Add status subcomannds: features and test-feature

This teaches the status command to work with features.
'status features' will show a table listing all known features and whether
they are currently on or off.
`status test-feature` will test an individual feature, setting the exit status to
0 if the feature is on, 1 if off, 2 if unknown.

src/builtin_status.cpp

@@ -10,6 +10,7 @@
 #include "builtin_status.h"
 #include "common.h"
 #include "fallback.h"  // IWYU pragma: keep
+#include "future_feature_flags.h"
 #include "io.h"
 #include "parser.h"
 #include "proc.h"
@@ -31,6 +32,8 @@ enum status_cmd_t {
     STATUS_LINE_NUMBER,
     STATUS_SET_JOB_CONTROL,
     STATUS_STACK_TRACE,
+    STATUS_FEATURES,
+    STATUS_TEST_FEATURE,
     STATUS_UNDEF
 };
 
@@ -40,6 +43,7 @@ const enum_map<status_cmd_t> status_enum_map[] = {
     {STATUS_FILENAME, L"current-filename"},
     {STATUS_FUNCTION, L"current-function"},
     {STATUS_LINE_NUMBER, L"current-line-number"},
+    {STATUS_FEATURES, L"features"},
     {STATUS_FILENAME, L"filename"},
     {STATUS_FUNCTION, L"function"},
     {STATUS_IS_BLOCK, L"is-block"},
@@ -54,6 +58,7 @@ const enum_map<status_cmd_t> status_enum_map[] = {
     {STATUS_LINE_NUMBER, L"line-number"},
     {STATUS_STACK_TRACE, L"print-stack-trace"},
     {STATUS_STACK_TRACE, L"stack-trace"},
+    {STATUS_TEST_FEATURE, L"test-feature"},
     {STATUS_UNDEF, NULL}};
 #define status_enum_map_len (sizeof status_enum_map / sizeof *status_enum_map)
 
@@ -66,6 +71,9 @@ const enum_map<status_cmd_t> status_enum_map[] = {
         break;                                                                              \
     }
 
+/// Values that may be returned from the test-feature option to status.
+enum { TEST_FEATURE_ON, TEST_FEATURE_OFF, TEST_FEATURE_NOT_RECOGNIZED };
+
 int job_control_str_to_mode(const wchar_t *mode, wchar_t *cmd, io_streams_t &streams) {
     if (wcscmp(mode, L"full") == 0) {
         return JOB_CONTROL_ALL;
@@ -82,6 +90,7 @@ struct status_cmd_opts_t {
     bool print_help = false;
     int level = 1;
     int new_job_control_mode = -1;
+    const wchar_t *feature_name;
     status_cmd_t status_cmd = STATUS_UNDEF;
 };
 
@@ -126,6 +135,15 @@ static bool set_status_cmd(wchar_t *const cmd, status_cmd_opts_t &opts, status_c
     return true;
 }
 
+/// Print the features and their values.
+static void print_features(io_streams_t &streams) {
+    for (const auto &md : features_t::metadata) {
+        int set = fish_features().test(md.flag);
+        streams.out.append_format(L"%ls\t%s\t%ls\t%ls\n", md.name, set ? "on" : "off", md.groups,
+                                  md.description);
+    }
+}
+
 static int parse_cmd_opts(status_cmd_opts_t &opts, int *optind,  //!OCLINT(high ncss method)
                           int argc, wchar_t **argv, parser_t &parser, io_streams_t &streams) {
     wchar_t *cmd = argv[0];
@@ -307,6 +325,24 @@ int builtin_status(parser_t &parser, io_streams_t &streams, wchar_t **argv) {
             job_control_mode = opts.new_job_control_mode;
             break;
         }
+        case STATUS_FEATURES: {
+            print_features(streams);
+            break;
+        }
+        case STATUS_TEST_FEATURE: {
+            if (args.size() != 1) {
+                const wchar_t *subcmd_str = enum_to_str(opts.status_cmd, status_enum_map);
+                streams.err.append_format(BUILTIN_ERR_ARG_COUNT2, cmd, subcmd_str, 1, args.size());
+                return STATUS_INVALID_ARGS;
+            }
+            const auto *metadata = features_t::metadata_for(args.front().c_str());
+            if (!metadata) {
+                retval = TEST_FEATURE_NOT_RECOGNIZED;
+            } else {
+                retval = fish_features().test(metadata->flag) ? TEST_FEATURE_ON : TEST_FEATURE_OFF;
+            }
+            break;
+        }
         case STATUS_FILENAME: {
             CHECK_FOR_UNEXPECTED_STATUS_ARGS(opts.status_cmd)
             const wchar_t *fn = parser.current_filename();

tests/status.err

@@ -6,3 +6,6 @@ status: Invalid combination of options,
 you cannot do both 'is-block' and 'is-interactive' in the same invocation
 status: Invalid job control mode 'full1'
 status: Invalid job control mode '1none'
+
+####################
+# Future Feature Flags

tests/status.in

@@ -48,4 +48,9 @@ function test_function
 end
 
 test_function
-eval test_function
+eval test_function
+
+logmsg Future Feature Flags
+status features
+status test-feature stderr-nocaret ; echo $status
+status test-feature not-a-feature ; echo $status

tests/status.out

@@ -1,3 +1,9 @@
 Not a function
 test_function
 test_function
+
+####################
+# Future Feature Flags
+stderr-nocaret	off	3.0	^ no longer redirects stderr
+1
+2