Add a fix for Symbol#inspect on 3.0, 3.1 (#361)

* Handle `Symbol#inspect` for Ruby `<3.2.0`
* Extract dedicated `Symbol` emitter
* Disable mutant on `Symbol#dispatch` as it depends on Ruby version and would fail on CI.
* Improve symbol detection logic
* Replace `regexp` check with `parse`

lib/unparser/emitter/primitive.rb

@@ -10,7 +10,7 @@ class Primitive < self
       # Emitter for primitives based on Object#inspect
       class Inspect < self
 
-        handle :sym, :str
+        handle :str
 
       private
 
@@ -20,6 +20,32 @@ def dispatch
 
       end # Inspect
 
+      class Symbol < self
+
+        handle :sym
+
+        private
+
+        # mutant:disable
+        def dispatch
+          if inspect_breaks_parsing?
+            write(":#{value.name.inspect}")
+          else
+            write(value.inspect)
+          end
+        end
+
+        # mutant:disable
+        def inspect_breaks_parsing?
+          return false unless RUBY_VERSION < '3.2.'
+
+          Unparser.parse(value.inspect)
+          false
+        rescue Parser::SyntaxError
+          true
+        end
+      end # Symbol
+
       # Emitter for complex literals
       class Complex < self
 

spec/unit/unparser_spec.rb

@@ -390,6 +390,11 @@ def noop
         "false"
       end
     RUBY
+
+    # Test Symbol#inspect Ruby bug: https://bugs.ruby-lang.org/issues/18905
+    assert_source(':"@="')
+    assert_source(':"$$$$="')
+    assert_source(':"8 >="')
   end
 
   describe 'corpus' do