Add support for feature flags

This introduces a new type features_t that exposes feature flags. The intent
is to allow a deprecation/incremental adoption path. This is not a general
purpose configuration mechanism, but instead allows for compatibility during
the transition as features are added/removed.

Each feature has a user-presentable short name and a short description. Their
values are tracked in a struct features_t.

We start with one feature stderr_nocaret, but it's not hooked up yet.

CMakeLists.txt

@@ -58,6 +58,7 @@ SET(FISH_SRCS
     src/postfork.cpp src/proc.cpp src/reader.cpp src/sanity.cpp src/screen.cpp
     src/signal.cpp src/tnode.cpp src/tokenizer.cpp src/utf8.cpp src/util.cpp
     src/wcstringutil.cpp src/wgetopt.cpp src/wildcard.cpp src/wutil.cpp
+    src/future_feature_flags.cpp
 )
 
 # Header files are just globbed.

Makefile.in

@@ -118,7 +118,8 @@ FISH_OBJS := obj/autoload.o obj/builtin.o obj/builtin_bg.o obj/builtin_bind.o ob
 	obj/parse_productions.o obj/parse_tree.o obj/parse_util.o obj/parser.o \
 	obj/parser_keywords.o obj/path.o obj/postfork.o obj/proc.o obj/reader.o \
 	obj/sanity.o obj/screen.o obj/signal.o obj/tinyexpr.o obj/tokenizer.o obj/tnode.o obj/utf8.o \
-	obj/util.o obj/wcstringutil.o obj/wgetopt.o obj/wildcard.o obj/wutil.o
+	obj/util.o obj/wcstringutil.o obj/wgetopt.o obj/wildcard.o obj/wutil.o \
+	obj/future_feature_flags.o
 
 FISH_INDENT_OBJS := obj/fish_indent.o obj/print_help.o $(FISH_OBJS)
 

src/fish_tests.cpp

@@ -45,6 +45,7 @@
 #include "expand.h"
 #include "fallback.h"  // IWYU pragma: keep
 #include "function.h"
+#include "future_feature_flags.h"
 #include "highlight.h"
 #include "history.h"
 #include "input.h"
@@ -1352,6 +1353,29 @@ static void test_utf8() {
 #endif
 }
 
+static void test_feature_flags() {
+    say(L"Testing future feature flags");
+    using ft = features_t;
+    ft f;
+    do_test(!f.test(ft::stderr_nocaret));
+    f.set(ft::stderr_nocaret, true);
+    do_test(f.test(ft::stderr_nocaret));
+    f.set(ft::stderr_nocaret, false);
+    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) {
+        counts[md.flag]++;
+    }
+    for (size_t c : counts) {
+        do_test(c == 1);
+    }
+    do_test(ft::metadata[ft::stderr_nocaret].name == wcstring(L"stderr-nocaret"));
+    do_test(ft::metadata_for(L"stderr-nocaret") == &ft::metadata[ft::stderr_nocaret]);
+    do_test(ft::metadata_for(L"not-a-flag") == nullptr);
+}
+
 static void test_escape_sequences() {
     say(L"Testing escape_sequences");
     if (escape_code_length(L"") != 0) err(L"test_escape_sequences failed on line %d\n", __LINE__);
@@ -4611,6 +4635,7 @@ int main(int argc, char **argv) {
     if (should_test_function("cancellation")) test_cancellation();
     if (should_test_function("indents")) test_indents();
     if (should_test_function("utf8")) test_utf8();
+    if (should_test_function("feature_flags")) test_feature_flags();
     if (should_test_function("escape_sequences")) test_escape_sequences();
     if (should_test_function("lru")) test_lru();
     if (should_test_function("expand")) test_expand();

src/future_feature_flags.cpp

@@ -0,0 +1,23 @@
+#include "config.h"  // IWYU pragma: keep
+
+#include <wchar.h>
+#include "future_feature_flags.h"
+
+/// The set of features applying to this instance.
+static features_t global_features;
+
+const features_t &fish_features() { return global_features; }
+
+features_t &mutable_fish_features() { return global_features; }
+
+const features_t::metadata_t features_t::metadata[features_t::flag_count] = {
+    {stderr_nocaret, L"stderr-nocaret", L"3.0", L"^ no longer redirects stderr"},
+};
+
+const struct features_t::metadata_t *features_t::metadata_for(const wchar_t *name) {
+    assert(name && "null flag name");
+    for (const auto &md : metadata) {
+        if (!wcscmp(name, md.name)) return &md;
+    }
+    return nullptr;
+}

src/future_feature_flags.h

@@ -0,0 +1,63 @@
+// Flags to enable upcoming features
+#ifndef FISH_FUTURE_FEATURE_FLAGS_H
+#define FISH_FUTURE_FEATURE_FLAGS_H
+
+#include <assert.h>
+
+class features_t {
+public:
+    /// The list of flags.
+    enum flag_t {
+        /// Whether ^ is supported for stderr redirection.
+        stderr_nocaret,
+
+        /// The number of flags.
+        flag_count
+    };
+
+    /// Return whether a flag is set.
+    bool test(flag_t f) const {
+        assert(f >= 0 && f < flag_count && "Invalid flag");
+        return values[f];
+    }
+
+    /// Set a flag.
+    void set(flag_t f, bool value) {
+        assert(f >= 0 && f < flag_count && "Invalid flag");
+        values[f] = value;
+    }
+
+    /// Metadata about feature flags.
+    struct metadata_t {
+        /// The flag itself.
+        features_t::flag_t flag;
+
+        /// User-presentable short name of the feature flag.
+        const wchar_t *name;
+
+        /// Comma-separated list of feature groups.
+        const wchar_t *groups;
+
+        /// User-presentable description of the feature flag.
+        const wchar_t *description;
+    };
+
+    /// The metadata, indexed by flag.
+    static const metadata_t metadata[flag_count];
+
+    /// Return the metadata for a particular name, or nullptr if not found.
+    static const struct metadata_t *metadata_for(const wchar_t *name);
+
+   private:
+    /// Values for the flags.
+    bool values[flag_count] = {};
+};
+
+/// Return the global set of features for fish. This is const to prevent accidental mutation.
+const features_t &fish_features();
+
+/// Return the global set of features for fish, but mutable. In general fish features should be set
+/// at startup only.
+features_t &mutable_fish_features();
+
+#endif