Cache if tracing is enabled

Like the comment said: That var lookup was kind of expensive.

So we simply use variable dispatch like we do for countless other things.

src/env_dispatch.cpp

@@ -53,6 +53,7 @@
 #include "reader.h"
 #include "screen.h"
 #include "termsize.h"
+#include "trace.h"
 #include "wutil.h"  // IWYU pragma: keep
 
 #define DEFAULT_TERM1 "ansi"
@@ -311,6 +312,10 @@ static void handle_read_limit_change(const environment_t &vars) {
     }
 }
 
+static void handle_fish_trace(const environment_t &vars) {
+    trace_set_enabled(!vars.get(L"fish_trace").missing_or_empty());
+}
+
 /// Populate the dispatch table used by `env_dispatch_var_change()` to efficiently call the
 /// appropriate function to handle a change to a variable.
 /// Note this returns a new-allocated value that we expect to leak.
@@ -337,6 +342,7 @@ static std::unique_ptr<const var_dispatch_table_t> create_dispatch_table() {
     var_dispatch_table->add(L"fish_history", handle_fish_history_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);
 
     // This std::move is required to avoid a build error on old versions of libc++ (#5801),
     // but it causes a different warning under newer versions of GCC (observed under GCC 9.3.0,
@@ -359,6 +365,7 @@ static void run_inits(const environment_t &vars) {
     update_wait_on_escape_ms(vars);
     handle_read_limit_change(vars);
     handle_fish_use_posix_spawn_change(vars);
+    handle_fish_trace(vars);
 }
 
 /// Updates our idea of whether we support term256 and term24bit (see issue #10222).

src/trace.cpp

@@ -6,13 +6,16 @@
 #include "flog.h"
 #include "parser.h"
 
-static const wcstring VAR_fish_trace = L"fish_trace";
+static bool do_trace = false;
+
+void trace_set_enabled(bool do_enable) {
+    do_trace = do_enable;
+}
 
 bool trace_enabled(const parser_t &parser) {
     const auto &ld = parser.libdata();
     if (ld.suppress_fish_trace) return false;
-    // TODO: this variable lookup is somewhat expensive, consider how to make this cheaper.
-    return !parser.vars().get(VAR_fish_trace).missing_or_empty();
+    return do_trace;
 }
 
 /// Trace an "argv": a list of arguments where the first is the command.

src/trace.h

@@ -16,6 +16,9 @@ void trace_argv(const parser_t &parser, const wchar_t *command, const wcstring_l
 /// \return whether tracing is enabled.
 bool trace_enabled(const parser_t &parser);
 
+/// Enable or disable tracing.
+void trace_set_enabled(bool do_enable);
+
 /// Convenience helper to trace a single string if tracing is enabled.
 void trace_if_enabled(const parser_t &parser, const wchar_t *command,
                       const wcstring_list_t &argv = {});