Allow setting feature flags on the command line

This introduces a new command line option --features which can be used for
enabling or disabling features for a particular fish session.

Examples:
  fish --features stderr-nocaret
  fish --features 3.0,no-stderr-nocaret
  fish --features all

Note that the feature set cannot be changed in an existing session.

src/fish.cpp

@@ -44,6 +44,7 @@ Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA
 #include "fallback.h"  // IWYU pragma: keep
 #include "fish_version.h"
 #include "function.h"
+#include "future_feature_flags.h"
 #include "history.h"
 #include "io.h"
 #include "parser.h"
@@ -61,6 +62,8 @@ Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA
 // container to hold the options specified within the command line
 class fish_cmd_opts_t {
    public:
+    // Future feature flags values string
+    wcstring features;
     // Commands to be executed in place of interactive shell.
     std::vector<std::string> batch_cmds;
     // Commands to execute after the shell's config has been read.
@@ -238,9 +241,10 @@ int run_command_list(std::vector<std::string> *cmds, const io_chain_t &io) {
 
 /// Parse the argument list, return the index of the first non-flag arguments.
 static int fish_parse_opt(int argc, char **argv, fish_cmd_opts_t *opts) {
-    static const char *short_opts = "+hilnvc:C:p:d:D:";
+    static const char *short_opts = "+hilnvc:C:p:d:f:D:";
     static const struct option long_opts[] = {{"command", required_argument, NULL, 'c'},
                                               {"init-command", required_argument, NULL, 'C'},
+                                              {"features", required_argument, NULL, 'f'},
                                               {"debug-level", required_argument, NULL, 'd'},
                                               {"debug-stack-frames", required_argument, NULL, 'D'},
                                               {"interactive", no_argument, NULL, 'i'},
@@ -277,6 +281,10 @@ static int fish_parse_opt(int argc, char **argv, fish_cmd_opts_t *opts) {
                 }
                 break;
             }
+            case 'f': {
+                opts->features = str2wcstring(optarg);
+                break;
+            }
             case 'h': {
                 opts->batch_cmds.push_back("__fish_print_help fish");
                 break;
@@ -375,6 +383,8 @@ int main(int argc, char **argv) {
 
     const struct config_paths_t paths = determine_config_directory_paths(argv[0]);
     env_init(&paths);
+    // Set features early in case other initialization depends on them.
+    mutable_fish_features().set_from_string(opts.features);
     proc_init();
     builtin_init();
     misc_init();

src/fish_tests.cpp

@@ -1363,6 +1363,11 @@ static void test_feature_flags() {
     f.set(ft::stderr_nocaret, false);
     do_test(!f.test(ft::stderr_nocaret));
 
+    f.set_from_string(L"stderr-nocaret,nonsense");
+    do_test(f.test(ft::stderr_nocaret));
+    f.set_from_string(L"stderr-nocaret,no-stderr-nocaret,nonsense");
+    do_test(!f.test(ft::stderr_nocaret));
+
     // Ensure every metadata is represented once.
     size_t counts[ft::flag_count] = {};
     for (const auto &md : ft::metadata) {

src/future_feature_flags.cpp

@@ -21,3 +21,37 @@ const struct features_t::metadata_t *features_t::metadata_for(const wchar_t *nam
     }
     return nullptr;
 }
+
+void features_t::set_from_string(const wcstring &str) {
+    wcstring_list_t entries = split_string(str, L',');
+    const wchar_t *whitespace = L"\t\n\v\f\r ";
+    for (wcstring entry : entries) {
+        if (entry.empty()) continue;
+
+        // Trim leading and trailing whitespace
+        entry.erase(0, entry.find_first_not_of(whitespace));
+        entry.erase(entry.find_last_not_of(whitespace) + 1);
+
+        const wchar_t *name = entry.c_str();
+        bool value = true;
+        // A "no-" prefix inverts the sense.
+        if (string_prefixes_string(L"no-", name)) {
+            value = false;
+            name += 3;  // wcslen(L"no-")
+        }
+        // Look for a feature with this name. If we don't find it, assume it's a group name and set
+        // all features whose group contain it. Do nothing even if the string is unrecognized; this
+        // is to allow uniform invocations of fish (e.g. disable a feature that is only present in
+        // future versions).
+        // The special name 'all' may be used for those who like to live on the edge.
+        if (const metadata_t *md = metadata_for(name)) {
+            this->set(md->flag, value);
+        } else {
+            for (const metadata_t &md : metadata) {
+                if (wcsstr(md.groups, name) || !wcscmp(name, L"all")) {
+                    this->set(md.flag, value);
+                }
+            }
+        }
+    }
+}

src/future_feature_flags.h

@@ -3,6 +3,9 @@
 #define FISH_FUTURE_FEATURE_FLAGS_H
 
 #include <assert.h>
+#include <unordered_map>
+
+#include "common.h"
 
 class features_t {
 public:
@@ -27,6 +30,12 @@ public:
         values[f] = value;
     }
 
+    /// Parses a comma-separated feature-flag string, updating ourselves with the values.
+    /// Feature names or group names may be prefixed with "no-" to disable them.
+    /// The special group name "all" may be used for those who like to live on the edge.
+    /// Unknown features are silently ignored.
+    void set_from_string(const wcstring &str);
+
     /// Metadata about feature flags.
     struct metadata_t {
         /// The flag itself.

tests/invocation/features-nocaret1.invoke

@@ -0,0 +1 @@
+--features 'no-stderr-nocaret' -c 'status test-feature stderr-nocaret; echo nocaret: $status'

tests/invocation/features-nocaret1.out

@@ -0,0 +1 @@
+nocaret: 1

tests/invocation/features-nocaret2.invoke

@@ -0,0 +1 @@
+--features    'stderr-nocaret' -c 'status test-feature stderr-nocaret; echo nocaret: $status'

tests/invocation/features-nocaret2.out

@@ -0,0 +1 @@
+nocaret: 0