase_word,
@@ -111,6 +112,17 @@ class char_event_t {
     /// The type of event.
     char_event_type_t type;
 
+    /// Hackish: the input style, which describes how char events (only) are applied to the command
+    /// line. Note this is set only after applying bindings; it is not set from readb().
+    enum input_style_t : uint8_t {
+        // Insert characters normally.
+        style_normal,
+
+        // Insert characters only if the cursor is not at the beginning. Otherwise, discard them.
+        style_notfirst,
+    };
+    input_style_t input_style{style_normal};
+
     /// The sequence of characters in the input mapping which generated this event.
     /// Note that the generic self-insert case does not have any characters, so this would be empty.
     wcstring seq{};

src/reader.cpp

@@ -1180,6 +1180,7 @@ static bool command_ends_paging(readline_cmd_t c, bool focused_on_search_field)
         case rl::backward_kill_path_component:
         case rl::backward_kill_bigword:
         case rl::self_insert:
+        case rl::self_insert_notfirst:
         case rl::transpose_chars:
         case rl::transpose_words:
         case rl::upcase_word:
@@ -3373,12 +3374,11 @@ void reader_data_t::handle_readline_command(readline_cmd_t c, readline_loop_stat
             }
             break;
         }
-            // Some commands should have been handled internally by input_readch().
-        case rl::self_insert: {
-            DIE("self-insert should have been handled by inputter_t::readch");
-        }
+            // Some commands should have been handled internally by inputter_t::readch().
+        case rl::self_insert:
+        case rl::self_insert_notfirst:
         case rl::func_and: {
-            DIE("self-insert should have been handled by inputter_t::readch");
+            DIE("should have been handled by inputter_t::readch");
         }
     }
 }