Allow turning off autosuggestions

This adds a variable, $fish_autosuggestion_enabled.

When set to 0, it will turn off autosuggestions/highlighting.
Setting it to anything else will enable it (which also
means this remains enabled by default).

src/env_dispatch.cpp

@@ -235,6 +235,14 @@ static void handle_fish_history_change(const env_stack_t &vars) {
     reader_change_history(history_session_id(vars));
 }
 
+static void handle_autosuggestion_change(const env_stack_t &vars) {
+    bool enabled = true;
+    if (auto val = vars.get(L"fish_autosuggestion_enabled")) {
+        if (val->as_string() == L"0") enabled = false;
+    }
+    reader_set_autosuggestion_enabled(enabled);
+}
+
 static void handle_function_path_change(const env_stack_t &vars) {
     UNUSED(vars);
     function_invalidate_path();
@@ -340,6 +348,7 @@ static std::unique_ptr<const var_dispatch_table_t> create_dispatch_table() {
     var_dispatch_table->add(L"fish_function_path", handle_function_path_change);
     var_dispatch_table->add(L"fish_read_limit", handle_read_limit_change);
     var_dispatch_table->add(L"fish_history", handle_fish_history_change);
+    var_dispatch_table->add(L"fish_autosuggestion_enabled", handle_autosuggestion_change);
     var_dispatch_table->add(L"TZ", handle_tz_change);
     var_dispatch_table->add(L"fish_use_posix_spawn", handle_fish_use_posix_spawn_change);
     var_dispatch_table->add(L"fish_trace", handle_fish_trace);

src/reader.cpp

@@ -603,7 +603,7 @@ struct layout_data_t {
 class reader_data_t : public std::enable_shared_from_this<reader_data_t> {
    public:
     /// Configuration for the reader.
-    const reader_config_t conf;
+    reader_config_t conf;
     /// The parser being used.
     std::shared_ptr<parser_t> parser_ref;
     /// String containing the whole current commandline.
@@ -2612,6 +2612,18 @@ void reader_change_history(const wcstring &name) {
     }
 }
 
+void reader_set_autosuggestion_enabled(bool enable) {
+    // We don't need to _change_ if we're not initialized yet.
+    reader_data_t *data = current_data_or_null();
+    if (data) {
+        if (data->conf.autosuggest_ok != enable) {
+            data->conf.autosuggest_ok = enable;
+            data->force_exec_prompt_and_repaint = true;
+            data->inputter.queue_char(readline_cmd_t::repaint);
+        }
+    }
+}
+
 /// Add a new reader to the reader stack.
 /// \return a shared pointer to it.
 static std::shared_ptr<reader_data_t> reader_push_ret(parser_t &parser,

src/reader.h

@@ -149,6 +149,9 @@ void restore_term_mode();
 /// Change the history file for the current command reading context.
 void reader_change_history(const wcstring &name);
 
+/// Enable or disable autosuggestions.
+void reader_set_autosuggestion_enabled(bool enable);
+
 /// Write the title to the titlebar. This function is called just before a new application starts
 /// executing and just after it finishes.
 ///