Setup SyntaxSuggest as default gem

Adds the `syntax_suggest` syntax error display tool to Ruby through the same mechanism as `error_highlight` and `did_you_mean`. Reference ticket: https://bugs.ruby-lang.org/issues/18159

close #4845

## What is syntax_suggest?

When a syntax error is raised by requiring a file, dead_end will use a combination of indentation and lexing to identify the problem.

> Note: Previously this tool was named `dead_end`. 

## Known issues

- SyntaxSearch's approach of showing syntax errors only works through integration with `require`, `load`, `autoload`, and `require_relative` (since it monkeypatches them to detect syntax errors). It does not work with direct Ruby file invocations https://github.com/zombocom/dead_end/issues/31.
  - This causes failure in the test suite (test_expected_backtrace_location_when_inheriting_from_basic_object_and_including_kernel) and confusion when inspecting backtraces if there's a different error when trying to require a file such as measuring memory (https://github.com/zombocom/syntax_suggest/issues/124#issuecomment-1006705016).
  - Discussed fix. We previously talked about opening up `SyntaxError` to be monkeypatched in the same way that other gems hook into `NoMethodError`. This is currently not possible and requires development work. When we last talked about it at RubyKaigi Nobu expressed an ability to make such a change.

1 files changed, 6 insertions, 0 deletions

diff --git a/ruby.c b/ruby.c
index 66feeb797e..26d263a1b1 100644
--- a/ruby.c
+++ b/ruby.c
@@ -94,6 +94,8 @@ void rb_warning_category_update(unsigned int mask, unsigned int bits);
     SEP \
     X(did_you_mean) \
     SEP \
+    X(syntax_suggest) \
+    SEP \
     X(rubyopt) \
     SEP \
     X(frozen_string_literal) \
@@ -307,6 +309,7 @@ usage(const char *name, int help, int highlight, int columns)
         M("gems",    "",        "rubygems (only for debugging, default: "DEFAULT_RUBYGEMS_ENABLED")"),
         M("error_highlight", "", "error_highlight (default: "DEFAULT_RUBYGEMS_ENABLED")"),
         M("did_you_mean", "",   "did_you_mean (default: "DEFAULT_RUBYGEMS_ENABLED")"),
+        M("syntax_suggest", "", "syntax_suggest (default: "DEFAULT_RUBYGEMS_ENABLED")"),
         M("rubyopt", "",        "RUBYOPT environment variable (default: enabled)"),
         M("frozen-string-literal", "", "freeze all string literals (default: disabled)"),
 #if USE_MJIT
@@ -1553,6 +1556,9 @@ ruby_opt_init(ruby_cmdline_options_t *opt)
         if (opt->features.set & FEATURE_BIT(did_you_mean)) {
             rb_define_module("DidYouMean");
         }
+        if (opt->features.set & FEATURE_BIT(syntax_suggest)) {
+            rb_define_module("SyntaxSuggest");
+        }
     }
 
     rb_warning_category_update(opt->warn.mask, opt->warn.set);